Manufacturing Methods for (U-Zr)N-Fuels

Technische Universität München Physics Department Chair E21

KTH Royal Institute of Technology

Reactor Physics division

Manufacturing methods for (U-Zr)N-fuels

Diploma thesis by

Tobias Hollmer

16 May 2011 I hereby declare that I have created this thesis independently, by using only the named sources and tools.

(place, date, signature) Abstract

In this work a manufacturing method for UN, ZrN and (U, Zr)N pellets was established at the nuclear fuel laboratory at KTH Stockholm/Sweden, which consists of the production of nitride powders and their sintering into pellets by spark plasma sintering. The nitride powders were produced by the hydriding-nitriding route using pure metal as starting material. This synthesis was performed in a stream of the particular reaction gas. A synthesis control and monitoring system was developed, which can follow the reactions in real time by measuring the gas flow difference before and after the reaction chamber. With the help of this system the hydriding and nitriding reactions of uranium and zirconium were studied in detail. Fine nitride powders were obtained; however, the production of zirconium nitride involved one milling step of the brittle zirconium hydride. Additionally uranium and zirconium alloys with different zirconium contents were produced and synthesized to nitride powders. It was found that also the alloys could be reduced to fine powder, but only by cyclic hydriding-dehydriding. Pellets were sintered out of uranium nitrides, zirconium nitrides, mixed nitrides and alloy nitrides. These experiments showed that relative densities of more than 90% can easily be achieved for all those powders. Pellets sintered from mechanically mixed nitride powders were found to still consist of two separate nitride phases, while nitride produced from alloy was demonstrated to be a monophasic solid solution both as powder and as sintered pellets.

3 This diploma thesis was created at the reactor physics division of KTH Stock- holm/Sweden within the framework of a study of physics at TU München. Contents

1 Introduction 7 1.1 Nitride fuels ...... 8 1.2 Fabrication of nitrides ...... 11 1.3 Our research and this thesis ...... 12

2 Nitride Synthesis 15 2.1 Background ...... 15 2.2 Set-up ...... 16 2.2.1 Testing ...... 19 2.3 Production of UN ...... 22 2.3.1 Starting material ...... 22 2.3.2 Synthesis ...... 23 2.3.3 Product ...... 25 2.4 Production of ZrN ...... 29 2.4.1 Starting material ...... 29 2.4.2 Synthesis ...... 29 2.5 Production of (U,Zr) alloy nitrides ...... 35 2.5.1 Motivation ...... 35 2.5.2 The U-Zr system ...... 35 2.5.3 Equipment ...... 36 2.5.4 Alloying ...... 38 2.5.5 Synthesis ...... 40 2.6 Summary ...... 50

3 Sintering 53 3.1 Background ...... 53 3.2 Experiments ...... 56 3.2.1 Zirconium nitride ...... 56 3.2.2 Uranium nitride ...... 60 3.2.3 Mixed nitrides ...... 63 3.2.4 Alloy nitrides ...... 67 3.3 Summary ...... 70

4 Conclusions and Outlook 71

1 Introduction

Facing the global climate change and dwindling resources, a sustainable energy supply is one of the major concerns of our days. Nuclear power can contribute to providing carbon dioxide neutral electricity; however, it faces the problems of limited 235U resources and the growing amount of nuclear waste. By now 442 nuclear power plants are operating [18], producing approximately 12000 tonnes of spent fuel every year. [13]

This waste consists on the one hand of fission products and on the other hand of minor actinides, which are produced by neutron capture reactions. These mi- nor actinides are mainly neptunium, americium, curium and californium. They are the primary contributors to the radiotoxicity of the spent fuel, since they have considerably longer halflifes than the fission products. A way of reducing this radiotoxicity is transmutation, where these minor actinides are separated from spent fuel and reused in a second fuel cycle. So they can be fissioned to less radiotoxic fission products and in addition the remaining energy of these nuclei can be utilised. Figure 1.1 shows a comparison of the ingestion radiotoxicity of used fuel before and after partitioning and transmutation (P&T), furthermore it also displays the radiotoxicity of the fission products and natural uranium ore. One approach to transmute minor actinides is the usage of an inert fuel matrix. Using depleted uranium oxide as a matrix, like it is done in today’s oxide fuels, would result in a production of more minor actinides while transmuting the previous ones. For this reason a matrix of light elements is favorable. Zirconium nitride shows quite promising properties as inert matrix, such as high thermal conductivity and a crystal structure which is compatible to plutonium nitride and uranium nitride.

Next to the capability to burn plutonium and minor actinides, another objective in the research of Generation IV fuels is to achieve a high breeding performance. Breeding describes the neutron capture process, which transforms non-ﬁssile materials like 238U to ﬁssile ones like 239P u, which can then be reprocessed and used for the production of new fuels. The main breeding reaction of 238U for example is: − − 238 1 239 β 239 β 239 92 U +0 n −→92 U −→ 93 Np −→ 94 P u Using this process in breeder reactors would tremendously enlarge the available

7 Partitioning and transmutation scenarios appropriate parallels with a repository. These studies The Inchtuthil Roman nails.6 The most northerly are called natural analogues. In the event that P&T is fortress in the Roman Empire at Inchtuthil in Perth- introduced, the timescales over which the waste must shire, Scotland had to be abandoned hastily in 87 ad. be isolated from the biosphere are much reduced and In an attempt to hide metal objects which could be one can have much greater conﬁdence in the engi- used for weapons, the Romans buried over one neered barriers by studying societal analogues—that million nails in a 5 m deep pit and covered them with is, society-built structures which have withstood the 3 m of compacted earth. These nails were discovered test of time over a couple of thousand years. in the 1950s. It was found that the outermost nails were badly corroded and had formed a solid iron oxide crust. The innermost nails, however, showed 5 Natural and societal analogues only very limited corrosion. This was attributed to the There are many radioactive materials which occur fact that the outer nails removed the oxygen from the naturally and can be found in rocks, sediments, etc. In inﬁltrating groundwaters such that by the time they particular, uranium, which is the main component in came into contact with the innerlying nails the waters nuclear fuel, occurs in nature. By studying the distri- were less corrosive. In the same way, the large bution in nature, information can be obtained on the volumes of iron in waste canisters are expected to movement of uranium in rocks and groundwaters. maintain chemically reducing conditions in an envir- Natural analogues provide a way of informing the onment which might otherwise become oxygen-rich wider public on the principles on which repositories due to the radiolytic decomposition of water. are built, without using complex mathematical demonstrations of safety and risk. One of the concepts The Kronan cannon.7 The Kronan was a Swedish which can be presented using analogues is the very warship built in 1668 and which sank in 1676 during slow degradation of materials over thousands of the Battle of O¨ land. One of the bronze cannons on years. Some notable analogues are discussed below. board the Kronan had remained partly buried in a (See also Fig. 2.) vertical position, muzzle down in clay sediments since

Dunarobba1 Introduction forest Natural and societal analogues

109 Inchtuthil Roman nails

108 Used fuel before P&T

107 The Kronan cannon

6 Oklo natural fission reactors 10

Nuclear Uranium ore reactor zones 5 Sandstone 10

Ore layer Sandstone

Granite 104 Used fuel

Ingestion radiotoxicity: Sv per ton spent fuel Ingestion radiotoxicity: after P&T Hadrian's Wall Fission The Needle's Eye products 103

102 101 102 103 104 105 106 Time: years

Figure 1.1: Ingestion radiotoxicity of spent fuel before and after P&T [22] Fig. 2. Will P&T make nuclear waste disposal publicly acceptable?. 265 resources since natural uranium consists of 99.3% 238U and just 0.7% 235U. Actually this process occurs in every nuclear reactor, but a breeder reactor is characterized REVISEby PROOFS the ability TH to produce f:/Thomas Telford/Ne/Ne42-5/NE-2222.3d more fissile material than Nuclear it consumes.Energy NE-2222 In order Page: to provide 265 KEYWORD sufficient excess neutrons, which can be used for the breeding process, most breeder reactors use a fast neutron spectrum. The ratio of the fissile material produced to the consumed fissile material is called breeding ratio. Another important parameter for the breeding performance is the doubling time, which is the required time to double the amount of fissile material. In order to achieve a high breeding ratio and a low doubling time a good neutron economy and a high linear heat rating are needed [7], which are two factors, among others, that make nitride fuels preferable over oxide fuels.

1.1 Nitride fuels

As shown in table 1.1, the melting point of uranium nitride is the highest among the possible ceramic fuels, but even more considerable is its thermal conductivity, which is at 1000 ◦C approximately eight times higher compared to oxide fuels.

8 1.1 Nitride fuels

These two factors together allow a much higher linear heat rating of approximately 700 W cm−1 compared to 450 W cm−1 in oxide fuels. Both values assume a so-called helium joint between the fuel and the cladding; with a sodium joint a heat rating of 900 W cm−1 can be achieved. [6] In respect of the neutron economy the approximately 30% higher heavy metal density is very advantageous, as well as the fact that there is just one nitrogen atom per uranium compared to two oxygen atoms. This reduces the amount of moderating atoms, and thus hardens the neutron spectrum, which improves the neutron economy in a fast reactor.

Property UO2 UN UC US UP Crystal structure Fluorite Rock.salt U density [g cm−3] 9.6 13.53 12.97 9.57 9.05 Lattice constant [Å] 5.470 4.889 4.960 5.490 5.589 Melting point [◦C] 2750±40 2850±30 2400±100 2460±30 2610±20 Thermal conductivity 5.86 17.99 19.66 12.13 12.55 at 200 ◦C [W m−1 K−1] Thermal conductivity 2.93 25.94 23.01 16.74 20.92 at 1000 ◦C [W m−1 K−1] U4O9, U2S3, U O , UN , U C , U S , U P , Other uranium phases 3 8 1.5 2 3 3 5 3 4 UO3 UN1.75 UC2 US2, UP2 etc. US3 P u2O3, P u3C2, P u3S4+x, Other plutonium phases P uO1.6, - P u2C3, P u2S3, - P uO2−x P uC2 P uS2−x

Table 1.1: Comparison between diﬀerent ceramic fuels [4]

A further advantage of nitride fuels is the simple phase system. Of plutonium nitride just one phase exists, P uN. Uranium nitride has three phases: uranium mononitride UN, uranium sesquinitride UN1.5 and UN1.75. However, UN1.5 and UN1.75 can easily be decomposed by thermal treatment. This decomposition occurs at approximately 1300 ◦C with a reasonable reaction rate. The simplicity in the phase system brings advantages in the production of the nitrides since the phase can easily be controlled and thus the product is more clearly deﬁned. Ura- nium mononitride, plutonium nitride and zirconium nitride have a rock-salt crystal structure, which means each of its two elements forms a face-centered cubic lattice as shown in ﬁgure 1.2. Due to this reason and their similar lattice constant it should be possible to form a solid solution of these compounds. The chemical compatibility of nitrides with the metal coolants sodium, lead and lead-bismuth is another advantageous characteristic making them usable in metal cooled reactor systems. [37]

9 1 Introduction

Figure 1.2: Rock-salt crystal structure [34]

Also their behavior under irradiation is quite promising. There has not been much experience so far; however, first experiments do indicate a considerably lower creep rate, radiation swelling as well as fission gas release compared to oxide fuels. Especially the release of corrosive fission gases, like cesium and iodine, is reduced. [41] Furthermore nitride fuels show a good solubility in nitric acid. This property makes it possible to recycle the spent fuel in the PUREX process. However, this advantage is very closely linked to one of the major disadvantages of nitride fuels. Under irradiation the radiotoxic nuclide 14C is formed mainly by the following reaction 14 1 14 1 7 N + 0n −→6 C + 1p

This 14C production can not be neglected taking into consideration the use of nitride fuels on an industrial scale. A solution to this problem would be the isotopic enrichment of the used nitrogen with 15N. As a result of the high costs of 15N and the impossibility to recycle all of it in the PUREX process [15], the costs of nitride fuels will be increased. Another possible way would be the isolation of the 14 C during the reprocessing as CO2 followed by a a conversion to cement. This product then could be stored as high active solid waste. Another drawback of nitrides is their strong aﬃnity to oxygen, especially in powder form. Therefore nitride powders have to be handled in protective atmosphere, which complicates their production.

10 1.2 Fabrication of nitrides

1.2 Fabrication of nitrides

There are several different ways of producing nitrides, which all have their specific benefits and downsides.

The carbothermic reduction is accepted to be suitable for industrial scale production of UN and (U, P u)N. Its major benefit is that it uses oxides as a starting materials, which have a high availability in today’s mainly oxide based fuel industry. It is represented by the following net reaction m MmOo+u + oC + N2 + uH2 −→ mMN + uH2O + oCO 2 where M is the used metal, U or (Pu,U). [40] The starting material can be in form of graphite and metal oxide powders or in form of micro spheres, which are obtained by the SOLGEL method. Due to its unfavorable thermodynamics, this reaction should be performed with a high nitrogen pressure while removing the produced carbon monoxide, in order to achieve a complete reaction. [33] The addition of hydrogen to the reaction gas is used to lower the carbon and oxygen residues from the final product. Nevertheless the carbon and oxygen levels still remain comparatively high, which, according to [41], has a negative influence on the fuels fission gas release and its compatibility with the cladding material. Technical problems of the carbothermic reduction are the high reaction temperature, approximately 1600 ◦C, and the difficulty to recover and clean the excess nitrogen. [39] The latter is necessary in order to keep the costs down in case 15N is used.

A rather new way of producing uranium nitride is the reactive milling, where UN1.5 is produced by ball-milling uranium powder in a nitrogen atmosphere. In this way high purity UN can be obtained at low costs; however, it is limited to small batches and thus merely suitable on a laboratory scale. [19]

The fabrication method used in our research is the hydride-nitride synthesis route. The starting material here is pure metal, which will be powderized by a hydriding reaction and then nitrided to the ﬁnal product. A detailed description of this process will be given in the next chapter. The reason why this method was chosen is the possibility to produce relatively large batches with a very high purity. Moreover in case of uranium as starting material the hydriding forms a ﬁne powder, wherefore no extra milling is needed. Due to this reason, and the fact that hydriding and nitriding can be performed in the same apparatus, powder handling, especially of the pyrophoric uranium hydride, is reduced to a minimum. This makes the production process easier and reduces the risk of contaminating the sample with oxygen.

11 1 Introduction

1.3 Our research and this thesis

This work is part of the Swedish research program GENIUS, whose objective it is to advance science and technology relevant for the Generation IV concepts to be demonstrated in Europe. [36] The purpose of the research at KTH fuel laboratory is to establish the production of UN as well as (U, Zr)N solid solution pellets. The experience and results gathered in this process will later be used for the establish- ment of a P uN and AmN production line at Chalmers fuel laboratory in Göteborg.

This research includes the nitride synthesis starting from uranium and zirconium metal and their processing into fuel pellets. Thereby two production procedures will be investigated. First a separate synthesis of UN and ZrN powder, which then will be mixed together and sintered and a second production route, where a U-Zr alloy will be produced and synthesized to nitride powder. Figure 1.3 visu- alizes those two production routes, where the gray boxes symbolize the synthesis procedures.

U (metal) Zr (metal) U (metal) Zr (metal)

+H2 +H2 alloying

UH3 (powder) Zr H2 (massive) U, Zr (alloy)

+N2 milling +H2

UN1.5 (powder) Zr H2 (powder) UH3, Zr H2 (powder)

+Ar +N2 +N2

UN (powder) Zr N (powder) UN1.5, Zr N (powder)

mixing +Ar

UN, Zr N (powder) UN, Zr N (powder)

SPS SPS

pellet pellet

Figure 1.3: Scheme of the pellet production

The ﬁrst part of this thesis will treat the nitride synthesis and the alloying experiments. First the installation of a real time monitoring and control system to the existing synthesis apparatus will be described. This signiﬁcant improvement gives the opportunity to automatically control the synthesis and to follow its kinetics in real time. Afterwards this apparatus was used to study the hydriding and

12 1.3 Our research and this thesis nitriding reactions in detail.

The second part will deal with the sintering of the produced powders by spark plasma sintering. Due to problems with our regular sintering furnace this alterna- tive sintering procedure was tested. It will be investigated if this technique is able to produce pellets with suitable properties for a nuclear application.

2 Nitride Synthesis

2.1 Background

The hydride-nitride synthesis route of uranium is performed in two steps. In the ﬁrst step uranium trihydride is formed from uranium metal according to following reaction :

−1 2U + 3H2 −→ 2UH3 ∆Hr = −254 kJ mol (2.1)

Uranium trihydride exists in two phases, α and β-UH3. Both have a cubic crystal structure, but the position of the atoms in the lattice is different. α-UH3 can be ◦ formed in a slow reaction at temperatures below 80 C, while β-UH3 forms rapidly at higher temperatures. Hausner et al. report a maximum reaction temperature ◦ −3 at 225 C. [16] The density of β-UH3 is 10.92 g cm and it appears as a black powder. [26] The reaction can be performed by heating the sample in a stream of hydrogen. This reaction is reversible and its reverse reaction can be achieved by keeping the sample at elevated temperatures in an inert gas or vacuum. In pure argon the maximum decomposition rate lays around 280 ◦C. [3] Using massive uranium the produced uranium trihydride falls off as powder. Thus, the end product does not need any further milling. This has the advantage that the powder can remain in the reaction chamber for the second reaction step and so the risk of contaminating the sample with oxygen is significantly reduced. Furthermore UH3 powder is pyrophoric and ignites spontaneously in air at room temperature. [21] In the second reaction step uranium nitride is formed by reacting the uranium hydride with nitrogen. There are three phases of uranium nitride UN, UN1.5 and UN1.75. This reaction can be carried out in flowing nitrogen at a temperature ◦ ◦ between 250 C and 500 C. Under this conditions UN1.5 is formed according to following reaction:

−1 4UH3 + 3N2 −→ 4UN1.5 + 6H2 ∆Hr = −1013 kJ mol (2.2)

This uranium sesquinitride is thermally unstable relative to UN, so UN1.5 can be decomposed to uranium mononitride in an inert gas at elevated temperatures:

−1 4UN1.5 −→ 4UN + N2 ∆Hr = 1879 kJ mol (2.3)

15 2 Nitride Synthesis

In this work a stream of argon and a temperature of approximately 1200 ◦C is used. Performing this reaction with a constant inﬂow of argon and thus a constant removal of the produced nitrogen should keep the decomposition temperature relatively low. In case of a nitrogen partial pressure less than 250 Pa the decomposition of the sesquinitride starts at approximately 1100 ◦C.

The synthesis of zirconium nitride is performed similar to the UN synthesis. A ﬁrst hydriding step produces a zirconium hydride:

−1 2Zr + xH2 −→ 2ZrHx ∆Hr < 0 kJ mol (2.4)

The value x can vary from 1 to 2. There are three phases, which diﬀer in crystal structure: γ (x = 1) with an orthorombic, δ (x = 1.5 − 1.65) with a cubic and (x = 1.75 − 2) with a tetragonal structure. The change in crystal structure decreases the Vickers hardness with increasing x. This makes it possible to powderize the zirconium hydride in a mill. This property is of advantage for the powder production since hydriding of zirconium unlike hydriding of uranium does not form powder. Also the density decreases from 5.9 g cm−3 (x = 1) to 5.56 g cm−3 (x = 2). Zirconium metal in comparison has a density of 6.52 g cm−3. Zirconium hydride powder has a gray dark to black color. [35] In the second reaction step, the zirconium hydride is converted to zirconium nitride by following reaction:

−1 2ZrHx + N2 −→ 2ZrN + xH2 ∆Hr < 0 kJ mol (2.5)

Zirconium nitride has a cubic crystal structure and a density of 7.09 g cm−3. It’s color is yellow-brown. In contrast to uranium, zirconium just forms one nitride phase; thus, a denitriding step is not needed in this case.

2.2 Set-up

The nitride synthesis is performed in a quartz tube, which is vertically mounted in a furnace. The quartz tube is connected to a gas mixer on the lower end, in which the diﬀerent reaction gases are merged. This gas mixer is fed by two gas lines, of which one is hydrogen and the other one can be switched between argon and nitrogen. Both gas lines are equipped with a gas cleaning system. The gas cleaning system is used to reduce the amount of impurities, mainly oxygen. This is done by conveying the gas through silica gel, ascarite and magnesium perchlorate. In case of nitrogen and argon an additional cleaning by heated magnesium granules and copper turnings is installed. The outlet on the top of the quartz tube is connected to a ﬁlter system, which retains small particles from leaving into the ventilation system.

16 2.2 Set-up

The sample is placed on a porous quartz ﬁlter by a half length of the quartz tube, in order to locate it in the heating zone of the furnace. Additionally this construc- tion can work as a ﬂuidized bed when working with powders, which enhances the gas-solid reaction. Figure 2.1 shows a schematic of the gas system and the quartz tube inside the furnace.

FM GC

N2 GM

SW GCS FC FC GCS H2

Figure 2.1: Left: Schematic of the gas system (SW: gas switch, GCS: gas cleaning system, FC: flow controller, GM: gas mixer, RC: reaction chamber, GC; gas cooling, FM: flow meter, grey area: furnace Right: Schematic of the reaction chamber, not true to scale (F: fittings, QT: quartz tube, Fu: furnace with heating coil, QF: quartz filter, S: sample .

A major disadvantage about this setup was the impossibility to observe the events in the reaction chamber, since a direct view to the sample is blocked by the surrounding furnace. This is unfavorable since uncompleted reactions cause a low quality of the end products and can even lead to a damaging of the equipment. For example an unsuccessful hydriding and nitriding would cause the uranium metal to melt at elevated temperatures in the denitriding step, what would block the quartz tube. In order to overcome this problem a gas measuring system was installed, which allows a real-time observation of the reactions in the quartz tube. The basic idea behind this gas monitoring system is the fact that the reactions (2.1) to (2.5) have a net production or respectively a net consumption of gas. This should allow to monitor these reactions by comparing the gas inﬂow into the

17 2 Nitride Synthesis reaction chamber and the outflow. For example the hydriding reaction of uranium should create a lower outflow compared to the inflow of hydrogen. In order to measure these gas flows one flow controller was installed on each feeding gas line and one flow meter on the outlet of the reaction chamber. A further improvement of this set-up was the installation of an pc interface to the furnace. This, together with the flow controllers, allows an automated control of the nitride synthesis, which means it is possible to program a sequence of temperature curves and different gas mixtures. This is realized by a Microstar DAP 1200e data acquisition hardware and a control program written in DASYlab. Figure 2.2 shows both the furnace and the control hardware. The furnace is a resistance furnace.

Figure 2.2: Synthesis Set-up .

Its heating coils are fed by a transformer which delivers maximal 50A at 48V. The power input is controlled by a thyristor in the primary loop of this transformer. In order to regulate the temperature of the furnace the thyristor is connected to the data acquisition hardware, where a software sided PID controller is regulating the thyristor output. As feedback this PID controller receives the temperature signal measured by a K-type thermocouple which is placed outside of the quartz tube in close proximity to the sample. The system can reach temperatures of approximately 1200 ◦C. As flow controllers the models Bronkhorst F-201CV-2K0 and as flow meter the model F-111B-5K0 were chosen. These flow meters measure a temperature difference along a heated capillary tube, which is placed parallel to the main flow. This temperature difference depends on the amount of heat absorbed by the gas and so it is proportional to the gas flow. Table 2.1 shows the properties of the chosen flow meters. This type of flow meters is quite insensitive to pressure and temperature changes of the inlet gas. This was necessary since the temperature and pressure of the

18 2.2 Set-up

Property F-201CV-2K0-H F-201CV-2K0-N F-111B-5K0 type controller controller meter gas H2 N2 N2 max. ﬂow [L min−1] 2 2 4 pressure [bar] 2 2 1 temperature [◦C] 20 20 20 relative error [%] ±(0.2 + 0.5 · F ) ±(0.2 + 0.5 · F ) ±(0.2 + 0.5 · F )

Table 2.1: Calibration data of the flow meters, F: actual flow in L min−1 gas after the reaction chamber changes during the synthesis. However, in order to minimize a possible temperature related effect, a gas cooling system in form of a metal spiral was installed between the reaction chamber and the flow meter. So as to give an assumption of the expected error realistic deviations of the calibrated properties were investigated with the aid of fluidat [17]. Fluidat is a database provided by Bronkhorst, which allows to recalibrate the instruments due to changes of external properties. Following table 2.2 shows the assumed deviations and the maximal relative error of the instrument. Property F-201CV-2K0-H F-201CV-2K0-N F-111B-5K0 temperature deviation [◦C] [15,25] [15,25] [20,40] pressure deviation [bar] [0.5, 4] [0.5, 4] [0.5, 4] max. relative error [%] ±(0.3 + 0.5 · F ) ±(0.5 + 0.5 · F ) ±(0.4 + 0.5 · F )

Table 2.2: Assumed deviations of the calibrated values and the maximal relative error, F: actual ﬂow in ls/min

2.2.1 Testing In order to demonstrate the usefulness and the limits of the gas monitoring system, the system was installed to a thermogravimetric analyzer (TGA). A thermogravimetric analyzer is an established apparatus, which allows to precisely measure the weight change of a sample and so to follow a chemical reaction. In this case a Setaram TGA 92 was used which allows a maximal sample size of approximately 3 g of uranium. As test runs two uranium samples were hydrided, since the simplicity of this reaction allows a direct comparison between the mass data and the ﬂow data.

Using the ideal gas law the mass increase of the sample m˙ and the ﬂow diﬀer- ence dF = Fin − Fout in reaction 2.1 are connected as follows:

mH · PFM m˙ F = −2 · · dF (2.6) R · TFM

19 2 Nitride Synthesis

Figure 2.3: Thermogravimetric analyser TGA 92 .

Here mH is the weight of atomic hydrogen, PFM and TFM the pressure and the temperature of the gas in the ﬂow meter and R the gas constant. The factor 2 is considering the fact that hydrogen atoms form H2 molecules under this conditions.

In order to be able to compare m˙ and dF , the latter one needs to be corrected. First an offset a of the flow measurement has to be removed, which is caused by a small leakage of the gas system. And second a temperature related effect has to be considered. A change in temperature leads to an expansion or a reduction of the gas volume in the heating zone, which causes a higher or lower outflow. This change in flow is linear to the derivation of the temperature with the proportionality factor b. Considering these two correction parameters one gets following formula:

mH · PFM m˙ F = −2 · · (dF + a + b · T˙ ) (2.7) R · TFM In the two test measurements uranium samples in form of discs were used. In preparation the surfaces of these discs were first ground in order to remove surface oxides and so to enhance the gas solid reaction between hydrogen and the uranium metal. Afterwards they were broken into smaller pieces to fit in the alumina crucible shown in figure 2.4, which was then mounted in the thermogravimetric analyzer. The two runs UA110203 and UA110204 differed in sample size and flow of hydrogen. A smaller sample was used in the second run since material spilled out of the crucible in the first run due to the high volume increase during the formation of UH3. In figure 2.5 the two measurements are shown. The red line is the temperature curve and the blue curve the derivation of the weight measurement. The green plot is the corrected flow difference according to formula 2.7. As correc-

20 2.2 Set-up

Figure 2.4: Uranium discs and alumina crucible .

Property UA110203 UA110203 −1 H2 ﬂow [L min ] 0.1 0.2 sample mass [g] 2.229 1.579

Table 2.3: Test run parameters tion parameters following values were determined: a = −1.67 × 10−3 L min−1 and b = 4 × 10−3 L min−1 s K−1. However, these values are just valid for this setup and so they were later determined again for the actual synthesis furnace. In the UA110203 run one can see a good accordance between the flow and the mass measurement. However, the negative mass change in the beginning of this run could not be detected with the flow sensors. This weight decrease was not caused by a chemical reaction of the uranium sample but by the drying of the alumina crucible, since it was not used for a long time. This evaporation of humidity has a low influence on the flow measurement but due to its high mass compared to hydrogen its visible in the weight measurement. The fact that this behavior was not observed in the UA110204 run, which was performed right after UA110203, sup- ports this explanation. The drop in the mass derivation curve at around t = 3200 s was caused by material which spilled out of the crucible, as mentioned before. In the UA110204 run the flow measurement showed a higher reaction rate than it was shown by the TGA. A reason for this behavior was not found. Unfortunately this problem could not be investigated further because of limited access to the equipment. But since such a drop of the reaction rate was not observed in other runs in the thermogravimetric analyzer before, an error of the TGA equipment is suspected.

21 2 Nitride Synthesis

Figure 2.5: UA110203 and UA110204 hydriding procedures .

In summary one can say that the flow measurement gives a rather good idea about the ongoing reaction. In both cases it showed the start and end of the chemical reaction in good accordance to the TG measurement. Additionally it is able to give an idea about process of the reaction and its reaction rate. One also has to consider that these two test runs were performed with very small sample sizes, which have a much smaller surface area than the samples used in later runs in the bigger furnace. This smaller surface area leads to lower reaction rates and so lower flow differences, which cause a higher error in the measurement.

2.3 Production of UN

In this work two production runs of pure uranium nitride were done, UP101027 and UP101124. This uranium nitride was later used to sinter pellets as described in chapter 3.

2.3.1 Starting material In the UP101027 run two cylinder shaped uranium rods with an approximate length of 8 cm and diameter of 4 mm were used. They had an overall weight of 39.487 g. These rods were already several years old and were not kept in a protective atmosphere during this time; thus, they showed a high surface oxidation. The surface of the samples was ground before the synthesis in order to remove this oxide layer. The starting material of UP101124 were the remains of an earlier unsuccessful synthesis. In this synthesis the hydriding of the uranium did not take place and so the two still metallic uranium rods, which were initially used, melted into one piece in the denitriding step. The total mass of this uranium piece was 36.313 g.

22 2.3 Production of UN

Figure 2.6: Unpolished uranium rods .

2.3.2 Synthesis

Hydriding

For the hydriding step a mixture of argon and hydrogen was used as an inlet gas. Argon was used as a security measure in case hydrogen would be consumed at higher rate than it is supplied to the reaction vessel, which could lead to a back suction of air through the gas outlet of the reaction chamber. This would lead to a contamination of the sample or in the worst case to a violent reaction between hydrogen and oxygen. In the two synthesis runs the inlet gas mixture was 0.4 L min−1 of argon and 0.5 L min−1 of hydrogen.

Figure 2.7: UP101027 and UP101124 hydriding procedures .

23 2 Nitride Synthesis

Figure 2.7 shows the synthesis procedures. The initial plateau temperature was chosen as 300 ◦C due to experience from earlier runs. In order to get a more complete reaction the temperature was then lowered to 250 ◦C, where it remained until the reaction was complete. The sudden temperature drop in run UP101027 at around t = 3000 s and the sudden temperature increase in UP101124 at t = 10 000 s was caused by an malfunction of the data acquisition hardware. As one can see the reaction rate strongly depends on the furnace temperature. This can especially be seen where the temperature drops from 300 ◦C to 250 ◦C. This behavior was described in [3] and can be explained by an increase of the decomposition pressure with increasing temperature. Neglecting this temperature dependency the reaction rate is decreasing over time while its derivative also decreases to zero.That can be seen in the areas with constant temperature especially in UP101124 between t = 1000 s and t = 7000 s. This behavior is in contrast with the two calibration runs UA110203, where the reaction rate stayed almost constant over a certain time period. The reason for this is the geometry of the sample, since the reaction rates depends on the surface area. In the UA110203 a ﬂat disc was used, which surface area stays almost constant in the beginning compared to a cylinder shaped rod, where the surface area is continu- ously decreasing. Another interesting fact is the diﬀerence in the temperature, where the reaction started. In the runs UA110203, UP101027 and UP101124 this temperature was between 280 ◦C and 300 ◦C, whereas it was considerably lower in UA110204. In this run the hydrogen reaction started at approximately 220 ◦C. A possible explanation can be a lower surface oxidation of the UA110204 sample.

Nitriding

The nitriding reactions were performed subsequent to the hydriding reaction by switching to a nitrogen ﬂow of 0.5 L min−1. As one can see the nitriding reaction is much more rapid than the hydriding reaction; a complete nitriding takes around 1000 s compared to 15 000 s of the hydriding step. An important reason for this is that the sample is now in powder form, which presents a much higher surface area than the original sample.

Furthermore the energy release of this exothermic reaction can be seen by the peak in the temperature curve. In fact the temperature increase of the sample itself is much higher, since the thermocouple is mounted outside of the quartz tube. In order to avoid any damage on the quartz tube this temperature increase should be limited. This can be done by a reduced nitrogen ﬂow.

The flow measurement indicates first a release of hydrogen in two steps followed by a nitrogen uptake. Effort was done to understand this reaction in detail with

24 2.3 Production of UN

Figure 2.8: UP101027 and UP101124 nitriding procedures . the aid of the thermogravimetric analyzer. However, these two data sets were not enough to determine the single steps of this reaction, since ammonia NH3 is formed under these conditions, which has a high inﬂuence on the ﬂow measurement. In order to get more detailed information about the single reaction step, one needs to analyze the outlet gas to determine the amount of NH3 formed during the reaction.

Denitriding In order to convert uranium sesquinitride, which was produced in the last step, to uranium mononitride the inlet gas was changed to argon with a flow rate of 0.5 L min−1. The temperature was then increased to 1150 ◦C. A higher temperature would be desirable so as to achieve a proper denitriding; however, the lifetime of the used thermocouple decreases significantly at temperatures above 1150 ◦C. In the measurements a small positive flow difference is visible, which is caused by the release of nitrogen. Especially in UP101027 it is noticeable that this flow difference decreases to zero. This indicates a complete denitriding of the sample. In UP101124, however, the reaction does not seem to be complete. The denitriding process is very slow at these temperatures, so it is difficult to measure. In order to get better measurements a higher temperature would be necessary. Then the denitriding would take a shorter time, which would also decrease the amount of used argon gas.

2.3.3 Product For analyzing the produced samples, the weight change was measured and a Siemens D5000 X-Ray diﬀractometer was used. Unfortunately UP101124 was sintered into

25 2 Nitride Synthesis

Figure 2.9: UP101027 and UP101124 denitriding procedures . pellets before an XRD analysis could be performed. An XRD analysis of one of those pellets will be shown; however, its informative value is limited, since some uranium sesquinitride would probably be decomposed to uranium mononitride due to the high temperatures used while sintering. The shown pellet was SPS-sintered at 1650 ◦C.

Figure 2.10: Uranium nitride powder from UP101027 .

The product appearance was in both cases a fine black powder, which can be seen in figure 3.6. The weight increase was 4.7% of UP111027 and 5.1% of UP101124. Both values are lower than the theoretical mass increase of UN compared to U of 5.88%. This difference can be explained by the adhesion of material to the reaction vessel walls and filters.

26 2.3 Production of UN

The comparison between the XRD data of UP101027 and a reference data set shows a presence of UN (figure 2.11). No sign of UN1.5 could be found, which agrees with the flow measurement of the denitriding step. Oxide impurities were also found, but may be over-represented, since the powder was not in a protective atmosphere during the XRD measurement. In contrast to UP101027, the XRD spectrum of UP101124 shows peaks indicating a UN1.5 phase (figure 2.12). This result is also in accordance with the flow measurement.

27 2 Nitride Synthesis

Figure 2.11: XRD spectrum of UP101027 .

Figure 2.12: XRD spectrum of a pellet sintered from UP101024 powder .

28 2.4 Production of ZrN

2.4 Production of ZrN

This section describes the performed synthesis experiments of zirconium nitrides.

2.4.1 Starting material As a starting material zirconium metal of reactor grade quality was used (ﬁgure 2.13). This means it is free of hafnium, which is a good neutron absorber, and thus undesirable in a nuclear reactor. The surface of the used zirconium is covered with a very thin oxide layer, since it was not kept in a protective atmosphere. However, this layer is very dense and protects the metal from further corrosion. So the material was not specially treated before usage.

Figure 2.13: Reactor grade zirconium .

2.4.2 Synthesis Hydriding The hydriding behavior of zirconium is illustrated by run ZA100902, which shows a typical reaction process. Here a cuboid shaped sample of 14.203 g was hydrided in a stream of 1.2 L min−1 hydrogen and 1.5 L min−1 argon. The hydriding reaction is already slightly visible at 450 ◦C. With increasing temperature the reaction rate moderately increases until around 560 ◦C, where the reaction then picks up speed quite rapidly. Here an exothermic reaction is detected by the increase in temperature. After a short time the reaction rate suddenly drops again. After the drop the reaction rate slowly decreases to zero. This behavior indicates a two step reaction, probably ﬁrst a formation of the γ phase and second, a further hydrogen uptake due to the formation of a δ phase.

29 2 Nitride Synthesis

Figure 2.14: ZA100902 hydriding procedure .

Figure 2.15: ZA100902 after hydriding .

30 2.4 Production of ZrN

After hydriding the sample was still in one piece (figure 2.15); however, its volume increased. The surface darkened, while it kept a shiny silver color in the bulk. Its hardness decreased quite noticeably, since it could be broken by hand. A pulverization, however, could not be seen. Also other test runs with different temperatures and hydriding-dehydriding cycles did not lead to a zirconium hydride powder. The dehydriding reaction occurred in a pure argon flow of 0.3 L min−1 at temperatures above 700 ◦C. The XRD analysis of the sample indicated the existence of a δ ZrH phase. No γ or phase was found.

Figure 2.16: XRD spectrum of ZA100902 .

Nitriding The nitriding of zirconium hydride is shown on two samples, ZA100915 and ZA101129. Both samples were hydrided like ZA100902 in the previous section. ZA100915 was then directly nitrided afterwards, whereas ZA101129 was ball milled for 1 hour before nitriding. ZA100915 had a mass of 10.370 g and was nitrided in a ﬂow of 1.5 L min−1 nitrogen. The mass of ZA101129 was 24.021 g and the used ﬂow rate was 1 L min−1. In ZA100915 it was tried to nitride the sample right after the hydriding process in a temperature ramp from 700 ◦C to 1200 ◦C. However, this just lead to a little nitrogen uptake at around 750 ◦C and then to a release of hydrogen at 1100 ◦C.

31 2 Nitride Synthesis

Figure 2.17: ZA100915 and ZA101129 nitriding procedures .

This was verified by the XRD spectrum of this sample, which showed the existence of both metal zirconium and a zirconium nitride phase ZrN0.28 with very little nitrogen content. ZrN, however, was not found. The color of the sample changed to a golden color on the surface, while it was still silver in the bulk. This indicates an incomplete diffusion of nitrogen into the sample. In order to get a more complete nitriding in ZA101129 lower temperatures were used. The flow measurement of this reaction was very noisy. This hints at very dynamical uptake and release reactions between the gas and the surface of the sample. Here a positive flow measurement between 450 ◦C and 900 ◦C was detected due to a release of hydrogen. The reaction was considered to be complete when the mean of the variations in the flow measurement reached zero. The XRD analysis confirmed the formation of the ZrN phase. The end product of this reaction was a yellow brown powder with some agglom- erations. Figure 2.19 shows the powder after sieving with 60 mesh.

32 2.4 Production of ZrN

Figure 2.18: ZA100915 after nitriding .

Figure 2.19: ZA101129 after nitriding .

33 2 Nitride Synthesis

Figure 2.20: XRD spectrum of ZA100915 .

Figure 2.21: XRD spectrum of ZA101129 .

34 2.5 Production of (U,Zr) alloy nitrides

2.5 Production of (U,Zr) alloy nitrides

This section will describe the production of uranium-zirconium alloys and their synthesis to nitride powder.

2.5.1 Motivation The original production route of (U,Zr) pellets consists of a separate production of uranium nitride and zirconium nitride powders. These powders are then mixed together before pressing and sintering them into pellets. This route involves a lot of powder handling: the transport of zirconium hydride powder after milling into the reaction chamber, the transport of zirconium nitride powder from the reaction chamber into the glove box and the same procedure for uranium nitride. Mixing these two powders in a mill causes extra work steps. The motivation of alloying uranium and zirconium together before the synthesis is to reduce the powder handling to a minimum, since there are several problems handling these powders. Uranium nitride, zirconium nitride as well as zirconium hydride have a high aﬃnity to oxygen, which leads to a strong oxygen contamination of the sample when these powders are exposed to air. They are also pyrophoric, so they can self-ignite in air under normal conditions. Furthermore there is a certain health risk, since uranium is an alpha emitter and due to the powder form easy to incorporate. An incorporation of such a substance can cause serious health problems, since alpha particles are strongly absorbed in the inner tissue and are very ionizing. Additionally uranium has a certain chemical toxicity, whose health hazard is comparable to the radio toxicity of depleted uranium. [30] If it is possible to get a ﬁne mixed powder directly after the synthesis the powder handling could be reduced to one transfer from the reaction chamber into the glove box. Also the mixing step would be omitted. Furthermore it should be seen if this process can lead to a better mixture between uranium and zirconium and so make it easier to form a solid solution during sintering. Or maybe it could be even possible to receive a solid solution already during the nitride synthesis.

2.5.2 The U-Zr system The U-Zr system was studied extensively due to its importance to the nuclear industry. Figure 2.22 shows the uranium rich part of the U-Zr phase diagram, which is interesting for this research. In the following experiments alloys from 5wt% to 27wt% were produced. As one can see the α-uranium phase, which occurs at room temperature, has a very low solubility of zirconium. According to [10] in equilibrium the maximal zirconium content is 0.19wt% at a temperature of 662 ◦C. However, studies of Chandrabhanu Basak et al. on uranium zirconium alloys with up to 10wt% Zr have shown that the as-cast structure is a single α-phase. [8]

35 2 Nitride Synthesis

Figure 2.22: U-rich part of the U-Zr phase diagram [8] .

This α-phase is supersaturated with zirconium. A formation of this phase would be very helpful in order to achieve a good mixture of uranium and zirconium. At higher zirconium contents uranium and zirconium form a δ phase, which corresponds to UZr2. However, the formation of this phase is very sluggish, and thus this phase will probably not be observed in the following experiments.

2.5.3 Equipment The alloying was tried in three different melting furnaces, which were available at the Material science department at KTH: • Single-arc melting furnace (Automatic Casting Machine): This furnace is an arc melting furnace with one tungsten electrode. The sample is placed underneath in a copper crucible, which consists of two separate pieces. After a melting period of maximally 60 seconds these two pieces open and the molten metal gets poured into a copper mold. This time can be extended by turning off the furnace before the crucible opens and repeating this process again. However, one has to wait until the copper crucible cools down, since its not actively cooled and further melting could cause damage. This machine is designed to keep the sample in an inert atmosphere; however, this worked very unsatisfactorily due to the high age of the machine. The maximal sample size is around 20 g. • Triple-arc melting furnace: This furnace (figure 2.23) is equipped with three freely movable tungsten electrodes, whose position could be adjusted also while melting. The sample is placed in a copper crucible inside a cylindrical quartz melting chamber. This chamber allows to observe the sample

36 2.5 Production of (U,Zr) alloy nitrides

during the melting process and to maintain a protective atmosphere. The copper crucible is water cooled and so there is no limitation of the melting time. The maximal sample size of this machine is at around 10 g of material. • Induction furnace (Controlled Rapid Quenching Machine): This furnace heats the sample by induction heating. The sample here is placed in a test tube shaped crucible made of quartz, which is surrounded by an induction coil. The crucible has a hole in the bottom, through which the molten sample is blown out by argon after the melting process. The melt can then fall on a rotating copper wheel to achieve a rapid quenching of the material or directly into a copper mold. The major advantages of this furnace are the good mixing of the material caused by the induced currents and the possibility to apply a very clean inert atmosphere. However, there is the possibility that the sample reacts with the quartz crucible, which would contaminate the sample.

After different alloying experiment with those furnaces it could be clearly shown that the triple-arc melting furnace yields the best results. The single-arc melting furnace had severe deficit in maintaining a protective atmosphere; thus, the samples and even the crucible were very oxidized after the melting process. Furthermore the furnace was not able to melt bigger samples (20 g) properly. Also the casting of the sample after the melting process did not work as intended due to the high viscosity of the melt. The shapes of the final products were very irregular. Also in the rapid quenching machine problems occurred, which made it not usable for our purpose. Firstly the quartz crucible reacted with the hot melt and secondly there were difficulties to melt zirconium. In a run with separate uranium and zirconium pieces, the uranium melted completely while the zirconium just melted partially. Furthermore there were problems with the casting process, since the melt was too viscous to be blown out through the hole of the crucibles. In contrast to those two furnaces, the triple arc melting furnace showed very good results. The inert atmosphere was much better than in the single-arc furnace. The samples had a shiny surface after the melting process without visible oxidations. There was also no chemical reaction observed between the sample and the copper crucible. Another advantage was the possibility to move the electrodes while melting, which allows to control the mixing of the metals. The obtained alloys had a drop shape and it was possible to melt them several times in order to achieve a better mixing.

2.5.4 Alloying The alloying itself was performed by Mr. Miquel Torres Oliver and further details about the experiments can be found in [32]. Interesting alloys were the samples

37 2 Nitride Synthesis

Figure 2.23: Triple-arc melting furnace .

AUZr101123, AUZr101124, AUZr101207 and AUZr101208. These samples were alloyed in the triple-arc furnace and diﬀered in zirconium content. Table 2.4 shows the the weight and zirconium content of the diﬀerent samples.

Sample Weight [g] U [wt%] Zr [wt%] U [at%] Zr [at%] AUZr101123 6.741 76.7 23.3 55.8 44.2 AUZr101124 5.684 73.1 26.9 51.0 49.0 AUZr101207 10.270 89.1 10.9 71.1 28.9 AUZr101208 10.065 94.9 5.1 87.6 12.4

Table 2.4: Sample weights and uranium/zirconium contents

As starting material for these alloying experiments uranium discs (figure 2.4) and zirconium metal pieces (figure 2.13) were used. The uranium discs were polished before with grinding paper in order to remove their surface oxidation as well as possible. Sample AUZr101124 was before unsuccessfully alloyed in the rapid quenching machine, where it reacted with the quartz crucible. In the alloying process the alloys were then melted up to four times so as to achieve a good mixing of the samples. The resulting products had a drop like shape with a flat side on the bottom. This side was then polished in order to analyze the samples by XRD and SEM. The surface of the drops had a shiny metallic appearance (figure 2.24). The XRD spectra of those alloys show good accordance with α-uranium peaks, but not with the α-zirconium. Also the delta-phase of the U-Zr system was not found, regardless of the zirconium content. Figure 2.25 shows exemplarily the XRD spectrum of AUZr101208.

38 2.5 Production of (U,Zr) alloy nitrides

(a) AUZr101207 (b) AUZr101208

Figure 2.24: Alloy drops .

Figure 2.25: XRD spectrum of AUZr101208 .

39 2 Nitride Synthesis

1 3 1 4 2 3

(a) AUZr101207 (b) AUZr101208

3 7 1 5 6 2

Figure 2.26: SEM images of the alloys .

In the SEM analysis (ﬁgure 2.26) all samples consisted mainly of uranium phases with dissolved zirconium in them (areas 1 and 3 in image a, 1 and 4 in b, 1 and 2 in c, 5 in d). The zirconium content in this phases varied from 3.6at% in AUZr101208 to 42.5at% in AUZr101123. Area 2 in a) and area 3 in b) were zirconium rich phases with some uranium dissolved in them. The zirconium rich samples AUZr101123 and AUZr101124 showed small dendrites of pure zirconium (3 in c, 6 in d). AUZr101124 additionally had a phase consisting of zirconium and silicon due to its reaction with the quartz crucible in the induction furnace (7 in d). After analyzing, AUZr101207 was heat treated in an argon atmosphere at 1000 ◦C and then slowly cooled to room temperature. In a new analysis no signiﬁcant change of the sample’s phase system could be observed.

40 2.5 Production of (U,Zr) alloy nitrides

2.5.5 Synthesis In this section the synthesis experiments UA101207, UA110223 and UA110301 are described. Their purpose was to produce nitride powders of the described alloys.

UA101207 In this synthesis the alloys AUZr101123 and AUZr101124 with zirconium contents of 44.2at% and 49.0at% were used. Together they weighed 11.942 g. The samples were three times hydrided (figure 2.27) and inspected through the quartz tube after each hydriding. In the first hydriding step the samples were heated up in a mixture of 0.7 L min−1 hydrogen and 0.3 L min−1 argon to a temperature of 350 ◦C. At approximately 300 ◦C a slight flow difference and so a slow hydrogen uptake could be observed, which decreased back to zero over a time period of 60 min at 350 ◦C. After this time the temperature was raised to 650 ◦C, where it stayed another 60 min. While heating the reaction rate increased again at around 450 ◦C and stayed almost constant. At approximately t = 15 000 s the hydrogen inflow stopped due to a hardware problem and the samples were cooled down to room temperature. The inspection of the samples afterwards did not show any visible changes, what appears reasonable, since the reaction rates were very low. In the second hydriding step the same gas mixture was used and the maximal temperature was raised to 950 ◦C in order to see if the sample reacts at higher temperatures. Here a slow hydrogen uptake could be observed until 400 ◦C, which was then followed by a hydrogen release peak and a second one was observed at around 850 ◦C. When the temperature was lowered the samples started to take up hydrogen. The observed reaction rates were considerably higher than in the hydriding run before. At 360 ◦C a peak in the reaction rate was visible. The inspection of the sample afterwards showed a partial powderization. This explains the higher reaction rate, since the powder provides a much higher surface area. In order to totally powderize the sample a third hydriding run was performed. Here the sample was heated up in a flow of 0.3 L min−1 argon to 950 ◦C and then additionally 0.7 L min−1 hydrogen were introduced. As expected the sample de- hydrided during the positive temperature ramp. Two dehydriding peaks were observed. The first one occurred between 350 ◦C and 550 ◦C. And the second one was between 650 ◦C and 950 ◦C. The second peak is in the same temperature region as the dehydriding peak of pure zirconium samples. This indicates that this peak is caused by the pure zirconium phases, which were observed in the SEM analysis. After hydrogen was introduced two hydrogen uptake peaks were observed. One small one, starting at 850 ◦C and one bigger one at around 360 ◦C, which occurred already in the last hydriding step. It is noticeable that the last peak increased quite considerably compared to the last run. This indicates that the powderization of the sample proceeded. This was confirmed by the observation of the sample after it cooled down.

41 2 Nitride Synthesis

Figure 2.27: UA101207 hydriding procedures .

In the nitriding step the sample was heated up to 1150 ◦C in a flow of 1 L min−1 nitrogen. A very rapid and exothermic reaction was observed at 260 ◦C. The temperature increased approximately 220 ◦C. The flow measurement showed a high similarity with the nitriding reaction of pure uranium. It first has two barely separated positive peaks, which indicate a release of nitrogen, and then a negative peak indicating the nitrogen consumption. Very different, however, is the speed of the reaction.While the nitriding of uranium hydride took around 1000 s (figure 2.8), this reaction happened in approximately 200 s. After this peak the reaction seemed to be complete, since no flow difference was measured afterwards. The nitriding reaction can be seen in figure 2.28. In the last step argon was introduced at 1150 ◦C with a flow rate of 1 L min−1 in order to decompose uranium sesquinitride. A slow denitriding could be observed during the first 2000 s. Due to the high fluctuations in the measurement it was difficult to determine if the denitriding was complete or not. After 120 min the

42 2.5 Production of (U,Zr) alloy nitrides

Figure 2.28: UA101207 nitriding and denitriding procedures . sample was cooled to room temperature.

Figure 2.29: Nitride powder from UA101207 .

The product appearance was a dark gray powder. The particle size was coarse, almost sand like. It had some flakes and bigger particles in it. The weight increase was 8.6%, which is slightly higher than the theoretical increase of 8.14%, assuming only the existence of uranium mononitride. This indicates the presence of uranium sesquinitride and so an incomplete denitriding step. This is confirmed by XRD analysis of this sample (figure 2.30). It shows clearly the presence of ZrN, UN and UN1.5. The presence of the ZrN phase means that at least some zirconium nitride did not form a solid solution with the uranium nitride. The other peaks in the spectrum are caused by impurities like oxygen and silicon from AUZr101124.

43 2 Nitride Synthesis

Figure 2.30: XRD spectrum of UA101207 .

UA110223

In this synthesis run the alloy sample AUZr101207 with a zirconium content of 28.9at% was used. Its weight was 9.940 g. After an initial hydriding at a temperature of 400 ◦C, six dehydriding-hydriding cycles with a maximal temperature of 600 ◦C were performed. For the hydriding a mixture of 0.7 L min−1 hydrogen and 0.3 L min−1 argon was used. The dehydriding was performed in 1 L min−1 of argon. Figure 2.31 shows these cycles, the different steps are divided by vertical lines. The behavior of the reaction is very similar to UA101207. As one can see the dehydriding peaks and hydriding peaks increase in every step. This indicates powderization of the sample and along with this increase of its surface area. The hydriding peaks occurred at 360 ◦C like in the previous run. When no significant increase was observed in the last hydriding peak compared to the previous one, the sample was nitrided in a flow of 1 L min−1 nitrogen. The nitriding reaction was again very rapid and exothermic. It caused a temperature increase of around 220 ◦C and was complete after approximately 200 s like the nitriding in the previous run. The starting temperature, however, decreased considerably to 165 ◦C, compared to 260 ◦C before. Afterwards the sample was denitrided at a temperature of 1200 ◦C for around 4.5 h. A low hydrogen release was detected starting at 1000 ◦C, which, however, stopped when the temperature reached 1200 ◦C.

44 2.5 Production of (U,Zr) alloy nitrides

Figure 2.31: UA110223 hydriding (H) and dehydriding (D) procedures .

Figure 2.32: UA110223 nitriding and denitriding procedure .

45 2 Nitride Synthesis

The product weighed 10.522 g after this synthesis. The weight increase is slightly lower than the theoretical one, which is probably caused by the adhesion of material to the reaction chamber walls and filters. The product appearance was a very fine dark powder. The powder was noticeably finer than the product from the previous run. The reason is probably the higher amount of hydriding-dehydriding cycles. However, this powder also contained several bigger gray lumps, which were very brittle. They could be easily crushed by a spoon. Figure 2.33 shows the produced powder and those lumps. The origin of this lumps and their composition is unknown, since there was no proper equipment to analyze them. Due to the lumps an XRD analysis could not be performed on this sample.

Figure 2.33: Nitride powder from UA110223 .

UA110301 In this run the alloy AUZr101208 was synthesized. This alloy had a zirconium content of 12.4at% and a weight of 9.940 g. For the dehydriding-hydriding cycles the same gas mixtures and temperatures were used as in the previous experiment. The cycling was repeated 9 times, in order to achieve fully powderized sample. The flow measurement was in good agreement with the previous run. As one can see in figure 2.34 the increase of the hydriding and dehydriding peaks can again be observed. Obviously this is a characteristic behavior of those alloys. The hydriding reaction started here at a temperature of approximately 340 ◦C, slightly lower than the other two alloys (360 ◦C). In contrast to the hydriding, the nitriding behavior of this alloy was very different to the previous runs. In a first try the hydride was heated in a flow of 1 L min−1 nitrogen to a temperature of 250 ◦C, where it stayed for 30 min. When no reaction was observed the sample was cooled down to room temperature. A second try was performed with the same settings as before. Surprisingly the sample started to

46 2.5 Production of (U,Zr) alloy nitrides

Figure 2.34: UA110301 hydriding (H) and dehydriding (D) procedures .

47 2 Nitride Synthesis

Figure 2.35: UA110301 nitriding and denitriding procedures . react at a temperature of just 75 ◦C. Even if the reaction was very rapid according to the flow measurement (duration 100 s), the temperature increase was very low compared to the increase in the previous two runs. It was just 27 ◦C compared to 220 ◦C before. The unusually low reaction temperature and temperature increase could be explained by a displacement of the thermocouple. If the thermocouple is not placed in close proximity to the sample, the temperature at the sample’s position would differ, since the temperature is not evenly distributed in the furnace. It would also lower the heat transfer from the sample to the thermocouple, and thus explain the low temperature increase during the reaction. An explanation, however, why the reaction did not take place in the first nitriding, was not found. After this nitriding step two more, one even with a maximal temperature of 750 ◦C, were performed. But no further reaction was observed in these two steps, so the reaction was considered complete. In order to denitride the sample, a temperature raise until 1200 ◦C was pro- grammed like in the last experiment. However, the thermocouple broke down before the denitriding could be finished, and thus the it was aborted early. In the beginning a gas release at very low temperatures between 20 ◦C and 350 ◦C was detected. It was assumed that this was caused by hydrogen, which was still left in the sample. Between 950 ◦C and 1150 ◦C a second peak was observed, which is similar to previous experiments. Due to the short time the denitriding was probably not complete. The product of this synthesis was a fine black powder. The XRD analysis (figure 2.37) showed, as expected, the presence of a uranium nitride phase as well as a uranium sesquinitride phase. Additionally uranium oxide was found, which is due to the fact that the powder could not be protected during the analysis. Interesting is the fact that no zirconium nitride peaks were found. And in addition the uranium

48 2.5 Production of (U,Zr) alloy nitrides

Figure 2.36: Nitride powder from UA110301 . nitride peaks seem to be shifted. This may indicate that the zirconium nitride is in solid solution with the uranium nitride.

Figure 2.37: XRD spectrum of UA110301 .

49 2 Nitride Synthesis

2.6 Summary

After this experiments following results about the synthesis set-up can be summarized:

• In the first section of this chapter was shown that the flow measurement system is able to give quite accurate real time information about the reactions. This is very helpful, since earlier only blind runs could be performed. With the aid of this system it was possible to determine the reaction temperatures and reaction rates of the different steps. This helped for example to determine if the powderization of the alloy samples was complete or not during the hydriding procedure. This spares the time consuming step of removing the quartz tube from the furnace after such a run in order to check the sample’s condition. That procedure is not just inconvenient, it also increases the risk of damaging the equipment or contaminating the sample. Furthermore it will be possible with this system to minimize the volume of gases used in the nitride production. This is especially desirable considering the 15N problem. In later runs, when 15N is used, a reduction of the used volume to a minimum will decrease the costs significantly. Another big advantage of this system is the automation of the nitride synthesis. Producing nitrides out of alloys was very time consuming. Because of the many hydriding-dehydriding cycles and the denitriding afterwards, the procedure took several days. The automated control and the possibility to observe and manually control the system over the Internet made it much more convenient for the operator. • There were also some drawbacks in the synthesis set-up. First the denitriding runs have shown that the maximum temperature of 1200 ◦C is too low to perform a proper decomposition of UN1.5. To overcome this problem a more powerful furnace and another thermocouple is necessary. Furthermore the quartz tube should be exchanged with another material, like alumina, to withstand the higher temperatures. In order to get a better understanding of the nitriding reaction it is planned to install a residual gas analyzer on the outflow of the reaction chamber. This might also help to quantify the oxygen impurities of the reaction gases. Another useful improvement would be the installation of a digital pressure gauge to the gas system. This would allow a better quantification of the reaction parameters, since the reactions also depend on the pressure of the gas. During all the experiments in this work the inlet pressure into the gas cleaning system was kept constant at 3.5bar; however, there was no information about the pressure drop in the gas cleaning system.

50 2.6 Summary

About the behavior of the materials the following be summarized:

• The hydriding of uranium depends strongly on the temperature. It was seen that the samples start to react at temperatures of around 300 ◦C. Lowering the temperature from 300 ◦C, after the reaction started, increased the reaction rate considerably. This is explained by the higher decomposition pressure at higher temperatures. Hydriding in a negative temperature ramp from 300 ◦C seems to be the best way to achieve a complete reaction, since this prevents a partial decomposition of already formed hydride. • Hydriding of zirconium does not form powder. Also cycling hydriding and dehydriding steps do not lead to a powderization. • The hydriding reaction differs very much between the different materials. Uranium on the one hand has a slow reaction with hydrogen, which strongly depends on the exposed surface area for uranium. Zirconium on the other hand reacts quite rapidly with hydrogen, even with the low surface area of a massive piece of zirconium. The latter reaction is diffusion controlled, while the first one is limited by the exposed surface area. • The cyclic hydriding-dehydriding of uranium zirconium alloys leads to a powderization of the material. The reaction rate depends on the surface area and so increases in every step. The hydriding temperatures seemed to be independent on the zirconium content at around 360 ◦C. • The nitriding of uranium and the alloys happens in a quite rapid reaction. The reaction is also very exothermic and lead to a noticeable temperature increase. The nitriding of zirconium hydride, however, also in powderized form, happens much slower. • The nitriding temperature of the alloys depends strongly on the zirconium content. It varied from 260 ◦C in a sample with approximately 47at% Zr to 80 ◦C with a zirconium content of 12at%. In pure zirconium the temperature lays at around 400 ◦C. • The alloying experiments showed quite promising results. It was possible to powderize the samples and achieve a good mixture between uranium and zirconium. This makes the production of mixed nitrides much easier compared to the original route, since it involves fewer powder handling steps. This could also make it easier to achieve a solid solution when sintering those powders. This will be seen in the next chapter, where the sintered pellets of traditionally mixed and alloyed nitride powders will be compared.

3 Sintering

This chapter will deal with the sintering of nitride powders into pellets. In this work spark plasma sintering (SPS) was used instead of traditional sintering methods like pressure-less sintering or hot-pressing. The sintering experiments were performed with the assistance of Mr. Pertti Malkki and in cooperation with Diamorph AB on a SCM Dr. Sinter SPS system.

3.1 Background

Experience with nitrides Early experiments with conventional sintering of uranium nitride showed that UN powders are difficult to densify. Evans et al. give an overview of works before 1963. [12] They showed that it is difficult to achieve relative densities of more than 90% by cold pressing and pressure-less sintering. In order to do so high temperatures of more than 1800 ◦C for several hours were necessary. These samples were sintered in vacuo. The high temperatures, however, led to a decomposition of uranium nitride, so that metallic uranium was formed. These difficulties led to methods for manufacturing dense UN such as arc melting of uranium nitride in an N2 overpressure or isostatic hot pressing. McLaren et al., however, showed that the oxygen content of the uranium nitride powder is a key factor, which is inhibiting solid state sintering. They achieved a relative density of 95% at 1700 ◦C by minimizing the oxygen content. Their production route included a preparative milling step of around 32 hours. Instead of vacuum they used an argon atmosphere which retarded the decomposition of uranium nitride. [25] Butt et al. performed sintering experiments on zirconium nitride. They used commercial ZrN powder with a purity of 99.5% and a particle size of < 325 mesh, which was first cold pressed into green pellets and then pressure-less sintered in an argon atmosphere. With a sintering time of 10 h at 1600 ◦C a relative density of 83% was achieved. [9] Arai et al. were able to produce (P u, Zr)N pellets in solid solution with 40wt% and 60wt% zirconium by regular sintering methods. However, this production route is very time consuming. First plutonium nitride was obtained by carbothermic reduction in a stream of nitrogen (10 h at 1823 K), which was then followed by a purification step in a mixture of hydrogen and nitrogen to remove excess carbon

53 3 Sintering

(20 h at 1723 K). Afterwards the ZrN and P uN powders were mixed and heated to 1673 K for 5 h in a hydrogen-nitrogen mixture for homogenization. This step was repeated three times. Then the pellets were cold pressed and sintered for 5 h at 2003 K in argon. In this way they reached high relative densities of more than 90%. [5]

There is not much experience in sintering nitrides for a nuclear application by SPS; however, performed experiments are quite promising. Muta et al. compare pressure-less sintered uranium nitride pellets with spark plasma sintered ones. [28] They achieved equal densities of around 90%, whereas the spark plasma sintering takes 40 min while the conventional way had a sintering time of 4 h. The latter one stayed the whole time at a temperature of 2100 K, while the ﬁrst one was exposed to a maximal temperature of around 1700 K for just 10 min. This short time is desirable when sintering fuel pellets containing americium nitride. Americium nitride easily decomposes and evaporates at elevated temperatures. [31] This evaporation is not acceptable due to the toxicity of americium. In experiments with ZrN and T iN, Muta et al. show that high densities can be achieved by SPS for these possible inert matrices. In a total sintering time of 20 min they achieve a theoretical density of 95.7% for T iN and 89.1% for ZrN. They also show that despite of the smaller grain size, which occurs in SPS due to the shorter sintering time, the thermal conductivity is higher compared to regular sintered pellets. Because of the smaller grain size also the Vicker’s hardness is increased. [29]

Working principle of SPS

Figure 3.1: Carbon die .

In the spark plasma process pulsed high DC current is used to heat the sample.

54 3.1 Background

Figure 3.2: Sintering set-up [38] .

Therefore the powder is ﬁlled into a cylindrical carbon die, which is closed at both ends with a carbon punch (ﬁgure 3.1). This carbon die is then wrapped into an thermally insulating carbon felt and mounted in the SPS set-up. Figure 3.2 shows a schematic and an actual photo of the spark plasma system. The temperature of the sample is measured by an optical pyrometer. During the sintering procedure the chamber is evacuated to a vacuum of around 2 Pa. In the following experiments the two rams provide an uniaxial pressure of 50 MPa. Compared to traditional sintering methods spark plasma sintering provides the advantage of high heating rates up to 1000 ◦C min−1, the application of pressure and the electromigration, caused by the electric current. [27] As seen before, it allows much shorter sintering times than conventional methods.

Density An important characteristic of the sintered pellets is their density. A high density results in better mechanical and thermal properties of the sample, such as higher thermal conductivity. [1] However, some porosity is also of advantage under irradiation, since it helps to reduce fuel swelling. [23] A relative density of more then 90% is desirable. The relative density is the actual density over the theoretical density of the material. As theoretical densities, following values are used in this work: Material Theoretical density [g cm−3] Source UN 14.33 [24] ZrN 7.09 [14]

Table 3.1: Theoretical densities of UN and ZrN

55 3 Sintering

The theoretical density of mixed nitrides is calculated by formula 3.1:

xU xZr −1 ρmixed = [ + ] (3.1) ρU ρZr

mi Here xi = is the weight content of i in the mixture and ρi is the theoretical mmix density of i. The density of the pellets was measured by the Archimedes principle according to the DIN EN 993-1 standard. After sintering, the pellets are covered with a carbon layer. In order to perform an accurate density measurement the pellets were polished before.

3.2 Experiments

In this section the sintering results of the diﬀerent powders will be shown. Fur- thermore the sintered pellets of regular mixed powders and alloy powders will be compared.

3.2.1 Zirconium nitride Table 3.2 shows the sintering parameters and the achieved relative densities of some representative zirconium nitride pellets. A typical zirconium nitride pellet after polishing can be seen in ﬁgure 3.3. Its appearance is a metallic golden color. Samples S100930B, S100611C1 and S110414A1 consist of commercial zirconium nitride, purchased from Sigma-Aldrich, whereas the other samples were made of "lab-made" powders (see chapter 2).

Sample Material Heating rate and Holding Relative max. temperature time [min] density [%] S100426C1 ZP090921 100 ◦C min−1 to 1600 ◦C 10 96.2 S100930B commercial 100 ◦C min−1 to 1500 ◦C 15 89.8 S100611A1 ZP09021 100 ◦C min−1 to 1600 ◦C 15 n.m. S100611C1 commercial 100 ◦C min−1 to 1600 ◦C 15 n.m. S110414A1 commercial 100 ◦C min−1 to 1200 ◦C 2 87.7 50 ◦C min−1 to 1650 ◦C S110414B1 ZA101129 100 ◦C min−1 to 1200 ◦C 5 99.7 50 ◦C min−1 to 1650 ◦C

Table 3.2: Sintering parameters and densities of the ZrN pellets

As one can see the lab-made powders ZP090921 and ZA101129 achieved a remarkably higher density compared to the commercial one. A reason for it is the

56 3.2 Experiments

Figure 3.3: ZrN pellet .

ZrN powder Nitrogen content [wt%] Oxygen content [wt%] lab-made 13.057 0.734 commercial 8.830 7.697

Table 3.3: Chemical analysis of the nitride powders particle size of the powders. The commercial powder had in contrast to the lab- made one a well defined particle size of 1 to 2 µm. The lab-made powder, however, had different particle sizes from a few µm up to the used mesh size from sieving the ZrN powder. These various particle sizes lead to a much better packing of the powder. A higher weight at same volume was already noticeable when filling the carbon dies. Another reason for the difference in density is the oxygen content of the powders. A previous chemical analysis of the commercial powder showed a much higher oxygen content compared to lab-made ones (see table 3.3). This leads to the formation of zirconium oxide phases and oxy-nitride phases, which have a lower density than zirconium nitride. This can be seen in the XRD measurements of the samples S100611A1 and S100611C1 (figure 3.5). Figure 3.4 shows the SEM analysis of S100611A1 and S100611C1. Here the higher porosity of the commercial powder can be clearly seen. Very remarkable is sample’s S110414B1 extremely high density of 99.7% at a sintering time of just 5 min. This high density was achieved by a powder with a low oxygen content, a slightly higher temperature than the previous runs and and a lower heating rate. The positive effect of the latter can be explained by the lower temperature gradient inside the sample. A high gradient would cause the outside of the sample to sinter while the inside still has a considerable porosity. [27]

57 3 Sintering

(a) S100611A1 (b) S100611C1

Figure 3.4: SEM images of ZrN pellets .

58 3.2 Experiments

(a) S100611A1 "lab-made"

(b) S100611C1 "commercial"

Figure 3.5: XRD spectra of ZrN pellets .

59 3 Sintering

3.2.2 Uranium nitride Table 3.4 lists some representative sintering experiments with pure uranium nitride.

Sample Material Heating rate and Holding Relative max. temperature time [min] density [%] S100426D1 MUP100421 100 ◦C min−1 to 1600 ◦C 15 96.6 S100426D2 MUP100421 100 ◦C min−1 to 1600 ◦C 2 95.6 S110223A1 UP101124 100 ◦C min−1 to 1200 ◦C 3 87.0 50 ◦C min−1 to 1650 ◦C S110323B2 UP101027 100 ◦C min−1 to 1200 ◦C 3 97.7 50 ◦C min−1 to 1650 ◦C

Table 3.4: Sintering parameters and densities of the UN pellets

A typical uranium nitride pellet after polishing can be seen in ﬁgure 3.6. Its apearance is a metallic dark gray color.

Figure 3.6: UN pellet .

The samples S100426D1 and S100426D2 were made of the uranium nitride powder MUP100421. MUP100421 was a mixture of several older UN powders. Unfor- tunately no XRD data and, thus, no informations about its uranium sesquinitride content were available. However, it is interesting that a relatively high density of 95.6% can be achieved already with a short holding time of 2 min. Increasing the holding time to 15 min increased the relative density to 96.6%. The pellets S110223A1 and S110323B2 were sintered of the powders described in chapter 2. S110223A1 had a relative density of 87.0%, while S110323B2 had a much higher density of 97.7%. The significant difference can be explained by the presence of the uranium sesquinitride phase in UP101124, which was shown before. The low density of this phase compared to uranium mononitride leads to a lowered density of the sample. The XRD analysis of this sample (figure 3.8) shows that the

60 3.2 Experiments

Figure 3.7: SEM image of S110323B2 . uranium sesquinitride phase is also present in the pellet after sintering, so the short sintering time is not enough to decompose it. A SEM analysis of pellet S110323B2 (ﬁgure 3.7) shows a very homogeneous single-phase microstructure.

61 3 Sintering

(a) S110223A1

(b) S110223B2

Figure 3.8: XRD spectra of UN pellets .

62 3.2 Experiments

3.2.3 Mixed nitrides Table 3.5 lists the performed sinterings with mixed nitride powders. The powders were mixed in a vibratory ball mill for 5 minutes.

Sample Material Heating rate and Holding Relative max. temperature time [min] density [%] S100426A1 31.8at% Zr: 100 ◦C min−1 to 1600 ◦C 2 88.6 ZP0990921 68.2at% U: MUP100421 S100426A2 31.8at% Zr: 100 ◦C min−1 to 1600 ◦C 5 88.8 ZP0990921 68.2at% U: MUP100421 S100426A3 31.8at% Zr: 100 ◦C min−1 to 1600 ◦C 15 91.9 ZP0990921 68.2at% U: MUP100421 S100426B1 16.1at% Zr: 100 ◦C min−1 to 1600 ◦C 15 94.1 ZP0990921 83.9at% U: MUP100421 S100426B1 16.1at% Zr: 100 ◦C min−1 to 1600 ◦C 5 92.2 ZP0990921 83.9at% U: MUP100421 S100426B1 16.1at% Zr: 100 ◦C min−1 to 1600 ◦C 2 90.1 ZP0990921 83.9at% U: MUP100421

Table 3.5: Sintering parameters and densities of the mixed nitride pellets

The appearance of mixed nitride pellets does not diﬀer from pure uranium nitride pellets. They also have a metallic dark gray color (ﬁgure 3.9). As one can see, the pellets with a higher zirconium content achieved a lower relative density at the same sintering parameters. Both S100426A3 and S100426B1 had a lower relative density than the pure zirconium nitride and uranium nitride pellets made of the same powders: S100426C1 and S100426D1. These two pellets had a relative density of approximately 96% (see the previous sections). An explanation for this fact can be the grain boundaries between uranium nitride and zirconium nitride phases, since they cause a lower density. This would indicate

63 3 Sintering

Figure 3.9: Mixed nitride pellet . that there is no complete solid solution in the mixed nitride pellets. The XRD spectrum of pellet S100426B1 (ﬁgure 3.12) shows the presence of a uranium nitride phase. However, the uranium nitride peaks are slightly shifted towards the zirconium nitride peaks, which indicates at least partial dissolution of the diﬀerent nitrides into each other. An XRD analysis of S100426A3 shows a larger shift of the uranium nitride peaks, which is caused by the higher zirconium content of the sample.

4 2

Figure 3.10: SEM image of S100426A3 .

The SEM analysis of S100426A3 showed three separate phases: two pure uranium nitride phases (areas 3 and 4 in ﬁgure 3.10) and one zirconium nitride phase (area 2). The phase in area 3 has a lower density than the one in area 4, which can be seen by its slightly darker color. This indicates that it consists of uranium sesquinitride instead of mononitride. The zirconium nitride phase had around 2.5 wt% of uranium in it. A line analysis of the boundary between a zirconium phase and its surrounding (ﬁgure 3.11) shows a partial dissolution of the nitrides into

64 3.2 Experiments each other. This shows that the sintering time was too short to achieve a solid solution.

Figure 3.11: Line analysis of S100426A3 .

65 3 Sintering

(a) S100426B1

(b) S100426A3

Figure 3.12: XRD spectra of mixed nitride pellets .

66 3.2 Experiments

3.2.4 Alloy nitrides Table 3.6 shows the two sintered alloy powders.

Sample Material Heating rate and Holding Relative max. temperature time [min] density [%] S110323C1 UA110301 100 ◦C min−1 to 1200 ◦C 15 95.3 50 ◦C min−1 to 1650 ◦C S110414C1 UA101207 100 ◦C min−1 to 1200 ◦C 15 89.1 50 ◦C min−1 to 1650 ◦C

Table 3.6: Sintering parameters and densities of the alloy nitride pellets

Pellet S110414C1 had a considerably lower density due to the uranium sesquinitride content of this sample (see chapter 2). Of special interest is sample S110323C1 (figure 3.13) with a zirconium content of 12.4at%, which makes it comparable to the mixed nitride pellet S100426B1. The XRD pattern of this pellet (figure 3.15) shows a good accordance with the one of S100426B1. However, the uranium nitride peaks are shifted further towards the zirconium nitride peaks. This indicates a better dissolution of zirconium nitride in uranium nitride. The lattice constant of the uranium nitride phase was calculated by Mr. Pertti Malkki in MAUD, an analysis software for diffraction spectra based on the Rietveld method. Table 3.7 shows the obtained value and the lattice constants of UN and ZrN. Substance Lattice constant [Å] Source UN 4.889 [20] ZrN 4.579 [2] measured value 4.853

Table 3.7: Lattice constants

Vegard’s law (equation 3.2) assumes a linear relation between the lattice constant of a solid solution and the concentrations of the constituents. [11]

aSS = xZraZrN + (1 − xZr)aUN (3.2)

Here aSS is the lattice constant of the solid solution, ai the lattice constant of constituent i and xZr the content of zirconium in at%. Using formula 3.2, one gets a zirconium content of 11.6at%, which is in good accordance to the actual value of 12.4at%. This is also conﬁrmed by the SEM analysis of the sample (ﬁgure 3.13), which shows that it consists mainly of one uranium nitride phase with a zirconium content of 11.9at%. The rest of the zirconium was found as a separate zirconium nitride

67 3 Sintering

Figure 3.13: S110323C1 . phase, which, however, just occurred to a small amount. This shows that in this sample a good solid solution of zirconium and uranium nitride was achieved.

Figure 3.14: SEM image of S110323C1 .

68 3.2 Experiments

Figure 3.15: XRD spectrum of S110323C1 .

69 3 Sintering

3.3 Summary

After these sintering experiments one can summarize the following:

• With spark plasma sintering very dense pellets can be achieved in a short time. Uranium nitride pellets with a relative density of more than 95% were achieved using a holding time of just 2 minutes at 1600 ◦C min−1. One zirconium nitride pellet reached a remarkable relative density of 99.7% after a holding time of 5 minutes at 1600 ◦C min−1. • The density of the sintered pellets strongly depends on the powder quality. As seen in case of the zirconium nitride, the particle size and particle size distribution as well as the oxygen content had a big inﬂuence on the achieved density. In case of uranium nitride the content of sesquinitride played an important role, due to its lower density. The short sintering time is not enough to decompose the sesquinitride to mononitride. • Additionally the heating rate had an inﬂuence on the density of the samples. A lower heating rate increased the density noticeably. • The sintering time in this method is too short to allow mixed uranium nitride and zirconium nitride powders to form a solid solution. This could be solved by heat-treating the pellets afterwards. Unfortunately this could not be investigated due to the lack of a proper furnace. • It was shown that solid solution (U-Zr)N pellets can be achieved by spark plasma sintering using nitrides made from uranium-zirconium alloys. There were indications that the alloying provides a good enough mixture of these two metals to get a solid solution already during the nitride synthesis.

70 4 Conclusions and Outlook

The synthesis control and monitoring system, which was developed within the framework of this thesis, performed very successfully. This probably unique system made it possible to monitor the synthesis very reliably. This not just gave a deeper understanding of the single reactions; it is also very helpful to assure the production of high-quality nitride powders. With the aid of this system it is possible, synthesizing uranium or uranium-zirconium alloys, to determine when the powderization is complete and to make sure that the uranium sesquinitride is decomposed. As it was shown, the latter is a key factor in order to sinter high density uranium nitride pellets. With these high-quality nitride powders it was possible to achieve single-phase uranium nitride pellets with a relative density of up to 98% by spark plasma sintering. This remarkably high density was attained at a maximum sintering temperature of 1650 ◦C and a holding time of just 3 minutes. It is noteworthy that no milling of the powder was involved. This shows on the one hand the high purity of the produced nitride powders and on the other hand the usefulness of spark plasma sintering. In contrast to traditional sintering methods, very high densities of nitrides can be achieved in a short time. This is especially useful considering the production of fuels containing AmN, since it avoids its decomposition and evaporation. Also high-quality zirconium nitride powders were produced, which had a very good sinterability due to their low oxygen content and good particle size distribu- tions. Here densities of more than 99% were achieved, which is considerably higher than other reported results. However, one milling step of the zirconium hydride was necessary, which its undesirable due to its pyrophoricity. Furthermore it was seen that the sintering time of SPS is too short to achieve a solid solution out of mixed UN and ZrN powders. In order to overcome these two problems experiments with zirconium-uranium alloys were performed. It was shown that in this way monophasic solid solution nitride powders can be obtained without milling of the powders. The homogeneity was good enough to form solid solution pellets by SPS sintering. In summation, one can say that with the alloying, the improved synthesis system and the spark plasma sintering, a manufacturing method was established which allows the production of high-quality solid solution (U, Zr)N pellets without any milling required.

71 4 Conclusions and Outlook

In order to prove the usefulness of the produced pellets as fast reactor fuel, further research will be done, such as the determination of their thermal conductivity and their compatibility to metal coolants. Furthermore irradiation tests will be performed. In order to do so, the production methods used for single pellets have to be adapted for production of larger batches. One challenge here is to ensure uniform quality. Another research topic will be the transfer of the knowledge gathered in this research to the plutonium nitride production line in Göteborg. One interesting point will be the synthesis behavior of plutonium, which will be investigated by the ﬂow measuring system developed in this work.

72 List of Tables

1.1 Comparison between diﬀerent ceramic fuels ...... 9 2.1 Calibration of the ﬂow meters ...... 19 2.2 Assumed deviations of the calibrated values and the maximal relative error ...... 19 2.3 Test run parameters ...... 21 2.4 Sample weights and uranium/zirconium content ...... 38 3.1 Theoretical densities of UN and ZrN ...... 55 3.2 Sintering parameters and densities of the ZrN pellets ...... 56 3.3 Chemical analysis of the nitride powders ...... 57 3.4 Sintering parameters and densities of the UN pellets ...... 60 3.5 Sintering parameters and densities of the mixed nitride pellets . . . 63 3.6 Sintering parameters and densities of the alloy nitride pellets . . . . 67 3.7 Lattice constants ...... 67

List of Figures

1.1 Ingestion radiotoxicity of spent fuel before and after P&T ...... 8 1.2 rock-salt structure ...... 10 1.3 Scheme of the pellet production ...... 12

2.1 Synthesis Set-up ...... 17 2.2 Synthesis Set-up ...... 18 2.3 Thermogravimetric analyser TGA 92 ...... 20 2.4 Uranium discs and alumina crucible ...... 21 2.5 UA110203 and UA110204 hydriding procedures ...... 22 2.6 Unpolished uranium rods ...... 23 2.7 UP101027 and UP101124 hydriding procedures ...... 23 2.8 UP101027 and UP101124 nitriding procedures ...... 25 2.9 UP101027 and UP101124 denitriding procedures ...... 26 2.10 Uranium nitride powder from UP101027 ...... 26 2.11 XRD spectrum of UP101027 ...... 28 2.12 XRD spectrum of a pellet sintered from UP101024 powder . . . . . 28 2.13 Reactor grade zirconium ...... 29 2.14 ZA100902 hydriding procedure ...... 30 2.15 ZA100902 after hydriding ...... 30 2.16 XRD spectrum of ZA100902 ...... 31 2.17 ZA100915 and ZA101129 nitriding procedures ...... 32 2.18 ZA100915 after nitriding ...... 33 2.19 ZA101129 after nitriding ...... 33 2.20 XRD spectrum of ZA100915 ...... 34 2.21 XRD spectrum of ZA101129 ...... 34 2.22 U-rich part of the U-Zr phase diagram ...... 36 2.23 Triple-arc melting furnace ...... 38 2.24 Alloy drops ...... 39 2.25 XRD spectrum of AUZr101208 ...... 40 2.26 SEM images of the alloys ...... 41 2.27 UA101207 hydriding procedures ...... 42 2.28 UA101207 nitriding and denitriding procedures ...... 43 2.29 Nitride powder from UA101207 ...... 44 2.30 XRD spectrum of UA101207 ...... 45 2.31 UA110223 hydriding (H) and dehydriding (D) procedures ...... 46

75 List of Figures

2.32 UA110223 nitriding and denitriding procedure ...... 46 2.33 Nitride powder from UA110223 ...... 47 2.34 UA110301 hydriding (H) and dehydriding (D) procedures ...... 48 2.35 UA110301 nitriding and denitriding procedures ...... 49 2.36 Nitride powder from UA110301 ...... 50 2.37 XRD spectrum of UA110301 ...... 50 3.1 Carbon die ...... 54 3.2 Sintering Set-up ...... 55 3.3 ZrN pellet ...... 57 3.4 SEM images of ZrN pellets ...... 58 3.5 XRD spectra of ZrN pellets ...... 59 3.6 UN pellet ...... 60 3.7 SEM image of S110323B2 ...... 61 3.8 XRD spectra of UN pellets ...... 62 3.9 Mixed nitride pellet ...... 64 3.10 SEM image of S100426A3 ...... 64 3.11 Line analysis of S100426A3 ...... 65 3.12 XRD spectra of mixed nitride pellets ...... 66 3.13 S110323C1 ...... 68 3.14 SEM image of S110323C1 ...... 68 3.15 XRD spectrum of S110323C1 ...... 69

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