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Manufacturing Methods for (U-Zr)N-Fuels

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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 es- tablished 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 partic- ular 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 5 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 lim- ited 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 previ- ous ones. For this reason a matrix of light elements is favorable. Zirconium nitride shows quite promising properties as inert matrix, such as high thermal conductiv- ity 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 ma- terials 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.
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