HYDRIDE BLISTER FORMATION IN ZIRCONIUM-ALLOY PRESSURE TUBE MATERIALS
R. N. Singh, R. Kishore and T. K. Sinha Materials Science Division, Bhabha Atomic Research Centre, Mumbai-400-085. India. Email : [email protected]
Abstract : The problem of hydride blister formation in zirconium alloy pressure tubes of Pressurized Heavy Water Reactors (PHWR) is well recognized. The hydrogen level in pressure tubes is initially much less than the solubility limit at operating temperatures but it increases during service and eventually crosses the threshold concentration for hydride blister formation. Hydrogen migration down the temperature gradient leads to the formation of hydride blisters at cold spots that may develop accidentally. This can severely limit the life of pressure tubes. In the present work, out of pile simulation experiments under controlled thermal boundary conditions were carried out to identify and study the effects of various parameters, which influence hydride blister formation and its growth. In this study, zirconium-alloy pressure tube test pieces of lengths up to 100 mm, widths in the range of 15 to 25 mm and thickness varying from 0.5 to 4 mm and zircaloy-2 pressure tube sections of 150 to 200 mm length were gaseously charged with different levels of hydrogen concentration in a manner such that the microstructure remained unaltered. Hydride blisters were grown on the hydrided specimens using an apparatus capable of producing required thermal gradients between the bulk specimen and a small localized region. Standard metallographic techniques were used to characterize the microstructure of blisters by sectioning the samples along Radial-Circumferential plane with respect to the pressure tube. By using diffusion equation time required to grow a blister of a given size was estimated and compared with times required in actual experiments. Hydride platelet orientations around the blister were explained qualitatively in terms of the stress field generated due to formation of hydride blisters and individual hydride platelets.
Nomenclature : PHWR Pressurized Heavy Water Reactor PT Pressure Tube CT Calendria Tube Zr Zirconium Nb Niobium O Oxygen H Hydrogen D Deuterium α-Zr Alpha zirconium having HCP crystal structure HCP Hexagonal Close Packed β-Zr Beta zirconium having BCC crystal structure BCC Body Centered Cubic D Diffusivity of hydrogen in α-Zr J Hydrogen flux
Cr Hydrogen concentration at a distance r
Cb Bulk hydrogen concentration BFT Blister Formation Threshold CF Cold Finger t Time in seconds
Tcs Absolute Cold spot temperature
Tb Absolute Bulk sample temperature
To Temperature distribution constant K Temperature distribution prelogrithmic coefficient R Gas constant Q* Heat of transport for hydrogen in α-Zr V* Volume of transport of hydrogen in α-Zr σ Stress r Distance
ro Blister radius b Blister depth w Specimen thickness DHC Delayed Hydride Cracking
1 Introduction : Dilute zirconium alloys find applications as the core structural materials of Pressurized Heavy Water Reactors (PHWRs) [1-5] because of their adequate aqueous corrosion resistance, low neutron absorption cross-section, and good elevated temperature mechanical properties. Though the hydrogen content of the core components is kept as low as possible by controlling the manufacturing process parameters [6], it can pick up hydrogen (deuterium) during service from any or all of the following sources [7] : 1. Deuterium / hydrogen evolved during the metal-heavy water coolant corrosion reaction. 2. Deuterium / Hydrogen evolved during the radiolytic decomposition of heavy water coolant. 3. Hydrogen added to coolant as scavenger for nascent oxygen evolved during radiolytic decomposition and 4. Deuterium/ hydrogen evolved during the corrosion reaction between the pressure tube and the moisture present in the annulus gas. Part of the hydrogen (deuterium) thus evolved is absorbed by the pressure tube material [5]. Hydrogen present in excess of solid solubility precipitates out as brittle hydride phase and can severely limit the life of in-core structural materials of PHWR [3-5]. The hydrogen-related problems associated with these components are hydride embrittlement [5, 8-12] mainly due to stress-reorientation of hydride [5, 13-18], delayed hydride cracking [5, 7, 19-24] and embrittlement due to the formation of hydride blisters [25-33]. The formation of hydride blisters in zirconium alloys pressure tube has to be visualized in conjunction with arrangement of core components in PHWR. Figure 1 [34] shows a typical coolant channel assembly of PHWR. Horizontally oriented pressure tubes act as a miniature pressure vessel in PHWR. Calendria tube surrounds the pressure tube from outside. Garter springs are provided at regular interval in the annular space to support pressure tube and prevent excessive sagging. Displacement of garter springs from the
designed location either during installation of the reactor or during service can lead to pressure tube-calendria tube contact thus setting up a steep thermal gradient across the pressure tube section. In the presence of a thermal gradient, hydrogen migrates down the temperature gradient. Once the local solid solubility is exceeded it precipitates out as hydrides at or around the low temperature spot (henceforth called cold spot). Since the transformation of zirconium metal into hydride is associated with increase in volume, a bulge will appear at the cold spot because of the bursting of hydride out of the surface. This hydride bulge is called hydride blister, due to its appearance. Attempts to contain the damage to the integrity of the pressure tubes due to hydride blister formation can be made from two angles [32]. From the basic materials research point of view, the blister is modeled with various boundary conditions to evaluate the blister size, time to grow hydride blisters of known size [29], growth rate [32], and hydrogen concentration profile [25] or to experimentally determine material property like cracking strength of hydride blisters [30], which can be used by designers. From engineers point of view attempts are being made to demonstrate the capability of the non-destructive techniques (both intrusive and non-intrusive) in detecting the contacting channels [35-37]. One aspect of the second objective is the development of techniques for hydride blister detection and their validation for adoption on actual reactor site [32]. Studies on the interaction between the hydride blister and delayed hydride cracking phenomena will be extremely useful for pressure tube life extension. Of late the embrittlement due to hydride blister formation has been receiving great attention due to its bearing on the safety and life of the pressure tubes of the PHWRs [25-33]. Hence, a research program has been started in Materials Science Division to study various issues related to hydride blister formation in dilute zirconium alloys. In this report, some of the results obtained from the out-of-pile hydride blister simulation experiments carried out under controlled thermal boundary conditions are being presented.
The objectives of the first phase of this research program were :
1. To design and fabricate blister simulation jigs for growing hydride blisters in laboratory. 2. To standardize the experimental procedure for growing hydride blister under controlled thermal boundary condition and in a reproducible manner. 3. Metallurgical Characterization of hydride blisters to understand the mechanism and kinetics of hydride blister formation in Zr-alloy pressure tube materials.
4. Modeling & experimental validation of hydride blister growth behavior. 5. To prepare samples for calibration of the BARCIS inspection head and its validation, before its adoption for in-service inspection to detect hydride blisters on actual reactor site.
2 Experimental Procedure : The hydrogen-charging facility was augmented to charge hydrogen in pressure tube spools of length up to 200 mm. The details of the hydrogen charging apparatus is given in 2.1.1. Two types of Blister simulation jig were fabricated and commissioned. These are Single specimen jig - for pressure tube spools of length up to 200 mm) and Multi-specimen jig for plate type specimens (dimension 100 x 25 x 5 mm) capable of holding up to six specimens at a time. The details of the jig are given in 2.1.2. Cold spot(s) was created on one surface of the specimen(s) by water-cooled point contact. The thermal gradient was adjusted by controlling the water flow. Two types of specimens were obtained from dilute zirconium alloy (Zircaloy-2 or Zr- 2.5Nb alloy) pressure tube material. The plate type specimens, obtained from cold flattened Zr- 2.5Nb tube sections, were of dimensions, length 35-100 mm, width 15-25 mm and thickness 0.5- 4.5 mm. The spool type specimens were sections of zircaloy-2 pressure tubes of length 200 mm. These specimens were polished successively up to 1200 grits and subsequently gaseously charged with controlled amount of hydrogen in a modified Seivert’s apparatus at a temperature in the range 350-400°C. A homogenization treatment was given to attain uniform distribution of hydrogen. Subsequently both the ends of the spool type specimens were threaded for attaching the extension tubes.
For plate-type specimens hydride blisters were grown following two methodologies. In case I the specimens were soaked at a predetermined temperature to take all the hydrogen in solution before applying the water-cooled point contact. This ensured that no hydride precipitate was present near would be cold spot region before the cold finger was applied. In case II the cold finger was maintained in contact with the specimens since the beginning of heating. This ensured that hydride platelets were present in the region around the cold spot before the arrival of the thermally migrated hydrogen.
For temperature measurement K-type thermocouples were placed on both the face of the samples at regular interval to record the temperature profile as a function of distance from the cold spot.
Hydride blister growth rate was taken as the ratio of the bulge height of the blisters to that of the time taken to grow it. The blister bulge height was measured using a micrometer of least count 0.01 mm (i.e. 10 micron). Standard metallographic techniques were used to reveal the microstructure of some of the hydrided specimens and a few hydride blister section along the radial-circumferential plane of the pressure tubes. The hydride blisters were also examined by neutron radiography.
2.1 Facilities set up : In order to achieve the objectives listed above, the gaseous hydrogen charging facility was augmented and upgraded. A blister simulation bench was specially fabricated for carrying out blister simulation studies under controlled thermal boundary condition. The details of the hydrogen charging system and the blister simulation jig are described below. 2.1.1 Hydrogen charging system : This system consists of two glass chambers to hold the specimen and the hydrogen source, a vacuum pumping system, a capacitance based manometer and a couple of resistance heated furnace to heat the specimen and the hydrogen source. The photograph of the gaseous hydrogen charging facility is shown in figure 2. Pressure tube spools of length up to 200 mm can be charged with controlled amount of hydrogen in a manner, which ensures that the initial microstructure remains unchanged. Pure zirconium hydride, obtained from Nuclear Fuel Complex, Hyderabad, is used as source for hydrogen. Unlike electrolytic charging and LiOH autoclaving, this is a contamination free process and ensures uniformity of hydrogen distribution.
2.1.2 Blister simulation jigs: It consists of a rod type resistance heater, aluminum jig, cold fingers of different sizes, temperature controllers and indicators and an insulation box. All these components are mounted on working table, called blister simulation bench. The aluminum jig is used for mounting the specimen(s) on which blister are to be grown and the thermocouples at the predetermined locations for temperature profile measurement. Two types of blister simulation
jigs have been setup for this research activity. These are single specimen jig and multi-specimen jig. Blisters are grown on pressure tube spools using single specimen jig. Three numbers of both curved and plate type test specimens can be loaded simultaneously on the multi-specimen jig for blister growth. Figure 3 shows the photographs of the both jigs mounted on the blister simulation bench.
3 Results : Some of the experimental results of the study on hydride blister formation in Zr-2.5 wt.%Nb pressure tube alloy are being presented in this section. The microstructure and texture of the pressure tube is such that only circumferential hydrides (oriented along the axial- circumferential plane of the pressure tube) form in the as-hydrided condition [5]. Figure 4 shows the optical micrograph of the as-hydrided Zr-2.5Nb pressure tube material containing (a) 35 ppm, (b) 60 ppm and (c) 85 ppm of hydrogen. Dark lines are the hydrides. With increase in hydrogen concentration hydride platelet length is found to increase, though there is no significant change in interplatelet spacing. Figure 5 shows the micrograph of the hydrided zircaloy-2 pressure tube spools at different axial and circumferential locations.
Two types of blister morphologies were observed. In case I soaking temperature and the time was so chosen that all the hydrogen was present in dissolved state. The cold finger was brought in contact with the specimen after at least one hour of soaking. This resulted in formation of a single blister at the cold spot, designated as type I blister (figure 6). In case II, the cold finger was kept in contact with the specimen since beginning of experiment. This ensured that during subsequent heating the region in contact with the cold finger remained at the lower temperature. This led to the formation of several small blisters on a ring around the cold spot, which were termed as type II blister (figure 6). With increasing time these small blisters joined to form a continuous ring. Thermal boundary conditions, alloy type and hydrogen contents of some of the test specimens are listed in table 1.
Hydride blisters of type I could be detected easily by neutron radiography. Figure 7 shows some of the neutron radiographs. Darker circular spot within a rectangular boundary is the image of hydride blister. However, the type II blister grown for the same duration could not be detected by neutron radiography.
The parameters like cold spot temperature, soaking temperature, specimen dimensions, contact conductivity, hydrogen content and alloy composition seem to affect the hydride blister growth rate. The effect of hydrogen content on the blister growth rate, for a given combination of bulk specimen and cold spot temperature, has been shown in figure 8(a). For the hydrogen concentration range studied, blister growth rate increase with the increase in hydrogen concentration. Figure 8(b) shows the effect of soaking temperature on blister growth rate for a given hydrogen concentration. With increase in soaking temperature, blister growth rate was observed to increase.
Figure 9 (a & b) show the radial-circumferential (RC) sections of types I and II, hydride blisters grown in Zr-2.5Nb pressure tube material. The metallographic examination of the section of blisters brought out the difference in their aspect ratio. Hydride platelets radiating from cold spot center can be seen. It is evident from this figure that a section of hydride blister has three regions (fig 9 c). Far away from the center of blister lies region I, comprising of matrix and circumferential hydrides. This region is similar to the as-hydrided micrograph (RC plane) shown in fig. 4. As one approaches the center of blister, a region comprising of matrix, circumferential hydrides and radial hydrides (normal to circumferential ones) can be seen. This will be called region II here. Region III is the region of single-phase hydride. The boundary between regions II and III appears darker under the optical microscope because of uneven etching [25-26]. Single- phase region III etches evenly and appears brighter. In region II (fig. 9 c), both radial and circumferential hydrides can be seen forming finer network as one approaches the boundary between regions II and III. Figure 9 (d) shows the montage of top view (AC plane) and RC section of the Type II blister.
3.1 Samples for BARCIS Calibration : Ultrasonic velocity ratio measurement technique can be used for hydride blister detection in pressure tubes [32]. In order to use this technique on reactors, further qualification using full size pressure tube specimens with known blister size and morphology is being carried out in collaboration with DRHR, B. A. R. C. inspection system (BARCIS) is the primary tool for in-service inspection of coolant channels of PHWR [35] in India to confirm their structural integrity and collect data related to changes in material property and component dimension. The goal is to calibrate the Mark III version of BARC Channel Inspection System (BARCIS). In order to accommodate the inspection head of the BARCIS-
Mark III, pressure tube sections of minimum length of about one meter are required. However, because of the practical difficulty of charging hydrogen in such a large piece, it was decided to use 150 – 200 mm long pressure tube pieces. These spools were gaseously charged with required amount of hydrogen and suitable threads were machined at both the ends. Subsequently, hydride blisters were grown under controlled thermal boundary condition. Finally, by attaching extension tubes the length was increased to full pressure tube length. Ten such spools of zircaloy-2 pressure tube have been prepared for DRHR for standardization of the hydride blister detection technique. Based on the feedback of DRHR further work will be carried out. Figure 10 shows the photograph of some of the samples on which hydride blisters were grown. Three blisters at same axial location but 120 ° apart along the circumference were grown in each specimen. Figure 10 (a - top) shows the photograph of sample number Blist.cal.07, containing blister grown using 2 mm diameter cold finger. The photograph of the other two blisters grown using 4 and 2 mm diameter is also shown in this figure (below). Figure 10 (b) shows hydride blister calibration sample nos. 11 to 13. Figure 10 (c) Shows the photograph of the blisters, grown on these samples. The details of some of the pressure tube spools on which hydride blisters were grown for BARCIS calibration are given in table 2. Remaining samples listed in table 2 are being used for hydrogen content estimation by both non-destructive examination and chemical analysis.
4 Discussion : Hydrogen is known to migrate down the temperature gradient in dilute zirconium alloys [5]. In case of garter spring displacement from the designed location, the pressure tube may sag due to creep and touch the cool calendria tube and thus setting up a thermal gradient around the outer surface of the pressure tube. When the hydrogen content is greater than blister formation threshold, it will migrate from the higher temperatures towards the cold spot. Hydrogen in excess of the solid solubility precipitates out as low-density brittle hydride. This led to the formation of a bulge on the surface, which due to its appearance is called hydride blister. These blisters were used for studying blister morphologies, blister formation threshold, parameters affecting hydride blister growth rate and hydride platelet orientations in the matrix surrounding the blister.
4.1 Hydride blister morphologies : The formation of more than one blister around or at the cold spot has been reported by Domizzi et. al. also [25]. This was attributed to improper
contact, which may be due to the large size of the cold finger (7 mm) used in this study. However, results of the present investigation indicated that the formation of single or multi- blisters, using a cold finger size of 2 mm only, is a function of soaking time, thermal boundary condition and hydrogen content. Two types of blister morphologies were observed in the present work. The specimens which were soaked for an hour, led to the formation of single blisters, whereas the one without any soaking resulted in the formation of multiple blisters around the cold spot. Multiple blisters could be more readily grown on thinner plates, at very low cold spot temperature and for high hydrogen content. In case I during soaking, all the hydride platelets dissolved, and when the cold finger is struck, the region just below the cold finger cooled first. At the lower temperature the hydrogen in excess of solid solubility precipitated as hydride. This hydride acted as the nucleus and kept growing with arrival of thermally migrated hydrogen from the bulk. However, in case II due to insufficient soaking and/or high hydrogen content all the hydrides were not dissolved. As a consequence of this, hydride platelets were already present in the region around the cold spot. Thus with the arrival of thermally migrated hydrogen, some of them could grow simultaneously like grain formation during solidification. With time these individual blisterets grow till they meet each other and then they fused to give the appearance of a ring around the cold spot. The blisters grown as case I resulted in the formation of a single blister at the cold spot (Figure 6 top row) whereas the blisters grown as case II led to the formation of several small blisters on a ring around the cold spot (figure 6 bottom row). With time these small blisters joined to form a continuous ring.
4.2 Blister Formation Threshold (BFT) : Hydrogen is known to migrate down the concentration and temperature gradient and up the tensile stress gradient [5]. The general diffusion equation using all the three parameters is given below :
* * − DCr d lnCr Q dT V dσ J = RT + − 1 RT dr T dr 3 dr
Where, Cr = Hydrogen concentration at any point r.
D = Diffusivity of hydrogen = 0.217 exp -(8380/RT) mm2/s for α-Zirconium J = Hydrogen flux. Q* = Heat of transport of hydrogen in metal. R = Gas Constant. T = Temperature. V* = Volume of transport of hydrogen in metals. σ = TENSILE STRESS (TAKEN AS +VE) COMPRESSIVE STRESS (TAKEN AS -VE)
As can be seen from the general diffusion equation given above, hydrogen migrates down the temperature gradient. Let us take the case when bulk hydrogen concentration is more than the TSS at the cold spot temperature. In such a case imposition of thermal gradient will also lead to the formation of concentration gradient and both thermal and concentration gradient will ensure hydrogen migration towards the cold spot. However, for the case when bulk hydrogen concentration is lower than the TSS at cold spot temperature, initially hydrogen migration due to thermal gradient will take place. However, this will set up a concentration gradient, which will allow hydrogen migration away from the cold spot. This has been illustrated by the schematics 1 and 2.
Tb Cb
TSS
To
Cold Hot end spot
Schematics 1 showing for Cb > TSS at cold spot, impression of thermal gradient results in setting up of concentration (TSS) gradient as well.
Hence, there must exist a critical hydrogen concentration for a given thermal boundary condition, below which there is no net migration of hydrogen. At this concentration of hydrogen a dynamic equilibrium is maintained and the hydrogen flux due to concentration gradient and thermal gradient acts against each other. This thermal migration threshold is called blister formation threshold (BFT).
Mathematically, the net flux, J, is given by
* − DCr d lnCr Q dT J = RT + = 0 2 RT dr T dr
Solving this partial differential equation, one can find the threshold hydrogen concentration as shown below.
From 2, we get d lnC Q * dT RT r + = 0 dr T dr
* 1 dCr Q dT ⇒ + 2 = 0 Cr dr RT dr
1 dC dT Q * dT r ⇒ + 2 = 0 Cr dT dr RT dr 1 dC Q * r ⇒ + 2 = 0 Cr dT RT Separating the variables and on integration we get,
* TSScs dC Q Tcs dT r ∫ = − ∫ 2 C T b Cr R b T
Limiting value of the hydrogen concentration, Cr, at the cold spot can be taken as terminal solid solubility of hydrogen in α-Zr at cold spot temperature.
* BFT Q ()Tb − Tcs ∴ = exp− 3 TSScs R TbTcs
T b
TSS
To
Cb
Cold At t = 0 Hot end spot
Tb
TSS
To C
Cb
Cold At t = finite value Hot end spot
Schematics 2 showing for Cb < TSS at cold spot, impression of thermal gradient does not result in setting up of concentration gradient at t =0 (top). However, with time as thermal migration of hydrogen starts, concentration gradient builds up which opposes the hydrogen migration due to thermal gradient (bottom).
Thus, blister formation threshold (BFT) can be evaluated using equation 3. From this equation it is evident that BFT depends on terminal solid solubility at cold spot temperature, heat of transport of hydrogen in α-Zr, cold spot and bulk specimen temperatures. Figure 11 shows the BFT variation with cold spot and bulk specimen temperatures. For a given bulk specimen temperature, as the cold spot temperature increases BFT is predicted to increase.
4.3 Swatzky’s model [29] : Model developed by Swatzky was applied to the blisters grown under laboratory conditions. By considering logarithmic temperature distribution, the above model was used to estimate the time required for blister growth in the present study.
4.3.1 Estimation of hydrogen flux : For cylindrically symmetric temperature distribution, as was the case during our experiments, the thermo-migration equation describing the hydrogen flux at any point is given by
dC DCQ * dT 4 J = −D − dr RT 2 dr
where, (the symbols have the same meaning as in equation 1)
D = Diffusion coefficient of hydrogen in α-zirconium = Doexp(-Q/RT), C = Hydrogen concentration participating in diffusion = TSS, for bulk hydrogen concentration > TSS (Terminal solid solubility) = Aexp(-H/RT).
Therefore,
(dc/dr) = (d(TSS)/dr) = (AH/RT2) exp (-H/RT)(dT/dr) 5
Assuming cylindrical symmetry, mid-wall temperature distribution can be approximated as:-
T = To + Kln(r)
T
Distance, r
=> (dT/dr) = (K/r) 6
Using 5 and 6 we can rewrite equation 4 as :
2 J = -DoAK/(rT ) ((H + Q*)/R)exp(-(H + Q)/RT) 7
r o
b
w
4.3.2 Estimation of time for blister formation : Amount of hydrogen in blister, M, is given by :
M = VCδ, where V is the volume of blister and Cδ is hydrogen concentration in δ- hydride.
2 2 Or M = ( /3) πro b Cδ. 8
If J(ro) be the flux at r = ro, M = 2πrowJ(ro)t.
From 8 we get,
t = (rob Cδ)/(3wJ(ro)).
As can be seen from table 3 the estimated time is higher than the experimental value. The reasons for overestimation of time for blister growth is being looked into but it appears that it could be due to the ignorance of diffusion through meta-stable beta-zirconium and contribution from the hydrogen migration up the thermal stress gradient due to the formation of cold spot.
4.4 HYDRIDE BLISTER GROWTH RATE : The growth of hydride blisters depend on the amount of hydrogen present in solution and the thermal energy associated with it. Hence, for high hydrogen content and high temperature the blister growth rate should be higher, which can be inferred from figure 8 also.
Since the kinetics of type II blister formation is slower, blisters grown for same duration were smaller compared to type I. This could be the reason for their non-detectibility during neutron radiography.
Since hydride blisters are formed in zirconium alloys due to the thermal migration of hydrogen down thermal gradient to the cold spot, any metallurgical/mechanical parameter, which will affect the diffusion (or temperature gradient) will have a significant effect on blister growth. Some of these parameter are bulk specimen temperature, soaking time, cold spot temperature, geometry of cold finger, thickness of plate, volume fraction of alpha and beta phases, mechanical and crystallographic anisotropy and alloying elements. Bulk specimen temperature: Blisters were grown with bulk specimen temperature in the range of 250-400 °C. Though the bulk specimen temperature seems to have no effect on the morphology of the blisters but it was found to affect the kinetics of the process. At high temperature it took
shorter time to grow a blister of given size. This is expected as amount of hydrogen participating in diffusion at high temperature is higher due to the endothermic nature of terminal solid solubility of hydrogen in zirconium alloys. Soaking time: Shorter soaking time favors the formation of Type II blisters whereas longer soaking time (>1 hour) favors the formation of Type I blisters. Cold spot temperature and plate thickness: Present experiments established that very low cold spot temperature favors the formation of Type II blisters whereas high cold spot temperature favors the formation of Type I blister at the cold spot. However the cut-off temperature is a strong function of plate thickness. For thinner plates the cut-off cold spot temperature for Type II blister formation is higher.
4.5 HYDRDE PLATELET ORIENTATION AROUND BLISTER : As can be seen in fig. 9 (c), radial hydrides appear only in a location around the deepest point (marked as ) of blister. Near the surface on both side of the blister (fig. 4) no radial hydrides could be seen. The composition, microstructure and texture of the matrix region surrounding the blister are expected to remain unchanged. Hence these factors could not explain the variation in hydride platelet orientation in the matrix around the blister. Since hydride platelet orientation in Zr-2.5Nb alloy is governed by stress [13-18], the stress field generated as a result of blister formation may explain the occurrence of radial hydrides only at selected locations. In this section a qualitative model is presented to explain the formation of hydride platelet network. During blister formation, initially only circumferential hydrides formed governed by the crystallographic criteria [5,38]. In the initial stages of blister growth, with the arrival of thermally migrated hydrogen, the inter-platelet spacing decreased and the matrix gradually transformed into single-phase hydride called blister. As the blister grew matrix experienced two types of stresses. These were local and global stresses. Local stress, σlo, originated because of the accommodation [39] of the individual hydride platelets in the matrix.
σlo is compressive for the matrix sandwiched between two hydride platelets. As the inter-platelet spacing decreases and/or the platelet thickness increases, the magnitude of the compressive stress
increases. The global stress, σgl, component [40] is generated due to the volume mismatch between the single-phase hydride (Region III of blister) and the matrix containing hydride platelets surrounding it (fig. 4). The hydride platelet orientation at any location in the matrix
around the blister is governed by sum of both the global and local stresses. Once the total tensile stress prevailing at any point in the matrix is greater than the threshold stress for reorientation of hydrides [5, 13-18], radial hydride will also precipitate out.
5 CONCLUSIONS :
1. The procedure for hydride blister formation in zirconium alloy pressure tube material has been standardized. Hydride blisters are being routinely grown, using gaseously charged zirconium alloy samples, under controlled thermal boundary condition.
2. Two types of hydride blister morphologies have been identified. Presence or absence of hydrides before the arrival of thermally migrated hydrogen is responsible for the development of these morphologies. Neutron radiography can detect type I blisters which is more deleterious to integrity of the pressure tubes.
3. The proposed model can qualitatively explains the formation of hydride platelet network. However, its quantification in terms of blister shape and size, hydride interplatelet spacing and platelet aspect ratio has to be carried out.
4. Fourteen number of zircaloy-2 pressure tube spools of length 150 mm have been gaseously charged with controlled amount of hydrogen. Out of these, blisters of different size could be grown successfully on ten samples for BARCIS inspection head calibration for blister detection. Remaining samples are being used for hydrogen content estimation by both non- destructive examination and chemical analysis.
Note : The calibration of BARCIS inspection head and its validation for blister detection on actual reactor site is being carried out in collaboration with Shri Manjit Singh, Head, Division of Remote Handling and Robotics, BARC.
Acknowledgements
Authors appreciate the constant encouragement provided by Dr. S. Banerjee, Director, Materials Group. Authors wish to thank Shri R. K. Sinha, Associate Director, Reactor Design and Development Group and Head, Reactor Engineering Division for his keen interest and for helpful suggestions during the course of this study. Authors are thankful to Shri. Manjit Singh, Head, Division of Remote Handling and Robotics for the financial support extended by him in augmentation of the hydrogen charging apparatus.
Authors are indebted to Shri K. C. Sahoo, Head, Post Irradiation and Examination Division for the financial support extended by him in the upgradation of the blister simulation jig.
Discussion with Shri B. K. Shah of Atomic Fuels Division was indeed helpful.
Zirconium hydride powders used as hydrogen source were obtained from Nuclear Fuel Complex, Hyderabad.
The efforts of Shri M. Unnikrishnan & Smt. P. Agashe of Materials Science Division, and Shri A. K. Sinha, AFD for some of the optical microscopy work cited in this report and Dr. A. M. Shaikh, SSPD for carrying out the neutron radiography are also duly acknowledged.
The assistance provided by Shri N. T. Parekh and Shri P. G. Adiga of Materials Science Division in carrying out the assembly and the electrical wiring of the blister simulation jigs needs special acknowledgement.
The help provided by Shri B. Dasmajumdar of Materials Science Division, and Shri. Ram Krishna Fotedar, of Materials Processing Division at many stages of sample preparation and blister simulation jig fabrication is also acknowledged.
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Table 1 : Details of hydride blisters grown in some of the Zirconium alloy coupons
Specimen Material Hydrogen Thermal boundary condition Duration in days Morphology type
Id. content in ppm °C (T b/Tcs)
B1 Zr-2.5Nb PT alloy 250 270-290/50-70 17 Type II
B2 Zr-2.5Nb PT alloy 195 300-330/50-70 30 Type II
B3 Zr-2.5Nb PT alloy 230 330-350/50-70 18 Type I
B4 Zr-2.5Nb PT alloy 110 380-390/50-70 31 Type I
B5 Zr-2.5Nb PT alloy 240 330-340/50-70 20 Type II
B6.1 Zr-2.5Nb PT alloy 218 300/50-55 5 Type I
B6.2 Zr-2.5Nb PT alloy 218 300/45-50 7 Type II
B7 Zircaloy-2 PT 118 300/60-80 28 Type II
B0 Zr-2.5Nb PT alloy 20 300/40 91 Type I
B8 Zr-2.5Nb PT alloy 135 270±10/100±10 20 Type I
B9 Zr-2.5Nb PT alloy 75 275±5/55±15 120 Type I
Table 2 : Details of hydride blisters grown in Zirconium alloy pressure tube spools for BARCIS calibration
Specimen Id. Material Hydrogen Tcs /Tb in °C CF tip radius Duration End use
content in ppm in mm in days
BLIST.CAL.01 Zircaloy-2 PT 32.4 50/300 2 30 Blister simulation
BLIST.CAL.02 Zircaloy-2 PT 86 60/350 & 50/330 4/1 33 Blister simulation
BLIST.CAL.03 Zircaloy-2 PT 104.3 50/400, 50/390 & 45 4/2/1 24 Blister simulation
BLIST.CAL.04 Zircaloy-2 PT 26.85 - - - Hydrogen analysis
BLIST.CAL.05 Zircaloy-2 PT 55.8 - - - Hydrogen analysis
BLIST.CAL.06 Zircaloy-2 PT 76.4 - - - Hydrogen analysis
BLIST.CAL.07 Zircaloy-2 PT 60 60/380, 55/360 & 50/400 2/4/2 53 Blister simulation
BLIST.CAL.08 Zircaloy-2 PT 104.6 60/385, 50/360 & 40/390 2/4/2 56 Blister simulation
BLIST.CAL.09 Zircaloy-2 PT 67 68/390, 54/344 & 56/397 2/4/2 12 Blister simulation
BLIST.CAL.10 Zircaloy-2 PT 69.4 - - - Hydrogen analysis
BLIST.CAL.11 Zircaloy-2 PT 71 65/400, 60/353 & 50/400 2/4/2 31 Blister simulation
BLIST.CAL.12 Zircaloy-2 PT 93.4 70/407, 61/362 & 61/406 2/4/2 27 Blister simulation
BLIST.CAL.13 Zircaloy-2 PT 80.9 76/401, 70/369 & 70/401 2/4/2 72 Blister simulation
BLIST.CAL.14 Zircaloy-2 PT 74.8 - - - Tensile property
Table 3 Comparison of time estimate to grow a blister of given size with actual experimental value
Id. No. Type r0 in mm b in mm t in days Swatzky’s model actual B3 I 1.57 1.17 59 18 B5 II 1.56 1.31 27.4 20