Patent Number: 
Section: description

A number of commissioning tests were performed to ensure that the furnaces used, which are radio frequency (RF) furnaces, function correctly and there is compatibility between the Zircaloy cladding and the molten stainless steel. In addition commissioning tests also involved resistance measurements on lengths of cladding which had been oxidised in air to produce an oxide layer having a thickness of approximately 20 xcexcm. The tests showed that the stainless steel started to melt at approximately 1250xc2x0 C., that is to say, at a considerably lower temperature than its specified melting point range of 1400-1455xc2x0 C. The tests also demonstrated that the high resistance of the zirconium oxide ( greater than 2 Mxcexa9) is eliminated, the resistance dropping to 0.1xcexa9 before the stainless steel begins to melt. A test carried out in a graphite crucible without the presence of stainless steel revealed that zirconium oxide is reduced on heating in a pure argon/carbon monoxide atmosphere. In addition the tests demonstrated that Zircaloy reacts strongly with molten stainless steel at 1500xc2x0 C. resulting in penetration of the stainless steel melt inside the Zircaloy cladding and severe interaction between the two alloys. The microstructure of both the stainless steel and the Zircaloy cladding is altered. The steel structure consisted of austenite grains in a complex eutectic mixture with inclusions of graphite flakes. The Zircaloy structure had an xcex1-Zr layer in the outer surface and columnar grains at the remaining inside thickness. There was also a layer of intermetallic compounds at the Zircaloy/stainless steel interface. In some tests the cladding had cracked at the stainless steel miniscus region due to the complex interaction in molten stainless steel, that is to say, differential contraction between the two alloys on cooling and hoop stresses generated in the Zircaloy cladding as a result of oxygen diffusion to form an xcex1-Zr layer. The shape of the Zircaloy cladding within the solidified steel was no longer round but heavily convoluted and the cladding was reduced in thickness. Tests were carried out at 1255, 1310, 1358 and 1415xc2x0 C. to determine the optimum temperature for reduction of the zirconium dioxide and to develop a good fusion bond between the Zircaloy and the stainless steel. The results showed that, although the resistance had dropped to a minimum value (approximately 0.02xcexa9) in all cases, there was a lack of fusion bonding at temperatures of 1255 and 1310xc2x0 C. The tests performed at 1358 and 1415xc2x0 C. show both elimination of the oxide layer and good fusion bonding between the two alloys. Accordingly, the optimum temperature was selected to be about 1385xc2x0 C. (midway between 1358 and 1415xc2x0 C.) to ensure good fusion bonding and minimised embrittlement of the cladding. Further tests were performed on single cladding lengths with differing oxide thicknesses (15, 19, 36 and 42 xcexcm) at 1385xc2x0 C. In all cases the high resistance due to the zirconium dioxide layer dropped to 0.1xcexa9 before the stainless steel began to melt. Subsequent to the stainless steel melting, there was a very small further drop in resistance probably due to dissolution of the residual oxide layer on the Zircaloy clad surface. A minimum resistance of around 0.025xcexa9 was attained in all cases before the temperature was raised to 1385xc2x0 C. There was no systematic relationship between the oxide thickness and the temperature to achieve a minimum; even the cladding with the maximum oxide thickness (42 xcexcm) achieved the minimum resistance at a similar temperature. A visual metallographic examination of the samples after the tests indicated complete melting of the stainless steel and good fusion bonding between the Zircaloy and the stainless steel. In a further test a small model fuel assembly (comprising a 3xc3x973 matrix of Zircaloy cladding lengths) was heated to a melt temperature of about 1385xc2x0 C. Movement of the rod assembly (supporting the nine cladding lengths) to one side of the graphite crucible occurred at about 1300xc2x0 C. indicating the bulk melting of the stainless steel block. A rapid drop in resistance from above 2 Mxcexa9 to less than 0.1xcexa9 occurred in the temperature range between 1000 and 1260xc2x0 C. Subsequently the resistance continued to drop very slowly with temperature until a minimum value of 0.025xcexa9 was recorded at 1350xc2x0 C. There was no further drop in resistance during heating to the target temperature of 1385xc2x0 C. After the test, it was found that the steel had melted around all the Zircaloy lengths and there was good contact between the two alloys. One cladding length had broken near the meniscus region and two lengths showed two cracks in the same region, the rest remaining intact. In order to limit embrittlement of the cladding melt interface, the experimental technique was modified to hold the cladding partially immersed in the stainless steel block during the heating and melting process. Immediately before cooling begins the cladding is fully immersed into the melt. In this way the top length of the cladding with limited embrittlement is encased by the stainless steel cast, thus reducing the tendency to fracture at the gas/melt interface. The minimum resistance criterion is met by the lower part of the Zircaloy cladding, which is fully fused in the stainless steel. Two Tests were Carried Out: Test 1: Heating to 1370xc2x0 C. at 200xc2x0 minxe2x88x921, hold time 2 minutes and cooling at xcx9c200xc2x0 C. minxe2x88x921 ie furnace turned off. Test 2: Heating to 1370xc2x0 C. at 200xc2x0 C. minxe2x88x921, hold time 2 minutes and cooling at 50xc2x0 C. minxe2x88x921. In order to allow partial immersion of the cladding during the heating and melting process and subsequent full immersion at the target temperature, some alterations were made to the RF furnace components. The stainless steel block was redesigned ie a 38 mm deep bore was drilled in the top length to immerse the cladding partially during the heating and melting process and the bottom 12 mm length was a solid block to allow the full immersion (further immersion by 10 mm) after melting. The graphite cubicle was dished at the top part to accommodate the molten metal ejected during the full immersion. The top plate of the furnace was provided with double seal entry to allow the gas tight movement of the cladding during the full immersion of the cladding at the final stage. The experimental technique involved heating the Zircaloy tube having an oxide layer xcx9c30 xcexcm, and stainless steel contained in a graphite crucible using a R F furnace. As open-ended tubes were provided, a plugging devise (alumina pellet) is inserted in the bottom of the tubes to minimise entry of the stainless steel melt during the test. The conductivity across the cladding/melt interface is measured in each case. The melt temperature (graphite inside temperature) was monitored by a thermocouple located inside a closed end alumina tube, inserted into a bore drilled in the crucible wall thickness just next to the stainless steel surface. Another similar thermocouple was set up to measure the temperature near the bottom end of the stainless steel block (at a mid position of 12 mm long solid stainless steel end). The inside of the cladding is flushed with helium gas during each test in order to exclude any residual oxygen. For both tests the resistance had decreased to a lower value of 0.1xcexa9 before melting of the stainless steel had initiated. The decrease in resistance occurred very rapidly in the temperature range of 1150 to 1300xc2x0 C. Below this temperature range the ZrO2 resistance was out of the resistance measurement range ( greater than 2 Mxcexa9), and the system produced erratic resistance values. In both the tests gas, possibly CO/CO2, was observed sparging through the molten stainless steel, and this effect was more pronounced and lasted longer for test 2. There were no surface cracks at the gas/melt interfaces or failure of the Zircaloy cladding during post test handling. However, a longitudinal crack appeared after test 1, 20 mm below the gas/melt interface; and a circumferential crack after test 2, 10 mm below the gas/melt interfaces (at the sites of cavities in the stainless steel cast). No stainless steel was observed inside the Zircaloy tubes after the tests. In test 1, the resistance dropped to 0.1xcexa9 at 1275xc2x0 C. and continued to drop very slowly with temperature until a minimum value of 0.032xcexa9 was recorded at the target melt temperature (xcx9c1370xc2x0 C.). The resistance value stayed almost constant on cooling the sample to room temperature. After the test, it was found that the steel had melted around the Zircaloy cladding and there was good contact between the two materials. There was no sign of cracking in the region of the gas/melt interface. The ejection of molten metal onto the dished part of the graphite crucible during the full immersion of the cladding indicated that the stainless steel had been fully molten. In test 2, the resistance dropped to 0.1xcexa9 at 1290xc2x0 C. and continued to drop very slowly with temperature until a minimum value of 0.025xcexa9 was recorded at the target melt temperature. The resistance value increased slightly to 0.029xcexa9 on cooling the sample to room temperature. After the test, it was found that the steel had melted around the Zircaloy cladding and there was good contact between the two materials. There was no sign of cracking in the region of the gas/melt interface. The ejection of molten metal onto the dished part of the graphite crucible during the full immersion of the clad indicated that the stainless steel had been fully molten. The samples for metallographic examination from each test were selected from two different positions; a transverse section at a position on the tip of the inner thermocouple and a longitudinal section at the gas/melt interface. The results of transverse sections show that there was good fusion bonding between the Zircaloy cladding and stainless steel (in both cases); however, in some areas (especially in the case of test 2) the contact between the two was lost due to the cavities in the melt. The photomicrographs show that the microstructure of the solidified stainless steel consisted of light and dark phases in the form of acicular (both light and dark phases) and polygonal (usually dark phases) grains. Coarse graphite flakes were also found within the mixture of the light and dark phases. The graphite flake size depends on the cooling rate, the faster the cooling rate the smaller the size. The Zircaloy had recrystallised with single grains traversing the cladding wall. The recrystallised grains also seemed to have a eutectic phase between them. The micrographs show three distinctive layers across the stainless steel Zircaloy interface; a thin layer (xcx9c5 xcexcm thick) next to the stainless steel surface, an adjacent smooth layer (xcex1-Zr(O) (xcx9c90 xcexcm thick) grown into the Zircaloy matrix and an intermetallic layer (xcx9c15 xcexcm thick) at the xcex1-Zr(O)/Zircaloy interface. The results of longitudinal sections show that there was a lack of fusion bonding between the Zircaloy cladding and stainless steel (in both cases, in a small depth studied up to 3.5 mm) near the gas/melt interface due to the presence of unreduced ZrO2 at the stainless steel/Zircaloy interface. The presence of ZrO2 was patchy and to a lesser extent in test 2 (than in test 1) which seems to be due to the slow cooling rate and hence the longer reaction time. However, the resultant improvement in embrittlement was better in test 1 as evidenced by topography of tiny cracks; the cracks being wider and longer in the case of test 2 than in test 1 at the Zircaloy cladding surface around the gas/melt interface. As has been indicated above, the present invention has application to the electric chemical dissolution of any metallic object, in particular to metallic assemblies where it is difficult to ensure good electrical conduct to all the parts of the assembly.