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Published: 1978

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The Reactivity 'Initiated Accident (RIA) test series to be conducted in the Power Burst Facility (PBF) has been designed.to determine fuel failure thresholds, modes, and consequences as a function of energy deposition, irradiation history, and fuel design. The RIA Scoping Test will be comprised of five single unirradiated rod sub-tests. The first rod will be subjected to a series of transient power bursts of increasing energy release to determine the energy deposition at cladding failure. The second and third rods will be subjected to energy depositions near that which caused failure of the first rod, to further define the failure threshold. Rods four and five will be subjected to large radially averaged energy depositions, 1990 and 2510 J/g respectively, to investigate facility safety concerns. Several analyses were performed to predict test fuel rod and system behavior during the five RIA Scoping Test phases. A reactor physics analysis was performed to obtain the relationship between test fuel rod and reactor core energy during a power transient. The calculations were made with the RAFFLE computer code. The thermal-hydraulic behavior of the test rod coolant was investigated for pellet surface energy depositions of 900, 1125, and 1350 J/g for the first three phases of the Scoping Test. The RELAP4 computer code was used for these thermal-hydraulic analyses. The results of the RELAP4 calculations provided input to the FRAP-T4 computer code for three fuel rod behavior analyses at pellet surface energy depositions of 815, 1020, and 1225 J/g. A cladding embrittlement analysis, using the results of the FRAP-T4 calculations as input, was made to investigate the cladding oxidation mode of rod failure for the lower energy phases. BUILD5 was the analytical tool used in this investigation. Finally, the pressure pulses generated as a result of failure of the test fuel rods in the final two high energy test phases were calculated using the SPIRT computer code. In previous reactivity initiated accident tests performed in the SPERT, TREAT, and NSRR facilities a pellet surface energy deposition of 12.350 x 103 J/cm3 was identified as the failure threshold for unirradiated fuel rods with the ambient test conditions of 300 K, 0.1 MPa, and no forced flow. This volumetric energy deposition is equivalent to a pellet surface energy deposition of 1190 J/g (284 cal/g) when the RIA-ST fuel pellet density of 10.365 g/cm3 is considered. ·For no-flow conditions, it was further observed that the presence of a flow shroud caused a reduction of up to 10% in the failure threshold. The modes of failure seen in the previous tests were cladding embrittlement and low pressure rupture as the zircaloy melting temperature was approached. In general, the rod failures occurred only when a peak cladding temperature of 2073 K or above was reached. Based on the analyses, it is predicted that the test fuel rod energy deposition failure threshold will be 1035 J/g (247 cal/g) at the pellet surface for the fuel rods used in the initial three phases of the RIA Scoping Test. The initial coolant conditions for these cases are equivalent to a fuel enthalpy of 69 J/g (16.5 cal/g) at the fuel surface over ambient conditions. When the difference in initial coolant conditions is considered, the total fuel enthalpy increase leading to cladding failure observed in the previous RIA tests is equivalent to 1122 J/g (268 cal/g) at the fuel pellet surface. The difference between the predicted failure threshold value and that observed in previous tests (87 J/g) is believed to be a combined result of the presence of a flow shroud and uncertainies in the computer codes used to make the predictions. The mode of failure according to the analyses will be rupture due to high temperature cladding weakening. The consequences of these failures are predicted to he minimal. The mode of failure for the high energy phases of the Scoping Test will be cladding rupture due to internal rod pressurizati ...