Decay heat evaluation of spent fuel of fast breeder reactor

Decay heat measurement of JOYO MkII spent fuel subassemblies was carried out using a decay heat measurement system installed in the spent fuel storage pond of JOYO. The generated heat from a spent fuel subassembly was evaluated by calorimetric method. The accuracy of the measurement system was determined by the sensor errors and heat loss which influences the heat balance in the system. Two types of heat losses were considered for calibration, one is the heat loss from the system, the other relates to radiation heating. The latter was generated by alpha, beta and gamma rays emitted from the radioactive nuclides as fission products, activated products and actinide nuclides. Neutrons moderated in the water may induce fission reactions with the fissile materials in the spent fuel. However, by the ORIGEN-2 calculations heat generation by fission reaction was approximately 5x10-5 W per subassembly and can be negligible for the system. Part of the gamma-rays penetrated outside of the system, therefore, gamma energy penetration was evaluated and measured heat was corrected to account for the gamma heat loss. Two subassemblies of the MkII spent fuel were used for this experiment. They reached the subassembly averaged burn-up of 58.2 GWd/t and 57.9 GWd/t respectively. The decay heat was measured during a certain interval of between 24 and 258 days. The decay heat was 1.23+0.03 KW - 0.26+0.0019 KW respectively. The measured decay heat was compared with the calculated values by 'ORIGEN 2' code using the cross section library of JENDL-3.2, and the decay data and fission yield data library JNDC-V2. Calculated to experimental (C/E) values for the measured subassemblies were 0.96-0.90. The decay heat generated by 235Pu and 241Am, which amount to 1% of initial composition of the fresh fuel, reached 6%-17% of decay heating during 24-258 days of cooling time. These results indicate that actinides are important to accurately evaluate the decay heat of spent fuel. (H. Itami)