(A) study on transport behavior analysis of fission products in a VHTR

Abstract

This study has attempted to sketch out the source-term generation in a VHTR and its release during normal and transient conditions, developing the integrated code, ACTOR. The sub-modules of ACTOR are MOTEX, TRIFA, DIFA, PLOTA, and LIOFA. It should be stressed that the evaluation of the source-term inventory is the first step on the path to nuclear safety. The study established the comprehensive methodology of the source-term estimation by using the combined code of MCNP, ORIGEN, MONTEBURNS, and MOTEX. The forty-one fission product generation with an increase of burnup has been simulated, and the results seem highly probable. Besides focusing on the source-term inventory, the actinide inventories were investigated and compared to that in a PWR. The results showed VHTRs are more attractive in terms of spent fuel management, because the amount of fuel required for the same power is less and VHTRs produce less plutonium. It would be informative to take into account an analysis of the spent fuel. In addition, the tritium inventory of in the coolant was also estimated. This information may provide the base knowledge about the heat exchanger maintenance exposure and the tritium permeation on the hydrogen production facility of VHTRs in future. After building up the source-term inventory, the study took into account the source-term release through core materials. The main process was to calculate how much activity would be release from fuels to coolant. TRIFA provided a fraction of the retention failure of TRISOs in pebbles. Although temperature and burnup were the main factors for the TRISO fracture, the higher temperature did not guarantee the higher fracture probability like the traditional models did. The result of TRIFA served as the input for DIFA, and DIFA estimated the fission product release rate from pebbles to the coolant. This fission product release model in core materials generally assumed that diffusion is an adequate representation of phenomena w...