Design Consideration of On-line Burnup Measurement System for High Temperature Gas-cooling Reactor: Influence of Cooling Time for Fuel Sphere on Burnup Measurement

Fuel spheres of the pebble bed high temperature gas-cooled reactor were non-destructively monitored on-line to ensure whether they reach their end-of-life burnup limit (80GWD/MTU).The core inventory Code (KORIGEN) and Monte-Carlo Code (MCNP4A) was used to simulate the burnup of fuel sphere and the response of HPGe detector, respectively. Uncertainty of burnup measurement was studied under different burnup values and cooling times. If indicator fissile nuclide of burnup measurement is 137Cs and counting statistical certainty of 5% for the bunrnup measurement system is met,the cooling time of the fuel shpere will be more than 6 d and the time of burnup measurement will be more than 15 s after the simulation results of the HPGe gamma-ray spectrometer for the irradiated fuel spheres are analyzed .