Analysis of Influence of Decay-Cooling Time on on-Line Burnup Measurement of HTR-PM

High Temperature gas-cooled Reactor-Pebble bed Modular（HTR-PM） requires a online burnup assay system to check whether a fuel pebbe should be discharged from fuel circulation according to its burnup value.Each pebble that has flowed out of the core is going to undergo decay and cooling before the burnup measurement, so the interval of decay-cooling time influences the burnup measurement. The paper analyzes this effect using the isotopic depletion and decay analysis code of KORIGEN and particle transport code of MCNP. The result shows that the decay-cooling time of 50 hours can meet the HTR-PM requirement well with the choice of suitable γ spectrum analysis method.