Effect ²³⁹Pu in Burnup Measurement Process in HTR-PM

The portion of ²³⁹Pu in the nuclear fuel cycle of gas cooled high temperature reactor-pebble bed module（HTR-PM） is very important at the high level of burnup,which certainly influences the online burnup measurement.This problem was discussed in the paper.When the expected average burnup was 80 000（MW.d）/t（U）,the calculation result showed that the energy release from ²³⁹Pu fission contributed to the burnup of fuel pebble by 26.7% and the acitivity of ¹³⁷Cs and ¹³⁴Cs from ²³⁹Pu fission is about 27.2% and 23.2%,respectively.Further analysis also showed that it is better to determinate the burnup by the activity of ¹³⁷Cs in the pebble than by the activity ratio of ¹³⁴Cs/¹³⁷Cs.