235U and 239Pu characterization in radioactive waste using neutron-induced fission delayed gamma rays

In view of long-term radioactive waste repository, the Differential Die away Technique (DDT) can be used to determine the amount of fissile materials, mainly ²³⁹Pu and ²³⁵U, in 225 L bituminized waste drums by detecting prompt and delayed fission neutrons between the pulses of a deuterium-tritium neutron generator. A complementary approach studied in this paper is to detect fission delayed gamma rays. Their yields were first measured with bare samples of ²³⁵U and ²³⁹Pu in REGAIN, a facility dedicated to Prompt Gamma Neutron Activation Analysis (PGNAA) at CEA Cadarache, France. Detectability in the 225 L bituminized waste drums is then assessed using the MCNPX model of MEDINA, another PGNAA cell dedicated to 200 L drums at FZJ, Germany. The performances in the bituminized drum are strongly limited by the high gamma count rate due to ¹³⁷Cs, which imposes the use of collimator and shield, to avoid electronics saturation, and makes the detection of the weak delayed gamma signal impossible. However, promising performances in lower-activity cemented waste drums produced in Germany are currently being studied.