HPGe/BGO Compton suppression system: Monte Carlo study of radiation monitoring system for failed fuel detection in sodium-cooled fast reactors.

Fuel failures detection and monitoring is a key issue in sodium-cooled fast reactor operation. This detection is based on monitoring the signal of fission products using dedicated radiation monitoring systems based on gamma spectrometry or neutron counting. In this context, the French Alternative Energies and Atomic Energy Commission (CEA) investigates and developpes a Compton suppression system composed by a high-purity germanium diode, a bismuth germanate scintillator and a dedicated digital signal processing allowing filtering coincidences. This approach enables the Compton noise to be filtered without impacting the useful signal from short-lived radioisotopes. Through a calculation scheme based on a validated MCNP6 model of the detector. It was demonstrated that the sodium degassing is a mandatory option reducing the minimal detectable activities of fission products by a factor of up to 27. The Compton suppression system leads to an additional minimization of minimum detectable activities up to a factor of 4 enabling the ease measurement of the Kr safety indicator.