How could fuel corrosion influence the delayed neutron signal time evolution in sodium cooled fast reactors?

Sodium cooled fast reactors (SFR) employ a fuel pin failure detection system to limit the release of fissile material into the coolant. The system monitors for a neutron signal in the coolant resulting from fission products released by cladding failure. The time evolution of the neutron signal in a delayed neutron detection system (DND) is related to the release mechanisms of delayed neutron precursors (DNP) as well as the fuel pin evolution. In order to interpret the delayed neutron signal’s time evolution and better identify the breached fuel area, the enhanced thermal diffusion release due to corrosion layer formation during the fuel evolution is modeled in this paper. As an experimental benchmark, one experimental case (RG 11 in the PHENIX reactor) is studied in this paper. By modeling the diffusive release fraction during corrosion layer formation and comparison with experimental data, we deduce that the enhanced diffusive release, which is influenced by temperature changes at the fuel rod periphery due to the corrosion layer formed, provides a good interpretation for the time evolution of the delayed neutron signal. In addition, the temperature rise due to the corrosion layer may also cause a pulsed gas release and a corresponding delayed neutron signal increase. •Delayed neutron detection introduction.•Description of an experiment case RG 11 in PHENIX reactor.•Enhanced thermal diffusion modeling during corrosion layer formation.•Results and discussions of the neutron signal evolution interpretation.