Simulation of delayed neutron precursor release due to wet rupture in a fuel pin and its detection methodology in PFBR

Detection of dry and wet fuel pin clad rupture is very important to ensure safe operation of fast reactors. An in vessel concept of delayed neutron detection system has been adopted for detecting wet rupture in a pool type MOX fueled 500 MWe Prototype Fast Breeder Reactor (PFBR) which is currently in the advanced stage of commissioning in India. It has 8 DND modules installed in the hot pool just above the intermediate heat exchangers inlet windows; each module has three 0.2 cps/nv sensitivity high temperature fission chambers which is surrounded by graphite moderator and B4C shield. In this paper, we propose a revised simulation methodology of DN signal estimation involving various steps of neutronics and thermal hydraulics calculations, which is considered to be a pre-requisite for the program of experimental demonstration of bulk DND system. A special DN source sub-assembly simulating a failed MOX pin of 1 cm2 defect area has been manufactured using natural uranium metal fuel slugs with a perforated clad. Prompt recoil model is employed to compute the release of fission products through the perforated clad. The DN source strength is estimated by solving 52 coupled radioactive decay equations for 52 parent and daughter (DN precursor) nuclides. The CFD analysis using PHOENICS code is performed to simulate mixing and decay of DN precursors in the sodium pool during their transport. Neutron transport calculation with distributed DN sources around the DND block is done to estimate the DN signal in the HTFC detectors using the fission equivalent neutron flux concept. The sensitivity of DN signal for different core locations is studied in detail at 20% thermal power level with 50% flow. The DN signal variation with power is also studied. This research work is performed to have a theoretical database prior to the actual calibration experiment of bulk DND system in PFBR after attaining first criticality.Y