An integrated analysis of DND experiments in the Indian experimental Fast Breeder reactor using prompt recoil and modified Non-Recoil DN precursor release models

The Indian 40 MWt experimental Fast Breeder Test Reactor (FBTR) operating at Kalpakkam has different systems for fuel pin failure detection. It uses a Gaseous Fission Product Detection (GFPD) system to detect the dry rupture phase of fuel pin clad failure, and it also has a delayed neutron detection (DND) system in each primary loop (east and west) for wet rupture phase detection. In 2011, a series of delayed neutron (DN) signal measurements were performed in FBTR to assess the sensitivity and localisation capabilities of the DND system. A special assembly with 19 perforated fuel pins of natural uranium-nickel metal alloy was used as a fission product source (FPS) for this test. In this paper, an integrated analysis has been carried out to simulate the experimental observations by using both neutronics and thermal hydraulics calculations. The Prompt Recoil Model (PRM) and modified Non-Recoil Model (NRM) with isotopic hold-up time are used to estimate the DN precursor release rate from the perforated fuel pin to the coolant sodium. The time-dependent activity is evaluated considering hydraulic dilution and decay of the DN precursors. To get the hydraulic dilution of DN precursors during their transport to the detector, a 3D CFD analysis of FBTR core with entire pool sodium has been performed using the commercial code ANSYS FLUENT. Monte Carlo modelling of the DND system is done for DN signal estimation by considering the spatial distribution of the DN source around the detectors. Results showed that a modified non-recoil DN precursor release model coupled with the neutronics-hydraulics simulation gives better prediction of DN signal in FBTR, and hence, this methodology can be extended for generating the contrast ratio for core locations where measurements are not performed.