Differential die-away instrument: Report on comparison of fuel assembly experiments and simulations

Abstract

Experimental results of the assay of mock-up (fresh) fuel with the differential die-away (DDA) instrument were compared to the Monte Carlo N-Particle eXtended (MCNPX) simulation results. Most principal experimental observables, the die-away time and the integral of the DDA signal in several time domains, have been found in good agreement with the MCNPX simulation results. The remaining discrepancies between the simulation and experimental results are likely due to small differences between the actual experimental setup and the simulated geometry, including uncertainty in the DT neutron generator yield. Within this report we also present a sensitivity study of the DDA instrument which is a complex and sensitive system and demonstrate to what degree it can be impacted by geometry, material composition, and electronics performance.

Differential Die-Away Instrument: Report on Benchmark Measurements and Comparison with Simulation for the Effects of Neutron Poisons

“…The Gd rods were positioned in the fuel assembly adjacent to water-filled guide tubes to be more effective thermal neutron absorbers. The experimental setup consisted of a water tank holding a DT neutron generator inside of a stainless steel pipe, three stainless steel pods holding multiple 12" active length 3 He detectors, and a 15x15 PWR fresh fuel assembly (Fig. 1).…”

“…Figure 1. The DDA experimental setup includes a DT neutron generator inside a stainless steel pipe, a 15x15 fresh fuel assembly, and three stainless steel pods holding multiple 3 He detectors inserted into HDPE, Cd-wrapped sleeves.…”

“…5). From previously performed calibration, the output of the DT neutron generator was estimated as 1.8•10 8 n/s ± 10% [3]. The die-away time of the simulated DDA signal was determined for multiple detector positions and fuel enrichments, with and without Gd rods (Table II).…”

“…The die-away time of the simulated DDA signal was determined for multiple detector positions and fuel enrichments, with and without Gd rods (Table II). Wrap-around effects from the previous neutron generator pulse were taken into account [3]. However, any effects from delayed neutrons were not due to the time cutoff of the tally.…”