Feasibility study for the adoption of multi-capsule irradiation protocol in the conduct of k0-based INAA using the GHARR-1 MNSR

Abstract

The Ghana Research Reactor-1 (GHARR-1) is currently a 23 cm length LEU core Miniature Neutron Source Reactor (MNSR) with a 13 % U-235 enrichment having 335 fuel rods, 15 dummy rods and a central control rod for neutron regulation. It has 10 accessible irradiation channels of approximate length of 17 cm and formed part of the shim tray structure which was considered for active routine experiment in the present work. Unlike the routine characterization of the inner irradiation channel of the Ghana Research Reactor- 1 facility which involves mainly the introduction of a single irradiation capsule of length, 5 cm loaded with samples, the present study adopted a multi-capsule scheme in which three (3) capsules each of length 5 cm were introduced into the 23 cm long irradiation channel to scientifically interrogate the feasibility and hence provide a sound basis for extending and improving the accessible irradiation space by 60% during the utilization of the GHARR-1, especially for irradiation involving intermediate and long lived radionuclides. The objective is also to achieve the characterization of neutron spectrum and determine their spatial distribution for close to the full length of the irradiation channel. Validation protocols based on k0 method were developed through the analysis of some reference materials for a careful study of irradiation, decay and counting scheme which achieved optimum radionuclide selectivity. The overall approach adopted for the flux characterization involves the preparation of flux monitors, packaging of three (3) capsules each of both bare and cadmium cover samples, irradiation at bottom, middle and top spatial demarcation of the irradiation channel, with each demarcation being the region of the 5 cm length of the bottom, middle and top irradiation capsules. Sample counting was undertaken using the HPGe detector after the samples were allowed specific decay time after irradiation and prior to counting. Flux monitors were used for the flux characterization and reference materials were used for the validation protocol. Spectrum acquisition was made possible through the use of the Gamma Vision Software and spectrum parameters (thermal to epithermal neutron flux ration (f), epithermal neutron shaping factor (α), thermal, epithermal, and fast fluxes) were determined. Results obtained showed increasing f-value across the irradiation column as, 18.5 ± 1.7, 21.0 ± 2.2 and 23.0 ± 7.08 respectively from the bottom capsule to the top capsules. The corresponding epithermal neutron shaping factor (α-value) varied as, −0.096 ± 0.029, −0.18 ± 0.036 and −0.20 ± 0.06 from the bottom to the top capsule. The experimental results determined in the bottom, middle and top capsule irradiation column for thermal, epithermal and fast fluxes are 4.60 × 1011±2.5 × 1010, 2.49 × 1010±5.98 × 108, 9.24 × 1010±2.2 × 109; 4.21 × 1011±1.01 × 1010, 2.01 × 1010±4.82 × 108, 4.81 × 1010±1.15 × 109; and 3.90 × 1011±9.36 × 109, 1.65 × 1010±3.90 × 108, 4.82 × 1010±1.16 × 109 respectively. The validation protocol using standard reference materials and treating each capsule with separate reactor characterization parameters indicated respective z-score distribution within a 95% confidence interval.