To prevent HEU from being used to manufacture nuclear weapons, a concerted effort was launched in 1978 to convert research reactors using HEU fuel to LEU. The conversion is intended to make test and research reactor fuel as unappealing as possible to groups/non-state actors interested in exploiting such highly enriched cores for non-peaceful purposes. The program's objective is to develop the technology required to convert research reactors from HEU to low enriched uranium (LEU) fuel. Since then, a worldwide effort has been made to restrict, and finally eradicate, the civil use of HEU (Travelli, 1993).Through use of High Enriched Uranium (HEU) to fuel research reactors instantly creates a slew of inescapable and obvious proliferation risks associated with material diversion and/or theft. Thus according sources, the US Department of Energy (DOE) established the Reduced Enrichment for Research and Test Reactors (RERTR) program in 1978 in response to rising concerns about proliferation and the ease with which state and nonstate actors may get nuclear weapons (Ibikunle et al., 2018, Feiveson et al., 2014 and Simon et al., 2021 ). A miniature neutron source reactor (MNSR) is one of numerous research reactors worldwide that use HEU as fuel. The Nigerian Research Reactor-1 (NIRR-1) is classified as a HEU because it is 90% enriched in fissile U-235 (Azande et al., 2010). It was created primarily for neutron activation analysis (NAA) and minimal radioisotope generation (Jonah et al., 2013). The NIRR-1 core was originally designed to run on HEU-90.2% fuel, but following a recent successful conversion, it now runs on LEU-13% fuel, light water as a moderator and coolant, and metallic beryllium as a reflector. The Beryllium shims, on the other hand, function as neutron reflectors for the reactor's longer operation. At the start of each operational cycle, the maximum allowable excess reactivity is 4 mk (Fig. 3.1, 3.2 and 3.3, Table 3.1) (Yahaya et al., 2017 and Simon et al., 2021). Previously, a feasibility study was conducted to estimate the enrichment necessary to convert a commercial Miniature Neutron Source Reactor (MNSR) (NIRR-1 in particular) from HEU (90.2%) to LEU (20%) fuel. Two LEU cores with uranium oxide fuel pins of varying diameters were investigated. According to the study, the findings obtained are equivalent to HEU core and indicate that it would be viable to employ any of the LEU choices for the conversion of NIRR-1 in particular from HEU to LEU (Jonah et al., 2009).
Omar et al., (2010) recently conducted a study of 235U burn-up for the HEU-fueled Syrian MNSR. They report strong agreement between WIMSD4-CITATION-based model predictions and the experimentally obtained measurements, with an optimum relative difference of no more than 5%.
Albarhoum (2010) used the WIMSD4 and CITATION codes to conduct an optimization study of uranium loading in MNSRs. Based on clad thickness and fuel meat radius adjustments, at least 31 fuel pins are saved while keeping safety criteria. The comparable core operation longevity, on the other hand, is predicted to drop by around 37%. The use of U–9Mo as an MNSR fuel has also been researched. According to reports, the preliminary excess reactivity of the 19.75% enriched monolithic U– 9Mo core of MNSR is 3.595 mk, which is roughly 10% less than the comparable figure for the HEU core. As a result, the predicted operating period of the U–9Mo-fueled MNSR is reduced.
Khattab (2005) calculated the fuel efficiency and radionuclide inventory in the Syrian miniature neutron source reactor at various operational power levels using the WIMSD-4 algorithm (10, 20 and 30 kW). The WIMSD-4 code was used to determine the changes in the fuel one group cross section and infinite multiplication factor for the MNSR along with the burn-up time. The Findings from this study indicate that after 10 years of reactor operation, the quantities of uranium-235 consumed and plutonium-239 created in the MNSR core are 7.708 g and 0.049 g, respectively. The Author also indicates that the proportion of uranium burn-up was 0.769\(\%\).
Earlier, Yahaya et al., (2017) used the programs WIMS and CITATION to determine the burn-up for the Nigerian MNSR HEU fuel and the core life time expectation after 10 years of operational period. The burn-up findings confirmed that the excess reactivity of NIRR-1 maintains a linear declining pattern with 216 Effective Full Power Days (EFPD) operations. The reactivity worth of top beryllium shim data plates was determined to be 19.072 mk. The results of the depletion study for the NIRR-1 core reveal that (7.9947 0.0008) g of U-235 was consumed during the course of 12 years of operation. The yield of Pu-239 build-up was determined to be (0.0347 0.0043) g.
Abrefah et al., (2013) evaluated the fuel burn up of the Ghana MNSR utilizing the ORIGEN2 and REBUS3 algorithms. They performed a fuel depletion study and found that the findings were in excellent accordance with publications for the HEU core for comparable computations using other approaches. For reference, they estimated the burnup and accumulation of U-235 and P-239 using both the REBUS3 and ORIGEN2 code modules. The ORIGEN2 code had a U-235 burnup percentage composition of 2.90%, whereas the REBUS3 code had a 2.86%. The magnitude of Pu-239 in the core is greatest at the end of the irradiation period, around 1.45E-01 grammes, so plutonium production in GHARR-1 is low. The accumulation of fissile Pu-239 and Pu-241 is insufficient to compensate for the reduction in reactivity caused by U-235 depletion, and the concentration of Pu-239 in spent fuel is insufficient to raise concerns about nuclear bomb production.
Recently, Khattab and Dawahra (2011) used the GETERA algorithm to generate the fuel burnup and radioactive inventory in the Syrian miniature neutron source reactor (MNSR) after ten years of operation. The GETERA code was used to determine the modifications in the fuel one group cross sections and infinite multiplication factor for the MNSR versus the burn-up time. They discovered that after 10 years of reactor operation, the quantities of uranium-235 consumed and plutonium-239 yielded in the MNSR core were 7.481 g and 0.0458 g, respectively. They also reveal a 0.747% uranium burn-up percentage. Winfrith Improved Multigroup Scheme-Argonne National Laboratory (WIMS-ANL) and REactor BUrnup System-Argonne National Laboratory (REBUS-ANL) computer codes were used in this investigation. The WIMS-ANL was used to generate the one group cross section data, and the REBUS-ANL was used to analyze core operational life.