International Conference on Research Reactors:
Safe Management and Effective Utilization

14-18 November 2011, Rabat, Morocco

The Nigeria Research Reactor-1 (NIRR-1) is a Miniature Neutron Source Reactor (MNSR), which is currently fueled by Highly Enriched Uranium (HEU). The reactor is equipped with ten irradiation channels mainly for Neutron Activation Analysis (NAA) and limited radioisotope production. A neutronics feasibility study has shown that it will be feasible to convert NIRR-1 from HEU (90.2%) to LEU (~12.5%) fuel. In this work, the impact of the proposed LEU fuel on neutron spectrum parameters in the experimental channels of NIRR-1 has been studied. The knowledge of the neutron spectrum parameters in irradiation channels of research reactors is required in several applications, especially for analytical works via the NAA technique. These include the thermal-to-fast neutron flux ratio, ƒ, and the non-ideality of epithermal neutron flux parameter, α, especially for the implementation of the k0-NAA method. Although, experimental methods have been recommended for the determination of the ƒ and α values, a parametric method was developed and used to calculate the neutron parameters in inner and outer irradiation channels of the current HEU core and the proposed LEU core of the reactor. In this regard, the MCNP code was used to calculate the neutron flux distribution in 640 neutron energy group structure and the cross section data of monitor reactions also in 640 neutron energy group structure were extracted from ENDF libraries. On the basis of the data, the Cd ratios of Au, Mn, Co, Th, and U were computed and used to calculate ƒ and α values. A comparison of measured data for the current HEU core with the calculated was used to verify the parametric method. The α-value in the inner channel of the HEU core was calculated to be -0.056±0.004 in comparison with a measured value of -0.052±0.002. However, the α-value in the inner channel of the LEU core was calculated to be -0.047±0.006. Similarly, the α-value in the outer channel of the HEU core was calculated to be 0.021±0.005 in comparison with a measured value of 0.029±0.003. The α-value in the outer channel of the LEU core was calculated to be 0.028±0.004. As expected, these results indicate slightly “hardened” neutron spectra distributions in the respective irradiation channels of the LEU core in comparison with the current HEU core

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