Design of an additional safety rod for Ghana's MNSR using MCNP code

In many reactors, there are both control rods and shutdown or safety rods with the latter specifically designed for rapid shutdown of the chain reaction. Normally, the MNSR will not be licensed due to its one control rod serving the purpose of both control of reactivity and safety of the reactor but for its relatively low output power. Hence, the design of the additional safety rod is to allow the reactor to meet standards. This thesis presents an investigative study into the design of an additional safety rod, which will be a crucial safety component in the Miniature Neutron Source Reactor (MNSR) and will provide an alternative and faster way to shut down the reactor. The additional safety rod for the Ghana Research Reactor-1 (GHARR-1), an MNSR, which is understudy here, was designed to provide sufficient shutdown margin under normal and accidental conditions for ensuring the safe shut down of the reactor. The Monte Carlo N-Particle (MCNP) code was used to model the reactor with the additional safety rod present. This is because MCNP uses the Monte Carlo method, which is a numerical procedure for solving mathematical problems based on statistical or probabilistic theory. The MCNP input deck for GHARR-1, which is available in notepad, was modified to include the additional safety rod positioned at the third inner irradiation site (site 3). In the first case, the safety rod was modelled with cadmium as the absorber material and the maximum radius of the rod was 0.95 cm. When inserted, the neutron multiplication factor (keff) was determined to be 1.00050±0.00006 and a corresponding rod worth of 2.9285 mk. This meant that the single Cd safety rod could not shut down the reactor. In the second instance, the third inner irradiation site was made deeper and when a longer Cd safety rod was inserted, the keff was 0.99997±0.00007 with a rod worth of 3.3101 mk. Then the deeper inner irradiation site was made larger by increasing its radius to 1.7 cm (radius of a large outer irradiation site). In this case, a larger Cd safety rod was inserted which resulted in a final keff of 0.99748±0.00007 and a rod worth of 4.3309 mk. In both cases, the results prove that the rods are capable of shutting the reactor when inserted but that of the larger site and rod having a higher shut down margin. In the last instance, the safety rod was designed with boron carbide as absorber material and the keff realized at the end of the simulation was 0.99814±0.00006 with a calculated rod worth of 5.2698 mk. These results were compared to two reference models made up of the central control rod and four cadmium rabbits. It was then concluded that the model of the safety rod with boron carbide as absorber material had the best parameters for shutting down the reactor than the other safety rod models. (au)