The effect of graphite thermal column around the newly designed large irradiation channel of GHARR-1 on neutron flux distribution

Monte Carlo N-Particle code (MCNP-5) was employed to simulate the neutron flux profile in a newly designed irradiation site surrounded by a graphite thermal column for Large Sample Neutron Activation Analysis, in the Ghana Research Reactor-1. The results shows that the average thermal neutron flux in the irradiation channel surrounded with 6 cm thick graphite was 5.45 × 1010 n cm−2 s−1 as compared with 1.74 × 1010 n cm−2 s−1 when there is no graphite around the irradiation channel. This shows an increase in the thermal neutron flux in the irradiation channel by a factor of 3.13. The thermal neutron flux gradient decreased at 7.0 × 109 n cm−2 s−1 per cm to 3.90 × 107 n cm−2 s−1 per cm when the channel is surrounded by 6 cm thick graphite which makes it very suitable for Large Sample Neutron Activation Analysis. The simulation agrees very well with the experimental keff of 1.00400 as compared with the final keff of 1.00390 recorded by this simulation.

► Irradiation site with graphite surrounding it was design and modelled with MCNP-5.
► Thermal neutron flux in the irradiation channel increased with presence of graphite.
► The thermal neutron flux gradient decreased along the diameter of the tube.
► Improved thermal neutron distribution in the channel making the site apt for LSNAA.
► The simulation results agree very well with available data such as final keff = 1.0039.