Study of the flux-trap optimum-size dimension in a radioisotope production reactors

The influence of the V sub(mod)/V sub(comb) ratio on the flux-trap optimum-size dimension in a research reactor was studied. For each V sub(mod)/V sub(comb) ratio value the thermal neutron flux on the axis of the flux-trap is a maximum, when the difference between the current of fast neutrons entering and the currents of thermal neutron leaving the flux-trap is a minimum. This condition is satisfied when the thermal neutron absorption rate by unit of volume, and the slowing down density to thermal neutron are equal in the flux-trap axis. The optimum-sized flux-trap increases when the V sub(mod)/V sub(comb) ratio decreases, but the relative variation is smaller than 20% for the range of V sub(mod)/V sub(comb) values studied (1.05 to 2.39). (author)