Comparison and Application of Variance Reduction Method Employed in Neutron Fluence Rate Calculation for Safety Rod of SPPR-300

When calculating the neutron fluence rate of safety rod in SPRR-300 (swimming pool research reactor), especially the top of safety rod, the problem will occur that neutrons transport in little volume at long distance. Variance reduction techniques used in MCNP were applied to solve the problem. By trial and error, several methods were tested, and efficient variance reduction method for this problem was found. At first, three techniques were chosen to do comparison according to the reactor model. The comparison results showed that an unsteady calculation phenomenon was observed along with the increase of neutron histories. Therefore, this didn’t reduce variance successfully. Then, the reactor model was modified to describe water with multiple cells, so that cell importance could be designated respectively. This variance reduction method induces more neutron tracks into the destination cell. Also, the neutron tracks and collisions in each cell can be checked from the output file. Finally, the calculation results of the updated model show that the relative error is less than 5%, and FOM (figure of merit) is steady, which imply that the variance reduction aim is accomplished.