Simulation of neutron imaging system of Tehran research reactor using MCNPX code and DXTRAN spheres variance reduction technique

Performing Monte Carlo simulation calculations for the neutron imaging system of the Tehran Research Reactor (TRR) as a complex system is usually very time-consuming. By using variance reduction methods, this time can be reduced to obtain results with sufficient accuracy. Each of the variance reduction techniques has its own advantages, problems, and characteristics, so the user is responsible for choosing the proper variance reduction method. In this research, the effect of using the variance reduction method of DXTRAN spheres in calculating the neutron and gamma dose rate at a point located in the neutron imaging system of the TRR has been selected and its results have been analyzed. For this purpose, by using the results of simulation with MCNPX code and calculation of neutron flux and dose at the E beam tube of TRR and simulating the imaging chamber, neutron dose rate and gamma rays using Tally F4 in two analog and non-analog ways were calculated. Also, for each mode, the Figure of Merit (FOM) for the number of different histories was compared. The results show that the use of this technique helps to reduce the calculation time until an acceptable variance is reached, although the effect of this method is not the same in the case of calculations related to neutrons and gamma, as well as increasing the FOM. This method also resulted in a significant increase in FOM in both neutron and gamma cases.