Neutronic uncertainty propagation for the SAFARI-1 research reactor benchmark: IAEA CRP T12029

Quantification of full core neutronic and sensitivity analysis of the SAFARI-1 research reactor was performed; the output parameter of interest was the neutron multiplication factor k_eff. Fuel element calculations were also studied to gain insights into the problem. The calculational approach in the Organization for Economic Corporation and Development benchmark for quantification of uncertainties in multiphysics calculations for LWRs was followed. The scope of the work is limited to reactor physics (neutronic calculations). Five core models were built in line with the SAFARI-1 benchmark specifications, of the 2016 IAEA Coordinated Research Programme. Model 1 was a Fuel Element (FE) model with reflective boundary conditions on the sides and void boundary conditions axially. Models 2 – 4 were full core models with control rods at 0 %, 50 % and 100 % insertion in the core. Model 5 was a full core model consisting of only fuel elements.

TSUNAMI-3D, which is part of the SCALE 6.2.3 code package, was used for uncertainty propagation. The uncertainty in the multiplication factor k_eff is calculated with the sensitivity coefficients and the covariance matrix using the so-called “sandwich rule”. It can be seen from the results that the uncertainty in k_eff is 0.490 % for the infinite fuel element model and about 0.640 % (highest value) for the full core model. It is also seen from the sensitivity analysis that the highest contributors to the uncertainty in the FE model are due to the average number of neutrons released per fission and due to the (n, gamma) reactions. For the full core model, the highest contributors are uncertainties due to the average number of neutrons per fission and that due to the fission spectrum, chi. A detailed behaviour of the individual contributors to the uncertainty is also investigated. It was also found that the size of the reactor core influences the value of the individual contributors. To a lesser extent, the heterogeneity of the reactor core also influences the value of the individual contributors. The neutron flux distribution in cores of different sizes was also studied since it contributes to the uncertainty due to chi, as well as the neutron importance, to observe the trend in the individual uncertainty contributions of chi and nubar.