Reactivity Analysis of Nuclear Fuel Storages : The Effect of ²³⁸U Nuclear Data Uncertainties

Sammanfattning:

The aim of this master thesis work was to investigate how the uncertainties in nuclear data for ²³⁸U affects the uncertainty of k_eff in criticality simulations for nuclear fuel storages. This was performed by using the Total Monte Carlo (TMC) method which allows propagation of nuclear data uncertainties from basic nuclear physics to reactor parameters, such as k_eff. The TMC approach relies on simulations with hundreds of calculations of k_eff with different random nuclear data libraries for ²³⁸U for each calculation. The result is a probability distribution for k_eff where the standard deviation for the distribution represents a spread in k_eff due to statistical and nuclear data uncertainties.

Simulations were performed with MCNP for a nuclear fuel storage representing two different cases:  Normal Case and Worst Case. Normal Case represents a scenario during normal conditions and Worst Case represents accident conditions where optimal moderation occurs. In order to validate the MCNP calculations and the libraries produced with TMC, criticality benchmarks were used.

The calculated mean value of k_eff for the criticality benchmark simulations with random libraries produced with TMC obtained a good agreement with the experimental k_eff for the benchmarks. This indicates that the libraries used in this this work were of good quality.

The TMC method´s drawback is the long calculation time, therefore the new method, fast TMC, was tested.  Both fast TMC and original TMC were applied to the Normal Case. The two methods obtained similar results, indicating that fast TMC is a good option in order to reduce the computational time. The computer time using fast TMC was found to be significantly faster compared with original TMC in this work.

The ²³⁸U nuclear data uncertainty was obtained to be 209 pcm for the Normal Case, both for original and fast TMC. For the Worst Case simulation the ²³⁸U nuclear data uncertainty was obtained to be 672 pcm with fast TMC. These results show the importance of handling uncertainties in nuclear data in order to improve the knowledge about the uncertainties for criticality calculations of k_eff.