Development of a hybrid neutron transport solver

Licentiate thesis, 2024

One of the most fundamental analyses of a nuclear reactor consists in solving an eigenvalue problem associated with the neutron transport equation and determining the effective multiplication factor and the distribution of neutrons of the system under static conditions. Two categories of methods can be used for this type of static core calculation: Monte Carlo and deterministic. Monte Carlo methods can reproduce near-real-physics characteristics of the problem at hand but with a high computational cost. On the other hand, deterministic methods lead to quicker but less accurate results after several approximations. In this work, a hybrid computational framework for static core calculations relying on the Interface Current Method (ICM) is developed.

The framework consists of three steps: 1) the whole computational domain is divided into subsystems, 2) a set of collision probabilities inside every sub-system is estimated using Monte Carlo, and 3) the set of collision probabilities is used to estimate the neutron scalar flux and the effective multiplication factor using the ICM. The framework is verified against Monte Carlo reference solutions for three cases based on data from a sodium-cooled fast reactor system, i.e.: 1) a hexagonal fuel-pin cell with simplified and detailed geometry, 2) a hexagonal arrangement of seven fuel-pin cells surrounded by coolant, and 3) a full-size hexagonal fuel assembly. 

In the first verification case, for both the simplified and the detailed fuel-pin cell, the framework was tested with and without production scattering cross sections, using 3 different coarse meshes, and considering collision probabilities estimated with different number of neutron histories. Good agreement with the reference solution is obtained for both simplified and detailed fuel-pin cells when using scattering production cross sections. The sensitivity analysis shows that increasing the number of neutron histories allows to minimize the uncertainty of the collision probabilities and thus improves the results. A coarse mesh with a combination of triangular and rectangular coarse nodes is used in the second and third verification cases. A relatively good agreement is obtained in terms of the effective multiplication factor and scalar neutron flux in fast systems.