Random effects on reactivity in molten salt reactors

• The problem of the effect of fissile lumps appearing randomly in a fluid fuel reactor is addressed.
• The effect on reactivity is evaluated by means of first-order perturbation theory.
• The estimation of the mean value and standard deviation is performed by Monte Carlo simulations and deterministic quadrature.
• The reconstruction of the probability density function of the reactivity is performed by polynomial chaos expansion.
• The results show that random reactivity effects constitute an important issue in the assessment of molten salt systems.

The problem of the effect of fissile lumps spatially appearing in a random fashion inside a fluid fuel reactor is addressed. The effect on static reactivity is evaluated by means of first-order perturbation theory. The analysis is carried out in diffusion theory with the presence of delayed neutron emissions, taking into account the fuel motion that introduces a distortion of the space distribution of the delayed neutron precursors. The method is applied to a one-dimensional configuration to investigate the general features of the random process. Afterwards, a more realistic two-dimensional cylindrical geometry is considered. The estimation of the mean value and standard deviation of the reactivity inserted is performed by Monte Carlo simulations and a deterministic quadrature approach, to compare the methods in terms of computational effort and accuracy of the results. The reconstruction of the probability density function of the reactivity is also performed by polynomial chaos expansion. The results presented show that random reactivity effects constitute an important issue in the assessment of these innovative molten salt systems.