THE NEW NEUTRON NOISE SOLVER OF THE MONTE CARLO CODE TRIPOLI-4

Neutron noise appears as fluctuations of the neutron field induced by vibrations of fuel elements,
control rods or any other mechanical structures in the core, as well as from global
or local fluctuations in the flow, density or void fraction of the coolant. Neutron noise
equations are obtained by assuming small perturbations of macroscopic cross-sections
around a steady-state neutron field and by subsequently taking the Fourier transform in
the frequency domain. Recently, a new Monte Carlo algorithm was proposed in order to
solve the transport equation in neutron noise theory with complex-valued weights and a
modified collision operator. This paper presents the new neutron noise solver based on
these methods and implemented in the reference Monte Carlo code TRIPOLI-4R  developed
at CEA. We illustrate the capabilities of the solver by considering the noise analysis
of a UOX PWR assembly.