Neutronics in reactors with propagating perturbations

Doktorsavhandling, 2012

This thesis contains several studies of the properties of neutron noise, primarily in Molten Salt Reactors and other reactors with propagating perturbations. Both one- and two-group diffusion theory are employed to investigate the differences that arise from the movement of the fuel. Data corresponding to a thorium-fuelled thermal reactor is used to investigate the properties of a more realistic possible realisation of an MSR system, as well as data corresponding to more traditional systems for contrast. Furthermore, the properties of the neutron noise from a vibrating absorber or fuel rod in a traditional reactor is investigated in a two-group, multi-region system. For the MSR, the Green's functions and the dynamic adjoint functions are investigated in the general case of arbitrary fuel recirculation velocity and in the limiting case of infinite fuel velocity which permits simplified solutions both in the static and dynamic case. It is found that the amplitude of the induced noise is generally higher and the domain of the point kinetic behaviour valid up to higher frequencies than in a corresponding traditional system. This is due to the differing behaviour of the delayed neutron precursors as compared to the traditional case. The MSR equations are not self-adjoint and the adjoint equations and adjoint functions have to be constructed, which is also done here. Finally, the space-dependent neutron noise, induced by propagating perturbations of the absorption cross section is calculated. A number of interesting properties that are relevant to full size MSRs are found and interpreted. The results are consistent with those in traditional systems but the domains of various behaviour regimes (point kinetic, space dependent etc.) are shifted to higher frequencies or system sizes.