Development of a Numerical Tool for Estimating the Neutron Noise in Nuclear Reactors

The neutron noise, i.e. the time-variations of the neutron flux around its meanvalue, can be used for obtaining important safety parameters in a nuclear reactor,e.g. the decay ratio for a boiling water reactor and the moderator temperature coefficientfor a pressurized water reactor. The main benefit of using the neutron noiseis that it can be taken from the existing detectors and there is therefore no need forperturbing reactor operation. It would thus be advantageous to be able to calculatethe neutron noise with a relatively simple model with a relatively small computationaleffort, a work initiated earlier at the Department of Nuclear Engineering,Chalmers University of Technology.The objectives of this report are thus to validate some of the assumptions inthe previously developed model and to evaluate various ways of improving it. Acomparison is thus conducted between P1 and diffusion theories with two energygroups for a 1-dimensional 2-region reactor. The previous model used the FiniteDifference Method, a method which is simple and easy to implement for spatialdiscretization of the equations. The drawback is that the accuracy is low unlessthe size of the nodes of the reactor model is small. To enhance the neutronics whilekeeping the size of the nodes in the range of 15-20 cm, theAnalytical Nodal Methodwas implemented for the calculation of the neutron noise.The results fromthis study are that there is no need in practical situations for usinghigher order theory for neutron noise calculations since diffusion theory givesresults that are accurate enough. The implemented Analytical Nodal Method alsoshows high agreement with the analytically calculated neutron noise