Numerical calculation of neutron fluxes at the exit of a complex neutron-guide system at IRI, Delft

A numerical method of calculation of the neutron flux for a complex guide system is described. A feature of the given calculation method is the “short section approximation” based on dividing the guide into many short straight sections, which simplifies the ray-tracing algorithm and the input of the complex geometry parameters into the calculation program. The accuracy of the numerical method is discussed. This method was used for the calculation of the neutron flux at the exit of a new neutron guide system at the Interfaculty Reactor Institute of the Hoger Onderwijs Reactor. Calculations showed that to minimize flux losses to 50% it is necessary that the reflectivity of a NiMo/Ti supermirror (m=2) at twice the critical angle of nickel should not be lower than 90%. The mean step between channels of the neighbouring optical sections should not exceed 0.02 mm and the waviness of the optical section walls should not exceed 0.5 mrad. The measured flux intensities at the exit of the neutron guide system showed quite a good correspondence to the calculated values.