Analysis for the Simulation of a Line Source by a 14 MeV Moving Point Source and Impact on Blanket Characteristics: The USDOE/JAERI Collaborative Program on Fusion Neutronics

Performing integral experiments with a 14 MeV line source offers better simulation to the neutron source conditions (in terms of energy and angular distribution) of the toroidal plasmas in Tokamaks. Because of the linearity of the neutron transport equation, a line source can be simulated by superimposing results from many point sources aligned on one line, provided the number of these point sources is large. The simulation was experimentally realized at the FNS facility within the USDOE/JAERI Collaborative Program on Fusion Neutronics. In this paper, the theoretical aspects of a line source simulation are discussed. Specifically, analytical results for achieving this simulation by continuously moving a point source of speed V within a distance of length 2L; [continuous operation (CO.) mode] are compared to results obtained from several point sources located at discrete number of locations within the distance 2L [stepwise operation (S.O.) mode]. In the C.O. mode it was shown that for activation measurements, ideal simulation to a line source of length 2L with a point source moving at speed V could be achieved, provided the decay constant λ of the activated product satisfies the condition λ.(2L/V) << 1. In the S.O. mode, the number of point source locations, the distance from the simulated line source where the neutron radiation effects are measured, and the type of reactions (threshold vs. non-threshold) considered are important factors in determining the degree of simulation. For example, it was shown that better simulation can be achieved if the source locations are chosen to be at points that are directly related to the roots x_i's of the Gauss-Legendre set P_N(x_i)=0, where N is the number of source locations. It was shown that larger number of point sources are needed to reproduce the line source effects on threshold-type reactions [e.g. ⁷Li(n,n'α)t] than on non-threshold reactions [e.g. ⁶Li(n,α)t]. Several transport calculations were also performed to study the degree of simulation from multiple point sources on the characteristics of the test assembly used in Phase IIIA of the program. In particular, the anisotropy of the incident neutron source arising from the structure of the target assembly, coolant channel, and water coolant was studied and compared to the case of isotropic point sources. It was shown that using N ≥ 20 points is adequate to the analysis of Phase III.A experiments.