Pulsed neutron experiments in thorium

This thesis presents the results of some experimental and theoretical investigations into the behaviour of fast neutrons in a pulsed thorium assembly. The experiments constitute the first detailed examination of the space-time characteristics of fast neutrons in a heavy metal system. The space and time dependent fission rates of 237Np, 235U and 239 Pu are measured following the injection of a short burst of fast neutrons of mean energy - 2.7 MeV into the centre of a thorium assembly of size 0.4 x 0.4 x 0.4 m3 . Analysis of the measured spatial distributions have been directed towards the extraction of the space independent . time dependent reaction rates .associated with the three-dimensional Fourier spatial modes. The time dependent reaction rates corresponding to the fundamental Fourier spatial mode and the associated values of the instantaneous decay constant have been compared with calculations from a multigroup diffusion theory code which uses the asymptotic reactor theory to describe leakage. The. thorium cross sections employed are the ABBN 26 group set and two 50 group sets from the WINFRITH data file and the ENDF/B-II data file. The calculated 237Np fission rates using the three data sets differ substantially from each other and decay faster than measurements for the first 30 ns or so after the initial neutron pulse. At later times measurements lie within the calculated curves. The calculated 235U fisslon'rates using the three data sets are in close agreement with each other. All three sets, however, predict too fast a decay rate for the first 70 ns or so after the initial neutron pulse compared with measurements, the disagreement being well outside the experimental errors. At later times the predicted values of the instantaneous decay constant are close to the measured values. Similar trends are observed for the 239Pu fission rates. The discrepancies between measurements and calculations are discussed and possible explanations are presented. The time dependent 237Np and 235U fission rates measured at several positions in the thorium assembly are also compared with calculations from a simple Monte Carlo code. Again the calculated reaction rates decay faster than the measured reaction rates. Some studies of the sensitivity of the measured parameters to physically reasonable changes in the thorium cross sections at various energies based on the multigroup diffusion theory calculations are presented. The measured parameters are shown to be insensitive to changes in the fission and radiative capture cross sections but are very sensitive to the elastic and inelastic cross sections.