Computer Simulation of Pulsed-Neutron Experiments Performed on the Fort St. Vrain High-Temperature Gas-Cooled Reactor

Abstract

Computer simulations were used to determine the optimum source location, detector location, and pulse rate prior to performing pulsed-neutron experiments on the 330-MW Fort St. Vrain high-temperature gas-cooled reactor (HTGR). The simulation procedure involved calculation of the amplitudes, decay constants, and modal shapes of the first few kinetic modes in the general expansion of the time response of the neutron flux following each pulse. By examining the nodes (zeros) of the first few harmonics (higher modes), source and detector locations could be determined that reduced or eliminated the contribution of these modes to the measured time response. Comparison of the simulated and measured time responses for the Fort St. Vrain HTGR demonstrates the effectiveness of the simulation.

The kinetic modes were calculated by the eigenfunction expansion method in two-dimensional geometry assuming two energy groups and six delayed-neutron precursors. The major limitation in the calculation is the use of two-dimensional core models, i.e., the assumption of separation of variables. For most power reactors on which pulsed-neutron experiments might be performed, this limitation should not be serious.