Subcriticality measurements using an epithermal pulsed-neutron-source technique in pebble-bed HTR configurations

The Pulsed-Neutron-Source technique, commonly used for the measurement of the subcriticality of multiplying systems, suffers from the fact that the values of reactivity obtained are dependent upon calculated correction factors and/or kinetic parameters. Calculations have been carried out to quantify the energy dependence of these correction factors in graphite moderated systems, and these indicate that epithermal neutron distributions are much less affected by kinetic distortion. Measurements subsequently made at the PROTEUS facility of the Paul Scherrer Institute in Switzerland have confirmed this effect and show that even in strongly reflected systems the reactivity deduced from epithermal measurements is, within experimental uncertainties, spatially independent