SUBCRITICALITY MEASUREMENTS USING AN EPITHERMAL PULSED-NEUTRON-SOURCETECHNIQUE 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 t hat the values of reactivity obtained are dependent upon calculated co rrection factors and/or kinetic parameters. Calculations have been car ried out to quantify the energy dependence of these correction factors in graphite moderated systems, and these indicate that epithermal neu tron distributions are much less affected by kinetic distortion. Measu rements subsequently made at the PROTEUS facility of the Paul Scherrer Institute in Switzerland have confirmed this effect and show that eve n in strongly reflected systems the reactivity deduced from epithermal measurements is, within experimental uncertainties, spatially indepen dent. (C) 1998 Elsevier Science Ltd.