The CESAR facility in Cadarache has been transformed to load in its centre fuel of the pebble-bed type (~ 10,000 elements) with 3.5% enriched uranium/41’ Some compari­sons between theory and calculations are given in refs. 42, 43 and 44. The relative conversion ratio was calculated with an error of 3%, spectral indices could also be calculated with good accuracy. Control-rod measurements on a single and a pair of rods could be recalculated with errors of the order of 1.5% of the rod worth.

A pebble-bed critical facility for high-temperature reactors (KAHTER) started operation in July 1973 at KFA Jiilich<45) with the scope of measuring effects which are peculiar of pebble-bed reactors (effect of upper air gap between core and reflector, effect of filling factor, etc.). The criticality of all configurations of the KAHTER facility have been calculated within errors of 0.3% in Д к Ik. The agreement between theoretical and experimental control-rod effectiveness lies within 2.5% for the central rod and within 5% for the reflector rods/46’ Also in Jiilich Th resonance integrals have been measured by means of a lead spectrometer in fuel compacts containing coated particles. The discrepancy between theory and calculations is of the order of 1 to 3%.<48)

Other minor contributions came from the RB-2 reactor—Bologna/49 50’ IRI (In­teruniversity Reactor Institute, Delft)15” and CCR Ispra/52’53’