THE ANOMALOUS FLUX PROFILE OBSERVED IN KAPP-1 DURING ITS FIRST APPROACH TO CRITICALITY

Unit 1 of the Kakrapara Atomic Power Project (KAPP-1), which went crit ical in August 1992, employs 35 Th bundles for radial flux flattening in the initial period of its life instead of the depleted U bundles us ually used for this purpose in pressurized heavy-water reactors. The r adial flux profile was therefore expected to be flat and theoretical p redictions had actually indicated a slight dip in the centre of the co re with two peaks about 120 cm on either side. Experimental measuremen ts of the flux in the central thimble during the first approach to cri tical, however, showed a single peak at the centre when the reactor wa s in a highly sub-critical state. The flux distribution showed a tende ncy to recover to its expected shape close to critical. In this note w e present an explanation of this intriguing behaviour after a brief de scription of the measurements and theoretical predictions.