Quarterly progress report on the NSRR experiments, (1)

In-pile experiments related with nuclear fuel behavior during reactivity-initiated accidents were started in the Nuclear Safety Research Reactor (NSRR), a thermal pulsed reactor, in October 1975, subsequent to the first criticality in June 1975. A total of 29 tests were carried out during the two quarters from October 1975 to March 1976. Experiments are in the three test series: (1) Scoping tests to study behavior of the normal, unirradiated PWR type fuel rods (standard test fuel rods) with change in the energy deposition, and to obtain the failure threshold, (2) wide-gapped fuel tests to study influence of the difference in radial gap width on cladding surface temperature and on the failure threshold, and (3) waterlogged fuel tests to study the relation between water volume in fuel rods and failure threshold. From these tests, the failure threshold for the standard test fuel was found to be in the energy deposition range of 214 cal/g.UO2 to 232 cal/g.UO2; at 232 cal/g.UO2 a fuel rod failed with circumferential cracks penetrating the cladding tube. At higher energy deposition of 292 cal/g.UO2, a fuel rod was broken into five large pieces with melting of UO pellets. At 334 cal/g.UO2 a fuel rod was broken into small particles with water-column jumping. The failure threshold for wide-gapped fuel rods was very close to that for standard fuel rods, and was in the range between 229 cal/g.UO2 to 240 cal/g.UO2. The waterlogged fuel rods with water filling all the space inside the rods failed with large longitudinal breach at the energy deposition of 101 cal/g.UO2. Described are the results of the tests, test procedures, and present status of analytical works. (auth.)