In-pile experiments on tight-pitch rod bundle integrity for power transients

The use of tight-pitch fuel rod bundles in high conversion water reactors causes concern for the cooling of rods during power transients. To quantify the DNB (departure from nucleate boiling) threshold and rod failure threshold of fuel energy deposited in fuel during a power burst, in-pile experiments with various coolant conditions were conducted. The fuel behavior was compared with that of conventional rod bundle. A small water inventory in the channel tended to reduce the DNB threshold, but the effect of coolant water inventory on the failure threshold was slight. However, a water velocity increase added significantly to both threshold energies at any water temperature. (orig.)