Separate effects study on sodium boiling and dryout under forced- and natural-convection conditions performed with a one-dimensional loop code

A comprehensive separate effects study has been performed with the one-dimensional code LOOP-1 on powers and times for sodium boiling initiation and dryout in a closed loop system. Two different kinds of transients were considered: loss-of-flow and loss-of-heat-sink. Loss-of-flow transients were studied under both forced- and natural-convection conditions. Loss-of-heat-sink transients were studied under natural-convection conditions. The results for loss-of-flow transients indicate that the boiling initiation time was reduced by a small amount, and the dryout time was reduced very significantly by increasing either the input power or the inlet temperature, or by decreasing the test section pressure for both forced- and natural-convection conditions. Under forced-convection conditions, a stabilizing effect occurred by either increasing the test section valve setting or by decreasing the bypass ratio with a pump head adjusted to provide the same steady state and initial transient flows; thus, longer boiling times could be maintained before dryout occurred. For natural-convection loss-of-flow conditions, increasing the test section valve setting or decreasing the bypass ratio reduced the test section inlet flow, which resulted in boiling inception and dryout occurring more rapidly. A larger flow before the loss of flow transient starts yielded longer boiling initiation and dryout times. Under loss-of-heat-sink conditions, the higher the inlet temperature, the lower the boiling and the dryout powers. The margin between boiling and dryout powers increases with increasing inlet temperature. Results have been verified with experimental data. These results indicate that a margin between several seconds and several hours (depending on the type of transient) is available before core damage may occur in an actual reactor. (orig.)