ATWS Characteristics of Super LWR with/without Alternative Action

Abstract

Anticipated-transient-without-scram (ATWS) of the supercritical-pressure light water cooled thermal reactor with downward-flow water rods (Super LWR) is analyzed to clarify its safety characteristics. At loss-of-flow, heat-up of the fuel cladding is mitigated by the water rods removing heat from the fuel channels by heat conduction and supplying their coolant inventory to the fuel channels by volume expansion. The average coolant density is not sensitive to the pressure due to the small density difference between ''steam'' and ''water'' at supercritical-pressure. Closure of the coolant outlet of the once-through system causes flow stagnation that suppresses an increase in the coolant density due to an increase in the temperature. Therefore, the increase in power is small for pressurization events. The coolant density and Doppler feedbacks provide good self-controllability of the power against loss-of-flow and reactivity insertion. An alternative action is not needed either to satisfy the safety criteria or to achieve a high-temperature stable condition for all ATWS events. Initiating the automatic depressurization system is a good alternative action that induces a strong core coolant flow and inserts a negative reactivity. It provides an additional safety margin for the ATWS events. Even the high core power rating of the Super LWR has excellent ATWS characteristics, providing a key reactor design advantage.

Transient Analysis of Supercritical Water-Cooled Reactor with Main Feed-Water Parameter Changing

“…Under steady operation, the flow rate of feed-water is equal to that of core coolant, and the decrease of feed-water pressure as well as temperature may also have influence on it, which may have influence for system operating. Under the non-coupling calculation condition, a certain study on transient and accident analysis for supercritical water-cooled reactor has been given [2]- [5]. But the calculation accuracy remains to be given further investigation with more detailed description.…”