Blowdown transients and implication for leak protection shutdown strategy

The creep-fatigue damage due to a blowdown transient is independent of the rate of water-side blowdown following isolation. The damage depends primarily upon plant system characteristics which set the rate of IHTS sodium cooldown after reactor scram and main pump trip. The time delay between a small leak alarm and reactor scram should be considered as a tradeoff between (1) providing time to confirm the existence of the leak to a reasonable level of assurance and (2) the potential for secondary tube wastage damage. In general, there appears to be no incentive to delay water-side blowdown following scram because the additional stress damage due to blowdown is insignificant and the potential for wastage damage should be eliminated as quickly as possible. One exception is the case of multiple evaporators feeding a superheater (as for CRBRP) where isolation and blowdown of one evaporator shortly after scram can cause significant additional stress damage to the superheater upper tubesheet. When initiated, the water-side blowdown process should be rapid. About 30 seconds blowdown time appears practical and adequate, based on the CRBRP design