The most effective means of describing natural circulation in the HTS, Calandria and containment is to integrate their operation in response to a large break loss of coolant accident (LOCA). The first phase of a LOCA is the subcooled blow down phase. During this phase, high pressure subcooled liquid is vented from the break under choked flow conditions. The primary HTS pressure and liquid inventory will decrease. When a LOCA is detected, the following automatic actions are initiated:

• Shut down the reactor,

• Initiation of the ECC system,

• Open the RWS valves to HTS,

• Crash cooldown (rapid depressurization of the steam generators),

• Isolate the broken loop of the HTS from the intact loop, and

• Trip the HTS pumps.

II-3.1. Decay heat removal — broken loop

Following reactor shutdown, the HTS loops are isolated and crash cooldown (rapid depressurization of the secondary side of the steam generators) is initiated. This depressurizes the HTS and facilitates refill of the broken loop by either the ECC system, or by gravity from the RWS (as a back-up to ECC).

Heat is removed from the broken loop by the LTC system in recovery mode (see Section II-2). If the LTC system fails (low probability event), the fuel and fuel channels of the broken loop will heat up until the pressure tubes sag into contact the Calandria tubes, resulting in direct conduction of heat to the moderator (refer to Section II-2.1). The moderator cooling system, assisted by natural circulation in the Calandria, serves as a back-up heat sink, preventing further core damage.

If the moderator fails as a heat sink (e. g. if moderator make-up from the RWS fails), the fuel channels will fail. The shield vault contains a large volume of water that can cool and contain the damaged fuel channels inside Calandria and halt the progress of the accident. Inventory is allocated in the RWS to make-up inventory boiled off from the shield vault during this time. As long as make-up can be provided to the shield vault to cool the fuel, Calandria vessel integrity will be maintained and corium — concrete interaction will be prevented.