A brief description is given here to a postulated large break loss of coolant accident (LBLOCA) followed by loss of ECC. During normal operation, the PMCS rejects heat deposited in the moderator to an ultimate heat sink that could be provided by elevated heat exchangers connected to the RWS. Following LOCA and reactor shutdown, the ECC system, which consists of the ECI and potentially a pumped system for long term cooling will remove decay heat. In the event that the ECC system fails, the fuel gets hot and heat is radiated from the fuel elements to the insulator and is then transferred to the moderator by conduction and convection. Preliminary simulations showed that the insulator thickness could be optimized to provide minimal heat loss to the moderator during normal operation (compared to the 5% loss due to gamma and neutron heating) but sufficient heat conduction during accident conditions.

X — 4. Conclusions

The CANDU-SCWR is expected to use existing CANDU passive safety features. In addition, the CANDU SCWCR employs an advanced fuel channel design that makes it possible to use a flashing — driven passive moderator cooling system to enhance the role of the moderator as a backup safety system. A flashing-driven loop can be designed to reject the moderator heat load under normal operating conditions. Since the moderator heat is comparable to the decay heat load immediately following reactor shutdown, this loop can also be used to reject decay heat under accident conditions through radiation to the insulator and conduction through the moderator-cooled pressure tube. This passive loop can be used during normal operation, which guarantees the functionality of the passive loop at all times, and can potentially eliminate scenarios that could lead to severe core damage.

REFERENCES TO ANNEX X

[1] U. S. DOE Nuclear Energy Research Advisory Committee and the Generation IV International Forum, A Technology Roadmap for Generation IV Nuclear Energy Systems, GIF002-00, December 2002.

[2] KHARTABIL, H. F., DUFFEY, R. B., SPINKS, N., DIAMOND, W., The pressure-tube concept of Generation IV Supercritical Water-Cooled Reactor (SCWR): Overview and status, Proceedings of ICAPP’05, Seoul, Korea, May 2002.

[3] LEKAKH, B., HAU, K., FORD, S., ACR-1000 Passive Features, Proceedings of ICONE14, Miami, Florida, USA, July 2006.

[4] KHARTABIL, H. F., A Flashing-Driven Moderator Cooling System for CANDU Reactors: Experimental and Computational Results, Presented at the IAEA Technical Committee Meeting on Experimental Tests and Qualification of Analytical Methods to Address Thermohydraulic Phenomena in Advanced Water Cooled Reactors, Villigen, Switzerland (1998).

[5] GILLESPIE, G. E., et al., Moderator Boiling on the External Surface of a Calandria Tube in a CANDU Reactor During a Loss-of-Coolant Accident, Proceedings, International Meeting on Thermal Nuclear Reactor Safety, Chicago, USA (1982).