IMR realizes ‘no cause of fuel failure’ concept by reactor design and the core is always submerged in primary coolant without safety injection. Therefore, only a heat removal function is required as the safety system. Figure XV-2 shows the configuration of the stand-alone direct heat removal system (SDHS). SDHS is a closed natural circulation system that removes residual heat directly from inside the RV to the atmosphere via SGs and passive SG coolers (PSGCs). The reactor is cooled down and depressurized without opening the primary system pressure boundary. Residual heat in the early stage of an accident is removed by latent heat of cooling water by submerging PSGCs. After the cooling water was dried out, residual heat will be removed by air-cooling, since the dried water pool and ducts form a wind tunnel. Therefore, the heat transfer mode in PSGCs automatically changes from water­cooling to air-cooling following the pool water evaporation and SDHS works a long time without any operator action and external support.

The reactor design and SDHS concept greatly simplify the safety system of IMR compared with conventional PWRs. There is no safety injection system and containment spray system, and SDHS makes support systems to change into non-safety systems, which are component cooling water system, essential service water system, and emergency AC power system. To show the feasibility of SDHS, the safety system of IMR, natural circulation and heat removal capability of the system including effect of non-condensable gas has been experimentally examined. SDHS is designed to accumulate non-condensable gas into the lower header of PSGC and the feed water tank of the SGs.

XIV — 4. Conclusions

The Integrated Modular Water Reactor (IMR) employs two natural circulation systems, the hybrid heat transport system (HHTS) for the primary system and the stand-alone direct heat removal system (SDHS) for the safety heat removal. The design concepts of IMR and these systems have been built and the test results showed the feasibility of the system. In the next design phase of IMR, 3D effects of two-phase natural circulation flow behaviour will be tested as a part of basic design study.

REFERENCES TO ANNEX XV

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[8] SUZUTA, T., et al., Steam-Water Natural Circulation Tests for Integrated Modular Water Reactor (IMR), Proc. of Japan-US Seminar on Two-Phase Flow Dynamics, Nagahama, Japan (2004).

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