Application of RELAP5/MOD3.2 to the Loss-of-Residual-Heat-Removal Event Under Shutdown Condition (NUREG/IA-0182)

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Publication Information

Date Published: April 2000

Prepared by:
K.W. Seul, Y.S. Bang, H.J. Kim, KINS

Korea Institute of Nuclear Safety
PO. Box 114
Yusung, Taejon
305–600, Korea

Prepared as part of:
The Agreement on Research Participation and Technical Exchange
under the International Code Application and Maintenance Program (CAMP)

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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The long-term transient following a loss-of-residual-heat-removal (loss-of-RHR) event during reactor shutdown was analyzed to determine the containment closure time (CCT) to prevent the release of fission products to environment and the gravity-injection path and rate (GIPR) to effectively cool the core. The thermal-hydraulic analysis was carried out using the RELAP5/MOD3.2 code and relevant modeling scheme, which were assessed with the LSTF experiment in a previous study (NUREG/IA-0143). Based on the plant-specific geometry data including various operating conditions, the possible event sequences were identified for the Yonggwang Units 3&4 plant (YGN 3/4), which is CE-typed PWR of 2,815 MW thermal power in Korea. As a result, the real plant simulation gives the similar calculation characteristics to the previous LSTF simulation, and then it was found that the RELAP5/MOD3.2 code is capable of appropriately simulating the loss-of-RHR event of the real plant.

From the five cases of the CCT analyses, it was estimated that the containment closure should be achieved within about 40 minutes to prevent the release of fission products in the large cold-leg opening case under the worst event sequence. However, it was also found that the first core uncovery could occur in the early phase of the event by the loop seal clearing phenomenon in the crossover leg. From the six cases of the GIPR analyses, it was revealed that the system was well depressurized and the core boiling was successfully prevented by the gravity-injection in cases with the injection point and opening on the different leg side. However, it was also found that the gravity-injection process could be ineffective in the case of relatively high pressurizer-manway opening because of the water holdup phenomena in the pressurizer. Also, it was estimated that about 54 kg/s of minimum injection rate was required to maintain core cooling and the core cooling could be provided for about 10.6 hours with the nominal water capacity of refueling water storage tank (RWST).

These results will provide useful information to operators to cope with the event. And, to apply them to the emergency and recovery procedures against the event, additional case studies will be needed for wide range of operating conditions such as reactor coolant system inventory, RWST water temperature, and core decay heat rate.

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