RELAP5/MOD3 Analysis of BETHSY Test 6.9c: Loss of RHRS: SG Manway Open (NUREG/IA-0187)
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Date Published: August 2000
G. R. Kimber, M. A. George
Dorchester, Dorset DT28DH
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
Accidents associated with a loss of the Residual Heat Removal System (RHRS) during PWR shutdown operation have received attention in recent years. Typically such events occur at low pressure and low core power.
A series of tests, 6-9a-d, has been performed in the BETHSY Integral Test Facility at Grenoble, France, to address the safety issues posed in this type of incident. The data from Tests 6.9a,c & d are available to the AEA Technology through its membership of the BETHSY Club.
Tests 6.9a & d have been analyzed in previous years. Test 6.9c, the subject of this report differs from previous tests as it simulates plant conditions following a loss of RHRS when the primary circuit is open in two places; one at the pressuriser manway and the other at the steam generator outlet plenum. There is thus the potential for a large inventory loss from the primary circuit and a deep core uncovery. This did in fact occur. The analysis of this test is directly relevant to PWR thermal hydraulics safety issues associated with loss of RHRS.
The analysis has been performed using the latest release version of RELAP5, i.e. MOD3.2. The main phenomena occurring during this rest are well represented by the calculation. The calculated primary circuit peak pressure matches the experimental value. The reduction in pressure following the peak is, however, delayed due to water draining from the steam generator and being entrained into the pressuriser.
The cladding temperature rise is delayed and the start of the gravity feed is about 150s late. Ultimately, however, the experimental peak clad temperature is matched to within 15°C. The mass error associated with the calculation is less than 10% for most of the transient.
It is concluded that the broad characteristics of the transient would be experienced in a similar scenario for PWR plant. The analysis will also form the UK submission for the International Standard Problem 38 being hosted by AEA Technology at Winfrith.