United States Nuclear Regulatory Commission - Protecting People and the Environment

Analysis of LOFT Test L5–1 Using RELAP5/MOD2 (NUREG/IA-0118, TD/SPB/REP/0130)

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Publication Information

Date Published: May 1993

Prepared by:
S. Cooper

Nuclear Electric
Barnett Way
Barnwood, Gloucester, GL4 7RS

Prepared as part of:
The Agreement on Research Participation and Technical Exchange
under the International Thermal-Hydraulic Code Assessment
and Application Program (ICAP)

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

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Summary

RELAP5/MOD2 is being used by Technology Division for the calculation of certain small break loss-of coolant accidents (SBLOCA) and pressurised transients in the Sizewell 'B' PWR.

To assist in validating RELAP5/MOD2 for the above application, the code is being used to model a number of small LOCA ind pressurised fault simulation experiments carried out in integral test facilities. The present report describes a RELAP5/MOD2 analysis of an intermediate break LOCA test in the LOFT facility. This test was designed to simulate the rupture of a single 14 inch diameter accumulator injection line in a commercial PWR with a 25% break in the broken loop cold leg. Early in the transient the pumps were tripped and the HPIS injection initiated; towards the end of the transient, accumulator and LPIS injection began.

RELAP5/MOD2 gave reasonably accurate predictions of the system thermal hydraulic behaviour but failed to accurately calculate the core dryout which occurred due to boil-off prior to accumulator injection. The error is due to the failure to calculate the correct core void distribution during this period of the transient. A separate calculction using the RELPIN code using hydraulic data from the RELAP5 analysis gave significantly improved predictions of the core dryout. However, the peak clad temperature was underpredicted, it is believed that the error is due to the fact that the core liquid inventory in this boildown was overpredicted in the RELAP5/MOD2 calculation.

A sensitivity calculation showed that when the core void distribution and core inventory were well predicted, both RELAP and RELPIN accurately predict fuel clad temperatures.

The work in this report was carried out on behalf of PPG and HSD under Agreement TSPBSW2001, PPG Task G212(22G).

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