TRAC-PF1/MOD1 Calculations of LOFT Experiment LP-02-6 (NUREG/IA-0027)

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Publication Information

Date Published: April 1992

Prepared by:
P. Coddington, C. Gill

Winfrith Technology Centre
United Kingdom Atomic Energy Authority
Winfrith, Dorchester
Dorset, England DT 28 DH

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

Published by:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555

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Abstract

The report describes four TRAC-PF1/MOD1 calculations modelling the OECD-LOFT experiment LP-02-6. This was a 200% double-ended cold leg break experiment performed at nearly full power (47 MW) and with a loop mass flow of 248 kg/s. In the experiment the pumps were tripped and then allowed to coast down naturally after the start of the transient. Two of the calculations compared the results of two versions of the code (12.2 and 13.0), one incorporated a reduced gap between the fuel and the cladding to reduce the initial fuel stored energy, and the other had the TRAC interface sharpener model switched off.

Following the opening of the quick acting blowdown relief valves to initiate the transient there is a net flow out of the vessel until about 4 seconds, at which time the broken loop cold leg flow out-of the vessel drops below the flow into the vessel from the intact loop cold leg, being driven by the pumps inertia. This net flow into the vessel, enhanced by flashing of subcooled liquid in the downcomer and lower plenum, causes a bottom-up flow of liquid and quenches about 2/3 of the core. Additionally a top-down partial quench, extending to about the 30 inch elevation, is observed at about 15 seconds. This corresponds to fluid running back into the upper plenum and down into the core as the fluid in the pressurizer and steam generator begins to flow back along the intact loop hot leg.

The nature of the observed quenching is not entirely clear: it may be genuine fuel pin quenching or simply localised quenching of the thermocouples.

At 17.5 seconds, the primary system pressure reaches 42 bars, at which point the Emergency Core Cooling System trips. Measurements suggest oscillatory flow immediately upstream of the accumulator injection point in the intact loop cold leg. Except for two slugs of liquid, totalling about 200-250 kg, compared to a total accumulator flow of about 1,690 kg, no continuous bypassing of the downcomer by the accumulator fluid occurs. Most of it finds its way to the lower plenum. As the water level here rises, it begins to quench the bottom of the core at about 37 seconds and the quench moves progressively upwards. The final quench of the uppermost elevations is coincident with the entry of accumulator nitrogen into the intact loop cold leg and the consequent rise in the primary system pressure.

All four TRAC calculations predict similar hydraulic behaviour to each other. The bottom-up liquid flow at 4 seconds extends to the top of the core, as opposed to just 2/3 of the way up as in the experiment. The TRAC modelling does not predict either the bottom-up or the top-down quench at 15 seconds and following the subsequent fuel rod dryout the calculated temperatures are too high, particularly at the top and bottom of the fuel rods. The reduced fuel-gap calculations (ZEROGAP and ISHARP) are considerably better in this respect due to their lower stored energy. The calculations predict no bypass of ECCS fluid in line with the experiment and all predict oscillatory core inlet flow. There were differences, however, in the behaviour of fluid in the intact loop cold leg, for in some of the calculations the production of liquid slugs was predicted, in others it was not. These differences are believed to be due to the sensitivity of the TRAC condensation model rather than any specific changes to the models.

All the calculations predict a surge of fluid into the core on the entry of nitrogen into the intact loop cold leg. However, the higher rod temperatures in the calculations mean that the final quench is delayed longer than in the experiment.

The main differences between the calculations are therefore restricted to the thermal behaviour of the fuel rods due for example to the different dispersed flow heat transfer used in versions 12.2 and 13.0 and the reduced fuel stored energy in the ZEROGAP and ISHARP calculations.