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# Numerics and Implementation of the UK Horizontal Stratification Entrainment Off-Take Model Into RELAP5/MOD3 (NUREG/IA-0096, AEA–TRS–1050, AEEW–R2501)

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## Publication Information

**Date Published:** June 1993

**Prepared by:** W. M. Bryce

Physics and Thermal Hydraulics Division AEA Thermal Reactor Services Winfrith Technology Centre Dorchester, Dorset DT2 8DH, United Kingdom

**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/MOD3 is a computer code for the the analysis of fault transients in light water reactor systems. It has been subject to an international development exercise.

Among the fault transients which can be analysed using RELAP5/MOD3 is a loss-of-coolant accident (LOCA) in a Pressurised Water Reactor (PWR), in which a small break occurs in one of the large diameter horizontal pipes forming the reactor inlet or outlet legs. In this situation the steam and water in the pipe may separate to form a liquid level. The quality of fluid discharged from the break (proportion of the mass flow which is steam) then depends on whether the break is above or below the liquid level. RELAP5/MOD3 contains a special model to calculate discharge from an off-take connected to a large diameter horizontal pipe in which there is stratified flow, referred to as the horizontal stratification entrainment (HSE) model. Apart from small break LOCAs, such a model is also needed for analysing other abnormal conditions in PWRs.

Early in its development, the RELAP5/MOD3 HSE model was the same as that of the RELAP5/MOD2 code. This was assessed by Ardron and Bryce [1] who found that the model tended to under-predict significantly the discharge quality in many cases. They selected improved correlations for the off-take quality and implemented them in a modified version of RELAP5/MOD2 Cycle 36.04. The modified code was shown to give much improved agreement with separate effects test data.

This report describes the numerics and implementation details for the modification of RELAP5/MOD3 to use the same improved correlations. These details include the areas where the use of the HSE correlation set are extended outside of its range. The modifications, which were incorporated into pre-release code version 411, have been retained in the first "frozen" release of the code (version *5m5*). Areas where changes have been made by EG&G in producing the release version which might affect the modelling are noted in the section on programming details. Particularly noteworthy is the restriction which apparently excludes the full control of the HSE model at the internal junctions of PIPE components.

A simple test problem is used to verify the RELAP5/MOD3 coding of the correlations, making use of the control system logic. This problem, run using the release code version, also demonstrates that this version seems to be more prone to instability than the pre-release version 411. The problem is not believed to be associated with the HSE model.

Physics and Thermal Hydraulics Division Winfrith Technology Centre

March 1991

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