Assessment of RELAP5/MOD3/V5m5 Against the UPTF Test No. 11 (Countercurrent Flow in PWR Hot Leg) (NUREG/IA-0116, KWU E412/91/E100)

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Publication Information

Date Published: May 1993

Prepared by:
F. Curca-Tivig

Siemens AG–KWU Group
D 8520 Erlangen
Federal Republic of Germany

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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Analysis of the UPTF Test No. 11 using the "best-estimate" computer code RELAP5/MOD3/Version 5M5 is presented.

Test No. 11 was a quasi-steady state, separate effect test designed to investigate the conditions for countercurrent flow of steam and saturated water in the hot leg of a PWR.

Without using the code's new countercurrent flow limitation (CCFL) model, RELAP5/MOD3/V5M5 overestimated the mass flow rate of back down flowing water up to 35 % (1.5 MPa runs) and 43 % (0.3 MPa runs). This is the most obvious difference to RELAP5/MOD2, which did not allow enough countercurrent flow. From the point of view of performing plant calculations this is certainly an improvement, because the new junction-based CCFL option could be used to restrict the flows to a flooding curve defined by a user-supplied correlation.

Very good agreement with the experimental data for 1.5 MPa - which are relevant for SBLOCA reflux condensation conditions - could be obtained using the code's new CCFL option in the middle of the inclined part (riser) of the hot leg. Using the same CCFL correlation for the simulation of 0.3 MPa test series - typical for reflood conditions -, the code underestimated by 44 % the steam mass flow rate at which complete liquid carry over occurs.

An unphysical result was received using a CCFL correlation of the Wallis type with the intercept C = 0.644 and the slope m = 0.8. The unphysical prediction is an indication of possible programming errors in the CCFL model of the RELAP5/MOD3/VSM5 computer code.

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