Coolant mixing in a PWR - de-boration transients, steam line breaks and emergency core cooling injection - experiments and analyses

Conference · Mon Jul 01 04:00:00 EDT 2002

OSTI ID:21167948

Prasser, H M; Grunwald, G; Hoehne, Th; Kliem, S; Rohde, U; Weiss, F P ^[1]

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1. Forschungszentrum Rossendorf, Institute of Safety Research, P.O.B. 510119 D-01314 Dresden (Germany)

The reactor transient caused by a perturbation of boron concentration or coolant temperature at the inlet of a Pressurized Water Reactor (PWR) depends on the mixing inside the reactor pressure vessel. Initial steep gradients are partially lessened by turbulent mixing with coolant from the unaffected loops and with the water inventory of the reactor pressure vessel. Nevertheless the assumption of an ideal mixing in the downcomer and the lower plenum of the reactor leads to un-realistically small reactivity inserts. The uncertainties between ideal mixing and total absence of mixing are too large to be acceptable for safety analyses. In reality, a partial mixing takes place. For realistic predictions it is necessary to study the mixing within the three-dimensional flow field in the complicated geometry of a PWR. For this purpose a 1:5 scaled model (the ROCOM facility) of the German PWR KONVOI was built. Compared to other experiments, the emphasis was put on extensive measuring instrumentation and a maximum of flexibility of the facility to cover as much as possible different test scenarios. The use of special electrode-mesh sensors together with a salt tracer technique provided distributions of the disturbance within downcomer and core entrance with a high resolution in space and time. Especially the instrumentation of the downcomer gained valuable information about the mixing phenomena in detail. The obtained data was used to support code development and validation. Scenarios investigated are: (1) Steady-state flow in multiple coolant loops with a temperature or boron concentration perturbation in one of the running loops. (2) Transient flow situations with flow rates changing with time in one or more loops, such as pump start-up scenarios with deborated plugs in one of the loops or onset of natural circulation after boiling-condenser-mode operation. (3) Gravity driven flow caused by large density gradients, e.g. mixing of cold emergency core cooling water entering the RPV through the ECC injection into the cold leg. The experimental results show an incomplete mixing with typical concentration and temperature distributions at the core inlet. which strongly depend on the boundary conditions. CFD calculations were found to be in good agreement with the experiments. (authors)

• Research Organization: American Nuclear Society, 555 North Kensington Avenue, La Grange Park, IL 60526 (United States)
• OSTI ID: 21167948
• Country of Publication: United States
• Language: English

Related Subjects

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS
BOILING
BORON
COOLANT LOOPS
COOLANTS
DENSITY
DISTURBANCES
ECCS
MIXING
NATURAL CONVECTION
PRESSURE VESSELS
PWR TYPE REACTORS
STEAM LINES
TRANSIENTS