Safety assessment and code validation for VVER-440 reactors

MTA EK operates several experimental facilities, in order to support code validation and safety assessment activities. The article presents the experimental capabilities of the thermal-hydraulic laboratory.

CERES- Vessel External Cooling facility

A research program has been going on at the Institute to give scientific support to the in-vessel corium retention in the Paks Nuclear Power Plant of VVER-440/213 type. The concept to be implemented in the plant is based on the external reactor pressure vessel cooling (ERVC) to preserve the reactor pressure vessel (RPV) integrity in severe accident sequences leading to core melt. The research program includes integral type experimental and computer code modeling of the severe accident sequence. For the experimental modeling of the cooling loop to be implemented in the plant, an integral type model, the CERES (Cooling Effectiveness on Reactor External Surface) facility was designed and constructed, containing a 1:40 section of the reactor vessel wall, with a scaling ratio of 1:1 for the vessel external surface and the elevations.

Several series of measurements indicated satisfactory heat removal on the vessel wall surface. On the basis of the experimental evidence the Power Plant introduced the safety enhancement ERVC concept in all units.

 

CIWH

Condensation-induced water hammer (CIWH) can happen, when a hot steam filled pipe is suddenly flooded by cold water. In that case, the steam might form a “pocket” which can survive for a while, but they abruptly collapses and the water – rushing into the empty space with very high speed – hits the pipe-wall with a big force.

The invoked pressure wave can break the clamps holding the pipe or even split the pipe itself, causing serious accidents. To avoid CIWH or at least to minimize its unwanted effects, experimental and theoretical investigations should be performed.

For experimental investigation, the EK has a facility physically connected to the steam-line of the PMK-2 facility. Compared to the other European devices, the CIWH facility of the EK allows very high pressure peaks (200 bars) in a large pipe. While presently only water can be injected into the hot steam filled pipe, in the future we are trying to improve the facility to be able to do inverse experiments, injection of hot steam into cold water. In the past, several experiments were done in the CIWH facility, some part of the results have been used to validate a computer program (the so-called WAHA-3 code) prepared by a big European collaboration to simulate condensation-induced water hammer phenomena. Recently this WAHA-3 program can be used to predict CIWH in various pipe-geometries for nuclear- and non-nuclear applications (like nuclear power plant, traditional – fossil – power plant, chemical factories, etc.).

 

PMK-2

The PMK-2 is a scaled-down model of the Paks Nuclear Power Plant equipped with VVER-440/213-type reactors of Soviet design is operating since 1990. It is a full pressure model of the plant with a volume and power scaling of 1:2070. Due to the importance of gravitational forces in both single- and two-phase flow the elevation ratio is 1:1 except for the lower plenum and pressuriser. The six loops of the plant are modeled by a single active loop. The coolant is water under the same operating conditions as in the plant, so transients can be started from nominal operating conditions. The core model consists of 19 electrically heated rods with uniform power distribution. In the core the heated length, spacer type and elevations, as well as the channel flow area are the same as in the Paks NPP. The main circulating pump of the PMK-2 serves to produce the nominal operating conditions and to simulate the flow coast-down following pump trip. The horizontal design of the VVER-440 steam generator is modeled by horizontal heat transfer tubes between hot and cold vertical collectors in the primary side. In the secondary side of the steam generator the steam/water volume ratio is maintained. From the emergency core cooling systems the four hydro accumulators of the Paks NPP are modeled by two vessels. They are connected to the downcomer and upper plenum similar to those of the reference system. The high and low-pressure injection systems are modeled by the use of piston pumps. To support the VVER reactor related analysis a wide range of accident scenarios has been performed. PMK-2 test results had continuously been applied to the validation of different versions of ATHLET, CATHARE and RELAP5, primarily in several domestic and international programs. Altogether, 28 PMK-2 tests were applied to different international code validations. A scientific school has been established with the participation of experts from 29 countries. The PMK-2 data base of 2007 contains 55 tests and can be downloaded from OECD/NEA web service.

 

PIV

Particle image velocimetry (PIV) is an optical method of flow visualization used in education and research. It is used to obtain instantaneous velocity measurements and related properties in fluids. The fluid is seeded with tracer particles which, for sufficiently small particles, are assumed to faithfully follow the flow dynamics. The fluid with entrained particles is illuminated so that particles are visible. The motion of the seeding particles is used to calculate speed and direction (the velocity field) of the flow being studied. The existing planar-PIV equipment of MTA EK was extended to Stereoscopic Particle Image Velocimetry (SPIV) system. It is an extension of planar-PIV to applications in which the presence of three-dimensional coherent structures and velocity gradients requires all velocity components to be measured.

In nuclear power plants during accident scenarios cold water may enter from the emergency core cooling system to the hot downcomer. The cold plum may cool the vessel wall unevenly, that may cause PTS (pressurized thermal shock), which should be avoided. The temperatures are very much depending on the mixing conditions of the cold and hot water streams. In order to form a validation database for the mixing phenomena measurements were conducted on a plexi-glass model consisting of three cold legs connected to a rectangular downcomer. Two of the cold legs were straight pipes, while the third one represented the bend typical for the cold legs of VVER-440 power plants. The ECCS line connects to this latter.

The measured data was compared with CFD calculations.

 

Contact
Attila Guba, MTA EK, Hungary
Head of Thermalhydraulics Department
attila.guba@energia.mta.hu