CoPool – Multidimensional Containment Pool-Model

Pressurized and Boiling Water Reactors

The security of pressurized and boiling water reactors (PWRs and BWRs) in nuclear power plants has gained a lot of significance since the incident in Fukoshima. Moreover, it will remain an important topic after the nuclear power phase-out in Germany. So, critical decisions should be promptly supported by computer simulations in the case of a conversion or even of an incident. In the last years the software COCOSYS has been developed by the Gesellschaft für Reaktorsicherheit (GRS).

Temperature Stratification in Water

The spatial area at the containment of PWRs and BWRs contains large hydraulic seals (condensation chamber at BWRs), which can also arise at an incidence (sump and reactor cavity at PWRs, control rod drive room at BWRs). These hydraulic seals have a distinct three-dimensional geometry, in which temperature stratification can occur in the water.

The global model of COCOSYS can only handle an averaged water temperature so far. Hence, hydrodynamic and thermal simulations are not sufficiently detailed in some cases.

Development of Compatible Softare CoPool

The aim of the BMWi project CoPool (1501369) is the development of a compatible software “CoPool”, which allows for a more precise simulation of flow and heat exchange without increasing the computational costs.

Therefore, we have to find a compromise solution between

• accuracy of the models
• discretization of equations
• the geometry
• the necessary simulation time

Interactive Preprocessor for Geometry Generation, Space Classification and Grid Generation

Since digital geometrical data is not available for almost all reactors, one has to construct the computational domain manually. This is one of the reasons for developing - in addition to the numerical solver - an interactive preprocessor for geometry generation, space classification and grid generation. Another important topic is an efficient and precise calculation of heat conduction in container walls.

The geometry generator allows for an interactive generation of different geometric objects using simple basic objects (cubes, cylinders, spheres, etc.) and Boolean operations. Container walls are designed analytically. The discretization of walls is realized by grid refinement. The enclosed space defines the flow area in which the 3D Navier-Stokes equations with Boussinesq approximation taking into account variable fill level on Cartesian grid are solved. The calculation of thermal conductivity is very efficient since the entire solution is calculated by a domain decomposition method.

Validation of the Software CoPool

Individual parts of the software were tested on the basis of model problems. In cooperation with Becker Technologies GmbH more complex phenomena like thermal layers are validated in real plant experiments.

All in all, CoPool is a simulation software, that can efficiently determine the local thermal differences in hydraulic seals for the purpose of reactor safety. In a follow-up project CoPool will be coupled with the software COCOSYS.