Modeling of a steam-line break in a boiling water reactor.

STAR-CD has been successfully employed for the CFD analysis of a steamline break in the boiling water reactor of Unit 2 at the Forsmark nuclear power plant. The calculation was motivated by the necessity of replacing the core grid of Unit 2 due to incipient damage of the welds between its components. A new design, consisting of a single continuous machined piece of metal without welds, was prepared.

Instead of using the traditional method for calculating the conditions and loads for the new design, a CFD analysis was performed using STAR-CD to simulate steam-line break scenarios. This enabled a dynamic analysis to be performed (similar to that used in earthquake simulation) with a detailed description of all transient loads on the reactor vessel caused by the break. The results were required for input into structural dynamics calculations.

The CAD geometry was directly imported into STAR-CD and the study assumed that the time scale of the transient analysis was smaller than the relaxation time of the water-steam system. Therefore, simulation was confined to the upper, steam part of the reactor, and two-phase effects (flashing) were not modeled. The analysis included a description of the dynamics of decompression waves propagating inside the reactor vessel. The calculated transient pressure fields acting on the reactor vessel inner walls and on the surfaces of various internal parts were transferred by direct data exchange to a structural mechanics code.

The results revealed the very complex nature of the behaviour of the non-linear decompression waves, best observed through animating a sequence of calculated flow fields. The computed instantaneous forces over the reactor’s internal parts were approximately twice the forces estimated in the past by less accurate methods.