June 1, 2009

Nuclear: Realistic Simulation Assists in Nuclear Power Plant Certification

Tim Webb

TÜV Models a Reactor Pressure Vessel with Abaqus

A typical FEA analysis of an RPV takes into account temperature transients, internal pressure fields, and the radiation embrittlement behavior of the vessel during a simulated LOC event. (The case described here is taken from an actual evaluation by TÜV at the recertification of a power plant in Germany.) The simulations examine stresses both at vessel walls and at entry points of the hot and cold water nozzles feeding into the RPV (Figure 10).

10. Nozzle flow. An exterior view of the nozzles (with red caps) through which hot and cold water circulate into and out of a nuclear reactor pressure vessel that houses the fuel rods. Courtesy: Westinghouse

To create their FEA evaluation models, TÜV engineers first obtained component condition data for the vessel and nozzles from nondestructive x-ray and/or ultrasound testing. "You must verify the initial condition of the RPV before setting up your models," says Hienstorfer. Every vessel is plant-specific. In the case described here, the material was ferrite steel coated with austenitic cladding to prevent the load-carrying ferrite layer from corrosion. The embrittlement of the metal over time was represented by end-of-life calculations based on existing data from irradiated material.

Next, Abaqus/CAE was used to build and mesh computer models of the vessel and the four water pipe nozzles that fed into it. Using larger, linear hexahedral elements reduced computation time for solving the global model (Figure 11), while smaller, quadratic hexahedral elements were used in the submodels (Figure 12) for more accurate depiction of stresses at the edges of nozzles. The global model had 22,650 elements and 76,650 degrees of freedom (DOF), whereas a typical submodel contained 10,900 elements with 144,000 DOF.