11. Shock treatment. This is a one-fourth cutaway view of a reactor pressure vessel at the start of a pressurized thermal shock simulation by TÜV, using Abaqus FEA from SIMULIA. The vessel, which normally operates at 300C (indicated in red), is shown as cooler water (30C) begins pouring in through the nozzle on the top right. (Units are degrees Celsius.) Source: TÜV
12. Thermal shock wave. This Abaqus FEA half model of a reactor pressure vessel shows the nozzle opening (shown as holes in Figures 11 and 13) through which cold water is quickly introduced to shut down the reactor, resulting in pressurized thermal shock. TÜV requires virtual testing of nuclear power plant components as part of the certification process. (Units are degrees Celsius.) Source: TÜV
Simulating Pressurized Thermal Shock
With their FEA models set up on a Dell Workstation PWS670 with a 3 GHz Xeon CPU, the TÜV team then used Abaqus/Standard for linear elastic simulation of the rapid cooling of the vessel, calculating the effects of a large increase in the tensile stresses on the inner vessel wall. This increase is the result of two phenomena.
First, the thermal conductivity of the two materials (cladding and base) is different, so each reacts differently to the rapid temperature change. Second, the emergency injection of colder water creates a temperature plume (temperatures inside the plume are significantly lower than those outside) that produces significant stress buildup at its leading edge (Figure 13).
13. All stressed out. The same reactor vessel as shown in Figure 11, but further into the pressurized thermal shock simulation, shows the stress distribution on the inner wall of the reactor pressure vessel (from red to blues and greens. (Units are degrees Celsius.) Source: TÜV
In addition to the simulation of such rapid temperature fluctuations, the effect of the high pressures under which the system would be operating was incorporated into the models. An elastic/plastic Abaqus simulation completed the virtual testing of PTS and predicted where the greatest surface and/or volumetric stresses would occur in the system. An average time transient for a full simulation was 8,000 seconds. The simulations were run well beyond the required tolerance levels, all the way to the point at which cracking would occur; such additional data are useful for fracture mechanics analyses and can be used in the future by inspectors, says Hienstorfer.
Compliance with Safety Regulations
The RPV in the example discussed here passed TÜV’s simulation testing, validating that its walls and nozzles would withstand the extreme conditions of a LOC event over the 40-year lifespan of the facility. "The Abaqus FEA calculations helped evaluate compliance of the vessel with regulatory safety requirements," says Hienstorfer.
The successful design, development, and lifetime maintenance of nuclear power facilities is a challenge that must be carefully managed from both an organizational and an engineering viewpoint, says Hienstorfer. He sees FEA as having an integral role to play in both operational evaluation and ongoing monitoring of nuclear facilities to assist in complying with regulations designed to ensure that the world’s growing energy needs can be met safely.
"We depend on FEA for computer modeling and virtual testing of reactor pipelines, vessels, and materials under extremes of stress and time," he says. "It definitely provides guidance to engineers to build both safety and longevity into their nuclear power plant designs."
— Contributed by Tim Webb (tim.webb@3ds.com), director of marketing communications and programs, SIMULIA Strategy and Marketing.
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