2015 IR&D Annual Report

Assessment of Thermal Fatigue in Light Water Reactor Feedwater Systems by Fluid-Structure Interaction Analyses, 20-R8434

Principal Investigators
Kaushik Das
Debashis Basu
Mohammed Hasan

Inclusive Dates: 12/13/14 – Current

Background — In a nuclear reactor, thermal striping, stratification, and cycling take place as a result of mixing pressurized hot and cold water streams. The fluctuating thermal load generated by such unsteady mixing may result in fatigue damage of the associated structures. The mixing is often caused by faulty valves and can potentially affect safety-related lines such as the pressurizer surge line, emergency core cooling injection lines, reactor clean-up systems, and residual-heat removal systems. Generally, thermal fatigue is considered to be a long-term degradation mechanism in nuclear power plants. This is significant, especially for aging power plants, and improved screening criteria are needed to reduce risks of thermal fatigue and methods to determine the potential significance of fatigue.

Though fluid mixing and thermal fatigue have been studied separately, a number of issues related to complex interaction between turbulent mixing and the mechanical structure of the light water reactor (LWR) have not yet been resolved. Key uncertainties in this area include the effects of solid walls on variations in the thermal load amplitude and frequency (often referred to as filtering). These effects determine the temperature spectrum transmitted from the fluid to the structure.

The primary objective of this ongoing project is to advance the use of numerical modeling techniques for reactor safety determination. This objective is being achieved by developing a proof-of-concept benchmark simulation that demonstrates that computational methods can be used to resolve the turbulent mixing-induced thermal fatigue in the context of LWR operations.

Approach — The project uses computational fluid dynamics (CFD) and numerical techniques to achieve the objectives. In particular, the modeling efforts primarily focus on the following:

  • Using CFD tools to resolve the turbulent thermal mixing process, to have accurate knowledge of the thermal processes in the fluid field,
  • Performing conjugate heat transfer (CHT) calculations within the fluid and surrounding solid structure to obtain a realistic estimate of the fluctuating heat load to the solid and assess the effect of a solid wall on thermal load modification,
  • Evaluating thermal stress, and
  • Estimating the structure fatigue based on the calculated thermal stress using an analytical approach.

Initially, the flow field is calculated using a CFD solver. At this stage the flow solution is compared against available experimental data for model confidence and benchmarking. Subsequently, temperature fluctuations on the structure are calculated using a CHT solver, and the thermal stresses are calculated from the temperature fluctuations. The CHT analysis is used for predicting fluid field and solid thermal fluctuation. The approach involves coupling the fluid and solid domains to predict the thermal stress generated by the thermal turbulence mixing phenomena. In this approach, the fluid field is modeled with the temperature-dependent incompressible Navier-Stokes equations. Turbulence is simulated using the standard Smagorinsky sub grid scale Large Eddy Simulation model. The heat equation is used to model heat transfer in the piping system. For thermal fatigue analysis, the temperature, thermal stress, and stress intensity factor are calculated separately. These variables are used to find the structure degradation (number of cycles to failure and crack length propagation) by applying fatigue crack propagation correlation.

Accomplishments — Numerical simulations of the T-junction experiment carried out at the Älvkarleby Laboratory of Vattenfall Research and Development AB were performed to validate the numerical simulation results. Excellent agreement was achieved between the simulated results and the experimental data. A technical paper was presented at the American Nuclear Society annual meeting in San Antonio in June 2015, and another paper will be presented at the American Society of Mechanical Engineers International Mechanical Engineering Congress and Exposition in Houston in November 2015. A MATLAB® code was developed for the fatigue analysis. The project developed a robust integrated computational methodology to assess thermal fatigue damage in T-junction configurations that involve mixing hot and cold fluids. A reduced order model proper orthogonal decomposition was used to capture the coherent structures and turbulence scales.

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