Analysis of a reactor pressure vessel subjected to pressurized thermal shocks

Abstract

The integrity of reactor pressure vessels (RPVs) of nuclear power plants is one of the most important topics in the field of nuclear energy production. Therefore, the integrity of RPVs has to be assessed for normal operation as well as for emergency transients. A critical transient concerning the RPV integrity is the emergency cooling of a pressurized water reactor, initiated by a leak in the hot leg. Such shock-like cooling in combination with the pressure, the so-called pressurized thermal shock (PTS), causes high thermal stresses in the RPV wall and stress intensities of pre-existing cracks which could exceed the remaining fracture toughness of the material, which is additionally embrittled due to neutron irradiation. This may result in a cleavage fracture of the most safety relevant reactor component.We present a PTS study of a reference reactor, starting with the calculation of the thermal-hydraulic system behaviour, followed by the simulation of the cold water temperature injection and mixing by means of computational fluid dynamics (CFD) method and the subsequent structural and fracture mechanics calculation. In the safety assessment, we compare the evolution of the stress intensity factors (SIF) during an emergency cooling transient with the fracture toughness at the tip of postulated cracks. Results and open questions will be discussed in the light of a realistic estimation of safety margins.

Study of the Stress Distribution Due to the Effect of Transient Analysis on a Vertical Pressure Vessel and Validation using the Mesh Independence Study

“…This value of is a Richardson extrapolation based on three grids. Equations ( 22) and ( 24 However, to standardise reporting numerical error estimates, GCI was defined as a grid-based on the right side of equation (8), and it has been fairly widely adopted in the CFD Community. Dividing equation ( 8) by 1 and multiplying by a factor of safety ( ) of 1.25 for convergence studies with a minimum of 3 grids to demonstrate the observed order of convergence [16].…”

“…Also, stress distribution was studied using the thermo-hydraulic computation method and Fracture Mechanics for the effect of cold-water injection mixing and temperature differential with hot water inside a PV. The mixing behaviour from the Computation Fluid Dynamics (CFD) shows that unilateral cooling produced a more stable cooling plume when compared with that of symmetry cooling and larger thermal shock was observed for two-phase flow when compared with a single-phase flow [8]. The transient thermal analysis of both temperature and heat flux distributions of a typical APR1400 PV under thermal loading was evaluated in the simulation of Maklago, et al [9].…”