PSI - Issue 13

Peter Trampus / Procedia Structural Integrity 13 (2018) 2083–2088 P. Trampus / Structural Integrity Procedia 00 (2018) 000 – 000

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Analyses (TLAA) valid for 30 years was also requested by the regulator. Among TLAA the most significant analysis relates to the Pressurized Thermal Shock (PTS) for the reactor pressure vessels (RPVs). The RPV is the only non replaceable component determining thus the plant’s technically feasible lifespan. PTS is an overcooling transient which causes a thermal shock to the RPV, while the pressure is either maintained or the system is re-pressurized during the transient. The thermal stress due to the rapid cooling of the vessel wall in combination with the pressure stress results in large tensile stresses which have their maximum value in the inside surface of the vessel. Also, a unique ageing phenomenon called irradiation embrittlement occurs in the RPV wall reducing the structural material’s fracture toughness and shifting the ductile -brittle transition temperature in the direction of higher temperature. In the case if a flaw (crack) would exist in an area of the vessel wall near to the inside surface where the material properties degraded due to fast neutron irradiation and a PTS transient would happen, the RPV integrity would be jeopardized, see the process in Fig. 1.

Fig. 1. PTS process and its impact.

The goal of this paper is to describe the methodology and the basic results of the PTS analysis performed for Paks NPP. It gives an example for the effect of loading and environment on the structural integrity of a large-scale pressurized component. Leading technical support organizations and both Hungarian and international consultants contributed to the accomplishment and independent validation of the analysis. The author chaired the expert committee dealing with RPV integrity as well as contributed to and edited the report on PTS analysis submitted to the regulator.

2. Inputs for PTS calculation

The PTS analysis followed the Hungarian regulation which is in line with the guidance of the International Atomic Energy Agency (IAEA 2006). The structural integrity against brittle fracture of the RPV is ensured if the factual ductile-brittle transition temperature, called critical temperature of brittleness k T , of its critical components is less than the maximum allowable component specific transition temperature allow k T . The analysis is based on the comparison of the static fracture toughness Ic K of the structural material and the stress intensity factor I K calculated from the given loading situation using the theory of linear-elastic fracture mechanics. Subjects of the PTS analysis were all possible RPV locations that could potentially be susceptible for service induced ageing. However, the focus of the analysis was placed on the RPV belt line region (base metal of forged ring and circumferential core weld) because of the intensive embrittlement process caused by fast neutrons. The major steps of the overall methodology of PTS calculations are as follows: • identification of the PTS transients, • thermal-hydraulic analyses of PTS transients, • neutron fluence calculations, • evaluation of material properties and irradiation effects on RPV structural materials,