PSI - Issue 16

Peter Trampus / Procedia Structural Integrity 16 (2019) 161–168 Peter Trampus / Structural Integrity Procedia 00 (2019) 000 – 000

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The ASME Inspection Code states that its application is allowed if the requirements of the ASME Construction Code (Section III) have been satisfied. Therefore, it was necessary to justify the compliance with the design, material selection, manufacturing, quality assurance criteria in accordance with Section III. In case of selected components (reactor coolant pressure boundary, main steam line, feedwater system, residual heat removal system, chemical and volume control system, emergency core cooling system and primary circuit purification system) a construction review was completed covering the following aspects:  justification of strength calculation for design, operation, maintenance and test, as well as accident loading;  justification of brittle fracture resistance by means of fracture mechanics calculations;  justification of over-pressure protection values;  fatigue calculations for critical components for the entire 50 years of operation;  determination of operational monitoring parameters when possible. As a result of the ASME Code requirements’ introduction the entire ISI/NDE activity of the plant was transformed to that one which could directly serve the service life extension. The RPV ageing mechanism of primary concern is the irradiation embrittlement of the structural materials in the beltline region. RPV structural integrity is jeopardized in transient regimes of the reactor (heat up, cool down) and when the RPV is exposed to Pressurized Thermal Shock (PTS) loading. PTS loading results from activation of the emergency core cooling system (ECCS) in response to the detection of a LOCA. In case of LOCA the relatively cold water of ECCS (20- 50 °C) is injected into the reactor down -comer, and this subjects the RPV inner surface to a severe thermal shock. Indeed, at the same time the pressure can be increased, and the superposition of the pressure induced stresses and of rapid cooling induced thermal stresses in a crack front result a high stress intensity factor. In case of an advanced embrittlement (low fracture toughness, high ductile-to-brittle transition temperature), there is an increased risk of unstable crack propagation. Irradiation embrittlement determines the operation life of the RPV thus PTS assessment is the key element of life extension and one of the most important Time Limited Ageing Analyses (TLAA), Trampus (2018). Conventionally, a crack type surface flaw of 1/4 wall thickness is postulated for the PTS assessment. The recent guidelines however tend to introduce the use of an embedded flaw because systematic investigations on non-used RPVs as well as ISI experience show an extremely low probability of existence of surface flaws. In accordance with this tendency, it is allowed to postulate an embedded (under cladding) flaw if the cladding integrity is assured by qualified NDE, and the cladding mechanical properties are known. Also, based on satisfactory results of qualified NDE, maximum crack depth of less than 1/4 of wall thickness may also be postulated. RPVs of the VVER-440 type reactors have a stainless-steel cladding with 9 mm thickness. Experimental results show that the cladding material after irradiation remains ductile enough. The results of the ISI completed so far has verified that no through-cladding defects exist. Both, postulation of under cladding crack and with a size less than the quarter of the wall thickness have extreme benefits when calculating the technically feasible lifetime of the RPV. As a consequence of this the qualification of cladding ET became a central issue in the life extension. The qualification requirements were: the probability of detection (POD = 100% for a > 3 mm in any direction), the sizing tolerance (Δa max =±2 mm), the position (ΔR < 10 mm in any direction) and the false call rate (very low probability), Trampus (2010). The qualification test block for ET can be seen in Fig. 5. 6.2. NDE system qualification pertaining to safety analysis

7. Conclusions

In this paper the role and the continually increasing importance of NDE was summarized in light of NNP operating life extension. It is unquestionable that human and environment have to be saved from potential impact of an NPP accident. This is exactly the so-called social goal of NDT/NDE. No need to justify that ageing assets, in our case the ageing NPPs, can be affected more frequently by known and eventual new, unknown degradations. Some examples of the operating life extension practice of Paks NPP, Hungary, were shown.