1. INRAG Public Conference, April 13 – 14, Aachen, Germany
Reactor pressure vessel embrittlement in Doel 3/Tihange 2
Ilse Tweer
INRAG Public Conference, April 13 – 14, Aachen, Germany

2. Reactor pressure vessel embrittlement D3/T2
The reactor pressure vessel (RPV) is the central component of a nuclear reactor, - for safe operation of the NPP throughout service life RPV failure has to be excluded, because safety systems are not designed to cope with RPV failure
Hazards for the structural intengrity of the RPV:
- development and/or growth of defects (cracks) - material degradation due to radiation embrittlement and thermomechanical fatigue
Demonstration of the RPV structural integrity (PTS analysis):
comparison of the calculated load on each detected crack for the most severe accident transients with the actual material properties (fracture toughness) of the RPV steel, considering the aging degradation
INRAG Public Conference April 13 – Aachen, Germany

3. Pressurized thermal shock (PTS) analysis
Source: SCK-CEN Eric van Walle, NENE 2013, Bled, Slovenia, September 11, The Detection of Hydrogen Flakes in the Belgian Doel3/Tihange2 Reactor Pressure Vessels
INRAG Public Conference April 13 – Aachen, Germany

4. Determination of the actual RPV steel properties
Surveillance irradiation experiments are supposed to monitor radiation-induced material embrittlement using plant-specific archive material
National regulations include embrittlement trend curves – empirical formulas based on irradiation experiments using similar steel samples
The trend curves allow to calculate the shift of the fracture toughness curve due to radiation-induced embrittlement (shift of the ductile-brittle transition temperature to higher temperatures) in dependence of the neutron fluence
The surveillance program results should be enveloped by the trend curves
INRAG Public Conference April 13 – Aachen, Germany

5. RPV embrittlement in Doel 3/Tihange 2
The detection of large amount of defects (hydrogen flakes/cracks) in the RPVs of D3/T2 has raised the question whether the defect-free surveillance program material can represent the defect-containing RPV steel
No defect containing archive material for D3/T2 is available
Electrabel performed irradiation experiments using samples from the AREVA shell VB 395, a rejected steam generator block containing hydrogen flakes, and samples from a German research project ; the representativeness of these samples for the RPVs D3/T2 is doubtful
The observed radiation embrittlement of the VB 395 samples was significantly higher than expected; Electrabel excluded radiation effects due to the defects and declared the high- embrittlement results as „abnormal outliers“ due to an unknown embrittlement mechanism
Electrabel defined new embrittlement trend curves and replaced the FIS curves
INRAG Public Conference April 13 – Aachen, Germany

6. Safety Cases 2015 Replacement of the FIS curve by a new trend curve (Electrabel 2015)
Safety Case 2012: Electrabel proposed an extra shift of 50°C on top of the FIS-curves to cover unknown radiation effects
Safety Case 2015: Electrabel replaced the FIS curve by a new trend curve
The new trend curve is in the SIA (structural integrity assessment) relevant fluence range (about 3x1019 n/cm2) less conservative than the proposed trend curve from 2012
INRAG Public Conference April 13 – Aachen, Germany

7. Conclusions
The radiation influence on the real defect-containing D3T2 RPV material has not been clarified due to the lack of representative samples
The declaration of irradiation results using VB 395 samples (that were supposed to represent hydrogen flakes containing materials) as „abnormal outlier“ is not acceptable
In the fluence range relevant for structural integrity assessment the new embrittlement trend curve is less conservative than the proposed embrittlement curve in the frame of the safety case 2012; thus the stated consideration of possibly higher embrittlement due to unknown processes in consequence of the irradiation campaigns is not existent
INRAG Public Conference April 13 – Aachen, Germany

8. Comment to the situation in Beznau
In the frame of the safety case Beznau-1 (2018) a Replika of the defect containing RPV shell was produced to study the effect of Al2O3 inclusions on material properties
No irradiation experiments were performed ENSI: „In the past, extensive research has been undertaken to investigate microstructural characteristics of embrittlement, develop models and provide databases. In comparison, there are only few investigations into the influence of non-metallic inclusions on irradiation embrittlement. This is because non-metallic inclusions are not bonded with the surrounding matrix and there is no doubt that non-metallic inclusions themselves are not subject to irradiation embrittlement. “
Remark: This argument is not sufficient, radiation- enhanced diffusion and radiation- induced segregation of impurities and radiation-induced defects cannot be excluded and could trigger defect growth by attachment at the existing inclusions
INRAG Public Conference April 13 – Aachen, Germany