Critical Reflections about Doel3 & Tihange2

Integrity reactor vessels Doel 3 and Tihange 2

Page: 34

3. Vibration levels are unknown Vibration levels are insignificant. 4. Opinion of Westinghouse AREVA, which has bought the Westinghouse license and which is also a nuclear power plant designer, has been strongly involved in the assessment. Moreover, as an answer to the return of experience of Doel 3 and Tihange 2, Westinghouse participated in the elaboration of [ 2 ] , which is a bounding safety assessment for a vessel postulated to have a large number of quasi-laminar indications in the beltline shell forgings. The bounding assessment included a Probabilistic Fracture Mechanic analysis of the U.S. PWR with highest mean reference temperature in a beltline ring forging at the end of a sixty-year license. This study determined that the incremental risk would be a factor of 10 below the risk criteria set by the USNRC. Therefore, the potential for vessel failure was considered acceptably low. This conclusion has been assessed and endorsed by the USNRC [ 3 ] . Minor correction In §1: the primary water is circulated by pumps through the steam generators with a volume flow of 21.000 m3 / h, instead of 21 m3 / h. References [ 1 ] ”Critical reflection about the integrity of the reactor vessels of the Doel 3 and Tihange 2 nuclear power plants”, René Boonen and Jan Peirs, 8 December 2016 [ 2 ] ”Materials Reliability Program: Evaluation of the reactor vessel beltline shell forgings of operat- ing U.S. PWRs for quasi-laminar indications (MRP-367)”, October 2013 [ 3 ] ”Technical assessment of potential quasi-laminar indications in reactor pressure vessel forgings”, USNRC, 8 September 2015, ADAMS ML15282A218 In the FANC answer (see page 30), a concentration of hydrogen should occur towards the crack sensitive zones. The reasoning in the FANC answer follow the viewpoint that the hydrogen migrates as if it would be saturated. However, the steel is far from saturated as the hydrogen was largely removed by a vacuum treatment of the molten steel. Consequently, such migrations of hydrogen did not take place as described in the FANC answer. Even in the case that the hydrogen migrates in the same manner as if it would be saturated, the hydrogen concentration gradient over the shell wall thickness as consequence of the temperature gradient during cooling of the ingot remains insignificant. This will now be demonstrated by the authors. The ingots were poured using a state-of-the-art vacuum technique, to remove the hydrogen from the molten steel. Before casting the ingot from which the lower core shell of the Doel3 reactor has been manufactured, the molten steel has been sampled and the hydrogen content was measured [ 14 ] p43. After cooling down the ingot in a controlled manner, samples of the metal are taken and the hydrogen content was measured again. The amount of hydrogen present in the steel of the Doel3 lower core shell was for all measurements 1.5 ppm [ 14 ] p44. During cooling of the ingot, impurities and alloying elements segregates into so called ghost lines which are sensitive to hydrogen cracking [ 9 ] . As solidification progresses from the outside wall Estimation of the hydrogen concentration gradient in the cracking sensitive areas.

R.Boonen & J.Peirs

May 18, 2017