The WENRA Findings in Doel-3 and Tihange-2

The question I have is:

1) If they would have just had a paperwork search in Doel-3 and Tihange-2, would they have discovered the flakes in their reactor vessel or inadequate inspection methods questioning if flaws were there?

2) In the USA, is the hydrogen concentration in the metal of such a concentration it would question if flakes could be in US reactors? 

3) What is the status of cladding flaws or corrosion in all USA vessels... 

Why hasn't the NRC required this from our licencees:  

"In response to the findings in the Belgian reactors, WENRA recommended in 2013 the nuclear safety authorities in Europe to request the licensees to verify the material quality and integrity of the RPV in a 2‐step approach: 

1.        A comprehensive review of the manufacturing and inspection records of the forgings of the RPV. 

2.        Examination of the base material of the vessels if considered necessary." 

The NRC stance to date has been, the licencees has scrupulously followed the 1970s NRC new vessel codes and regulations proving they have no flakes. It's as if the agency believes following any old rule or code proves the vessels are safe. I saying, are the 1970s codes and regulation adequate to disclose similar hydrogen flakes in USA vessels?

 

The NRC believes it is not limited to the Belgium vessel manufacturer...it is a similar forging process worldwide.  

NRC INFORMATION NOTICE 2013-19: QUASI-LAMINAR INDICATIONS IN REACTOR PRESSURE VESSEL FORGINGS

 September 22, 2013: While the forgings in the European NPP were manufactured by Rotterdam Dockyard (using ingots supplied by Friedrich Krupp Hüttenwerke (Krupp)), there is no evidence of any factors unique to the forging practices of the Rotterdam Dockyard, or the practices used by Krupp in making the ingots, which suggest an increase in the likelihood of developing quasi-laminar indications during the fabrication process in comparison to other forging manufacturers.

This crux of the issue with my 2.206: The NRC didn't specifically state the licencees would have positively detected hydrogen flakes in new forgings (1970s) and they did a full safety analysis saying there is no safety issues. Course, now we have new inspection technology and we have massively increased our knowledge of metallurgy since first startup. 

I am saying also, it is a unacceptable with our aging fleet of nuclear plant vessels we aren't doing UT inspections in areas outside the weld strips. This might include a representative sample of the vessel walls in the end, after we understand what is going on. I think having greater than 99% of the vessel walls not ever being ud't once operations began is not acceptable.      

2013-19: ASME Code, Section III, requires the UT examination of all forgings during the fabrication process and specifies the acceptance criteria. Per Section III, a forging is considered to be unacceptable if the UT examination detects the presence of reflectors that produced indications resulting from discontinuities in the material accompanied by a complete loss of back reflection from the far side of the structure. The applicable Codes required that the examinations be performed in accordance with ASTM SA-388; this document defines the recording criteria implemented for the RPV forging examinations. Licensees are required to maintain all fabrication records, including NDE and acceptance records; therefore, licensees should have records of the NDE performed on RPV forgings during fabrication. If recordable quasi-laminar indications were detected during the fabrication of any RPV, Section III of the ASME Code would require the indications to be compared to the examination acceptance criteria.

Unacceptable indications would require repair in accordance with the ASME Code, Section III.

So the material characteristics, the strength and ductility of the metal flakes though laboratory testing as we understand it, is being questioned...    

Doel3/Tihange 2: next steps of the review process

"Safety Case 

Before being allowed to restart both Doel 3 and Tihange 2 reactors, Electrabel shall first submit a Safety Case to the FANC in which it convincingly demonstrates that the presence of hydrogen flakes in the walls of the reactor pressure vessel (RPV) does not compromise its structural integrity. This Safety Case must be structured around three major topics, corresponding each to a chapter of the action plan that Electrabel is due to implement:  

1. The ultrasonic inspection technique of the RPVs: detection, measurement and location of hydrogen-induced flaw indications.

2. Material properties of specimens containing hydrogen flakes: radiation effects and transferability of the test results to the Doel 3 and Tihange 2 RPVs.

3. Structural integrity of a RPV containing hydrogen flakes.

The results of the actions related to theme 1 and theme 2 will provide the input for theme 3."    

Next steps of the review process

After completing every action related to themes 1, 2 and 3 and interpreting the results, the licensee Electrabel will submit its Safety Case to the FANC. The FANC and its technical subsidiary Bel V will thoroughly review this Safety Case using the specific expertise of the recognised inspection organisation AIB Vinçotte (for theme 1), the International Review Board (for theme 2) and an external research team (for theme 3). The FANC will collect opinions from all these parties and take them into account to decide whether Electrabel is allowed to restart the Doel 3 and Tihange 2 units. This process will take another few months. 

 Excerpts: 

Report: Activities in WENRA countries following the Recommendation regarding flaw indications found in Belgian reactors

17 December 2014

01 Background

01.1 The findings in Doel-3 and Tihange-2

In 2012 a new type of in‐service inspection (ISI) of the reactor pressure vessel (RPV) by ultrasonic testing (UT) was introduced in Belgian nuclear power plants. These inspections were introduced earlier in France to search for underclad cracks that may be present in the base metal directly below the interface to the cladding. These underclad cracks, if present, have perpendicular orientation to the surface and were created by the welding process of the austenitic strip cladding onto the ferritic base metal. 

Yet, in the RPV wall of Doel‐3 and Tihange‐2 these inspections did not find any underclad cracks but a large number of flaw indications, located at different distances from the surface in the lower and upper vessel forged rings. As this technique is not suitable to find any flaws far from and nearly parallel to the surface, additional UT with straight beam (0°) was applied. With this technique, thousands of nearly laminar indications were found at larger depths of the base metal, mostly planar and nearly parallel to the surface of the RPV. 

Following a number of investigations and evaluations, the UT indications in the RPV of Doel‐3 and Tihange 2 were unambiguously assigned to hydrogen induced flaws (“hydrogen flakes“). 

01.2       Metallurgical considerations 

According to current knowledge hydrogen flakes may only form during manufacturing of the base metal. The formation of hydrogen flakes is a phenomenon well known to the steel manufacturers and may happen after cooling down the steel from high to ambient temperature, e.g. in the ingot after pouring or in the forged part after the forging operation and heat treatment. Flake formation is driven by the accumulation of hydrogen at segregations or inclusions in the metal. This accumulation of hydrogen is diffusion controlled, so the formation of flakes may have an incubation time of some days or even a couple of weeks at room temperature. 

Due to the main deformation direction during the forging operation, these segregations or inclusions are preferentially stretched in planes parallel to the surface of the forging leading to the formation of laminar hydrogen flakes of the same orientation. The formation of hydrogen flakes depends on a number of factors, the most important being the hydrogen concentration and the size of the ingot, both determining the possible accumulation of hydrogen. This makes large forgings most prone to flaking. Further important factors are a “sensitive” microstructure and the stress state. Despite these known dependences it appears difficult to exclude the formation of flakes in a large forging on the basis of these factors. Therefore, acceptance tests of the base material including appropriate UT is considered the most important step to assure that the parts are free of hydrogen flakes. Therefore the WENRA recommendations as well as the WENRA questionnaire specifically asked for the results of these tests.

Plate material is generally considered much less prone due to smaller ingot sizes and higher degrees of deformation during the rolling operation compared to forging. This results in a less sensitive microstructure. Therefore, components made from plates are outside the scope of further analyses and are not addressed in the recommendations by WENRA referred to below.  

The “flakes” are not considered as “cracks” however they represent a detachment or separation within the material that is assumed to have a similar detrimental effect on the mechanical behaviour of the component. In assessments of the structural integrity of the RPV the flakes are always modeled as cracks. 

01.3       The role of different inspections 

According to international practice, semi‐finished products, i.e. “forgings” or “plates”, are subjected to an acceptance tests before they are assembled (mainly welded) to a component. Considering the possible incubation time of the formation of flakes the acceptance tests of forgings are generally not performed before one month after completion of the forging operation and the “quality heat treatment”. According to international practice of the manufacturers parts showing clear indications of flakes are discarded and will not be assembled. 

These acceptance tests generally comprise UT with different inclinations of the beam to find flaws of any orientation or character. UT with straight beam (0°) is the most appropriate to find planar flaws parallel to the RPV surface such as hydrogen flakes. Besides, UT with angle beam, surface testing (e.g. with magnetic particles) and destructive mechanical tests are performed. This testing appears to be common practice of all manufacturers, at least since the 70ies. 

In general, more UT is performed after each welding operation, e.g. after joining the forgings by circumferential butt welds and after welding of the cladding onto the internal surface of the RPV. These post‐weld tests aim to check for flaws in the welding, including the interfaces and the heat affected zones in the adjacent base materials. These inspections do not repeat testing the full volume of the base metal again as no change is expected compared to the acceptance test of the semi‐finished parts. 

After completion of the components more inspections by UT are performed in the framework of ISI. In all countries the full volume of all axial and circumferential welds and the adjacent heat‐affected zones are inspected. In general the volume of the base metal is not inspected again during ISI, except at VVER plants, where some parts of the base metal are covered by UT (see chapter General Observations). 

Regarding the UT techniques, different inclinations of the beam may be used in order to find planar flaws in different orientations. UT with angle/straight beam is applied to search for flaws orientated nearly perpendicular/parallel to the RPV surface. Furthermore, the techniques may focus on certain zones within the component, e.g. zones close to the surfaces or close to mid‐wall. Any of the special techniques applied may also find flaws in other orientations or other zones not focused on, however with lower sensitivity and probability.  

In case of Doel‐3, the UT dedicated to find underclad cracks with angle beam and focus near the interface to the cladding accidentally found some of the hydrogen flakes that were relatively close to this interface. Yet, it did find only a minor part of all the flakes found later by the dedicated UT using straight beam focussing on various depths. The latter is the technique of choice to find hydrogen flakes and was also used for the acceptance tests of the semifinished parts. Other techniques are considered less appropriate to find any flaws parallel to the RPV surface and in the centre of the wall, where most of the hydrogen flakes are expected, if any. This has to be born in mind when evaluating the UT results of the pre‐ and inservice inspections (PSI and ISI).

02 The WENRA recommendation 

In response to the findings in the Belgian reactors, WENRA recommended in 2013 the nuclear safety authorities in Europe to request the licensees to verify the material quality and integrity of the RPV in a 2‐step approach: 

1.        A comprehensive review of the manufacturing and inspection records of the forgings of the RPV 

2.        Examination of the base material of the vessels if considered necessary. 

Furthermore, it may be considered by the national regulators to extend the scope of analysis to large forgings of other primary equipment.  

Early in 2014, the WENRA Technical Secretariat sent out a questionnaire to the nuclear safety authorities in order to receive some feedback on the actions taken in the member countries. After receiving information from all relevant member countries the status of the actions taken has been summarized

The issue has no relevance for NPP in Romania (pressure‐tube reactor) as well as for Lithuania and Italy (no NPP in operation). 

The following general conclusions can be drawn from the different answers.  

Regarding step 1, a comprehensive review of the manufacturing and inspection records of the forgings of the RPV: 

•       Most member countries had the manufacturing records checked for all or some of those RPV made from forgings. In case the records of some RPV were not checked yet, they are planning to do it until 2016 at the latest.  

•       Some operators checked the records of all forgings of the RPV, others only those of the cylindrical rings of the RPV beltline. 

•       In all cases where the documentation was checked, it contained sufficient information to conclude that acceptance tests were performed that were capable to find hydrogen flakes. 

•       From all the documents that have been checked, member countries responded that either “no flaws”, “no notable (registered) indications”, “no notable indications similar to flakes” or “no unacceptable indications” were documented or found during UT.

Regarding step 2, from an additional examination of the base material of the vessels can be concluded that: 

•       Most member countries performed or planned to perform some kind of additional inspections in response to the findings in Doel‐3 during the upcoming regular ISI taking place every 4 to 10 years.  

•       Most member countries decided to have inspected some sample of the cylindrical rings.  

•       As far as the inspections were already performed, no indications similar to flakes were found with the engaged inspection technique.

Exceptions were Slovakia and Bulgaria operating VVER units with different kind of ISI programs covering also some parts of the base metal. They do not plan any additional inspections in response to Doel‐3. Yet, Slovakia is considering a re‐evaluation of the regular ISI program. Apparent differences in the ISI program of these countries with respect those following Western regulations are addressed in the following. 

From the information received from Bulgaria and Slovakia, it appears that there is some significant difference in the scope and periodicity of the ISI performed at RPV of VVER plants (and possibly still following the original inspection plans) on the one hand and of Western PWR plants and those following Western regulations on the other hand: 

•       While UT at RPV of Western type reactors is either performed from the inside (PWR plants) or from the outside (BWR plants) (with periodicity 4 to 10 years), UT is performed from the inside and outside in VVER plants. Periodicity at VVER 440 units is every 8 years for both sides with 4 year shift between both inspections, periodicity at VVER 1000 units is every 6 or 8 years from the inside and every 6 or 4 years from the outside. 

•       Even more important appear the differences in the area covered by the UT: While all circumferential welds and the adjacent heat‐affected zones are inspected at the RPV of all units, some parts of the base metal are also covered in Slovak and Bulgarian VVER type units. 

In the following section 03.2 an overview of the activities is given country by country. A plant specific overview can be found in Annex 1.