Antwerp, Belgium, corrosion, Doel, Electrabel, fracture toughness test, GDF-Suez, hydrogen cracks, hydrogen damage, hydrogen embrittlement, hydrogen flakes, mechanical damage, neutron embrittlement, nuclear energy, Nuclear hazards, nuclear reactor pressure vessel, nuclear reactors, Nuclear Regulatory Authority, nuclear shutdown, nuclear waste, reactor pressure vessel walls, reactor pressure vessels, steel, Tihange, ultrasonic testing
The Belgian Nuclear Regulatory Authority released clarifications regarding the detection of the flaws-cracks in the Doel and Tihange Nuclear Reactor Pressure Vessels, on February 25, 2015. The increased number of defects found are said to be due to increased precision in measurement, rather than increased numbers. They show 3 tables indicating the increase in numbers found. However, due to the inversion of dots and commas in French and English numbers, we are only reproducing the final with comment. The defects are now believed to range from as small as 12.3 x 13.7 mm (roughly 1/2 x 1/2 inches) to as large as 179mm x 72.3 mm (7 inches x 2.8 inches), which seems huge. Electrabel is a subsidiary of French GDF-Suez. UC Berkley professor-corrosion expert, Digby MacDonald, has recently warned that this “disease” could well afflict all nuclear reactors in the world, compared it to ebola or measles of nuclear reactors, and said that it could lead to their shutdown. Local media spoke of it as a scourge.
From the February 25, 2015 Press release by the Belgian Nuclear Regulatory authorities:
“Doel 3/Tihange 2: clarifications regarding the detection, the position and the size of the flaw indications
Following recent reports in the press, the FANC wishes to shed some light on the data currently available regarding the number and size of the flaw indications in the steel walls of the reactor pressure vessels of Doel 3 and Tihange 2.
2012: discovery of the flaw indications
In the summer of 2012, Electrabel conducted ultrasonic inspections on the reactors of Doel 3 and Tihange 2 using a measuring device called “MIS-B”. These inspections resulted in the discovery of flaw indications in the walls of the reactor pressure vessels,…” [Table at original]
“When the FANC gave permission for the restart of Doel 3 and Tihange 2 on May 17 2013, it imposed a series of requirements that had to be met before the restart and a series of requirements to be met in the course of the first reactor cycle (read the “final evaluation report” of May 2013 here http://fanc.fgov. be/GED/00000000/3400/3429.pdf). Featured in this second set of requirements was the complete qualification of the measuring instrument “MIS-B”. Electrabel had to be able, in other words, to prove that:
This device is capable of detecting all flaw indications in the steel walls of the reactor pressure vessels;
The method used for the interpretation of the ultrasonic signal is capable of determining the correct dimensions of the detected flaw indications.
At the beginning of 2014, the analysis of the results of this qualification demonstrated that the detection parameters of the device MIS-B did not allow to detect all flaw indications, and that the method used for the interpretation of the signals tended to underestimate the dimensions of a small part of the detected flaw indications.
Reinterpretation of the 2012 results with a conservative measurement
Therefore, the method was adapted in order to ensure a conservative measurement of the flaw indications, i.e. to ensure that the measured length of the flaw indications is equal to or bigger than their actual length. The application of this improved procedure implies that flaw indications which are located in close proximity to one another can be interpreted as one single, longer flaw indication. Therefore, the application of this new method slightly reduces the number of detected flaw indications, but also leads to an increase in their average and maximum length, because of their artificial regrouping.
Electrabel has used this new method to reinterpret the results of the ultrasonic inspection of 2012, in order to refine the size of the flaw indications that were detected in 2012. The results of this reinterpretation are the figures that have circulated in the press in recent days…” [Table at original]
“New ultrasonic inspection with the lowered detection threshold and conservative measurement
In addition, Electrabel has increased the sensitivity of the ultrasonic detection device MIS-B to ensure that all flaw indications were detected. In May and June 2014, the operator conducted new ultrasonic inspections in Doel 3 and Tihange 2 with these adapted detection parameters. The results of these inspections were then interpreted using the aforementioned new measurement method.
The combination of these two improvements has resulted in the detection of about 60% more flaw indications in the steel walls of the reactor pressure vessels of Doel 3 and Tihange 2. The new method implies that signals with a very low amplitude, which is characteristic for small flaw indications that wouldn’t be detected with the detection threshold of 2012, are being regrouped. This regroupment leads to an increase of the average and maximum length of the flaw indications.
Electrabel passed a final synthesis of these new data to the FANC in February 2015“…[see Table at top of page]
“The FANC wishes to stress that these are ‘raw’ data that still need to be checked and validated by the Belgian nuclear safety authorities (FANC and Bel V) and by the accredited inspection organization AIB-Vinçotte.
Furthermore the FANC wants to specify two important issues:
The newly detected flaw indications are located in the same areas of the walls of the pressure vessel as the formerly detected flaw indications.
Their presence primarily leads to an increase of the local density of the flaw indications.
These are laminar (parallel with the wall) and are located in the steel of the walls, at a depth of 5 to 150 millimeters, and not in the cladding.
It was the increased sensitivity of the ultrasonic inspection device that led to their discovery.
The flaw indications that were detected in 2012 have not evolved between 2012 and 2014. To exclude a possible evolution between 2012 and 2014, Electrabel applied the old detection threshold and the new interpretation procedure on the measurements of 2014. In that way, the operator could compare the data of the two measurements in an identical way.
Finally, the FANC wants to remind that:
The research on the impact of the flaw indications on the mechanical properties of the walls of the vessels is still ongoing (see our message of February 13, 2015). This new information does not allow in any way to already draw conclusions on the structural integrity of the walls of both vessels, and by extension, on the eventual outcome of this case.
The safety cases that Electrabel has to submit to the FANC in order to justify a possible restart of both reactors will be thoroughly examined by the Belgian safety authorities. The safety case will also be analyzed by an independent foreign laboratory.
Afterwards the FANC will publish the definitive safety cases it will have received from Electrabel on its website.
* The FANC has not yet received all information on the lower core shell of Tihange 2. These data are available from Electrabel. 25 Février 2015”
http://www.fanc. be/fr/news/doel-3/tihange-2-clarifications-regarding-the-detection-the-position-and-the-size-of-the-flaw-indications/753.aspx (Emphasis our own)
From the February 13, 2015 News Release:
“In February 2015, Electrabel completed the actions related to the theme of the ultrasonic inspection technique.
This technique was originally designed for the control of the welding and the cladding of the RPV, but it also proved to be able to detect flaw indications in the wall of the RPV. Electrabel had to qualify the technique, i.e. prove that all hydrogen-induced flaw indications can be found and can be measured correctly using the ultrasonic inspection. By doing so, Electrabel found that the inspection procedure had to be slightly modified and that the detection threshold of the probes had to be lowered to ensure the proper detection of all flaw indications.
In 2014, a further inspection was carried out based on the improved procedure and the modified settings of the machine, resulting in the detection of a greater number of flaw indications than was measured in 2012 and 2013. This means that Electrabel now has to take into account 13 047 flaw indications for Doel 3 and 3149 flaw indications for Tihange 2 in its calculations. These additional flaw indications are similar to those which were previously considered and are located in the same area of the RPV…
After an analysis of the safety cases of both reactors, the FANC and Bel V decided on May 17, 2013 that Doel 3 and Tihange 2 could be restarted. Linked to this agreement, however, was the condition that Electrabel had to perform a series of medium-term actions to consolidate the hypotheses of its Safety Case. These actions were divided into three major themes:
1. The ultrasonic inspection technique of the RPVs: detection and measurement of hydrogen-induced flaw indications
2. Material properties of steel containing hydrogen flakes
3. Structural integrity of a rpv containing hydrogen flakes
The results of the actions on issues 1 and 2 provide the input for theme 3.
In carrying out tests related to theme 2 during the spring of 2014, a fracture toughness test revealed unexpected results, which suggested that the mechanical properties of the material were more strongly influenced by radiation than experts had expected. As a precaution both reactors were immediately shut down again. Electrabel launched a test campaign to find an explanation for the unexpected test results.” Original in English: http://fanc.fgov. be/fr/news/doel-3/tihange-2-new-update/745.aspx (Emphasis our own).
Based on the measurements in this 2013 report, which discusses 10 mm, the defect-crack sizes are apparent now ranging from 12.3mm x 13.7mm for the smallest to 179 mm x 72.3 mm for the largest. http://fanc.fgov. be/GED/00000000/3300/3393.pdf
Best general primer on hydrogen damage. Short, clear and with great images to help understand the topic: http://faculty.kfupm.edu.sa/ME/hussaini/Corrosion%20Engineering/04.07.03.htm
Note that nuclear reactors are also damaged by neutron bombardment, which can lead to sudden failure.
Map locations exported from wikipedia: http://en.wikipedia.org/wiki/Doel_Nuclear_Power_Station http://en.wikipedia.org/wiki/Tihange_Nuclear_Power_Station