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RADON

Doel 3/Tihange 2: new update


After a large number of flaw indications was discovered in the walls of the reactor pressure vessels (RPVs) of Doel 3 and Tihange 2 during a scheduled maintenance in the summer of 2012, the Belgian nuclear safety authorities (FANC and Bel V) decided that Electrabel had to submit a Safety Case to justify the restart of both reactors. Electrabel had to demonstrate specifically and convincingly in its Safety Case that the flaw indications in the walls of the RPVs do not compromise its structural integrity.

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.

At the same time, the licensee continued the execution of the medium termed-action plan. In the mean, this has led to the following results:

 

More accurate information about the flaw indications

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.

 

New sequence of material testing

At the same time, Electrabel also continues its research on the material properties of the RPV and the unexpected results of the previous fracture toughness test. Currently a 4th irradiation campaign is being executed in the research reactor BR2 of the SCK, where, next to hydrogen-flaked samples of the French VB395 test material, other hydrogen-flaked samples of another test material of German origin are also being irradiated. The results of this irradiation campaign and of the subsequent material tests are expected by April 2015.

 

New meeting of the international review board

Electrabel provides the FANC and Bel V with results of ongoing tests and analyses on a regular basis. The Belgian security authorities need time to look into this new information and will continue their analysis during the first months of 2015. Therefore, they call in the help of international experts who are specialized in damage mechanisms caused by radiation and in mechanical resistance tests. This international expert panel (International Review Board) met for the first time in Brussels at the start of November 2014. The main conclusion of this meeting was that the methodology used by Electrabel was not yet sufficiently developed to make a well-grounded judgment. The international experts have formulated some suggestions for further actions and studies. Based on these suggestions and on the documents already analyzed, the Belgian security authorities have passed a series of additional demands and suggestions to Electrabel, so that the licensee can adjust its methodology and validate the underlying hypotheses of its arguments.

In April 2015, the FANC will organize a new meeting of the international panel of experts to obtain their advice on the results of the new material tests and on the new data provided by Electrabel.


13 Februari 2015


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