Doel 3 & Tihange 2 – Proven Safe

Published: July 31, 2013

Doel 3 NPP-reactor reloading (2005) © Electrabel/BECKERS Raf

 

After months of investigation of Belgium’s Doel 3 and Tihange 2 NPP reactor vessel Safety Cases, in May 2013, FANC approved both units’ safe restart. Tractebel Engineering played a pivotal role in Electrabel’s mission to explain and define detected flaws and confirm the structural integrity of both PWR vessels.

HIGHER SAFETY CHECKS EXTEND SHUT DOWN

In June 2012, Doel 3 NPP shutdown for its mandatory 10-year overhaul; as is the Electrabel (GDF SUEZ) way, due to earlier Operational Experience reports on reactor vessel anomalies, investigations went further than regulations required… Read more

TOP EXPERTISE IN ACTION

Justifying many 1,000’s unknown indications in the 2 NPP reactor vessel walls, one of the most urgent investigation and innovative safety review programmes in nuclear began… Read more

A PROGRAMME OF 5 KEY STEPS

Achieving a seemingly impossible task covered 5 key steps requiring long hours, proactive action and intuitive application of Tractebel Engineering’s nuclear expertise…
Read about

  1. Application and verification of the ultrasonic tests (UT)
  2. Identifying the origin and nature of the indications
  3. Establishing the impact of the defects due to hydrogen (DDH) on material strength
  4. Calculating the structural integrity of the vessels
  5. Confirming the safety case – pressure/acoustic and post-ultrasonic testing

NEW VALUE FOR THE INDUSTRY

The green light to restart Doel 3 and Tihange 2 was official acknowledgement of the industry’s acceptance of the innovative thinking and expertise that helped resolve the issue… Read more


The challenge was clear; with Electrabel, we had to prove that these indications had not, did not and would not impact the reactor vessel’s ability to withstand any potential incident or threat – or face the permanent shut down of the NPPs.Luc Goossens, Head of Projects – Belgian Nuclear Power Plants, Tractebel Engineering Nuclear

HIGHER SAFETY CHECKS EXTEND SHUT DOWN

In June 2012, Doel 3 NPP shutdown for its mandatory 10-year overhaul. As is the Electrabel (GDF SUEZ) way, due to earlier Operational Experience reports on reactor vessel anomalies, investigations went further than regulations required.

DEEP INVESTIGATION REVEALS INDICATIONS
Ongoing safety checks are a given of NPP operation, but the 10-year safety revision involves no less than 10,000 maintenance tasks and safety checks – one being the inspection of the stainless steel cladding of the reactor vessel wall.

However, related to OE feedback on Tricastin NPP in France where “cold-cracks” under the vessel cladding were found some years before, in 2012 Electrabel decided to also inspect the first 2.5 cm of all its reactor vessel shells using top ultrasonic testing (UT) technology; Doel 3 being the first. No cracks were found. But signals indicated other anomalies deeper in the wall.

A second UT investigation of the full 20 cm inner wall showed these to be a large number of “quasi-laminar” (parallel to the surface) oval shaped indications embedded in the metal, but not impacting the inner or outer wall surfaces, mainly concentrated in 2 bands encircling the vessel’s mid-section.

FIRST DOEL 3 – THEN TIHANGE 2
The Tihange 2 10-year review followed in September 2012. Repeated UT investigations showed similar indications in the reactor vessel wall, although to a lesser extent. With safety always of utmost priority, Electrabel elected to extend the shutdown of both units until proven unquestionably safe to restart. With the Federal Agency for Nuclear Control (FANC) and international nuclear experts waiting for justification – it took close to a year of 24/7 effort to achieve.

  • June 2012 – First ultrasonic test Doel 3
  • July 2012 – Second ultrasonic investigation Doel 3
  • July 2012 – Programme for investigation set in motion
  • September 2012 – Tihange 2 reactor vessel inspected
  • December 2012 – Safety Cases submitted to Nuclear Authorities
  • January 2013 – Authorities respond positively but request further confirmations
  • March/April 2013 – Addendums to Safety Cases delivered to Authorities
  • May 2013 – FANC gives go-ahead for restart of both NPP units

  • Doel 3 NPP-reactor reloading (31/05/2005) © Electrabel/BECKERS Raf
  • Doel 3 NPP-reactor reloading (31/05/2005) © Electrabel/BECKERS Raf
  • Doel NPP-aerial view (12/07/2006) © Electrabel/DE BARSE Rudy
  • Doel NPP (29/04/2008) © GDF SUEZ/ABACAPRESS/GUIBBAUD CHRISTOPHE

 

TOP EXPERTISE IN ACTION

Justifying many 1,000’s unknown indications in the 2 NPP reactor vessel walls, one of the most urgent investigation and innovative safety review programmes in nuclear began.

CALLING ON THE BEST IN THE INDUSTRY
Nuclear safety being at the heart of Nuclear Power Generation; within an interdisciplinary team led by Electrabel, bringing in Laborelec (GDF SUEZ) and organisations including SCK-CEN (the Belgian Nuclear Research Centre), AREVA and leading academics, Tractebel Engineering played a pivotal role: deploying over 200 nuclear engineers, with 50 experts working full time on the mission.

Between July 2012 and May 2013, Tractebel Engineering developed, with the team, the full project road map. Planning and covering the many different aspects; Tractebel Engineering:

  • Tracked, traced and reviewed 100’s of original manufacturer’s documents in order to establish the origin of the indications – many found in Group archives.
  • Carried out 50,000 hours of specialised studies; primarily in the areas of non-destructive examination, material behaviour and fracture mechanics. This included the definition, follow-up and interpretation of 600 mechanical tests performed in laboratories.
  • Developed a new, specific flaw screening and grouping methodology as regular ASME codes could not be applied due to the vast number of indications.
  • Defined and performed numerous mechanical calculations and numerical simulations in 2D and 3D requiring over 3,000 hours of calculations on the supercomputers of Tractebel Engineering, CENAERO, ESI and Areva.
  • Delivered the majority of documents of the technical files
  • Contributed to drafting the 2 Safety Case reports and addendums, and supported Electrabel in discussions with the Belgian Safety Authorities and their international safety review experts.

The development of new methodologies that classic computations proved valid and valuable, allowed us to demonstrate the reality of the situation to all international experts and nuclear authorities. It was an important achievement for all involved.Valéry Lacroix, Design Engineer & Specialist Fracture Mechanics, Structural Integrity, Tractebel Engineering Nuclear


A PROGRAMME OF 5 KEY STEPS

Achieving a seemingly impossible task covered 5 key steps requiring long hours, proactive action and intuitive application of Tractebel Engineering’s nuclear expertise.

1. APPLICATION AND VERIFICATION OF THE ULTRASONIC TESTS (UT)

AN UNUSUAL EVENT FROM THE START
Managing the first UT, carried out by Intercontrole (AREVA), was an unusual task for the inspections team. Evidence of a few small indications led to the complex re-planning and reprogramming of the UT device to inspect the whole vessel wall. With the full extent of the indications revealed; an essential step was to verify the UT investigation – only then could Electrabel and Tractebel Engineering be sure that what the UT showed in Doel 3 and Tihange 2 was a reality.

2. IDENTIFYING THE ORIGIN AND NATURE OF THE INDICATIONS

A KEY ASSUMPTION
The indications appeared to be a result of metallurgical reaction during manufacture. Intensive review of archived construction files of the vessels; many of them microfilm files studied on 30 year old projectors, turned up key information related to the chemical content of cast steel (ingots) and the heat treatment of the metal during vessel manufacture.

THE FACTS REVEALED
A reactor vessel is constructed using separate cast steel parts which are then forged into one seamless unit. In the case of Doel 3 and Tihange 2, the steel parts were cast in Germany by Krupp and sent to Holland to be forged by Rotterdamsche Droogdok Maatschappij (RDM). Files indicate that between the 2 stages the steel components would have cooled down substantially and then been reheated during the forging process.

DDH – A STABLE BUT UNPROVEN ISSUE
According to metallurgy experts, in the case of Doel 3 and Tihange 2, the defects due to hydrogen (DDH) do not threaten the stability of the steel. But this still had to be checked by multiple mechanical and chemical tests and, in consultation with the AIA (control body for pressure equipment), be fully confirmed for Electrabel.

3. ESTABLISHING THE IMPACT OF THE DDH ON MATERIAL STRENGTH

CHEMICAL AND MECHANICAL TESTING
To identify the impact of the DDH on the strength and resistance of the material and the RTndt (reference temperature leading to the transition of brittle to ductile steel), needed for the Structural Integrity assessment calculations, a long series of physical; chemical and mechanical tests was done.

DIGGING DEEPER
The Mechanical Works team defined and supervised tests on pieces (flawed and flawless) cut from a rejected steam generator with similar indications found at AREVA – submitting the samples to a wide variety of destructive tests to measure the flawed material’s ability to resist stresses.

4. CALCULATING THE STRUCTURAL INTEGRITY OF THE VESSELS

BEYOND THE NORM
Assessing the structural integrity of a component checked according to regulatory inspection is usually done by applying ASME (American Standard of Mechanical Engineering) codes; tables providing material properties and acceptable flaw dimensions for that component. In regular reactor vessel inspection ASME codes call for the justification of each flaw not falling into acceptable limits.

THINKING OUTSIDE OF THE CODE
Applying the ASME coding philosophy to the unique and unprecedented situation, our experts developed a completely new code, methodology and criteria for screening and grouping flaws; based on measurements such as location and radiation exposure and a more subtle grouping criteria for grouping flaws that influence one another and singling out flaws that don’t.

5. CONFIRMING THE SAFETY CASE – PRESSURE /ACOUSTIC AND POST- ULTRASONIC TESTING

All findings concluded that the indications do not impact the safe operation of the reactor vessels and the Safety Cases were presented to the authorities early December 2012. Mid-January 2013, the authorities replied that they believed a restart could be envisaged, but asked for additional information.

STILL UNDER PRESSURE
This required a pressure test on both vessels, taking the pressure up to 175 bars (nearly 15% above normal), with additional acoustic testing by Cegelec AE Tech applied to the vessel walls.

A SAFE CONCLUSION
A final post-pressure test UT was also carried out; confirming that no indications had evolved under high pressure. All final results and reports were submitted to FANC in April 2013. Subjected to a stringent control by independent nuclear experts, satisfied that the issue had been resolved and all safety criteria met, on 17th May, FANC authorised the safe restart of Doel 3 and Tihange 2.


  • Tihange NPP-reactor building (03/10/2008) © GDFSUEZ/MONTIGNY PHILIPPE/ABACAPRESS
  • Tihange NPP-reactor (03/10/2008) © GDF SUEZ/MONTIGNY PHILIPPE/ABACAPRESS
  • Tihange NPP-aerial view (12/05/2005) © Electrabel/DE BARSE Rudy
  • Tihange NPP-reactor (19/09/2007) © Electrabel/PEROT Alain
  • Tihange NPP (03/10/2008) © GDFSUEZ/MONTIGNY PHILIPPE/ABACAPRESS
  • Tihange NPP (04/10/2004) © Electrabel/PEROT Alain

 

NEW VALUE FOR THE INDUSTRY

The green light to restart Doel 3 and Tihange 2 was official acknowledgement of the industry’s acceptance of the innovative thinking and expertise that helped resolve the issue.

INDUSTRY IMPACT
RDM supplied goods and services for 20 other NPPs worldwide operating reactor vessels. While the problem discovered may have affected other forgings, the decision to investigate all reactor vessels to the Belgian level rests with the nuclear operators and authorities of each country.

But a new step in NPP safety has still been made. Tractebel Engineering’s contribution in developing the methodologies, calculations and tests that led to the resolution of the Belgian issue has been noted by many in the industry. Much of the knowledge gained will be widely shared in the interest of global nuclear safety. Should a reactor vessel anywhere now have a similar issue, based on an approved Safety Case, the starting point is known.


I’m particularly proud of what was achieved in meeting this challenge alongside Electrabel. It is added recognition of our high expertise level and the very strong relationships we enjoy with our Customers. Nuclear is definitely a fascinating business; with ultra-professional people – and very brilliant engineers!Christian Pierlot, Executive Vice President-Nuclear, Tractebel Engineering Nuclear

Professionalism

The important decision taken by the Federal Agency of Nuclear Control (FANC) on May 17 is the result of exhaustive effort by our teams in Electrabel as well as our colleagues in Laborelec and Tractebel Engineering. For over ten months, we worked together in a complementary manner, with rigor and a great deal of professionalism”. Sophie Dutordoir, Administrator-General Manager Electrabel (in MIX – internal magazine of Electrabel, June 2013)

NEED TO KNOW

In a NPP, the reactor vessel is the main part of the second barrier between the fuel and the outside world.  Designed to withstand extreme pressure and the stresses of all thermal transient phases; 12m high by 4m in diameter, the solid steel vessel is the one component whose integrity must be guaranteed for all time.

More information

Luc Goossens, Head of Projects – Belgian Nuclear Power Plants
Valéry Lacroix
, Design Engineer, Structural Integrity
Philippe Dombret, Chief Engineer
Frederic Somville, Surveillance Engineer
Anne-Sophie Bogaert, Inspections Engineer
Pierre Dulieu, Design Engineer
David Lacroix, Design Engineer

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