Standardized reviews of all European reactor pressure vessels have been recommended by the Western European Nuclear Regulators Association (WENRA). The call follows the discovery of inclusions at two Belgian units last year, both of which have since restarted.
Ultrasound tests conducted in summer of 2012 suggested the possible presence of cracks in the vessels of Belgium's Doel 3 and Tihange 2 units. Further investigations by owner Electrabel indicated that these were so-called hydrogen "flakes" introduced during the manufacturing process. The plants remained offline until Belgium's Federal Agency for Nuclear Control concluded that the inclusions were of no safety significance and approved their restart in May this year.
The vessels for Doel 3 and Tihange 2 were produced by Rotterdam Drydock Company (Rotterdamsche Droogdok Maatschappij, RDM). As many as 21 reactor pressure vessels made by RDM are located around the world. National nuclear safety authorities immediately ordered inspections of those units, but no similar inclusions were found.
WENRA - a non-governmental organisation comprised of senior nuclear safety regulators from across the Europe Union and Switzerland - has now called for all reactor pressure vessels at European plants, regardless of their manufacturer, to be subjected to a standardized review in order to verify materials quality and structural integrity.
It noted that regulatory authorities in several countries have already decided to demand safety reviews to check for hydrogen-induced forging defects in reactor vessels from the operators of the plants under their supervision. However, WENRA has recommended that "measures are implemented on the basis of the same criteria." It has called for a standardized, two-step review procedure. Firstly, operators should conduct a comprehensive review of the vessel manufacturing and inspection records. Secondly, if the national nuclear safety authority considers it necessary based on the results of first stage, examination of the vessels should be undertaken using non-destructive testing (NDT) technology.
These NDT examinations - which can be carried out during scheduled outages - should cover a representative volume of vessel forging base material in areas known to be potentially susceptible to hydrogen flaking, WENRA suggested. If these inspections reveal evidence of hydrogen flaking, the inspections should be extended appropriately, it said.
WENRA stressed, "It is up to the national nuclear safety authorities to define the necessity, testing scope, volume and non-destructive method, depending on the available information on the vessels."
WENRA chairman Hans Wanner commented, "We consider it important and necessary to be aware of this issue and that appropriate measures will be taken at every European plant."
Researched and written
by World Nuclear News