Improvement of the fuel canal at Ikata 1 in Japan using adhesively bonded steel patches was completed in late 2002. This marked the completion of six preventive maintenance projects to improve the stainless steel liners of the spent fuel pools and fuel canals at Ikata’s three PWR units.

Framatome ANP’s Coating and Adhesive Technology group has developed a wide range of innovative techniques using organic materials for nuclear applications. Adhesive bonding, for example, is a fast, inexpensive and low-dose alternative to the conventional repair technique of welding, and even enables repairs to be carried out underwater. Coatings, on the other hand, have been used to develop accident-resistant plastic liners for concrete containments as a substitute for steel containments.

History

During reracking work in 2000 in “Pit B,” a spent fuel storage pool at Ikata 3, chloride-induced weld corrosion was discovered at two locations. These were repaired in the conventional manner by welding. However, the plant operator, Shikoku Electric Power Corporation (SEPCO), wished to go a step further by ensuring that the leak-tight integrity of the pool liner would nevertheless be guaranteed if such damage should later recur when spent fuel was being stored in the pool. For this reason SEPCO was interested in a prophylactic measure which would provide suitable protection at reasonable cost for the remainder of the plant’s service life. Thanks to Framatome ANP’s impressive portfolio of references, SEPCO chose the company’s adhesive bonding technique, first deployed more than 10 years ago for fuel pool repair and preventive maintenance at a German nuclear power plant. Since then the technique has been deployed with great success to repair steel fuel pool liners at a number of European plants.

Improvement Projects Cover Fuel Pools and Fuel Canals

In the summer of 2000, only three months after the corresponding feasibility study had been commissioned, adhesively bonded steel patches were applied over a total of 600 meters of welds on the walls and floor of Pit B at Ikata 3. In Pit A which, unlike Pit B, already contained fuel assemblies, adhesively bonded steel patches were installed over a total of 900 meters of welds in December 2001 after the pool had been decontaminated. Framatome ANP additionally submitted a study based on improvement of the fuel pools in Ikata 1 and 2 (which are almost completely full of spent fuel assemblies) using the same technique underwater.

Other activities included installing adhesively bonded steel patches over the welds in the reactor cavity at Ikata 3 as well as in the fuel canals of Units 1, 2 and 3. Only those sections of the welds that were most at risk were improved; for example, all fuel canal corner joints. Despite considerable spatial constraints – the fuel canals in Ikata 1 and 2 are particularly hard to access and are very narrow – all six projects were completed well ahead of the specified deadlines.Materials for All Needs

In spent fuel pools, adhesively bonded steel patches are exposed over the plant’s remaining service life to high levels of radiation from the spent fuel stored in the pools. Also, patches on the pool floors have to withstand severe mechanical loading induced by the fuel storage racks and the fuel assemblies they contain. The cumulative dose for these areas was calculated to be around 16 MGy. The selected adhesive – Framatome ANP EP 130 – has a verified radiation resistance of 20 MGy, thus providing a sufficient safety margin. For the materials employed in the reactor cavity and fuel canals, however, the main criterion was that they must adhere strongly to the electropolished steel liner plates, without mechanical surface treatment beforehand (to prevent the ingress of foreign materials such as blasting agents). For this reason Framatome ANP recommended use of Framatome ANP SG 460 – a material with a radiation resistance that is lower than that of Framatome ANP EP 130 but is still several times higher than the plant lifetime dose anticipated at these locations.

Framatome ANP EP 130 is an epoxy-based material while Framatome ANP SG 460 is silicon-based. In both cases, a thick layer of adhesive (approx. 4 mm) was applied to either the liner or the steel patch. Once all patches had been properly positioned, a continuous pressure was applied to the patches until the adhesive had fully cured. Then the work was completed by finish-machining all the joints and edges.

Contractual Setup

Each of the six improvement projects was performed by a consortium comprising Yonden Engineering Company, Inc. (responsible for detailed engineering, supply of steel patches, provision of maintenance personnel and performance of work in situ) and Framatome ANP (basic engineering, supply of adhesives, technical expertise, training and quality management). Marubeni Utility Services, Framatome ANP’s agent in Japan, provided interpreters and coordinated all activities.

General view of “Pit B” welds after improvement

Detailed view of adhesive bonded steel patches at fuel canal corners

Advantages of Framatome ANP’s adhesive bonding techniques compared to welding repair:

• The techniques are faster, less expensive and result in lower doses
• Exact locations of existing leaks do not have to be precisely pinpointed because large areas can be covered using adhesively bonded patches
• The structure of the existing material is not altered; i.e. there is no material oxidation or sensitization and no change in component dimensions
• Use of organic materials for repairs prevents any further propagation of corrosion
• Framatome ANP’s patches and adhesives have been qualified and approved by the competent authorities in Germany, Spain and Switzerland, not only for preventative maintenance but also for repair
• The materials available can satisfy nearly all requirements