Forging ahead in BWR internals replacements29 April 1999
Swedish BWR owners have started mid-life replacement programmes for their reactor pressure vessel internals. A major replacement was completed for the oldest unit, Oskarshamn 1, in 1998. In an ongoing project to be completed next year at Forsmark, new core shrouds and top guides are fabricated in forged materials without any structural welds, thus reducing economical risks and costs of future inspections to the plant owner.
Many nuclear power reactors have reached mid-life and have begun to experience mechanical failure. The most frequent problems in Boiling Water Reactors (BWRs) have been mechanical wear and corrosion, cracks propagating and embrittlement.
Whereas much attention has been paid to maintaining the primary system in as near as possible perfect condition, considerable concern of its integrity developed in the 1990s when cracking of internals was uncovered in several BWRs around the world. In particular, circumferential cracks were observed in the core shrouds of BWRs in Europe, the US and Asia.
THE OSKARSHAMN 1 EXPERIENCE
In Sweden and Finland, there are 11 operating BWRs designed by ABB Atom. Six of them are of an advanced design with internal recirculation pumps. The oldest BWR, Oskarshamn 1, completed a comprehensive repair and improvement programme, known as the FENIX project, some years ago. After a system decontamination, the Reactor Pressure Vessel (RPV) and internals were inspected from the inside under dry conditions. Although the pressure boundary system was in perfect condition, several cracks were found in the RPV internals, due to intergranual stress corrosion cracking (IGSCC) and thermal fatigue. Replacements made during the FENIX project included internal feed and emergency core cooling water risers and the core shroud support (see Nuclear Engineering International, December 1994, pp12-14 and June 1995, pp32-33). A decision was also made to replace the core shroud, the shroud head with the core spray system, the steam separators and feed water spargers. In 1996, ABB Atom won the contract to make these replacements, which took place during a prolonged plant outage in 1998.
The design of the new internals for Oskarshamn 1 were based on the original drawings. However, several improvements were introduced. Materials requirements were improved, the number of welds was reduced with much consideration given to their location. An improved core spray design and higher performance steam separators were also introduced. The design was optimised for easy installation and to facilitate future inspections.
APPLICATION TO NON-ABB PLANTS
At Oskarshamn 1, the removal and installation activities for replacement of the core shroud were carefully recorded. The combined time for these two activities was four weeks, including handling of radiation shielding, cutting, weld preparation, welding and inspection activities. While this is indicative of the time needed to complete an operation for other ABB BWRs, studies made for non-ABB BWRs based on the Oskarshamn 1 experience indicate a critical refuelling floor time of approximately 14 weeks for undertaking the following activities:
• Removal and storage of control rods and control rod guide tubes.
• Removal and storage of the top guide and core plate.
• Transportation and handling of new components.
• Segmentation and disposal of old components.
• Replacement activities.
REPLACEMENTS AT FORSMARK 1 & 2
In Sweden, new codes and requirements for the inspection and repair of nuclear power plants have been introduced in recent years. In particular the costs of qualification of inspection methods have gone up remarkably, while at the same time prices of mechanical components have fallen substantially. For RPV internals, the cost of one inspection could be as high as one third of the price of the component. Based on Life Cycle Cost (LCC) considerations, it has become more attractive to make internals replacements before even more costly inspections, repairs or other problems occur.
ABB Atom has recently received contracts for replacement of upper core support grids (“top guides”) and core shrouds for the twin BWRs, Forsmark 1 and 2, to be delivered in the year 2000. For both contracts, two alternatives were evaluated by the customer: machining from forged material (ie no structural welds) or welded design. In both cases the plant owner chose the forged version, after considering future inspection costs. The main part of the shroud is made from a single forging, the top guides from two forgings, with one centre piece and an outer ring joined together by guide pins. In both cases the design life time is 40 operational years.
Advances in the manufacture of forgings have made these new designs possible. Ring forging and disc forging processes have been developed by steel makers with material specifications and sizes adequate for BWR internals. These forgings are machined according to specifications. By using the same manufacturer for both core shrouds and upper core grids, the compatibility of these components can be tested at the works before shipping to site.
Today’s experience indicates that plant lifetime for most BWRs can be extended significantly beyond the original design goal of 40 years. It should be recognised, however, that some components, like the core shroud, need to be replaced once during a plant’s lifetime. The reactor internals in ABB Atom BWRs, therefore, incorporate design features to facilitate easy replacements. This has been a design principle for all operating ABB Atom BWRs which the company has maintained for its most recent design, the BWR90+.
Based on experience from Oskarshamn 1, studies made for other vendors’ BWRs indicate that core shroud replacements can be made in a reasonable critical time during a prolonged outage. Considering the recent cost development for hardware versus software in fabrication technology, advanced designs like forged versions may become the most attractive solution for replacements, as well as a preferred choice for BWRs built in the future.
Depending on other activities during plant shutdown, critical time path for replacement is one important parameter of the financial evaluation made by the plant owner. Recent ABB Atom efforts are aimed at bringing down the total time for shroud replacements for all BWR designs.