Plant life management
2,300MW on the line22 May 2009
Corrosion in the boiler closure units in four AGRs in the UK led to a 17-month, £155m engineering project to get the reactors back online. By Will Dalrymple
In a March presentation to the World Association of Nuclear Operators in March, British Energy CEO Bill Coley called the effort to restart Hartlepool 1&2 and Heysham 1 reactors 1&2 ‘British Energy’s most significant technical, engineering and management challenge.’
The project, which consumed three million man-hours, generated 500,000 printed pages of safety case documentation. The work was carried out with the daily pressure of 2300MW of electrical generation capacity, a quarter of BE’s production capacity, offline. All the reactors had returned to full operation in March.
“Perhaps it was a leap of trust for the CEO to believe that I could sort this out,” project director Stuart Crooks tells NEI. “And he didn’t know how much it was going to cost. The capital cost of the project in the end was about 10% of the amount of lost
revenue. The alternative was to close the plant down. We didn’t want to concede that that option was realistic for BE, or for the UK.”
The trouble was found during an inspection of a boiler closure unit, a plug of concrete 2.4m (8ft) in diameter and 1.8m (6ft) high that supports a 200t and 15m (50ft) pod boiler beneath it. The BCU is a primary boundary between the reactor gas coolant circulating beneath and the atmosphere. Steam and cooling water tubes pass through penetrations cast in its concrete. The bottom surface of the BCU is a 19mm (3/4in) metal plate. To help the BCU cope with the high forces it is subjected to, and to help it resist the forces imposed by a reactor incident, the entire outer circumference is wrapped with nine layers of 2.6mm wire. During testing, a corroded wire was discovered.
“We knew that we have had water in the BCUs for some years,” Crooks says, a result of leaking cooling water. “We had developed an inspection technique using radiography to demonstrate the integrity of the component.” Inspectors later found corrosion in the wires in all four of the Hartlepool and Heysham AGRs, ie all the reactors with the pod boiler design in which the BCU is employed. Because corroded wires might compromise the integrity of the BCU, which, if it failed, could breach the reactor enclosure, BE shut down the reactors as a precaution.
“When this defect was identified we did not have a repair plan,” Crooks says. “We had started work on options in early 2007 at principle level but none of these had completed feasibility or design studies. We had set up research into the field and had some ideas of locking in the prestress and fitting a restraint, but those principles had no detail at all.”
After dusting off this preliminary research, Crooks assembled a group of 30 experts: contractors, consultants, suppliers and other experts for three days of brainstorming – called optioneering – in October 2007 to flesh out these ideas.
Crooks says: “I can remember having dinner at Lancaster House, and some of the engineers drawing on napkins. That’s where we started. Once we had done the early optioneering, we could tell that some ideas were not working, or that we couldn’t underwrite them with a safety case, or they weren’t going to be demonstrable. The principles that the ideas had to meet were a 20-year design life, inspectability – the reason why we got into this problem was that the component was not designed to be inspected- and reversibility. That ruled out solutions such as gluing or filling with grout and narrowed the field.”
The limited space around the BCU units also blocked many potential solutions. Surrounding the BCU is a metal annulus only 20mm (1 inch) away from the winding. Surrounding that is a concrete well, with only a 70 mm gap from the annulus.
Over the next year, the British Energy team started, and managed, a total of seven streams of work to investigate and solve the problem. Crooks explains how the project worked: “The day-to-day was that every morning at 8:00am all the project leaders were on a call, sharing direction and strategy, the issues that the week held, problems, and the issues of that day. Then there were separate side meetings during the day and a daily call at 5:30pm. In addition there was an in depth review held once a week that took all morning. On top of that, then-Heysham station director Matt Sykes and I had a weekly meeting with the CEO Bill Coley and the financial director Stephen Billingham to give us the support we needed.”
The biggest single technical issue was to find a way to maintain tension on the BCU in case the wire windings failed. Three alternatives were developed and run in parallel. “Risk management in the project dictated that in each of the key strands of work, we wanted three solutions. If one had a problem with the safety case, we could fall back on another,” says Crooks. They were a Freyssinet plan to install pressurised bags of grout between the walls of the concrete well; a Doosan-Babcock plan to install wedges over the wire terminations, which it judged to be their weakest point; and an Atkins plan to clamp bands on top of the wires. The three designs were developed on until they began to interfere with BE resources. In April 2008, the bands won out.
The final design of the band was a compromise between space, the material and the operating parameters of the BCU, says Simon Cross, Atkins project manager. For example, the bands needed to be tight enough so that in case of a gas release, during which the bands would heat up and expand, they would still be tight enough to hold the BCU together. On the other hand, they would need to be loose enough to not be damaged, or damage the BCU, when the BCU expands (it expands slightly as the reactor heats up after a refuelling outage).
Doosan Babcock, which was the prime contractor at Hartlepool, had the job of manufacturing and installing the bands. “We had to compromise on the initial tolerancing of Atkins. The theoretical tolerance wasn’t achievable in modern precision machining. We did what could be done, and they did the calculations to see if it was okay,” explains Ceri Green, director of nuclear business at Doosan Babcock. The bands were eventually made in four sections so they could be assembled around pipework extending out of the top of the BCU. The main bands are only 3mm thick, and the clasp that joins the sections together had to be less than 20mm to be able to fit inside the gap between the wires and the metal annulus. Five threaded bolts run vertically through two wedges mounted in each clasp. Turning the bolts tends to push the wedges down into the clasp, which tensions the clamp.
“We had three different machine shops making the clasps,” Green says. “We welded parts in Renfrew. We set up a dedicated workshop area to deal with putting the strain gauges on, carried out in a shop where the boilers were originally manufactured. It was difficult because of the quality and tolerancing required, the exceptional cleanliness required for the brazing. They were closely monitored and inspected.”
Because of the urgency of the project, the design and manufacturing were done partly in parallel. “In effect, we didn’t fully prove that they worked until they had started manufacturing them,” Simon Cross at Atkins says. Green at Babcock adds: “This was a fast-track project. We were proceeding at risk, and we made various things that at the end we discarded, which is common to any fast-track process.”
Doosan Babcock installed a total of 288 bands, or 1152 segments, (nine per BCU, with eight BCUs for each of the four reactors), between August and December 2008. Green says that installation teams began practicing on full-scale mock-ups that they had made, using television cameras and cut-outs to see inside the metal shell. “After we started the mock-ups, we kept the one-inch gap, and gradually built up tooling and guides so that we could drop the bands in.
“We had handling tools, and four guys, and cameras to ease the band down in. We aimed to do a band a shift. That was to assemble the band, position it, take it down into the right location and orientation, and then tighten it. Each band had two strain gauges on each of the four quadrants, so there are eight gauges with trailing cables. Just to make life more difficult, the cables had to be put into a data logger.”
Doosan Babcock also fabricated, and installed, another device that mounted on top of the BCU, called the external steel restraint, from an EASL design. The restraint, which weighed 8t, consisted of bolted sections of 60mm-thick high-impact carbon steel plate that mounted on top of some of the 48 studs, 1.5m (5ft) long, that run in a ring near the outer edge of the BCU. The studs hold the bottom plate down; in the event of a reactor accident, or in case both wires and the bands failed, the new restraint will allow the plate to rise less than 7mm (if pushed by forces of 2500t) until it locks completely. Keeping the plate down keeps the lid on the radioactive CO2 in the reactor.
Another work stream in the project investigated the effects of friction between wire windings. “We wanted to demonstrate to ourselves that if all the anchors failed, the wire would not unravel,” Crooks says. BE commissioned Taylor Woodrow to use the original drawings to manufacture a BCU (it was only half-height, but otherwise identical). Taylor Woodrow wound wire on to the BCU from a winding machine that it custom-built. Then it measured how the wire tension was affected when chunks were cut off, when the BCU was shaken on a seismic table (also specially-built), or when the BCU was cooked in an oven. “We demonstrated that it was quite a robust component, and wouldn’t unravel,” Crooks says. Taylor Woodrow eventually built nine replica BCUs.
At the same time, wire inspection efforts were redoubled. The initial X-ray inspections that first flagged up the problem were not able to image the entire rope length because of space restrictions. Instead, a new visual inspection technique was developed that passed a camera through a 17mm bolt hole in the BCU, to gain access to a wall of wire 6ft high. A total of 192km of wire was visually inspected.
Other work streams were managing the instrumentation that connected the 8000 sensors measuring band section strain, restraint strain, bottom plate temperature and strain, and new sensors measuring cooling water flow and temperature. Doosan Babcock was also involved in a work stream to upgrade the boiler cooling water system, to help preserve the BCU concrete. In another work stream, contractor Sir Robert McAlpine did finite element analysis and computer modelling on the BCU itself. Altogether, the restart project involved 150 companies across the UK.
Crooks says that the multiple layers of BCU fixes – restraints on top of the BCU, and bands on top of wires – was the fundamental safety strategy. “This combination of different systems, each capable of providing the required integrity to the BCU, produces an incredibly robust and safe solution to the BCU problem,” he says.
Despite its cost, the repair project is an investment in the future of the plants, Crooks says. “Technically, we have made sure that the BCUs will not be limiting factors for the life extension. There will be other issues in the AGRs that are life-limiting, but not the BCUs. But they could well have been life-limiting.”
From an accounting point of view, the Hartlepool and Heysham plants are now expected to run until 2014. But BE will soon gear up for an economic review of life extension at the plants. And later this year, both plants are due to complete a major 10-year Periodic Safety Review, which is a licencing requirement. The next review would not be due until 2019.
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