EDF Energy has invested in the reactor sea water system at Torness in Scotland to reduce the risk of blockage or system failures while the reactor runs to its new final shutdown date in 2030. Corrina Thomson reports.
Torness is a twin AGR station on the southeast coast of Scotland, 33 miles east of Edinburgh. The site employs over 700 people and began generating in 1988. Its two reactors have a nominal full load of up to 600MWe each. Following a recent seven-year life extension announcement by operator EDF Energy, Torness is scheduled to be shut down in 2030.
EDF Energy has been upgrading its reactor sea water system in a £7.5 million project, which has improved the plant’s pipework for the remainder of its extended life.
The reactor sea water (RSW) system provides a way of dissipating heat from various parts of the AGR reactor system. These are: the pressure vessel cooling system (PVCS); the reactor auxiliaries cooling system (RACS) which includes the diagrid support skirt cooling system (DSSCS) and the bypass gas plant cooling system (BGPCS); the circulator auxiliaries cooling system (CACS); the pond water cooling system; the generator transformer cooling system; and the turbine house drains sump cooling. The system is designated as safety class 3.
Each reactor has four PVCS coolers, two BGPS coolers, two DSSCS coolers, pond water preschoolers (which are not usually required), and eight CACS coolers. Pairs of coolers are arranged in parallel, normally one in service and one on standby. The RSW system flow in each reactor totals around 450kg/s. This comprises: PVCS – 290kg/s; BPGPCS – 40kg/s; DSSCS – 3kg/s; and CACS – 110kg/s.
Torness has one fuel pond which holds about six months’ worth of irradiated fuel from the two reactors. The pond water-cooling system continuously circulates a proportion of the pond water through a pair of 100% duty redundant coolers, which are in turn cooled by the PVCS system.
A spokesperson for EDF Energy discussed the pros and cons of using a sea water cooling system: "The main reason for choosing sea water as a cooling medium is its availability and simplicity. Being situated on the coast, Torness, like all the AGRs, uses sea water cooling as an ultimate heat sink. A demineralised water- cooling system would be more complex, requiring intermediate heat exchangers and perhaps air-cooling towers.
“The downside of using sea water is obviously its corrosive nature, which we mitigate by material choice and inspection. It also has to be dosed with low levels of sodium hypochlorite to prevent fouling of the system by marine organisms."
The RSW pipework in the reactor building basement is aseismic high-density polythene. Unit 1 and 2 RSW pipework is high density polythene, apart from the strainer pipework on both units. The RSW pipework between the pump house and the reactor basement is glass reinforced plastic encased in concrete.
In each cooling loop, RSW pumps are fed from cooling water drum screen outlets. Sea water is pumped to strainers, then routed to the reactor building where it feeds several coolers. It discharges from coolers connected to return mains, to the head tank arrangement. The turbine house drains are fed from a RSW return main, into a surge chamber then to an outfall.
Strainers reduce the entrained solids in sea water, which help reduce the chance of blockages in the coolers downstream. Each coolant loop has a pair of strainers, normally with one in operation and one on standby. The standby strainer can be started manually if required.
The strainers are an auto-backflush type and water for continuous back-flushing is from the strainer body. Dirty water is discharged into the RSW discharge pipework. Changeovers are manual. Strainers are sized so that they can operate for 24 hours after a loss of backflushing facilities.
The RSW system provides essential cooling to a number of plant areas. In the past on occasion essential cooling has been lost because the carbon steel vylastic lining has become detached and blocked the pipe. Defects initiated by pipework corrosion have also led to a partial loss of cooling.
“We have experienced rubber lining detachment in the sea water cooling for the conventional turbine plant on a small handful of occasions. For example, turbine lubricating oil coolers have become blocked – but never the reactor sea water cooling system," said the EDF Energy spokesperson.
“Flow blockage in the conventional plant is a commercial risk: for example, we might have to reduce output temporarily and change over to a spare cooler pending repairs. But we don’t want to run the risk of flow blockage in the reactor sea water system.
“Inspections of the reactor sea water cooling pipework in the mid-2000s showed the lining was deteriorating in some areas and we made repairs using glass flake and other coating products," they continued.
“However, we recognised that these repairs would not last the entire station life and made the decision to replace the pipework in its entirely, to eliminate the risk. Our strategy for the conventional plant is similar but we have placed a lower priority on that."
They went on to say corrosion can also cause a problem: "Essentially this is about leakage at the corrosion site. We deal with this by isolating the defective section of pipework and bringing the spare cooler, or whatever plant item is affected, into service. We can then perform a temporary repair, such as a clamp or a fibreglass patch, until a window becomes available for a permanent repair, which usually means one of the three- yearly, statutory outages."
A scheme to replace the carbon steel pipework with high-density polyethylene was completed on unit 1 in 2007 and similar work was completed on some parts of unit 2 in 2012. The remaining section of at-risk vylastic-lined carbon steel pipework within the unit 2 strainer pits A/B and C/D has now been replaced with new high-density polythene pipework.
All system valves were also replaced, new by-passes installed to address flow issues, new pipe supports installed, plant labelling replaced and existing system defects cleared where possible. The area was also cleaned and painted to improve the facility for staff.
EDF Energy plans to replace the unit 1 strainer pit pipework in the two-month 2017 outage, scheduled to start in April. This last part of the work is estimated to cost £1.3 million out of the total £7.5 million upgrade.
Some 469m of pipework has been replaced so far and after the unit 1 outage in 2017, this figure will reach 499m. Over 80 valves have been replaced on unit 2 and in 2017 the same number will be replaced in unit 1. The new pipework has a design life of at least 25 years, so EDF Energy expects it to cover the entire projected lifetime of Torness.
The RSW seal pit outlet pipework to the surge shaft is designed to prevent damage to the concrete surge shaft surface, foaming and water spray. The pipework is constructed from carbon steel, 500mm, reducing to 400mm, outside diameter, externally painted and internally lined. Each outlet has a sluice-gate-type valve to prevent discharge from the surge shaft into the seal pits and back into the outfall, or for isolation if work is being carried out.
EDF Energy staff had noted degradation of the discharge pipework and its flange bolt fastenings in visual inspections. This raised the question of the existing pipework and fasteners, and their adequacy for continued use.
Initial options to remedy the problem included replacement of the current carbon steel pipework with high-density polythene but a cost benefit analysis concluded this was not feasible as it would also require significant civil and mechanical design works.
EDF Energy said replacing the surge shaft pipework would have been more invasive than in other sections and access more difficult. Staff assessed the condition of the surge shaft pipework via ultrasonic non-destructive thickness checks, internally and externally, and determined the most cost-effective solution was to shot-blast and recoat it.
Outage work encompassed the visual and ultrasonic checks of the existing pipework, as well as replacing corroded fixings where required, and the shot blasting and coating of the pipework. The key outcomes of the assessment were to inspect and repair where possible and to manufacture replacement pipework as mitigation if required.
The repairs to the surge shaft pipework system are also expected to last the remainder of Torness’ operating life.
The RSW system undergoes a range of periodic functional tests and inspections. The tests on the operation of key pumps, valves and alarms, are done at intervals of between three and 12 months. Inspection of the passive components, such as pipework and supports, is at three-yearly intervals, during statutory outages. EDF Energy uses intrascopes and cameras during these inspections shift operators tour the plant at least twice each day and are trained to spot and report any early signs of degradation or component defects.
EDF Energy stated that the maintenance regime has not changed since the life extension announcement. The spokesperson said: "The use of high-density polythene in place of steel pipework has removed the corrosion risk but it’s prudent to keep checking the new pipework whenever we have the opportunity. There are no specific ageing mechanisms that would drive us to change the regime as the plant gets older.
“The regime of functional tests and inspections means that most of the maintenance we do on this system is preventive. Occasionally the functional tests or operator tours will reveal minor defects, which are repaired promptly by our onsite maintenance team. But this is no more than about 10-15% of the work we do."
EDF Energy said the cost of a three-yearly outage is typically around £20 million. The RSW inspections are a small part of that, running to tens of thousands of pounds. The company explained the saving made through the introduction of high-density polythene
is in the cost of repairs, which was not a big proportion of the outage spend. The most important benefit of the work is removing the risk of failure or blockage of the system.
Project manager Tom Finney discussed the lessons from the RSW work: "EDF Energy is gradually adopting high-density polythene pipework for many of its nuclear and conventional sea water systems in most of its AGR fleet.
“We have developed detailed technical standards for design and inspection of this type of pipework and these are overseen by our central engineering organisation. Our projects organisation ensures that learning from each installation is captured in a ‘lessons learned’ database.
“As an example, one of the lessons learned was about the change in position of some of the valves in the system. Due to the radius of the bends in the high-density polythene being greater than steel, it meant that some of the valve positions were slightly different to existing. This was not a problem in the majority of cases, but when it came to valves operated from above the floor grating, we found that the extended spindles did not fit. Modifications had to be made to these spindles so that the valves could be operated."