How do you close a nuclear power station?
By Steven Green, BBC Knowledge & Learning
As the UK embarks on building what could be a new generation of nuclear power plants, work continues to decommission the first generation of nuclear power stations at sites including Trawsfynydd in Snowdonia which will take an estimated 90 years to complete.
Eryl Pritchard is at the controls of a robotic arm engaged in the painstaking process of retrieving radioactive resin from a dark, water-filled vault. “It’s not as simple as it looks,” he says.
The UK’s first generation of nuclear power stations
- Britain’s Magnox nuclear power stations were designed in the UK and built in the 1950s and 60s
- The first Calder Hall came online in 1956
- The pressurised, gas-cooled reactors used natural uranium as fuel, and also yielded weapons-grade plutonium
- Nuclear power generation peaked in 1996, contributing 26% of the UK’s electricity
- Wylfa in Anglesey is the last operational Magnox reactor in the world – set to continue until at least September 2014
Nothing about it looks simple. Radioactivity inside the vault means everything has to be done remotely using tools mounted on the robotic arm.
A bank of monitors show the arm from various angles. A control panel shows a mind-boggling array of levers, switches and buttons.
“Once you get your head around it, you soon figure out how it works.”
Eryl Pritchard is leader of the Rotary Deployment Arm (RDA) Test Team at Trawsfynydd. While he controls the RDA, a colleague is also watching the monitors to warn of potential hazards.
The umbilical wire connected to the RDA could get wrapped around its mast. There are dead zones in the tank from which the robot could only be recovered manually – a dangerous and time-consuming task.
“When you’re doing delicate operations at the base of a tank, trying to wash underneath, it might take you maybe ten minutes to move the arm all the way around to get the tool into the right orientation. It can get a bit frustrating and stressful, but it’s manageable.”
The UK’s only inland nuclear power station
The resin was used as a cleaning agent to remove radioactivity by ion exchange from water used at the site before it could be discharged. Trawsfynydd has more resin waste than other nuclear plants because there were tighter restrictions on any radioactive discharges into the adjacent lake. Britain’s other nuclear power stations are on the coast, where tide and current can disperse any radioactivity.
The radioactive resin is classed as Intermediate Level Waste (ILW) and will be placed in a new storage building on site along with other similar waste, from parts of the plant which came into contact with the nuclear fuel.
When the plant was operating, no consideration was given as to how to eventually dismantle and decommission the station. So as a consequence, a lot of operational waste was put in the vaults and is now difficult and dangerous to get at, hence the need for robotic equipment.
Christopher Rogers, Senior Site Engineer is one of only a handful of staff to work at Trawsfynydd on the decommissioning and when it was generating power.
“The concentration at the time was ‘Let’s get the megawatts out.’ Nowadays we’re looking at getting the place finally closed off.
“The technical challenges are very different. When we were running, our deadlines were a lot, lot tighter – it could be the difference between a couple of hundred thousand pounds a day lost for the company if we didn’t get back online again.
“Now there isn’t quite the immediate challenge, obviously we’ve still got to meet our deadlines, but I think there is a little less pressure than there was in those times. The regulations we have now are a lot tighter and stricter than when we were running”.
Some of the decommissioning work is considered too dangerous or uneconomic to carry out in the current phase of decommissioning.
The two steel pressure vessels which contained the nuclear reactors are staying in the reactor buildings until the radioactivity of the steel has decayed to a safer level.
The twin reactor buildings are having their height lowered to reduce the visual impact of the plant on the surrounding Snowdonia countryside.
From 2026 that is how the site will be left with the ILW still stored on site alongside the reactor buildings.
Where does the radioactive waste go?
The final decommissioning at Trawsfynydd and elsewhere depends on finding a safe long term solution for where to deposit the ILW as well as the High Level Waste (HLW) currently stored at Sellafield. This includes the spent nuclear fuel which was removed when the plant closed.
The most likely solution is storage deep underground but a site has yet to be identified. It is hoped that will be established by the 2040s allowing the ILW at Trawsfynydd to be removed.
- 1959: Construction started
- 1965 to 1991: Electricity generation
- 1993 to 1995: Decommissioning starts – fuel removed and sent to Sellafield
- 1995 to 2016: Recovery of waste and preparations to put the plant into a ‘passively safe’ state
- 2020-26: Reduction in height of reactor buildings
- 2040s: Scheduled removal of Intermediate Level Waste to deep geological storage
- 2074: Final site clearance starts
- 2083: Site returned to pre-existing state
Final site clearance at Trawsfynydd is not projected to start until 2074 with the land returned to its original state by 2083. It is at that stage that the reactor buildings and the steel pressure vessels will be demolished.
“The principle (radioactive) isotope is cobalt 60, with a half-life of 5.3 years, so if you leave it for 70 to 100 years, you’ve got about 0.001% radioactivity left. By capitalising on that radioactive decay, leaving it for that length of time, you can have operators use their hands to dismantle it”, explains Vic Belshaw, Programme Delivery Manager.
Another issue will be what to do with the graphite reactor cores which are still inside the steel pressure vessels at Trawsfynydd.
“One of the problems the UK has with its commercial nuclear power reactors, with the exception of Sizewell B, is that they all have a large graphite core which is used as a moderator, that’s inside the pressure vessel and that’s used to slow the neutrons down.
“There is no real disposal route for the graphite in the UK, the volumes are huge. If you look at the carbon, and the sulphur as well, certain isotopes, the half-lifes are hundreds of thousands of years,” says Belshaw. While the radioactivity will be very persistent, the graphite is only classed as Low Level Waste (LLW), so will probably not be stored deep underground.
Could the decommissioning be done faster?
Twenty years after it closed, many hundreds more people are employed at Trawsfynydd than when it was generating power. And it will be many decades before the site can be fully restored.
Could it be done more quickly? Not economically says its owners, Magnox. Earlier decommissioning would involve more expensive remote operations and still carry the risk of exposing workers to radiation.
Others would rather see a quicker solution. “In an ideal world, the Society would prefer the reactor building to be decommissioned, and the site returned to its original condition, as quickly as possible,” says the Snowdonia Society, a local charity. “Obviously the decommissioning needs to be done safely.”
Source: BBC News