Everything but the fuel20 August 2014
Five spent fuel ponds have so far been drained as part of a UK national project to decommission redundant magnox power stations before they start a long care and maintenance (C&M) phase to accommodate radioactive decay. A roving team using simple tools and innovation techniques carries out the work. By Will Dalrymple
Once the magnesium-oxide-clad, natural uranium fuel elements from magnox plants were removed and shipped to Sellafield's magnox reprocessing plant, the oblong, poured reinforced concrete ponds, all different shapes and sizes (but all 6m deep), were left empty. Empty of fuel but full of other potentially hazardous material; the skips that held the fuel, cooling water, accumulated sludge at the bottom of the pond, and caesium and strontium contamination in the water, the sludge and the walls.
As part of the decommissioning process these 12 redundant ponds across eight sites must be emptied of equipment (or furniture), drained of water, the surfaces stabilized and the structures readied for the long period of care and maintenance. Five ponds are fully drained, and two (Hinkley Point and Hunterston) are nearing completion. Since April 2011, 12,697m3 of water has been drained, leaving 13,231m3.
About 250 people work across the magnox ponds programme, directed by 10 staff at Magnox headquarters at Berkeley.
The scope of the pond decommissioning programme officially also includes the active drains that carry active effluent in a site, mobile active effluent treatment of the water itself and decontaminating the empty waste vaults that hold fuel element debris (FED) from operational processes. The ponds programme sits alongside other projects FED retrieval, intermediate level waste (ILW) retrieval, treatment and packaging work and plant and structures modifications, at the carbon dioxide-cooled, graphite-moderated magnox reactors, shut down in the 1980s, 1990s and 2000s.
After some preliminary operations in 2006, the ponds at Hinkley Point A were temporarily left, as the Magnox Optimised Decommissioning Plan came into force and attention shifted to higher-priority sites at Bradwell and Trawsfynydd. But from early work at Hinkley Point A a small team of experienced personnel emerged; the programme has been keen to move them, and their ideas, around to the fleet's other pond projects. They call it 'lead and learn', and the idea is to try to share experience and knowledge.
“The concept is that you meet a challenge, go through a first-of-a-kind of experience on one site, develop a process to deal with it and then apply that to the next site, and develop it further. The idea is that you never have to go through the same learning curve twice; there is a constant evolution as you progress through the programme," said Steve Walters, Ponds Programme Director.
Pond clean-up at five sites
Now the ponds clean-up programme is being carried out at five sites: Hinkley Point A, Bradwell (the lead magnox site, expected in 2015 to enter the 80-100 year care & maintenance stage to reduce radioactivity levels before final demolition), Hunterston A, Chapelcross and Trawsfynydd; all are at different stages. Of the remaining sites, Dungeness ponds will be next, then Sizewell's, then Oldbury's in 2017. (Wylfa is the only magnox site with a dry fuel store; Berkeley, which ceased generation in 1989, had its ponds cleared in the 1990s).
Since each pond differs in dimensions, extent of structure below ground, pond water treatment equipment and proximity to other major structures (such as reactor buildings), the exact end-state of each pond varies. A flowchart was developed to support the decision process (Figure 1).
The general pond decommissioning process has been the same at every site: remove furniture, retrieve sludge, drain and stabilize surfaces, and finally prepare for C&M. During that latter period, the intervals between site entries will be measured in years, and site work will be limited to maintaining the integrity of the pond and weatherproof barrier. Eventually the ponds will be demolished to ground level.
The 'furniture' consists of submerged equipment and tools. These can be significant: some sites, such as Hinkley Point A, performed desplittering (cutting cooling fins off fuel elements to better pack them into fuel flasks) and the associated equipment is large, cumbersome and several metres underwater. This called for some clever thinking by ponds personnel, who modified simple tools with long window- cleaning handles so they could work through the pond water, using it as shielding, while they unbolted and manually size-reduced the furniture.
Sludge is primarily corroded magnesium from the fuel cladding, which settles at the bottom of the pond. It is vacuumed up using innovative techniques and transferred to settling tanks. In some ponds, sludge removal and processing has been complicated by embedded particulates and fuel-containing materials. In some parts of the Hinkley Point A pond, sludge was up to six inches deep and contained fuel fragments that had to be separated. These were picked out of the sludge using various long-reach tools before the sludge was gathered using a remotely operated vehicle (ROV) and vacuumed out of the ponds using a hydro-cyclone. Elsewhere, the sludge was filtered in a rock basket in a submersible filter unit, or passed through a perforated skip with 5mm holes.
Once the sludge is cleared and furniture removed, the pond water - called 'liquor'- is drained and treated using the plant's active effluent treatment system, or a mobile one (see box).
Wherever possible, clean-up tasks are carried out from the side rail using modified tooling. However, in the less contaminated ponds, such as Chapelcross, dose assessments found that the best method for cleaning the pond floor was manual access and hand cleanup, using every day cleaning tools.
Options based on contamination levels
The degree of pond contamination depends on the ponds' operational history, particularly how well they were managed and how long the fuel was kept in the pond.
In some cases, such as at Bradwell, contamination in the walls is so great (500 mSv/hr at the water level) that even with positive-pressure air suits dose limits would make work intervals impracticably short.
“We take measurements to help us predict the likely dose rates and contamination conditions following drain-down. In some cases this has determined that more aggressive decontamination techniques are needed during the drain-down of the pond," Walters said.
In these cases, workers have removed the top surface of the pond walls to a depth of 4-6mm. Core samples have generally shown that almost all (99%) of the contamination found in pond walls has penetrated less than 10mm deep, and the majority is in the first 3mm. At Hunterston, the largest pond in the fleet, workers used high-pressure water jetting to take off the paint and some of the concrete (to a depth of 6mm). On the other hand, that technique tends to generate lots of sludge to treat and is not effective on floors (the moving effluent tends to recontaminate areas).
Most recently, at Hinkley Point A, personnel have begun to dry-shave a 10mm layer of concrete off the reactor one spent fuel pond walls using a five-disc rail cutter, which bolts on to the wall, cuts a 2m x 2.3m area, and generates a powder waste with no wet sludge. That waste, categorized as low-level, will be sent to the National Low Level Waste Repository.
Some of these operations are made much easier thanks to the brainwave of a Hunterston A worker, who suggested installing pontoons to use as a working platform. Since the pontoons float on top of the water, as its level falls they give access to deeper sections of wall that emerge. They have been used at Hunterston and Bradwell and are now being used at Hinkley Point A.
Once the pools are drained and contamination levels stabilised, attention turns to readying the structures for C&M. Assessments are made and according to contamination levels further decontamination may be required.
At Trawsfynydd, a plant next to a lake in a national park, a dry scabbling system was used to cut out up to 40mm of wall surface. The aim is to remove all of the contamination in the walls to qualify for out-of-scope designation (although decontaminating joints and cracks has been more difficult).
Alternatively, as at Bradwell, walls and floors may be coated with a sealant. For the first time in the UK, a polyurea sealant was used on the walls and floor of the Bradwell pond to stabilize the surfaces ready for care and maintenance; it is a thick and hard sprayed-on polymer.
Remotely operated vehicle (ROV)
This standard £10,000 remote-controlled digger, specially adapted for underwater use by removing electrical drives and converting the hydraulics to run with water instead of oil. It is used to consolidate and retrieve sludge at the bottom of ponds, and was developed and used at Hinkley Point A and later Bradwell. Because it was an off-the-shelf product adapted by Magnox staff, it saved thousands of pounds. A unit has also been sent to Sellafield to be deployed in one of its legacy ponds as part of its clean-up programme.
Mechanical concrete removal requires a contact surface. Some pond floors, such as Chapelcross, are not flat, so they cannot be shaved. In contrast, liquid nitrogen shoots at high pressure through a delivery gun and cuts surfaces of varying geometries, and once it has done that, it evaporates. Ponds Programme Director Steve Walters says, "It is a wet cutting system that is dry. One of the side effects of the process is that when it hits the surface it freezes it, so that it breaks up more readily. The majority of contaminants in concrete are in the cement paste surrounding the rock aggregate, which is very low-level waste. It might be possible to separate the cement paste from the rock aggregate. That is part of the economic evaluation. Its benefits and costs are currently being evaluated."
At Hinkley A 1700 fuel skips, each 3ft long by 18 inches wide, were cleaned using high-pressure water jetting and sent to parent company EnergySolutions' facility in Oak Ridge, Tennessee for reuse in US accelerator projects as shielding.
That technique did not work on Bradwell skips, whose caesium and strontium contamination was more deep-seated. For 511 other fuel skips in the magnox fleet (and perhaps 4000 at Sellafield), a new decontamination method is needed. Operational trials have begun with a CNC lathe that cuts 1.2 mm off of the metal walls of the skip, which has been laser-cut into five flat steel plates. Although the shavings are intermediate level waste, the vast bulk (80%) of the 110kg skip has been categorized as free-release and can be sent off site for recycling.
Mobile active effluent plant
A submersible ion-exchange filter that removes Cs-237 and Sr-90 has been tested at Hinkley A for possible use with off-the-shelf water treatment filtration plants for other pond clean-up projects.