How Babcock plans to decommission UK nuclear submarines14 February 2014
An environmental impact report provided for public consultation about the decommissioning of seven UK nuclear submarines planned in Scotland has published detailed information about how the submarines’ reactor pressure vessels are planned to be removed.
An environmental impact report provided for public consultation about the decommissioning of seven UK nuclear submarines planned in Scotland has published detailed information about how the submarines' reactor pressure vessels are planned to be removed.
Rosyth Royal Dockyards Ltd. (RRDL), a subsdiary of Babcock International Group, has applied for consent to begin the dismantling project in January 2016. The UK nuclear regulator initiated a formal, three-month consultation on the project in late January (for which comments must be receved by 21 April 2014.)
After what it calls 'considerable consultation', the UK Ministry of Defence has chosen to remove radwaste in-situ at Rosyth and Devonport Nuclear Licensed Dockyards. But because of the delays expected to develop an intermediate-level waste store for the RPVs of ultimately 27 decommissioned nuclear submarines, MOD and Babcock have decided to split the d&d operation into two stages. (The ILW site is now expected to be identified in 2016, and operational in 2019.) Stage one is docking the submarine and removal of low-level waste, primarily within the reactor compartment. Stage two will involve removing the reactor pressure vessel and surrounding primary shield tank. Stage 2 will only commence when the ILW interim storage solution is agreed. The plan is to dismantle a first demonstration submarine entirely, and study the process, before contracting for dismantling the remaining units.
The project is expected to take 12 years.
In July 2013 TM Engineers announced it had successfully trial-assembled the steel structure for a crane to be used at the Devonport dockyard, in Plymouth, for similar submarine decommissioning operations. The crane's structure is 10m long, 10m high and more than 5m wide, and weighs almost 60 tonnes. More than 8000 hours of welding have gone into the structure, with each weld being fully inspected after the joint is complete to ensure quality requirements of a high integrity, seismically-qualified nuclear facility. The crane will run on rails within a new Reactor Access House (RAH) in 14 Dock at the dockyard. Based on a similar facility to that used for refuelling Vanguard class submarines in 9 Dock, the RAH is in effect a large 'high tech' moveable steel construction that spans the dock and is mounted on rails supported directly from the concrete dock walls, along which it moves using a hydraulic motion drive system. When in use during defueling operations the RAH, which weighs 600 tonnes, is moved in place over the submarine reactor compartment where it provides a stable, enclosed, low-level transfer bay, housing the crane, defueling tools and equipment, and providing a platform from which the operators can work safely.
Operation in detail
The submarine is docked down on a supporting cradle and the In Dock Installation (IDI) fitted and commissioned alongside. A hull access cut is made to connect the south end of the IDI to the submarine RC, this will provide a route for operator access and plant removal. The north end of the IDI is connected to a dock access stairwell.
Routine procedures are also part of this process step, including:
- Flooding the dock and de-watering after dock-down. Undertake radiation survey external to the pressure hull to determine whether a radiologically controlled area needs to be established.
- Establish brows, connect alarms and services.
- Install alter gantries.
- Jet wash hull and wash dock.
- Erect scaffolding to provide safe access.
- Install environmental containment, i.e. bunding and tenting, to enable ground, surface and rainwater management.
- Tile removal on RC cut lines. A limited number of acoustic tiles will be removed to enable hull cuts to be made.
Reducing dose by SG removal
In order to reduce ambient dose rates within the RC the steam generators are removed early in the process. Hull cuts are made above the SGs to allow them to be extracted vertically. A lifting frame arrangement with integral containment is fitted to the top of the submarine hull to extract the SGs through the penetrations. It is noted that the SGs will be delagged prior to removal. Once a SG is inside the lifting arrangement containment the penetration to the RC is sealed. Finally the crane is used to transfer each SG to a waiting transport vehicle. A turning frame is used to rotate the SG from vertical to horizontal orientation for loading on the vehicle.
Prior experience has been gained at Rosyth Dockyard, when SGs were removed on a previous occasion, with a new set being fitted into a sea-going submarine.
Once the SGs are removed, de-planting of all Nuclear Steam Raising Plant (NSRP) pipe work and small items of plant will commence. All items will be bagged and appropriately tagged/labelled prior to leaving the RC via the hull access cut into the IDI or the SG containment.
All radioactive material removed from the RC is characterised and packaged into suitable containers or directly into ISO freight containers for dispatch to an appropriate recycling / disposal facility. It is likely that these activities will be undertaken in the active waste accumulation facility (AWAF); the covered dock area.
Removing and replacing LLW items within the RC is a routine refit activity. The scale of the work in removal of all the LLW however, is significantly greater.
Vessel head removal
The existing RPV closure head is classified as LLW and is to be segregated from the RPV body which is classified as ILW and has a different disposal route. The RPV is filled with water prior to removal of the head to reduce dose rates in the working environment to acceptable levels.
The RPV closure head is then removed vertically through the tunnel plug and out through a hull cut as has previously been done in refuelling campaigns. A replacement head (that is required to retain the RPV core barrel in place and provide shielding and containment of the RPV internals) is imported through one of the SG hull penetrations and manoeuvred into place on the RPV using a rail / trolley arrangement. Once the replacement head is secured the shielding water is pumped out and the RPV internals dried using a dehumidifier. The original closure head is transferred to the AWAF for size reduction, packaging and transport off site to an authorised waste treatment facility.
Primary shield tank and RPV work
The primary shield tank (PST) contains approximately 25 tonnes of potassium chromate 0.2% solution which must be removed for disposal. The potassium chromate is extracted through a penetration in the top of the PST and pumped to a transport container (swap tank) on the dockside. The full swap tank is transferred to the AWAF on a trailer and the contents decanted into Intermediate Bulk Containers (IBCs) for final disposal off site. Two swap tank transfers will be required to remove all of the potassium chromate and associated flushing water. See Sheet 6 of Reference 65 for further details.
During Stage 1, approximately 200 litres of potassium chromate will be removed from the PST header (expansion) tank. This will be transferred into a carboy, sealed and labelled and stored within the submarine. During Stage 2, the bulk potassium chromate will be removed from the PST, and the Stage 1 arisings will be disposed of along with the remainder of the solution.
The RPV is separated from the PST on-board the submarine. The RPV is suspended in the PST supported by a substantial metal skirt. In order to separate the RPV it is necessary to cut the support skirt. During cutting the RPV will be supported on a lifting frame attached to the RPV via the lid securing bolts. The lifting frame is in turn supported by hydraulic jacks reacting off of the top surface of the PST. This support arrangement ensures that the RPV does not fall into the PST when the support skirt is cut.
Once the support skirt is cut and the RPV is supported on the hydraulic jacks, a test lift is made using the jacks to demonstrate that the RPV is completely disconnected from the PST.
Prior to removal of the RPV a large series of hull cuts is required to allow crane access to the top of the RPV and provide a removal route. Following demonstration that the RPV is disconnected from the PST the crane is connected to the RPV lifting frame and the RPV is transferred to a transport container within the dock bottom. Once loaded and secured in the transport container the complete package is rotated to a horizontal orientation using a turning frame provided with the container. The horizontal package is then lifted to a waiting transporter on the dockside and transferred to the AWAF building for temporary storage on the transport trailer.
The PST is an integral part of the submarine structure and is separated by cutting the lower hull and RC forward bulkhead which make up the front and bottom of the PST. The PST is then exported via the same access penetration as the RPV but is lifted, by the crane, directly onto a transporter on the dockside. From the dockside the PST is transferred to a size reduction facility within the AWAF where it will be cut up, characterised, packaged and consigned for disposal / recycling as appropriate.
The RPV will be stored in its transport container on the road trailer / Self Propelled Modular Transport (SPMT) in the AWAF. If necessary, temporary shielding will be erected around the RPV to reduce dose uptake by workers in the area.
After arrival of the PST in the AWAF, samples will be taken from the complex structure of the PST for analysis to further inform its radionuclide inventory. Any residual potassium chromate solution will be removed. The PST will then be reduced to suitable sized pieces to allow disposal via approved recycling / disposal routes. The initial assumption is that dry diamond wire technique (cold cutting) will be used. Precautions will be implemented to minimise the risks associated with residual chemicals, including working within a fully bunded area. Radiation protection measures will be implemented as necessary. During PST size reduction operations the feasibility of size reducing the PST within the geometry of an empty RC will be investigated.
Large items such as the pressuriser and the SGs will leave the RC through specific hull cuts. The remainder will leave via the IDI, where practicable dispatch will be direct from the IDI. Radioactive material will be routed via the AWAF for any required size reduction, assay, packaging according to a loading plan and removal of any resin or free liquid.
Significant volumes of waste
SDP will generate a significant increase in volume of soft trash; namely, gloves, caps, swabs etc. Existing practice at Rosyth Dockyard of co-disposal of LLW and VLLW soft trash is not considered to be consistent with the national policy with regards to the waste hierarchy. An alternative, more compliant disposal route for VLLW will be assessed.
Significant volumes of metallic LLW, estimated to be in the order of 400 tonnes, will be removed from the vessel during RC de-planting and transferred to the AWAF to be assayed, weighed, labelled and dispatched. The LLW will be sorted, segregated, size reduced if necessary and packaged in to appropriate containers i.e. drums and/ or International Standards Organisation (ISO) containers.
LLW will either be sent for further waste treatment i.e. shot blasting, decontamination, melting etc. or directly to the LLWR for final disposal if unsuitable for recycling. Redundant RPV heads recently disposed of by Devonport Royal Dockyard have all been successfully recycled by Studsvik of Sweden.
Metal scrap, such as stainless steel, carbon steel, copper, aluminium or lead is melted. The end-product is metal ingots that can either be immediately free-released as conventional scrap metal or released after a period of decay storage. The small amount of residual products (slag, sorted material, cutting and blasting residues and dust from the ventilation filters) retain the radioactivity and are sent for disposal to the LLWR.
It is currently assumed that the PST can be processed as LLW and will be acceptable for disposal at the LLWR.
For more information, or to comment on the consultation please visit http://www.hse.gov.uk/consult/condocs/rosyth-royal-dockyard/index.htm