J B Campbell
BNFL

Building B203 is located on the British Nuclear Fuels Plc (BNFL) Sellafield site in England and operated between 1954 and 1987 to purify plutonium nitrate and recover plutonium residues. The plant consists of approximately 7,000 metres of stainless steel process pipework and 200 stainless steel vessels, housed within four storey brick built cells within an overbuilding.

Due to plant modifications within the cells, leaks had occurred from the stainless steel primary containment into the cells and as a result, the cells had become highly contaminated. The cells were served by an unfiltered extract system and, despite no operations being carried out within the plant, the plant was contributing approximately 60% of the sites airborne alpha discharges by 1993, due to gradual drying out of the cell.

To allow decommissioning to proceed, it was necessary to firstly install a new modern filtered cell extract to reduce the risk presented from the plant. Ancillary equipment, external to the cells was then removed to provide adequate free space to allow installation of size reduction and waste handling facilities to support in cell dismantling. These include a plasma arc cutting station, waste monitoring and packaging equipment and pressurised suit access facilities.

Decommissioning of the cells is currently underway, with vessels and pipework being removed from the cells, size reduced and exported in 200 litre waste drums. Removal of vessels is being carried out in a strictly criticality safe sequence.

Located on the British Nuclear Fuels Plc (BNFL) Sellafield site in England, Building B203 commenced operation in 1954 to purify plutonium nitrate from the primary separation plant, prior to finishing in downstream plutonium finishing plants. B203 was constructed adjacent to the B204 primary separation plant (Ref. 1), allowing direct pipeline feed of Pu nitrate feedstock from B204 into B203. Plutonium purification within B203 was carried out using a continuous extraction process, based on pulsed columns, housed within brick built cells.

In 1964, B204 was replaced by a new primary separation plant, which incorporated a plutonium purification cycle. At this time B203 was no longer required to carry out plutonium purification, but continued to operate for recovery of miscellaneous plutonium residues.

By 1987 it was recognised that B203 was no longer suitable for continued operation and was shutdown.

Modifications to the plant over its operating life had led to some leakage from the primary containment into the in cell areas, leading to cell internals becoming highly contaminated. Through the gradual drying out of the cell and the unfiltered cell ventilation system, alpha discharges from the plant were a significant proportion of the total site aerial discharge and contamination was also penetrating the cell walls to a significant depth. Furthermore, continued care and maintenance costs for the plant were becoming substantial. The decision was therefore taken to decommission the plant at an early stage.

The scope of the decommissioning project is to upgrade the plant ventilation systems, remove and dispose of all redundant in cell and out cell plant and equipment and to ultimately demolish the building.

The ventilation systems has been installed and commissioned, redundant out cell plant and equipment has been removed and disposed of, new plant and equipment has been installed and commissioned to support in cell dismantling and in cell dismantling is underway.

B203 comprises two similar adjacent brick built cells (north and south) surrounded on three sides by an operating annulus, all of which is housed within a steel framed main building. The main building measures approximately 22 m by 17 m on plan and rises four stories to a height of approximately 18 m. There is also a penthouse measuring approximately 12 m by 11 m rising a further 6 m above the roof of the main building.

The cells are formed from brick panels, built within a structural steel frame, with the brick panels varying in thickness from 225 mm to 355 mm. The west wall of the cells also forms the external wall of the building. The roof over the cells is formed from pre cast concrete slabs, while the roof over the remainder of the building, including the penthouse, is formed from profiled steel decking.

The cells housed the process plant, which was used for plutonium purification and residue recovery, consisting of approximately 200 stainless steel vessels and 7,000 m of stainless steel process pipework.

Access to the cells was via a pressurised suit entry facility and shower at first floor level. The pressurised suit access facility provides direct access into the south cell; access to the north cell being from the south cell.

The annulus areas housed gloveboxes associated with sampling of in cell process streams and preparatory stages of the plutonium residue recovery process, process pipework, workshops, offices and switch rooms. The penthouse housed gloveboxes associated with preparatory stages of the plutonium residue recovery process.

At the time of plant shutdown, the ventilation system consisted of a filtered vessel extract system and unfiltered cell extract system. The original cell extract system was based on an extract duct from the cell routed directly to the adjacent Building B204 stack. The original vessel extract system was based on ducted extract from each vessel, into a manifold, which was discharged to the B204 stack via two stage HEPA filtration. The arrangement of the ventilation system at the time of plant shutdown is shown in Figure 1.

Operational problems led to the plant within the north cell being shutdown in 1972, with all subsequent operations being carried out in the south cell plant. There were no records of any Post Operational Clean Out (POCO) of the north cell plant having been carried out on its shutdown. Furthermore, during the period from 1972 to 1987, items of plant were recovered from the north cell to replace identical items within the south cell which had failed. The physical and radiological status of the plant in the north cell was therefore unknown on final plant shutdown in 1987.

Not all sections of the south cell plant were used in the plutonium residue recovery process. Records suggest that those sections of the plant which were used for plutonium recovery were subject to POCO on shutdown. There is no record of POCO being carried out on the remaining sections of the plant in the south cell.

Due to the numerous modifications which had been carried out on the plant over its operational life, leaks of process material had occurred from the primary stainless steel containment into the cell. This had led to levels of alpha contamination within the cells in excess of 100,000 cps and localised radiation sources in excess of 10 mSvh^-1 g . Sampling of the cell walls showed that by the time of plant shutdown, in cell contamination had penetrated the cell wall brickwork to a substantial depth.

At this time, the cells in B203 were the only plutonium cells on the Sellafield site which did not have a filtered ventilation system. B203 was therefore contributing a significant proportion of the total alpha aerial discharge from the site. To ensure discharges from B203 were minimised, entries into the cell were prohibited in 1988.

The gloveboxes in the out cell areas had been used to prepare plutonium residues prior to recovery processing within the cells and to sample in cell process streams. The gloveboxes therefore contained plutonium contaminated equipment.

Also, contamination had migrated into a number of out cell areas. These included the electrical panels and containment area surrounding the plutonium residue preparation gloveboxes on the third floor.

At the time of initiating planning for decommissioning of B203 a number of principal problems were identified, which introduced constraints on how the project could be carried out.

It was known that the plant had been modified during operation and it was suspected that the plant drawings had not been updated to reflect all modifications. The physical status of the plant on shutdown was therefore unknown. Furthermore, it was not known which sections of the plant had been the subject of POCO and the extent to which POCO had been carried out. The radiological inventory and liquor hold up within the plant was therefore unknown.

The embargo on access into the cells, due to the unfiltered cell ventilation system, meant that it was not possible to survey the cells to determine their physical and radiological status.

This lack of plant knowledge and the restrictions preventing access to better information, played a significant part in the development of the initial stages of the project strategy for the decommissioning of B203.

The overall strategy adopted to carrying out the decommissioning of B203 was based around improving plant containment and reducing risk to the environment through its existing unfiltered ventilation, followed by dismantling using manual techniques.

A phased approach was also adopted to implementation of the project strategy to provide improved ventilated containment, allowing a more detailed knowledge of the plant status to be defined. Planning for dismantling of in cell equipment could then be carried out, based on known requirements for the work. This allowed preparation for dismantling to proceed with confidence that the minimum cost approach had been adopted to allow the plant to be dismantled in a safe manner.

The scope of work to be carried out under each of the phases is outlined briefly below.

Phase 1 of the project addressed provision of a filtered ventilation extract system for the B203 cells. The scope of work undertaken in phase 1 included:

• Design studies to determine the preferred engineering solution.
• Detailed design and manufacture of the new ventilation system.
• Plant preparation to allow installation of the new system, including decontamination.
• Installation and commissioning of the new system.

Phase 2 of the project addresses removal of the redundant plant and equipment from the annulus and in cell areas and disposal of waste arising. The scope of work to be undertaken under phase 2 includes:

• Structural and radiological surveys of the plant.
• Design and engineering of waste processing, packaging and export facilities (the conditioning facility) to support in cell dismantling.
• Clearance of redundant plant and equipment from annulus areas.
• Installation and inactive commissioning of the conditioning facility to support in cell dismantling.
• Breakthrough between the conditioning facility and the south cell and active commissioning of the facility.
• Dismantling of in cell plant and equipment, disposal of waste arising and ongoing decontamination as work proceeds.
• Dismantling of the conditioning facility.

Phase 3 of the project will address final decontamination and demolition of the B203 cell and building structures.

Design studies were carried out to consider and assess alternative options for improvements to the cell ventilation system. The options considered included siting of a new ventilation plant serving the plant, located within the adjacent building B204 and upgrading the existing system. The preferred option was to provide a new system, based on new ductwork, fans and filters, with the fans and filters located within the B203 fourth floor penthouse and the exhaust discharging to the existing B204 stack. The arrangement of the preferred option is shown in Figure 2.

The fourth floor penthouse had been constructed as a modification to B203 to provide preparatory facilities to support the plutonium residues recovery process and contained two large gloveboxes and other ancillary equipment. To allow the new ventilation plant to be installed within the penthouse, the redundant gloveboxes had to be removed, along with the ancillary equipment. A number of redundant inactive process tanks also had to be removed from the fourth floor of B204 to provide a laydown area for receipt of the fans and filter units into the building.

Removal of the gloveboxes within the penthouse was carried out manually by personnel dressed in pressurised suits, within a ventilated Re-locatable Modular Containment (RMC). The ancillary equipment within the penthouse was removed first to provide space for construction of the RMC, Mobile Filtration Unit (MFU) and portable pressurised suit shower unit. The MFU included High Efficiency Particulate Air (HEPA) filters and the shower incorporated a re-circulation system to minimise liquid effluent arisings.

The gloveboxes were assayed using a Decommissioning In-Situ Pu Inventory Monitor (DISPIM), to determine their plutonium inventory. DISPIM assay showed that the plutonium inventory within the gloveboxes exceeded 450 g. To ensure criticality safety, the waste was monitored using a Plutonium Contaminated Material (PCM) piece monitor, before being placed into 200 litre drums for disposal. The piece monitor used was an early model and was only capable of monitoring individual pieces of waste. This increased the time taken to complete dismantling work.

Thick perspex shielding windows formed an operating face which surrounded the gloveboxes. The perspex panels, which were approximately 150 mm thick and measured 1.2 m by 1.5 m, were size reduced for loading into the 200 litre waste drums. All dismantling was carried out successfully using standard hand tools.

The new plant installed within the penthouse included two (duty and standby) 34,000 m³h^-1 centrifugal type extract fans and two stage (primary and secondary) HEPA filters, with three banks of filters for each stage.

Structural surveys had shown that, due to the additional loads (each fan weighed 5 te) being introduced into the building at fourth floor level, structural strengthening of the building was necessary.

Space constraints within the penthouse meant that the fans and filter banks had to be stripped down following manufacture to allow them to be imported into the penthouse where they were reassembled in their permanent location.

The exhaust duct from the fans was routed from the fourth floor of B203 to approximately 10 m above the tenth floor level of B204 via the external B204 staircase. The ductwork is approximately 1.0 m square and is of stainless steel construction in external areas, where the duct is exposed to the elements and mild steel construction in internal areas.

The exhaust duct was connected to the existing B204 stack manifold, utilising two existing redundant connections to the manifold. The existing connections had become redundant some time ago and blanking plates had been fitted. As there were no records of contamination levels within the ductwork, considerable care had to be taken when making the new connections.

Discharges from B203 were accounting for approximately 60% of aerial alpha discharges from the Sellafield site. It was therefore recognised that timely installation of the system was essential to ensure filtered extract from B203 was provided as soon as was practicable.

The new ventilation system was made active when the extract connection to the north cell was made, at third floor level, during November 1993. The new system has operated fault free since that time, with only one filter change having been carried out. However, the extent of in cell work carried out to date is limited to clearance of the ground floor of the south cell. It is anticipated that more regular filter changing will be required during in cell dismantling work.

With the new filtered ventilation system operational, personnel entry into the cells became possible to carry out visual and radiological surveys.

The radiological surveys showed levels of alpha contamination within the cells in excess of 100,000 counts per second (cps) alpha, although levels were typically less than 5,000 cps on the first, second and third floor walkways. Radiation levels within the cells were typically of the order 30 to 100 m Svh^-1 b g and 2 m Svh^-1 neutron, although hot spots were found with levels to 450 m Svh^-1 b g and 4 m Svh^-1 neutron. Additionally a hot spot was found from a stain on the cell wall giving off a dose rate of up to 20 mSvh^-1 b g , 10 mSvh^-1 g .

A campaign of vacuuming was carried out to remove loose dust and debris from the floors of the cells in an attempt to reduce levels of contamination. This work served to substantially reduce the levels of airborne activity generated within the cell during personnel entries. However, contamination levels remained in excess of 3,000 cps alpha.

During the personnel entries, video surveys were also carried out to provide additional information to support detailed planning for dismantling.

Options were considered to determine the preferred approach to dismantle the in cell plant and equipment.

The preferred solution was to construct new engineered facilities within the ground floor annulus area on the south side of the south cell. The new facilities were to include; a recirculating pressurised suit shower facility with ion exchange treatment unit; lifting and handling facilities for large plant items; a plasma arc cutting station, with a dedicated fume extraction unit; PCM piece monitor, capable of handling 200 litre drum quantities of waste with fluoride contamination; and waste export facilities. The new waste conditioning facility is shown in Figure 3.

In cell dismantling would commence at ground floor level within the south cell with dismantled items being transferred into the conditioning facility for processing and export.

Siting of the main conditioning facility within the ground floor annulus area and the plasma arc fume extract system within the first floor annulus area, meant that clearance of these out cell areas was necessary to provide the required free space.

A programme of out cell clearance work has been undertaken to remove redundant plant and equipment from the annulus areas at all floor levels. This programme of work is now largely complete. The redundant plant and equipment which has been removed has included:

• Inactive process pipework located at all levels.
• Active sample cabinets located at all floor levels.
• Inactive support frames and access platforms located at all levels.
• Active solvent pumps located at ground floor level
• Instrument cabinets located at the third floor level.
• Active gloveboxes located at the third floor level, which formed part of the plutonium residue recovery process.

Clearance of the annulus areas commenced at ground floor level followed by the first floor level to provide clear space for installation of the conditioning facility and plasma arc filtration unit. Work then continued on the second floor level, before being completed on the third floor. The clearance work commenced with clearance of the north side annulus on the ground floor to provide a temporary storage area for waste drums.

Removal of the active cabinets was carried out manually by personnel working in pressurised suits, within a RMC. The RMC utilised was that which had been procured for glovebox removal in the penthouse, which had been designed on a modular basis to allow flexibility of re-use. The RMC was also served by the portable pressurised suit shower which had been procured for penthouse clearance.

Prior to removal, the active cabinets and gloveboxes were assayed by DISPIM to determine their plutonium inventory. It was found that all sample cabinets had a plutonium inventory which was significantly less than 450 g, allowing them to be dismantled and packed directly into 200 litre drums, without the requirement for PCM piece monitoring. The filled drums were then exported for drum monitoring to provide an accurate plutonium inventory for each drum, prior to being placed in interim storage.

The decommissioning team were closely involved with the design and manufacture of the conditioning facility, to ensure previous decommissioning experience was incorporated within the design. The conditioning facility was manufactured and assembled at an off site location, with the decommissioning team continuing to be involved during the works assembly and testing. This ensured that they had become familiar with the plant and its assembly requirements before installation of the facility within B203.

The conditioning facility was installed in B203 adjacent to the south wall of the south cell, with no engineered access between the facility and the cell. This allowed the facility to be commissioned inactively before the access was formed to the highly contaminated cell.

Following inactive commissioning, an opening was formed through the south wall of the south cell into the conditioning facility. Formation of the opening was carried out from within the conditioning facility. After formation of the opening, an overhead runway system was installed between the conditioning facility and the south cell to allow large items of waste to be easily handled into the facility for size reduction.

The initial in cell dismantling task carried out was to remove plant and equipment from ground floor level of the south cell. This task was used to carry out active commissioning of the conditioning facility and to prove the adequacy of the decommissioning safety case.

Active commissioning has been completed with all systems operating in accordance with design expectations. Dismantling works is currently continuing at first floor level within the south cell. The sequence of dismantling plant items is predominantly driven by ensuring that criticality safety is maintained at all times.

It is estimated that the dose uptake to personnel during in cell dismantling will be 419 man mSv. With the limited dismantling experience to date, a review of the accuracy of that estimate, based on actual dose uptake, has not yet been possible.

It has been estimated that the dismantling work carried out under phases 1 and 2 will generate approximately 240 m³ of PCM waste and approximately 460 m³ of Low Level Waste (LLW).

On completion of in cell dismantling work, phase 3 of the project will be undertaken in the following manner:

• Remaining redundant plant and equipment will be removed from the out cell areas.
• The redundant and operational B203 ventilation plant located in B203 and B204 will be removed.
• The electrical distribution system, which is contaminated, will be removed.
• The cells will be demolished.
• The building superstructure may also be demolished.

Although demolition of the cells is included within phase 3, the plant may be maintained under an extended period of care and maintenance before demolition is carried out. A study is currently being carried out to determine the preferred overall approach to phase 3 which offers the most cost effective solution, while ensuring the risk to the public is maintained as low as is practicable.

When the preferred approach has been identified, detailed planning for phase 3 will be carried out.

The project is currently in its main in cell dismantling phase of work. All work which has been carried out to date has been completed successfully, within programme and financial targets. Installation of the filtered cell extract system has succeeded in reducing the Sellafield site alpha aerial discharges by a substantial proportion.

In the work which has been carried out to date, lessons have been learned and reinforced which are appropriate to decommissioning projects generically and specifically to projects similar to decommissioning the B203 plutonium purification and residue recovery plant.

The problems and complexity of decommissioning can be significantly reduced if plant POCO has been carried out at the end of its operational life, while the plant is configured to carry out the task in the most cost effective manner. This is particularly important when decommissioning is delayed by an extended period after completion of operations.

Ensuring that the status of the plant is fully and accurately documented on completion of operation, including POCO, allows detailed planning for decommissioning to select the most cost effective approach, with greater confidence of achieving success.

The project is demonstrating that even those plants which present significant radiological problems can be cost effectively decommissioned, without the need to deploy highly sophisticated equipment.

1. S F Challinor, BNFL, WM&D, "Operational experience of decommissioning the first Primary Separation Plant at Sellafield U.K." WM97, March 5 1997, Tucson, Arizona.

© British Nuclear Fuels plc

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