Small space, big problem

The Dounreay fast reactor complex in northern Scotland is being decommissioned but a problematic seized mechanism in the site's fast reactor reprocessing plant Active Filter Change Facility (AFCF) had reduced ventilation capacity since 2009. During a filter change operation workers found the mechanism on one of the filters had seized and they were unable to change it.

Decommissioning of the D1206 plant was being held up by the air filtration problem but access to the AFCF is difficult and the area is subject to heavy radiological contamination after decades of operations in the reprocessing plant.

Many of Dounreay's facilities were built and operated without decommissioning in mind, which has posed numerous problems to staff now dismantling the old, experimental complex.

History

The design of D1206 was drawn up in the 1950s to address the need to reprocess irradiated fast breeder reactor fuel onsite with the aim to alleviate the issues associated with transporting spent fuel across the UK. Construction began in 1957 and installation of the actual plant systems in 1958.

D1206 was originally built to reprocess Dounreay Fast Reactor fuel but was refurbished to carry out dissolution and reprocessing of MOX fuel for the Prototype Fast Reactor. The method used was dissolution in nitric acid followed by solvent extraction.

Active commissioning of the plant began in 1959. It was operational until 1997, at which point it was deemed to be in care and maintenance. Limited post-operational clean out began almost a decade later in 2006.

Purpose

The main purpose of the D1206 active extract ventilation system is to stop the spread of contamination by providing a constant negative pressure across all the containments. When access ports are opened to deposit and retrieve items or equipment, the ventilation ensures that air will flow in to the containment, preventing contamination from leaving.

The system was the only one affected by reduced ventilation capacity as most buildings in the site's Fuel Cycle Area (FCA) have self- sufficient dedicated ventilation systems.

The AFCF filters the extract to ensure contamination is not discharged to the atmosphere. When the plant was operational, the other function of the vents was to extract the aerosol by-products of the chemical processes from the various vessels. The vessel extract used to be sparged, or flushed, when the plant was operational to stop active liquors from entering ducts.

The current primary function of the ventilation is to prevent contamination releases to general plant areas when containment is deliberately breached in a controlled manner to provide access for decommissioning work.

Depression gauges are used to ensure the ventilation system is working properly. Flow measuring instrumentation is not fitted on individual ventilation branches and the combined D1206 active extract is the only one with a flow meter. The total volumetric flow rate for the active extract is about two cubic metres per second at all times.

Contamination

The AFCF is heavily contaminated and the dose rate in the filter chamber is estimated to be several hundred millisieverts per hour. A hole in the wall was made to provide access to filter clamps and the dose rate coming from that was one millisievert per hour.

The design of the AFCF is such that when its filter-changing robot rotates the filters from a horizontal to a vertical position, in order for them to fit a waste container, contamination falls onto the floor of the chamber. The filters sit behind a mobile shield wall during normal operations. The robot grasps a filter and rotates it from a horizontal to vertical position, then taking it to a maintenance bay that has an overhead flasking port.

The robot had to be serviced in 2009 because the filter-grasping mechanism had loosened, raising the risk that a filter would be dropped. At the time, unsuccessful attempts were made to service the robot. This aspect of the work was carried out as part of the AFCF repair package.

The maintenance regime is such that filters should be changed if they fail a DOP test, which tests integrity every ten years, unless a compelling argument can be made that the plant does not warrant it. The filters have previously been changed when the plant was still operational.

Following that, the fourth secondary filter could not be changed because the clamps were seized in place. None of the other filters were changed while the robot was out of commission.

Layout

The D1206 active extract system is made up of five main ventilation branches, each one fitted with a vortex amplifier, which ensures that depressions are maintained during losses of containment.

The glovebox branch originally served about 20 gloveboxes, but close to half have been decommissioned. Most were directly connected to the main glovebox vent which becomes progressively wider the closer it gets to the AFCF. Air enters the system through bleed-in filters with one fitted on every glovebox.

The two caves in D1206 have interconnected voids but there are two separate branch extracts for each one. The posting ports and a HEPA-filtered inlet used to be the primary sources of air. The damper for the HEPA-filtered inlet is now closed so there is no routine ingress of air.

The fourth main branch is dedicated to eight process cells as well as several ancillary extensions. The cells are connected to the branch in the same way as the gloveboxes. However, most of them are also interconnected like the caves.

Air intake is limited and most of it comes through a bleed-in filter located on a centrifugal contactor facility, interconnected with a medium-active cell.

The fifth branch is dedicated to extracting from the various vessels.

The glovebox extract feeds into the first AFCF primary filter. The cells, the caves, and the vessels all have dedicated ducts. They join a common duct before connecting to a manifold which feeds the second, third, and fourth AFCF primary filters. The AFCF has a common void where all the extracts mix before going through four secondary filters.

Seized mechanism

The filters are kept in place against ducting using rotating clamps, mounted on cams. As the cams rotate the clamps catch the filter from underneath and pull it up towards the mating face. A 'bush' separator sits between the stainless steel cam and stainless steel shaft, used to drive the rotation of the assembly.

In the past a mild steel bush was used contrary to design specifications. When it seized in 2009, too much torque was applied to the shaft which shattered a split pin in the flexible connection to the external face. The fault was isolated to the mechanism which keeps the filters in place.

Approach

The repair team decided that due to the high radiation and contamination in the area the repair inside the heavily shielded chamber would have to be carried out semi-remotely. The team used a mock-up of the project to create and trial the tools needed for the repair, ensuring workers received a minimal dose.

Before work in the radioactive filter cell could start the repair area had to be opened. A redundant dissolver cell control panel and office were removed. In addition a section of a dividing wall between the cell and the inactive feed area was also removed to improve access.

Using techniques developed and proved on the mock-up, the team cored a hole into the cell shielding and identified the cause of the problem - the use of the mild steel component and too much torque when it corroded and seized.

The problem was replicated on the mock- up and staff tested ways to repair it, before completing the work in the radioactive area using the now proven method and equipment.

The filter clamp repairs took a year to get from the concept design stage to implementation. Three engineers worked alongside a fitter to develop the methodology. Estimated cost of the AFCF project is about £20,000 which includes the materials and machining costs. The new filters are like-for-like replacements.

The alternative was constructing a bypass and a new filter change facility, estimated to cost about £10 million and could have delayed the decommissioning programme.

Successful repair of the AFCF means that staff can proceed with the D1206 programme of cell clean-up work with the ventilation system working at a greater capacity.

Gordon Tait, senior project manager, said: "This is another excellent demonstration of the benefits of mock-ups in decommissioning and also highlights the engineering talent and teamwork existing within the DSRL workforce.

“There are no processes currently taking place in the areas affected by the reduced capacity. This is why AFCF issues were not addressed when they arose in 2009."

However, he continued: "The reduced capacity severely limited the ability to carry out decommissioning activities in cells and caves due to the possibility of blinding the filters. This is a credible scenario which could occur due to the agitation of contamination during decommissioning operations. These activities can now proceed."

Mr Tait outlined some of the knowledge gained from the AFCF work, saying: "As many nuclear sites are reaching the end of their lifespan, many difficult situations will develop where the courses of action are severely limited by the historical design. Lateral thinking is vital when tackling these challenges. The methodology should also be robust.

“The AFCF work showed that assumptions based on historical data are not reliable. It is important to quickly adapt in response to the
multiple unknowns at an inherited plant.

"Remote tooling needs to be utilised to its full potential because a keyhole surgery methodology is very adaptable. It allows for work to be safely suspended while newly discovered challenges are resolved."

Steve Beckitt, FCA project director said: "This is an excellent example of the use of mock-ups and innovation. Its success has resulted in significant savings of over £10 million."

Ventilation

There are about 50 facilities at Dounreay where airflows are partly, or wholly, ventilated to prevent unnecessary exposure of the workforce. Some buildings have their own ventilation stacks while others, such as the older plants in the FCA, share a common network of air ventilation systems and stacks.

Dounreay's operators found several years ago that the existing ventilation system in the FCA was not fit for purpose or up to modern standards.

Due to its construction in the 1950s, it had not been built as the integrated, controlled and properly alarmed system that would be currently acceptable. Many decades of operations on radioactive materials had also affected its ability to perform its intended function.

Ventilation systems aim to prevent dispersion of airborne radioactive substances within the plant, as well as controlling releases of radioactive material to the environment so they are below authorised discharge limits. In Dounreay's case these limits are set by the Scottish Environment Protection Agency.

Key improvements have been made as decommissioning progresses, including replacement of the old main 55m high FCA stack with a new ventilation system comprising two 30m high stacks. More than 300 tonnes of steel was used to create a network of ducts and chambers that formed the new ventilation system, needed to decommission more than a dozen of the most hazardous facilities at the site.

The experimental nature of many of Dounreay's redundant facilities, such as the reprocessing plants, reactors and labs, means the decommissioning of the site requires innovation as well as a measured and proven approach.

Site closure involves: cleaning out and demolishing redundant nuclear and non- nuclear facilities; segregating and packaging the radioactive and non-radioactive wastes for long term storage or disposal; and removing nuclear fuels and other nuclear waste.

Site operator DSRL is a wholly owned subsidiary of the Cavendish Dounreay Partnership Ltd, a consortium of Cavendish Nuclear, CH2M and AECOM. It is funded by the UK's Nuclear Decommissioning Authority to deliver the site closure programme.