Two decades after the world's worst nuclear disaster struck the Chernobyl nuclear reactor in the then Soviet republic of Ukraine, efforts to clean up its toxic legacy are gathering pace.
Engineers have produced outline designs for a massive, $1bn (£600M) prefabricated aircraft hanger style building which will enclose the crumbling containment structure erected over the reactor.
The structure, known as the sarcophagus, was hastily erected in the nine months following the disaster to contain radiation leaking from the damaged reactor.
It is now at the end of its 20 year design life and needs to be demolished safely while allowing the damaged reactor to be decommissioned.
Its replacement, known as the New Safe Confinement has been designed as an arch roofed enclosure that will cover the damaged reactor.
'There is an important difference between connement and containment, ' says European Bank for Reconstruction & Development (EBRD) technical manager Ian Heriot. The EBRD is administrating Chernobyl's new Shelter Implementation Plan on behalf of the G7 group of industrialised nations and other donors to the project.
'A containment shelter would be a rugged structure that could completely enclose the hazard, protecting it from any outside interference and preventing any leakage, ' says Heriot.
'A con nement shelter separates the hazard, offers protection from weather and attempts to limit leakage but is not indestructible, it could not, for instance, withstand an aeroplane impact, ' he adds.
Detailed design, procurement and construction of the shelter is currently out to tender so its al appearance is not yet known. But designs will have to conform to the 2004 concept produced by Bechtel, EDF, Battelle and subcontractor KCK.
Heriot claims that the arched roof gives the best cost to space ratio providing a large, exible working area underneath.
The arch will have an internal height of 92.5m, an internal span of 245m and an external span of 270m. Its 150m length will comprise 12 bays and 13 arch frames at 12.5m centres.
Heriot says the walls at either end of the roof structure will be built around the power plant buildings which surround the damaged Unit 4 reactor. But it will not be supported by them.
Walls at each end are instead likely to be buttressed for additional strength.
'An important part of the conceptual design was the assembly of the structure. Radiation levels mean that working above the reactor is hazardous and work times limited, ' says Heriot.
'The plan is to fully assemble the arch, kit out all the infrastructure a short distance from Unit 4 and then drag the whole thing over.
'This means workers won't be exposed to unacceptable radiation levels and they will have a head start to get out should the old shelter collapse.' Large sections of the arch will be fabricated off site and transported to an assembly area west of Unit 4, where sections will be individually erected and connected to form an arch bay.
The larger buildings surrounding Unit 4's eastern side will form part of the eastern wall of the arch structure. Gaps between the arch and the building will be in lled with preinstalled steel sheeting.
Roof arch units will be assembled west of the power plant and mounted on wheels or rollers which will run on rails to allow the completed structure to be moved across the power plant buildings and into position, probably using strand jacks.
All the infrastructure, electrics, plumbing, lighting and ventilation systems required for the decommissioning of Unit 4 will be installed during the erection of the arch bays. Gantry cranes capable of reaching every point within the enclosed area will also be tted.
Massive piled foundations will support the roof arch footings and the 330m of rails which will carry wheels or rollers under the arch base.
Piling will be complicated by the fact that there is a layer of radioactive material just below the topsoil, which was dumped on the site to make safe waste scattered by the 1986 explosion.
'The launch system was derived from incremental bridge launching systems. It's well established technology and should take little more than 48 hours, ' says Heriot.
When in its final position the western wall will be constructed and buttressed, before the nished structure is handed to the Ukrainian authorities for decommissioning.
Strategies for decommissioning, waste removal and for the eventual demolition of the arch itself at the end of its 100 year design life are all in Ukrainian hands.
Heriot says work on the site is currently focused on improving and maintaining the Sarcophagus to prevent a collapse.
Work is also underway to install the necessary infrastructure for arch construction, improving radiological, structural and seismic monitoring and bee ng up site security to prevent the continuing theft of tools and, on occasion, radioactive waste.
'I imagine that in decades to come, the site will be levelled with a small mound covering the reactor and a plaque on top saying: 'do not dig'' says Heriot.
The project is currently out to tender and the successful bidder is expected to be announced by the end of the year.
The winner will have just five years to engineer, procure and construct the enclosure.