United Kingdom Nirex Limited aim to develop a single mined repository for the safe disposal of low- and intermediate-level radioactive waste The Companv is exploring a number of repository concepts, including an offshore facility. This chapter describes the results of design work which has addressed the feasibility, constructability, operational safety and costs associated with developing a subseabed repository accessed from a man-made structure.
The objective of UK Nirex Ltd is to develop a single mined repository for the safe disposal of low- and intermediate-level radioactive waste. In doing so the company will have due regard for the environmental impact and cost of such a development Accordingly the current programme of work being undertaken by Nirex embraces
the assessment of long-term radiological impact,
the assessment of the non-nuclear environmental impact,
the design of waste packaging and transport systems,
assessment of the costs and constraints associated with repository Construction
Nirex has explored three basic concepts for a deep disposal facility
a land-based repository,
a subseabed repository excavated from a coastal setting,
a subseabed repository excavated from an offshore structure
This chapter describes the third concept and it presents the conclusions of design work undertaken on behalf of Nirex by Sir Robert McAlpine and Sons Ltd in association with NEI International Research and Development Ltd and James Williamsons Associates This has addressed the feasibility, constructability, operation safety and cost of a repository constructed beneath the seabed accessed from a man-made structure. Reference sites and reference waste packages have been assumed for the purpose of this work
Two methods of constructing a repository have been considered.
a network of caverns excavated at depth, accessed by shafts sunk from structures founded on the seabed, and,
a series of large diameter, deep shafts (referred to as boreholes) excavated from an artificial island
The exploitation of the hydrocarbon reserves in the North Sea in the 1960s and 1970s required the development of structures for a variety of locations, water depths, sea conditions and functions. The procedures which have been developed for their design have been adopted as offshore industry standards, world-wide, and have applied to ever more demanding locational circumstances.
Two offshore platforms would be required for a cavern repository. One would be dedicated to waste disposal operations, and the other to all non-nuclear activities.
The offshore platforms will have a service life of at least 50 years. They must be able to withstand environmental forces, ship collision, fire and explosion and provide security against malicious interference. Critical parts of the structures may also need to be seismically qualified The structures must support decks to give stable working platforms at a level effectively clear of wave action, and carry facilities for constructing and operating the repository. These facilities include construction plant, headtowers and winders, handling equipment for spoil and backfilling materials, radioactive waste receipt, storage and handling facilities, ventilation intake and exhaust equipment, power supply, offices, workshops; stores and accommodation.
