An earlier article (Ref.1) in this journal discussed mainly the management of UK ILW in surface facilities; the disposal of vitrified HLW in a hillside was only briefly mentioned. This article expands the latter concept, emphasising an engineering approach.

Background

Some thirty years ago, a group of engineers and technologists at Risley, UK, responsible for the design of radioactive plants, began a feasibility and order of cost study of the storage and underground burial of vitrified HLW. This was published in 1980 in Ref. 2. A synopsis of the conclusions is set out below together with some more recent observations.

A. Two main types of disposal were identified. A ‘wet’ repository implied location of waste containers below the water table with long pathways of any activity in the water back to man as in current UK studies. On other hand, a ‘dry’ (hillside) repository, the concept preferred in Ref. 2, set out to inhibit water access to the canisters by drainage of the host rock. (In the UK, there are few suitable natural dry locations). Section 11.2 (Ref. 2) discusses the integrity of this system. Any groundwater naturally flowing through undrained rock is very highly filtered in doing so; artificial extra drainage would not block but merely remove some of the resistance of flow to the biosphere. A variation, preferred for coastal or other saline groundwater sites, is to direct any slight flow near the waste down to a sump, from where large boreholes (200mm) would extend to deep saline permeable strata.

B. The preferred option for storage of containers (say for 100 years or so) before transfer to a repository was a surface store with natural air cooling. Heavy shielding of containers (300 mms of malleable iron) was suggested, allowing ‘hands on’ manipulation in tunnels and open field storage. The extra cost of the thick shielding was offset by savings in storage and disposal operations.

C. The main drainage for a ‘dry’ system is by the access tunnels which slope upwards gently towards load zones; any seepage of activity would then be safely directed into the drains; (in a ‘wet’ repository, activated water would be dispersed randomly, rendering remedial action impracticable). Drainage from the load tunnels could be mainly of two types (other schemes may eventually be preferred incorporating features of either type). The first is where the bottoms of load tunnels are used to allow downflow of any in-leakage towards the junction with the main access tunnels. Small particle material such as sharp sand or gravel and perhaps containing absorbers for transuranics and/or sacrificial iron turnings would fill the voids. Deflectors such as overlapping granite slabs or titanium sheet could prevent direct seepage on to waste packages underneath. In the second, auxiliary boreholes just below the load tunnels would take away groundwater to the access tunnels; backfill round the waste would be impermeable material such as bentonite. In both cases, boreholes above the load tunnels could lead away most of the (inactive) groundwater to weathered zones to seep away to the surface of the hillside; any water pressure towards the load tunnels would thereby be greatly reduced.

D. There are several safety features against activity migration with either form of drainage – the backfill, the long corrosion resistance of the thick shielding (particularly as little air would be brought in by any water reaching the load tunnels), the actinide absorbers (possibly also in the access tunnels, mixed in gravel filling) and the absorption in the permeable deep strata of any traces of activity passing the above barriers. There could be inspection for a long demonstration period in realistic closure conditions.

Conclusions

There is promise in the above concept, which is better than ‘wet’ systems in safety, cost and simplicity to merit further study. Because of the difference from naturally ‘dry’ systems, however, such as salt mines or Yucca Mountain (now abandoned), it is perhaps more appropriate to rename it a ‘drained’ system. There is sufficient similarity to the original UKAEA concept to use this as a basis for updating and to include Spent Fuel disposal. The main novelty would be the enclosure of Spent Fuel in thick shielding. Revising the costing should be straightforward. Siting of a hillside repository should be easy, since its requirements are not very restrictive and the site could be chosen for other reasons, such as ease of transport. It would seem worthwhile, for example, to drill a few boreholes in Black Combe near Sellafield and use realistic ground data for the assessment. Looking at other locations need only be necessary if Black Combe held some unusually bad characteristic. An assessment of the relative hazards of surface burial of non-active toxic wastes versus radioactive wastes should be followed up as recommended in Ref.1 to establish how much radioactive waste need go to a hillside location rather than the cheaper surface alternative.

Current UK disposal research has contributed little to world knowhow that is original and relies heavily on that of overseas countries such as Sweden. Overall, the above ‘drained’ and surface disposal concepts offer solutions to radwaste disposal well known to the engineering industry and are much cheaper and more practical and so should replace current work on ‘wet’ repository systems, particularly in the present UK economic climate. Moreover, there are so many possible safety features that some operations might well be omitted e.g. vitrification could be replaced by direct cementation of High Active effluent.

References

1. W. R. Burton, Store ILW as Toxic Waste, Nucl, Eng. Int. (October 2009).

2. W. R. Burton and J. R. Griffin, Long-Term Storage and Disposal Systems for Highly Active Waste, ND-R-514(R)(1980).


Author Info:

Bob Burton is author of Nuclear power, pollution and politics (1990, ISBN: 978-0-415-03065-6)

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Date: Friday, 16 July 2010
Original article: neimagazine.com/news/newsengineering-is-the-key-for-nuclear-waste-disposal