Store ILW as toxic waste

27 October 2009

The oceans have been formed from countless storms and the flow of vast quantities of water and sludge. The Amazon alone pours out billions of tonnes of chemicals every year, including carcinogens such as beryllium, cadmium, nickel and uranium. The inventory of arsenic in the world’s waters, in particular, is several billion tonnes and this is a worse carcinogen in water than plutonium. Natural uranium and radium in the seas amount to around 5 billion and 34 million tonnes respectively. On land, in the top 500 metres of UK rocks there are nearly a billion tonnes of uranium and 300 tonnes of radium, which are being steadily leached into the sea. In coal-fired power, wastes have been casually spread in enormous heaps from mine spoil and coal refining; ‘fly’ ash from burning coal (containing uranium and its decay products) is collected in lagoons and made into blocks for building construction. Radioactive waste would therefore be an insignificant extra hazard if dispersed in the sea or on the land surface. However, international opposition on political grounds would probably cause a long delay before there was general agreement on such direct dumping.

Attention in the past has been directed towards disposal by deep burial of nuclear waste inland, but there is no method of remedial action after backfilling once any activity has begun to spread. Recent predicted costs of this system have been astronomical (GBP70 billion). Furthermore, the large excavations required (at unspecified sites) would demand much energy and high carbon emissions.

To date, double standards have been used in disposing radioactive and non-radioactive (infinite half-life) toxic wastes. However, in the last few years, EU directives on sealing up non-radioactive toxic waste sites have indicated that the end objective should be to reduce leakage to surface and potable waters as far as possible. To make the criteria consistent for all types of wastes (except spent fuel), it should be sufficient therefore to seal up nuclear waste sites after a few hundred years, when they have decayed to a similar level of hazard as toxic waste sites. Since the radioactive waste would then continue to decay, the site would become safer than non-active sites. This timescale raises the possibility of building artefacts for dry storage to last at least until the time of sealing, in particular, above ground at Sellafield.

Nearly 30 years ago, the UKAEA published a comparison of designs for burying vitrified high active waste blocks in the UK, one of which involved ‘dry’ disposal in a hillside, with groundwater being drained to sea without contacting activity. A possible site for this design is under Black Combe, a short distance south from the main source of the waste, Sellafield.

“Radioactive waste would be an insignificant extra hazard if dispersed in the sea or on the land surface.”

A typical artefact, suitable for intermediate-life waste (the preponderance by volume), could be constructed of concrete with waste in stainless steel cans stored retrievably in ‘dry’ cells, so that withdrawal to alternative storage or disposal was always feasible. Each can would be capped with an overlapping dome, allowing gases to vent downward from under the edges. The whole setup could be sealed and vented but kept under nitrogen at slightly above atmospheric pressure and monitored to keep out oxygen and water vapour. (Oxygen- and water-getters might be useful here). In this environment, corrosion of the cans would be negligible (perhaps even if made of mild steel) and retrieval or shuffling would then be always feasible. In a later phase, voids could be filled with sharp sand or gravel, with drainage through the floor between cells, so that a catastrophic collapse or accumulation of water would not arise. Even at this stage, the backfill could easily be removed by (say) vacuuming it out. After an accepted demonstration period, the main entrance could be filled inter alia with large blocks or concrete; the residual waste would be low-activity and tamperproof, requiring long operations with heavy lifting equipment to regain access. The concrete roof would be waterproofed with (say) clay or bentonite, then soil on top would enable landscaping to be arranged; the whole would appear similar to inland toxic waste sites and could be known for convenience as a low fell.

Around this, a set of boreholes near the artefact could connect to saline water in permeable strata below. Any drainage from the artefact would then become saline, thus fulfilling EU requirements concerning groundwater. Another variation could be a small island, with a convenient attached harbour, built offshore on rocks to allow seawater to flow under surface waste buildings. In each of the above variations, with suitable heat removal during the monitoring phase, high active waste might be stored therein, rendering a hillside facility unnecessary or small; existing structures might be suitably modified for dry storage. Early sealing up (less than 100 years) should be possible for short-lived wastes.

A hillside facility could have similar internals in tunnels or caverns with mud and/or bentonite between concrete and rock. Boreholes in the rock above and in the floor below to permeable rocks in tidal contact with the sea could provide bypass routes to sea for groundwater. Little erosion would take place after sealing before activity levels became extremely small.

A comparison of designs and ground tests should begin urgently, related to locations in and around Sellafield. In fact, experiments on borehole connections to saline water and sealing up some existing waste buildings under slight pressure and venting could be arranged straightaway. This should begin to resolve the largely synthetic problem of existing nuclear waste disposal.

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By Bob Burton

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