Burying The Problem: The Long Term And Nuclear Waste

ANothing else made by industry has to be thought about on the timescale that spent nuclear fuel demands. A fuel assembly withdrawn from a reactor is thermally hot for decades and radiologically hazardous for something closer to a hundred thousand years, while the dry steel casks in which most countries now keep such fuel above ground are licensed, at best, for sixty. Almost every national programme has converged on the same outline of an answer - a repository four to five hundred metres down in rock that has been quiet for a very long time, the fuel sealed in metal canisters, each canister packed round with compacted clay - and the outline is not what specialists argue about. They argue about which of those barriers is actually carrying the safety case, and about how much weight the numbers can bear, since every one of them has been obtained from an experiment lasting years and is then asked to describe a hundred millennia.

BGreta Bianchi, a corrosion chemist at the Fjordane Institute of Materials, has spent her career arguing that the answer is not the canister. Between 1997 and 2019 her group placed 240 copper coupons in boreholes in a test tunnel 420 metres below ground and recovered them in batches, weighing each one to establish how much metal had gone. The median loss was 0.4 micrometres a year, a figure from which the arithmetic is easy and comforting: a canister wall of fifty millimetres would survive well beyond the period the regulator asks about. Bianchi's point is that the median is the wrong statistic. In eleven coupons, every one of them recovered from a borehole where the surrounding clay had been colonised by sulphide-producing bacteria, the metal had not been removed evenly across the surface but concentrated in pits some thirty times deeper than the average loss. A canister does not fail on average; it fails at its worst point.

CThe account yields a prediction that can be tested, and Bianchi has tested it. If the pitting is the work of microbes living in the buffer, then a buffer hostile to microbes should show none of it. In a second series, coupons were surrounded by bentonite compacted to different densities and left for four years. Where the clay reached a dry density above 1,550 kilograms per cubic metre, no coupon pitted at all; below 1,450, nine of twenty did. Pitting was confined to the cases in which the clay had been emplaced at the lower densities, and the boundary between the two regimes was sharp rather than gradual. Bianchi presents the finding as a constraint on how a repository must be built rather than as an objection to building one, and observes that the densities which suppressed the bacteria are demanding to achieve uniformly at depth.

DWilhelm Chaudhary, a hydrogeologist at the Val Rendena Underground Laboratory, does not dispute the pits but rejects what is made of them, on the ground that a design resting on several barriers cannot be judged by testing one of them to destruction. In her tracer experiments, water carrying dissolved caesium was injected into fractured granite and followed to a collection borehole; the caesium moved so slowly, being repeatedly caught and released by mineral surfaces along the way, that the modelled travel time to the surface came out at about 40,000 years. Feeding that retardation into a model of the whole system, Chaudhary found that even if every canister were assumed to be breached at 2,000 years, the peak dose at the surface reached only 0.9 per cent of the regulatory limit. She concedes two things. Her tracer sorbs strongly, whereas the isotope that dominates the calculated dose, iodine-129, barely sorbs at all; and her retardation figures come from one rock type at one site.

EThe disagreement has not stayed in the seminar room. Statutes passed in 2019 in several countries require that the waste remain retrievable for a hundred years after emplacement, and the municipalities asked to host repositories have made their consent conditional on that undertaking. Bianchi points out, uncomfortably for her own position, that nothing in her pit measurements bears on whether a repository as a whole would release anything, since a breached canister and a safe repository are not alternatives. Chaudhary replies that the two cannot be kept apart in practice, because a hearing shown a corroded specimen will read it as a failed system whatever the modelling says, and because retrievability itself keeps a repository open, and therefore oxidising, through the very period in which the canister is doing most of the work. Both regret the habit of advertising a lifetime for a single component rather than a performance for the whole.

FSomething like a settlement is emerging in which the two accounts are given different periods rather than ranked against each other. For the first few thousand years, while the fuel is hot and its radiotoxicity at its highest, the metal is what stands between the waste and the water, and how the clay is emplaced around it matters more than anything in the rock. Afterwards, when no one can any longer promise that the canisters are intact, the safety case rests on how slowly the rock and the clay allow dissolved material to move. What nobody can answer is whether sorption measured over four years still describes rock that has been loaded and unloaded by ice sheets several times, its groundwater chemistry driven back and forth; no laboratory method now available can settle it. What both researchers reject is the framing that produced the quarrel, namely that the problem must be either a container problem or a geology problem.