Safety_Series_015_1965 - gnssn - International Atomic Energy ...

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This publication is not longer validPlease see http://www-ns.iaea.org/standards/the stress. Owing to this plastic behaviour it is essentially impermeablesince any cracks that may develop tend to be self-healing.Another attractive feature of rock salt is that its thermal conductivityis higher than that of most rocks, so it is better suited to dissipateheat generated by radioactive waste. Because of its chemical andphysical homogeneity it is dissimilar from all other soils althoughthe bedded deposits frequently contain interbeds of shale with a higherpermeability; it is nevertheless considered essentially as a storagemedium.In investigating its suitability for high-level liquid storage, experimentswere conducted in which solutions, chemically similarto the waste, were introduced into 16 m3 pits in the floor of a saltcavern (Fig. 8a) [39]. Electrical heating was installed to simulatethat of the actual waste, causing the solution temperature to increaseover a period of one month until equilibrium was achieved (50°C)(Fig. 8b). From these results the in-situ thermal conductivity anddiffusibility were measured and found to agree, within 10-20%, withlaboratory results on single crystals.A complication with intensely radioactive aqueous solutionsarises from their instability under high radiation. Aqueous solutionsare radiolytically decomposed, producing a hazardous mixture ofhydrogen and oxygen, and pressures of several atmospheres maydevelop. It is probably not feasible to expect salt formations to containsuch pressures for long periods. Laboratory irradiation experimentshave shown that radiolytic stability may be assured fordoses below 109 rad if the waste solution is introduced into granulesof crushed salt and allowed to permeate the interstices without forminga free surface above. For doses approaching 109 rad a liquidphase forms over the salt accompanied by a build-up of pressure.The formation of vapour and gases may also cause a hazard to storage.A serious problem is caused by the interaction between the nitratesof the hot radioactive solution and the chloride in the salt, whichtogether form a highly corrosive condition in the vapour space above.Acid waste produces NOC1, C 02 and oxides of nitrogen at temperaturesabove 50°C but this effect may be avoided if the acidity is r e duced to below 4M and the temperature maintained below 60°C.In liquid storage, isolation is best assured by introducing thesolution into specially prepared chambers in the salt, but at presentnumerous difficulties remain. In addition to the corrosion and offgasproduction, the self-heating problem is assuming greater im portance because it has increased by at least an order of magnitude28
This publication is not longer validPlease see http://www-ns.iaea.org/standards/Fig. 8aExperimental disposals, below pyramid shaped covers,in Carey Salt M ine, Hutchison, Kansas, USA.Average temperature rise in waste.since the Oak Ridge experiments, as a result of higher fuel burn-up.It is believed that the combination of self-heating and the density differencebetween the solution and the salt could result in an upward29
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