Recent proposals have suggested that radioactive wastes can be isolated 1 as dilute solid solutions in a crystalline, titanate assemblage. One titanate assemblage, SYNROC, consists of zirconolite (CaZrTi207), perovskite (CaTiO3) and "hollandite" (BaAl2Ti6016) with additional accessory phases. There are two major problems in the evaluation of the long term stability of any crystalline wasteform such as SYNROC: 1) it is difficult to assess the long term stability of materials from short term laboratory experiments that are not necessarily valid simulations of complex geochemical processes, and 2)the corresponding titanate minerals are uncommon, making it difficult to study long term alteration and radiation effects on a significant number of specimens from different localities and geologic environments. There has even been considerable controversy concerning the stability of the reasonably common and simple phase, perovskite (CaTiO3)

The overall objective of disposal of high-level and alpha bearing wastes in deep geologic formations is to isolate these wastes from the biosphere for such a period of time that a subsequent future release of radionuclides from the buried wastes will not result in undue radiation exposures to man.

When radioactive waste is disposed in a salt dome, it is important to evaluate the hydrologic stability of the dome. It depends on the dissolution rate of the dome, which again is determined by the transport and dispersion properties of the cap rock and the other formations surrounding the dome. The same properties are also required in safety assessment work for calculation of migration of radionuclides, which might be released from the dome to the surrounding strata.

The purposes of this report are: (1) to discuss the manifestation of dissolution processes (such as land subsidence and method of formation of breccia pipes and brine reservoirs) in the bedded salt of southeastern New Mexico in the vicinity of the WIPP site, (2) to postulate conditions under which breaches of the repository might occur, (3) to discuss radiological consequences of such breaches, and (4) to identify areas that need further investigation.

Deep-seated collapse and brecciation is a feature common to bedded salt in a number of evaporite basins. The origin of these features is poorly understood. The relatively young geologic age of uplift and exposure of evaporites in the Delaware Basin, and a still-active hydrologic system, provides a unique opportunity to examine the pathways of water movement related to collapse features and salt dissolution. In addition, salt deformation structures are closely associated with brine, dissolution, and collapse. This report briefly discusses evidence for deep-seated dissolution,considers the relationship between deformation, dissolution, and brine, and examines the potential for dissolution at the WIPP (Waste Isolation Pilot Plant) site with respect to stages in the deformation-dissolution process.

As the thickest and primarily monomineralic saline horizon in the Federal Republic of Germany, investigation of the Zechstein-2-Series, Stalfurt-Halite (Na2) is being emphasized for the final disposal of HLW. This horizon is of particular importance, especially since it is the base of the whole saline formation in northern Germany

In the ultimate storage of high level wastes in rock salt thermomechanical phenomena caused by the temperature rises play an important role for elaborating the storage concept. Important factors are both large space effects concerning the repository and the geological formation and small space phenomena influencing to a considerable extent the interaction between waste blocks and the host rock. While the large space aspects are largely inaccessible to an experimental investigation, the small space effects, especially the convergence behavior of individual heated boreholes, are eligible for a systematic experimental and computational study allowing verification of available computational methods and thermomechanical material laws for rock salt.

The German concept of high level waste final storage by the aid of waste glasses is based on rock salt formations as deposits. Investigations consider the accidental case of water entrance into the deposit where the waste forms then are exposed to highly concentrated salt brines at elevated temperatures and pressures.

Even in the early 1960's it had already been decided to place high-level radioactive waste in the salt formations of the Federal Republic of Germany. However a major concern is the formation of highly corrosive brines in the event of water filling the repositories. In order to estimate the risks of waste storage in salt repositories it is necessary to investigate the physicochemical properties of the waste materials and of those radionuclides extracted into brine. The extraction of radionuclides from borosilicate glasses or spent fuel elements has been discussed, taking into consideration the two transport mechanisms: diffusion and convection.

Safety studies for final disposal of nuclear waste at the Gorleben site in Germany are performed by the project "Safety Studies Entsorgung (PSE)" supported by the Federal Ministry for Research and Technology. The ain of the project is to do consequence assessments for selected site-specific scenarios. The restriction to consequence assessments, instead of performing an overall risk assessment,is based on the expectation that the safety of a given repository site can be demonstrated by showing that the consequences of each scenario, which might reasonably occur, are acceptable.

Laboratory experiments to determine the sorption and the rate of diffusion of cesium and strontium in pieces of granite have been performed. The effective diffusivity, Dp ep was found to be I-2, 10-12 m2/2 for both cesium and strontium.

A migration experiment with the objective to investigate the existence of a connected pore system in undisturbed rock has been performed in the Stripa mine at the 360 m-level.

A model is proposed which describes the transport to and from a waste canister in a repository. The model includes flow and diffusion in the fractures in the bedrock as well as diffusion in the backfill. Calculations have been made on the inward transport of corrosive agents to a buried canister from the surrounding bedrock. The oxidants are transported from the flowing groundwater through the backfill to the surface of the canister, were they are assumed to react instantaneously with the canister material. The rock is modeled as a discrete fracture system. The fractures are assumed to be evenly spaced and the groundwater movement is described by potential flow. The model is three dimensional.

A transport model of radionuclide migration in fissured rock is presented. It includes advection and hydrodynamic dispersion in the fissures and diffusion and sorption in the rock blocks. 1 and 2D analytical and 3D numerical solutions of the model have been developed. A large number of calculations for the approximate range of variation of the input parameters have been performed. A large impact of hydrodynamic dispersion and micropore diffusion on the amount of radioactive material reaching the biosphere is found.

The release rate of radionuclides from a degraded canister containing radioactive waste is controlled by diffusion through the backfill and by diffusion into the water flowing past the canister. In low porosity low permeability bedrock like the Swedish granites the amount of water which can be contaminated may be very small. Two sample cases are calculated. One uses the conditions for a repository for vitrified high level waste, the other applies to a repository for spent fuel. The small amount of water which can carry the waste in combination with the low solubilities for the major longlived actinides limit the release rate from the repositories to very small values.

One of the options being considered for the disposal of radioactive waste is deep burial in crystalline rocks such as granite. It is generally recognised that in such rocks groundwater flows mainly through the fracture networks so that these will be the “highways” for the return of radionuclides to the biosphere. The main factors retarding the radionuclide transport have been considered to be the slow water movement in the fissures over the long distances involved together with sorption both in man-made barriers surrounding the waste, and onto rock surfaces and degradation products in the fissures.

In July/August 1981 and February/March 1982 we sampled water for isotope analysis from water-bearing fractures of two mines in Elliot Lake. Permission for sampling was given to Ontario Hydro by Denison Mines Ltd and Rio Algom Ltd, Quirke Mine in conjunction with the CANMET Mine Safety Tests. The purpose of the investigation was to find out whether there is a relationship between the water present in some fractures in a deep mine and the shallow groundwater in the far-field of the mine. The investigation was done in co-operation with University of Waterloo.

The possibility of permanent burial of radioactive waste from nuclear power plants, is studied in Sweden at the KBS (Nuclear Fuel Safety) - project. Definite repository sites have not yet been selected, but the general principles of construction regarding the layout have been devised (KBS).