It has been almost three decades since the hiqh level radioactive wastes were proposed to be stored into underground caverns. It had been propicius at first to direct the search towards the sedimentary salt beds and later extension were made to others kind of lithologycal types such as granite, basalt and shale. The mainly two geotechnical aspects which must be included in this kind of design have been pointed out as the thermal behavior of rock mass facing a temperature elevation and the mechanical response due first to induced field stress by underground excavation and then added by the thermal inducedstresses due to temperature field change, It has been presented in this paper analytical solution including the two mentioned qeotechnical aspects which allow the usual design of vitrified waste cylinders individually installed into conveniently spaced holes. It should be mentioned that in this solution the effect of the stored energy decay with time has been included. This consideration obviously allow a better long-term analysis for the mechanical and thermal behavior of rock mass. The resulting model has been applied to simulate the expected brazilian conditions by estimating type of the radioactive waste and the qeomechanical features of the considered gneissic type of rock.
Archaelogical and geological evidences have Shown that, at the current technological level, the soluction that best takes into account; the essential safety requirements and practical viability for nuclear high level waste storage is burial in conveniently selected rock formation.
This wastes are basically constituted by nuclear spent fuel or reprocessing operations wastes. They have as main characteristic a considerable residual thermal energy. This energy whose intensity decreases with time, is the most important aspect to be considered on the storage facility design, due to the induced temperature, stress and strain for thousand of years.
This paper presents analytical solutions that permits a theoretical eval1lation of this effects, in which are included the storage energy decay with time.
The wastes are packed into metalics cylinders which have length greater than its diameter.
With these assumptions, Carslaw and Jaeger showed that the mathematical problem is reduced to the soluction of the general thermal conduction equation: where: (equation in full paper).
This form permits to correlate the adimentional therms (KQ~T). (7) c (+), by means of which was obtained, by numerical guadrature integral, the values plotted in figures (1) to (4), corresponding to source length cylinders varying from lm' to 4m.
Figures (1) to (4) permits to obtain the temperature at any point of the equatorial plane (z=O), at any time, by means the thermal properties of the rock and the thermal power of each cylinder.
With the medium values of the brazilian gneissies thermal properties, found of Hamsa, and the probables thermal characteristics of the cylinders gived by U.S. Department of Energy, showed in table I, and using numerical integration of equation (8) it was possible to obtain temperature evaluation in the equatorial plane of the source, as shown in the figure (5) •