136. 



e. The comparatively high thermal conductivity of salt and its suf- 

 ficiently high melting point -would permit the storage of wastes at 

 moderate temperature without effect on the walls of the cavity, pro- 

 vided the plasticity of salt is not increased by long -continued exposure 

 to elevated temperatures. 



12.2 These advantages would not exist to the same extent if the salt 

 cavities were produced by pumping water into the salt formations and 

 the removal of the salt as brine. The large extent of cavities formed 

 by this method, the absence of roof support, and the lack of control 

 over the underground distribution of radioactive waste introduced into 

 such cavities are disadvantages -which make it inadvisable to consider 

 the use of such space for disposal. The possibilities of collapse of 

 such cavities are considerable and instances of surface subsistence 

 from such collapse are known to the salt industry. 



13. PROBLEMS OF UTILIZATION OF MINED-OUT SPACE 



13.1 The storage of high-level radioactive waste in underground salt 

 space presents several problems of an engineering character. These 

 problems differ in some respects depending upon the physical form and 

 characteristics of the waste as it -would be produced by reactors or 

 processing plants. 



13.2 High-level waste no w being produced from these sources is in 

 liquid form. The liquid as produced is chemically active, radioactive, 

 and produces heat through radioactive decay. It is therefore desirable 

 that the -waste be treated before storage to minimize these hazardous 

 characteristics. It is also, in some cases, diluted in the course of the 

 chemical separation process so that the volume is materially increased. 



13.3 The activity of the waste is now chemically neutralized by treat- 

 ment -with alkaline solutions before it is placed in surface storage tanks 

 for aging. This process results in an increase of about four times in 

 the volume of the waste but this can be reduced by evaporation to a 

 point where the slurry contains about 35% solids. ( ' Waste so neutra- 

 lized would apparently not have any chemical effect on the walls of a 

 salt cavity -with which it moves into direct contact but further study 

 should be given to this problem. 



13.4 The storage of the waste in surface tankage for a period of six 

 months or more permits the decay of some of the fission products that 



