132. 



8. CHARACTERISTICS OF RADIOACTIVE WASTE 



8.1 If natural or enriched uranium is used in metallic form in a heter- 

 ogeneous reactor, the fissioning process proceeds to some point limited 

 by economics, corrosion or mechanical stability. It is probable that 

 large quantities of fissionable and fertile material ■will remain in the 

 irradiated fuel. Thus spent fuel elements are still very valuable since 

 they contain part of the initial charge of fissionable and fertile material 

 along with any new fissionable material produced. They are trans- 

 ported, usually in solid form, to a chemical processing plant for re- 

 covery and separation of fissionable and fertile material from fission 

 products in diluent. This is accomplished by dissolving the elements 



in an acid such as nitric acid, followed by selective solvent extraction 

 of valuable components from diluents and fission products. This leaves 

 the fission products in the bulk of the depleted processing stream or 

 raffinate. This raffinate stream is the high level waste and poses the 

 principal disposal problem. 



8.2 After irradiation in a reactor, the metallic elements in which un- 

 consumed fuel and waste are mixed are highly radioactive and they are 

 accordingly stored before processing for a period of time, during which 

 further decay of fission products occurs. Cooling periods vary. How- 

 ever, the rate of decay of fission products is approximately the same; 

 e.g. , after 135 days the activity of the fission products is reduced by a 

 factor 10" 4 from their activity level at the time of discharge from the 

 reactor. At the time of discharge from reactor, the gross fission prod- 

 uct activity is 5.7 per cent of the rated power of the reactor. 



8.3 If the fuel is fed to a reactor of homogeneous type in liquid form, 

 the spent fuel must also be processed in liquid form. Because it is, 

 under present conditions, more difficult to transport the waste in liquid 

 than in solid form, the chemical processing for the removal of the 

 waste from the fuel will presumably be accomplished at each reactor. 

 Future developments may make it feasible to transport such liquid 

 waste economically and safely. 



8.4 In either case, the waste products of the reactor, except for those 

 disposed of to the atmosphere in gaseous form, will be presented for 

 disposal as liquids. The characteristics of the liquid waste are deter- 

 mined by the particular method of chemical processing used. Wastes 

 resulting from the operation of nuclear reactors are classified as high- 

 level wastes . 



