TRANSURANIC ELEMENTS IN SPACE NUCLEAR POWER SYSTEMS 85 



and since the fuel inventory required in the system could be contained in multiple 

 structures, the extent of release per container is controlled within the heat-source design 

 by a probabiUstic scaling factor, where the release probability, including the occurrence 

 probability of the accident, is inversely proportional to the quantity of fuel released. Risk 

 assessments of current space systems have indicated that a source term to the biosphere 

 of 1 to 10 Ci of respirable fuel might be expected with a probability of from 10"^ to 

 10~^. This source term could be either an atmosphere release at altitude or a ground 

 point-source release. Releases to the hydrosphere are controlled by the integrity of the 

 fuel containers and the dissolution rate of the fuel form. On the basis of release-rate 

 experiments, plutonium concentrations in seawater are generally in the picocurie per 

 millihter range at distances of 10 m or more from the point source. 



In summary, progress has been made in the space program to virtually eliminate the 

 release of radioactive fuel during normal operations and launch aborts. As future trends 

 require larger systems with liigher electrical power levels and larger fuel inventories, 

 more-stringent system safety requirements and more-sophisticated analytical and test 

 methods to improve the quality or risk assessments and source-term evaluations are being 

 developed and enforced. 



