The Relevance of Chernobyl to LWR and PHWR Source Term Experimental Programs

Abstract

A number of mechanisms and complex processes have been invoked in an attempt to explain the fission product releases during the Chernobyl accident. Unfortunately, the published information about fuel temperatures, environment and activity releases is sparse and sometimes inconsistent. As a consequence, a comprehensive analysis of the event is inconceivable. However, it is possible to set some bounding conditions within which the activity releases from the fuel elements took place. For example it can be assumed conservatively that: (a) the fuel was subjected to temperatures between 400-2700 °C, and (b) the fuel environment ranged from reducing to oxidizing.

The experimental programs of Oak Ridge National Laboratory (ORNL) and the Chalk River Nuclear Laboratories (CRNL) were conceived to study fission product release and behaviour under a variety of conditions. In summary the main objectives are: to quantify fission product release from irradiated fuel under conditions from reducing to oxidizing; to relate fuel morphology changes to release data; and to determine the physical and chemical forms of the released species.

The ORNL studies involve the annealing of fuel rod segments in a horizontal or vertical induction furnace in dry air, steam-helium-hydrogen and purified helium at temperatures from 500 to 2425 °C. To date 30 tests have been done on irradiated fuel rods plus other control tests with tracers and fuel simulants. Future tests are planned to core melt- down temperatures, on the stability of certain chemical forms and on the influence of reactor structural/control materials.

The CRNL program consists of annealing of irradiated UO₂ in air, and steam environments, at temperatures between 500-2700 °C. To date 45 tests have been done at temperatures up to 1668 °C. Future work will involve release measurements during UO₂-Zircaloy interaction, and tests with short fuel rod segments at temperatures from 1000 to 2700 °C.

By combining the results from both the ORNL and CRNL programs we are able to present a more complete picture of the fission product release from UO₂ fuel in postulated reactor accidents. The bounding conditions estimated for the Chernobyl accident lie within those already considered by these programs and no additions to these experimental programs appear to be required.