Damaged fuel, fission product release

Since for reactors in this group the potential of fuel damage and fission product release is related to the adequacy of the shut-down heat removal system, the requirement for and extent of an emergency plan feasibility has to be established on a case-to-case basis. 

At the present state of the art, a corner of an article about evaluation of population hazards is hardly an appropriate place in which to attempt an exposition of reactor safety. Nevertheless, we may contrive a brief description of these types of reactor accidents which, it is thought, could lead to fission product release. The intention is to illustrate ways in which fuel could be damaged and then release fission products ultimately to the atmosphere. Though gas-cooled reactors, water-cooled reactors, and sodium-cooled fast reactors will be discussed, no comparisons, invidious or otherwise, are intended between the safety of these systems. 

Figures 29 and 30 show the peak generated as the containers on the pontoon are finally breached in the year 2305. Five hundred GBq of fission products and 1600 GBq of actinides are immediately released to the Kara Sea from the cracks and porosity of the damaged fuel. In the following year, the rate of release reverts to the calculated corrosion rate of the oxide fuel fission product and actinide release rates are 1.7 GBq-a and 5.7 GBq a -, respectively. The fuel slowly corrodes away and the activity of the fuel itself decreases in the year 3300, the release rates for fission products and actinides are 0.05 GBq-a and 2.7 GBq a, respectively. The fuel is finally corroded away by the year 4570.

Summarizing the results obtained from the theoretical studies as well as from the experimental investigations, it can be expected that, in the course of a severe core damage accident, fission product iodine released from the fuel is converted to... 

Osetek, D. J., Cronenberg, A. W., Hagrman, D. L., Broughton, J. M., Rest, J. Behavior of fission products released from severely damaged fuel during the PBF Severe Fuel Damage Tests. Proc. 5. Intemat. Meeting on Thermal Nuclear Reactor Safety, Karlsmhe 1984, Report KfK 3880/3, p. 1367-1376... 

Suh, K. Y., Hammersly, R. J. Modeling of fission product release and transport for severe fuel damage analyses. Nucl. Sci. Engng. 109, 26— 38 (1991)... 

Vinjamuri, K., Osetek, D. J., Hobbins, R. R. (b) Fission product release rates measured during in-pile fuel damage tests. Trans. Am. Nucl. Soc. 46, 480-482 (1984)... 

Release Category BP events are accident sequences in whieh frssion products are released directly from the reactor coolant system to the environment via the secondary system or other interfacing system bypass the containment. The fission-product release to the environment begins approximately at the onset of fuel damage, and there is no attenuation of the magnitude of the... 

The retention of fission product iodine and xenon by unirradiated and irradiated pyrolytic-carbon-coated (Th,U)C2 fuel particles has been studied in annealing experiments and has been compared with similar studies of the release (or retention) of barium and strontium. The objective was to study the effects of irradiation on the retention of the two types of fission products and to determine the mechanism of release which could account for the observed behaviors. In both unirradiated and irradiated particles, iodine and xenon were found to be retained highly by the impervious isotropic pyrolytic coating which was unaffected by the irradiation. In contrast, the fuel kernel which controls the release of the metallic species is damaged severely by the irradiation, resulting in a marked decrease in its ability to retain the metals. 

Similar measurements on the alkaline earth fission products have shown increased releases that may depend functionally on the level of irradiation. The increased releases may be interpreted as indicating that damage to the fuel kernel has occurred since the kernel has been shown to control the steady-state release of these metals. 

B. The Release of Fission Products from Damaged Fuel... 

In any reactor, as must be obvious, fuel is prevented from rising to damaging temperatures, which could cause release of fission products in many cases, by maintaining a sufficient balance between the rate of nuclear heat generation and the rate of heat removal by the coolant system. Clearly,... 

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