Partial Fuel Meltdown Events
Partial fuel melting events have occurred intentionally and unintentionally in the 50 year history of nuclear reactors.
Intentional tests to characterize fuel melting events were conducted using the BORAX, SPERT, and TREAT reactors at the National Reactor Testing Station in Idaho during the 1950's and 1960's. In some cases, these reactors were destructively tested.
Unintentional partial fuel melting events have occurred in the following cases:
Only in the Windscale case was the radioactivity released from the fuel not contained either by the reactor cooling system or the containment. Other than Chernobyl, fuel damage in the Three Mile Island 2 case was the next most extensive.
Following are descriptions of the EBR-1 and Lucens events. For additional information on fuel melt events, please see the Google "nuclear reactor fuel melt event" search results.
The following explains what happened in the 2 events without links to further explanation of the events.
2. EBR-1 (National Reactor Testing Station, Idaho, United States)
EBR-1 was a small (1 MWt) breeder reactor that used a liquid Sodium-Potassium (NaK) mixture for cooling. The fuel in the second installed core was highly enriched uranium-zirconium alloy. On November 29, 1955, power oscillations were noted during low core flow conditions. During testing to evaluate the effect on core reactivity with temperature, the temperature rose to 720C. At this point, the uranium metal fuel and stainless steel cladding around the fuel began to interact resulting in melting. Bowing of the fuel was caused by higher temperatures which, in turn, made the reactor more reactive resulting in more power and a higher temperature.
Other than the fuel melting, no explosion , plant damage, or radiation releases occurred. The core was replaced. The reactor continued operation until December 1963.
6. Lucens (Lucens, Switzerland)
Lucens was a small (30 MWt) reactor that used carbon dioxide gas for cooling and heavy water as a moderator. On January 21, 1969 during a startup, the following happened:
The fission products were contained by the containment. The reactor was subsequently dismantled and extensive investigations conducted.
Fuel melting, whether partial or extensive, is not expected to present a hazard to the public with the redundant emergency core cooling systems, reactor protection systems, and containment design. Fuel melting, however, can, and has been, a significant economic effect because of the resulting radiation effects within the reactor cooling system and the cost of replacing the fuel. Also, the NRC has required utilities to conduct probabilistic safety assessments for the plant-specific design configuration. The NRC's goal nominally to keep the total probability of core melt to less than 1 x 10-4 (or once per 10,000 reactor-years). To esnure that the plant equipment is maintained in accordance with that probablistic safety assessment , the NRC developed the Maintenance Rule (10 CFR 50.65). Performance criteria must be established that take the results of the probablistic safety assessment into account. If the performance criteria are exceeded, the utility must establish goals and corrective actions to bring the defective equipment into conformance with the probablistic safety assessment results.
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