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Spent Fuel Pool Accident Simulator

In a conversation with emergency preparedness personnel in the U.S. northeast, a concern about terrorist attack to the spent fuel pool (SFP) was raised. This is a more realistic threat and would cause greater harm than dramatic attacks such as aircraft crashing into the containment, particularly because the SFP building is not as strong as the containment while storing some four times more radioactive materials.

The event of a spent fuel pool draining or evaporating followed by zirconium fire has been analyzed. The USNRC report NUREG-1738, titled "Technical Study of Spent Fuel Pool Accident Risk at Decommissioning Nuclear Power Plants," was published February 2001, prior to the tragic events of September 11th. The sort of scenarios analyzed are not so far-fetched. A few years ago, a loss-of-cooling event at one US nuclear plant heated up the area floor to the point of melting the "rubber boots."

As EP professionals, we may want to think hard about how to prevent and mitigate these kind of events.

PCTRAN/SFP screen shot

PCTRAN/SFP is a highly advanced simulator that conducts thermal-hydraulic and radiological calculations for a loss-of-cooling event. All modeling is plant specific, taking into consideration pool inventory of cycle burn-up, geometry, and cooling system design. In a typical scenario, the time to bulk boiling could be as short as a few hours, then followed by several days to boil off and uncover the fuels. In the meantime, a heavy load drop, such as the casks, may crush the fuels and add sufficient positive reactivity to reach criticality. The simulator can perform either accelerated run for practical exercise purposes or instanteous projection, jumping to the degraded state. By opening a drain valve at the bottom of the pool (which doesn't really exist in the actual pool), an operator can simulate an intentional drain-down or damage caused by an earthquake. According to the NUREG, it takes 2 to 40 hours to heat the fuel to 900°C. This again, will be simulated an accelerated pace for training purposes. During this course, the fuel clad will swell and burst. The breach in the clad releases radioactive gases present in the gap. When the gases reach the point of rapid oxidation in air or steam, the reaction is highly exothermic. The energy released from the reaction, combined with the fuel's decay heat, can cause the reaction to become self-sustaining and ignite the zirconium. An ensuing fire could result in significant release of spent fuel fission products, dispersing them far from the reactor site. PCTRAN/SFP will reproduce all of these events quantitatively.

In the PCTRAN/SFP mimic for a typical SFP shown above, there is a circulation cooling system with heat exchangers relieving the decay heat to the environment, regular makeup pumps and emergency diesel-driven firewater pumps. Any of the functional component or system can be enabled and disabled by simple point and click with the mouse. When the fuels are exposed and heated up, their temperatures will be indicated in color. In addition to fission gases in the gap, damaged fuel aerosols such as alkali metals, tellurium, barium, cerium, lanthanides, etc., will be traced. We will use NUREG-1465 "Accident Source Terms for Light Water Power Plants" for the release mechanism. Their contribution to the Fuel Handling Building radiation monitors and release path through the vent and wall leakage will form the site boundary doses. PCTRAN/SFP also simulates radiation monitor readings in the surrounding region.

As compared to typical nuclear power plant accidents, an SFP accident is slow in its evolution. A loss of cooling situation could take days to evaporate the pool water and hours to heat-up to the point of burning. However, events such as a catastrophic earthquake or intentional sabotage can be very quick in evolution. Should water supply be resumed following such events, the consequence can be mitigated. But if fresh water rather than the required borated water is used, either by administrative error or intentional sabotage, reduction of pool boron concentration may cause a return to criticality. Another consideration is that when water level reaches 3 feet above the top of the fuel, the radiation level may become high enough to prohibit human access.

Using PCTRAN/SFP for training and exercise will give the staff a quantitative feel and realistic appreciation for spent fuel pool accident events. Should an event occur in real life, PCTRAN/SFP can be an invaluable tool for making instant projections of the time to pool boiling, fuel uncovery and dose release, which are essential for determining protective actions such as notification, shielding and evacuation.

© 2007 Micro-simulation Technology