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 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.
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