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Courtesy of US Nuclear Regulatory Commission
In nuclear power plants, large pools are used for storage of spent fuel
bundles after they are removed from the reactor. A recent study by the
office of Nuclear Reactor Regulation (NRR) at the US Nuclear Regulatory
Commission (USNRC) examined the risks that would accompany an accident
during which the pool coolant is lost in some catastrophe. This task was
undertaken as part of a larger program of decommissioning nuclear sites
around the United States. The aim of the project was to predict the peak
temperatures in the spent fuel after this type of coolant accident and
to examine the buoyancy-induced natural circulation flows that provide
the bulk of the cooling during this type of scenario. Engineers from the
office of Nuclear Regulatory Research (RES) provided the CFD analysis.
Path lines colored by temperature enter the building on the left, drop
into the pool region, and are warmed by the fuel before rising and exiting
from the containment building on the right.
A three-dimensional model of a generic pool was developed in FLUENT.
The simulated pool is housed in a large containment building with an operable
ventilation system above the pool surface. The pool is filled to capacity
with fuel in high density racking which occupies the lower third of the
pool. A total of 4200 fuel bundles of various ages are contained in the
pool. A complete reactor core consists of roughly 800 fuel bundles. It
is assumed that the racks are filled from right to left over several years
of reactor operation. The 800 bundles on the left of the pool are the
freshest bundles assumed to come from the final core off load. These release
the highest amount of energy. The energy released from the fuel decreases
over time. A steady flow of air is assumed to flow into the upper building
through the ventilation system.
Steady state simulations were performed using FLUENT to examine the thermal
and flow characteristics two, three, four, and six years following a reactor
shutdown. At each of these decay times, the peak temperatures in the hottest
fuel were of interest. As the assumed time from the reactor shutdown increased
from two to six years, the decay energy from the fuel decreased and the
maximum predicted temperatures were reduced. Sensitivity studies were
performed that examined variations in fuel burn up, flow resistance, ventilation
flow rate, and heat transfer coefficient on the walls and ceiling of the
containment building as well as other buildings. The studies reflected
the impact of these parameters on the peak fuel temperatures and the natural
circulation flows within the building. Thermal effects due to radiation
and exothermic cladding reactions, such as oxidation, were ignored in
the simulation because their effects are only significant at elevated
temperatures.
Temperatures in the containment building and near the fuel rods as a function
of decaytime. The shape of this curve closely resembles the expected decay
heat curve.
A number of simplified codes, such as SHARP, SFUEL, and COBRA-SFS are
usually used for pool heat-up predictions. Some of these codes have sophisticated
physical models tailored specifically to this task. The codes typically
rely on simplified flow assumptions in and around the fuel racks, however,
and do not model the entire building surrounding the fuel pool. These
simplified flow field assumptions do not account for variations in pressure
and temperature in the rack region resulting from the three-dimensional
nature of the flow. The CFD results, on the other hand, focus on the global
three-dimensional flow field in the racks, pool, and surrounding buildings.
These predictions have helped reduce the overall uncertainty associated
with this important aspect of spent fuel pool cooling. For example, the
results of the analysis showed that for spent fuel with a decay time of
more than about 4 years, the air alone can keep the fuel temperature below
600°C.
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