POWER May 2010 - 36

NUCLEAR POWER
6. Pool party. Storing spent fuel assemblies
underwater in a storage pool. Source: DOE
7. Limited nuclear fuel pool capacity. This chart shows the cumulative number of
filled pools at nuclear power plants. All operating nuclear power reactors are storing used fuel
under NRC licenses in spent fuel pools. Some operating reactors are using dry cask storage.
Source: NRC
100
80
60
40
20
The water-pool storage option involves
storing SNF assemblies under at least 20
feet of water to provide shielding from the
radiation and removal of decay heat (Figure
6). About one-fourth to one-third of the
total fuel load is removed from the reactor,
typically every 18 months, and replaced
with fresh SNF. You may recall that early
in the development of commercial nuclear
reactors, the government was expecting to
construct a nuclear fuel reprocessing plant
and the pools were sized to hold and cool
SNF until it could be transported to the reprocessing
facility. On April 7, 1977, President
Jimmy Carter banned the reprocessing
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Spent fuel pools at capacity
1990 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 2015
Year
of commercial reactor fuel in the U.S. Since
then, many of the nuclear plant spent fuel
pools have either reached or are nearing capacity
(Figure 7).
Current regulations permit re-racking of
the storage pool grid and fuel rod consolidation,
subject to NRC review and approval, to
increase the amount of SNF that can be stored
in a pool. However, both of these methods are
constrained by the size of the pool.
In the early 1980s, utilities began looking
at using dry casks to increase on-site
storage capacity. The process of loading a
cask, consisting of a steel cylinder designed
to hold typically two dozen SNF assemblies,
takes place underwater in the storage
pool. Once the assemblies have cooled for
given period of time, they are transferred
underwater from the storage racks to the
submerged cask. Next, the cask is removed
from the storage pool, where excess water
is removed. Then it is backfilled with an
inert gas to enhance decay-heat transfer capabilities,
welded or bolted closed, inserted
into a concrete overstructure (depending on
design), and stored vertically on a concrete
pad. The cask itself provides the necessary
radiation shielding. Other above-ground designs
seal the SNF inside a steel cylinder,
which is then inserted either vertically into a
concrete silo or horizontally into a concrete
vault. The concrete provides the radiation
shielding (Figure 8).
The NRC approves dry-storage systems by
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evaluating each design for resistance to accident
conditions such as floods, earthquakes,
tornado missiles, and temperature extremes.
Some cask designs can be used for both storage
and transportation. The dry-storage casks
are located in an independent spent fuel storage
installation (ISFSI). Such storage may
be either at the reactor site or elsewhere (see
" How to Solve the Used Nuclear Fuel Storage
Problem, " POWER, April 2008).
Number of filled pools

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POWER May 2010

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