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| Author: |
John Cohen |
| Course: |
EEE 460 |
| Date: |
Spring 2002 |
Decommissioning Of A Nuclear
Submarine
Overview--History
At the end of World War II the United States shifted its focus from
nuclear bombs to nuclear power. Nuclear power aboard sea vessels allowed
sea-crafts to travel far longer distances than their predecessors at higher
speeds without refueling. In addition, because the reactor power plant
does not require oxygen the vessel can remain submerged. These qualities
sparked interest in the U.S. Navy and the first nuclear driven vessel was
operational by the mid 1950's. In 1954, the Nautilus became the first nuclear
submarine to succeed with this new technology. Soon afterwards, the rest
of the world caught up in producing nuclear propelled vessels. In 1959,
the first Soviet submarine was operational, while the British (1963), French
(1971), and Chinese (1974) had too succeeded in developing this technology.
Until about 1991, the United States produced on average 3 submarines per
year. "All together, a total of about 500 nuclear-propelled vessel's have
been constructed since 1954" [1].
Nuclear powered technology is approaching its 50th anniversary. There
is no set time for service of a nuclear submarine, however, from experience
the United States nuclear submarines operate 25-30 years before having
to be decommissioned. Each country has their own life expectancy per nuclear
ship. What many countries failed to recognize in racing to build these
highly sophisticated machines is that upon taking them out of service the
vessels cannot just be junked. In fact, because of the nuclear capability,
the decommissioning process has become an important problem that has not
yet been solved. Both technically and financially, this issue is gaining
more and more attention around the world.
The Dismantlement Process
A nuclear submarine is heavily radioactive at the end of its lifetime.
After a few decades of use (see Overview for the
lifetime of a nuclear submarine) large parts of the surrounding area of
the reactor have become radioactively contaminated, in addition to the
reactor itself.
The ships last journey is to the naval shipyard where upon arrival
the reactor must be shut down for a period to allow the immediate radioisotopes
to decay. Some of these include Iodine-131, half-life 8.04 days, and Xenon-133,
half-life 32.50 days, which can cause a disaster if an accident were to
occur during removal [2], [3]. After this time has past, the reactor is
now ready to be broken down. Usually, the hull above the reactor in the
submarine is opened up and serviced. Defueling is accomplished using similar
procedures as those of the refueling process.
The dismantling of a nuclear submarine is an extremely delicate process,
which can lead to devastating catastrophes if done improperly. Each individual
task is a vital to the whole procedure. Detailed attention is necessary
to avoid having the reactor become critical, thereby causing an explosion.
In addition, a damaged reactor compartment has additional consequences.
In such accidents, the reactor compartment becomes so damaged that it is
unsafe to finish the dismantling process by standard techniques. As a result,
dismantling the submarine must be halted to allow time to gradually decrease
the amount of radioactivity contained in the compartment.
Under normal dismantling processes, the final step of the breakdown
is transporting the spent fuel to a storage facility where it is either
reprocessed or listed as radioactive waste. Defueling removes over 99 percent
of the radioactivity associated with the reactor [1]. The remaining radioactive
elements remain toxic for thousands of years. Some, but not all include
radium-226 (half-life 1599 years), carbon-14 (half-life 5715 years), and
iodine-129 (half-life 17,000,000 years). After all the fuel has been removed,
the heavily radioactive reactor compartment is discarded one of three ways:
sea disposal, shallow land burial, and deep land burial 1].
Reactor Enrichment
The reactors aboard nuclear powered submarines are far more highly enriched
than the civilian reactors used for instance in local power plants. The
amount of enrichment is proportional to how long the submarines can travel
before having to refuel the reactor. In most cases, 20-45 percent Uranium
235 has been used as fuel in submarine reactors, as opposed to four percent
in civilian reactors [1]. The type of reactor also plays a large role in
the level of enrichment, however, it can vary from 20 percent to 90 percent
(weapons grade) Uranium 235. Most nuclear submarines are equipped with
pressurized water reactors (PWR) [4]. Each reactor contains approximately
250 fuel assemblies, each having up to a few tons of fuel rods. Under normal
operating conditions, the PWRs must be refueled every seven to ten years
[1].
In addition to the large level of enrichment of Uranium 235 in nuclear
submarines, many submarines, mostly those of the Russian fleet, contain
two reactors in the same compartment aboard one submarine [5]. Nuclear
Propelled Aircraft carriers can be powered upwards to approximately eight
reactors.
Pressurized Water Reactor Shown Above [2]
Generated Radioactive Waste aboard Nuclear Submarines
Waste generated by nuclear reactors containing radioactive atoms are
classified as radioactive waste. Radioactive waste contains alpha particles,
beta particles, gamma rays and neutrons. Waste is broken down into various
categories depending on the radioactive level and type of material. For
instance, Waste containing heavy isotopes, known as Transuranic wastes
(TRU), consists of alpha emitters and has long decay time [2], [3]. Fission
products fall under the high-level waste (HLW) category. HLW contains Beta
and gamma emitters and are highly radioactive [2], [3]. There decay is
less than those of TRU, however, they are more active and has a further
range of radioactivity. A third type of waste is called low-level waste
(LLW). LLW is less densely populated with radioactive atoms [2], [3]. Under
United States policy, all removed material from a nuclear reactor is deemed
as waste. Other countries, are able to reuse portions of the removed material
using reprocessing techniques.
References
[1] Susanne Kople, "Nuclear Submarine Decommissioning and Related Problems,"
Bonn International Center for Conversion, Germany, August 1997.
[2] "Nuclear Concepts for the 21st Century," class notes for EEE
460, Department of Electrical Engineering, Arizona State University, Spring
2002.
[3] Raymond L. Murray, Nuclear Energy, 5th edition, Butterworth-Heinmann,
2001
[4] "The Russian Northern Fleet," Bellona Foundation, 1996, http://www.bellona.no/imaker?id=10090&sub=1.
[5] "Should further US submarine production be postponed?," RAND
Organization, http://www.rand.org/publications/RB/RB7102
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