Uranium-238 (U-238) is the most common
isotope of
uranium found in nature. When hit by a
neutron, it eventually becomes
plutonium-239 (Pu-239).
Around 99.284% of
natural uranium is
uranium-238, which has a half-life of
1.41 × 10
17 seconds
(4.46 × 10
9 years, or 4.46 billion years).
Depleted uranium consists mainly of
the 238 isotope, and
enriched
uranium has a higher-than-natural quantity of the
uranium-235 isotope.
Reprocessed uranium is also mainly
U-238, but contains significant quantities of
uranium-236, and in fact all the isotopes of
uranium between
uranium-232 and
uranium-238 except uranium-237.
Nuclear Energy applications
In a fission
nuclear reactor,
uranium-238 can be used to breed plutonium-239, which itself can be
used in a nuclear weapon or as a reactor fuel source. In fact, in a
typical nuclear reactor, up to a third of the generated power does
come from the fission of plutonium-239, which is not supplied as a
fuel to the reactor, but
transmuted from uranium-238.
Breeder reactors
Breeder reactors use the waste uranium-238 from fissile reactors as
a fuel source.Uranium-238 is not usable directly as
nuclear fuel, though it can produce energy via
fast fission, where a "fast" fission neutron that has an energy in
excess of 1 Mev can cause the U
238 to fission (but the
resulting ~1.7 neutrons produced have too few high-energy neutrons
to carry on this reaction as a chain reaction, but they can
contribute some 1% - 10% of all fission reactions in a reactor,
depending on design); however,
238U can be used as a
source material for creating the
element plutonium.
Breeder reactors carry out such a
process of transmutation to convert
fertile isotopes such as uranium-238 into
fissile plutonium. It has been estimated that there is anywhere
from 10,000 to five billion years worth of uranium-238 for use in
these
power plants [55596]. Breeder technology has been used in several
reactors
[55597].
As of
December 2005, the only breeder reactor producing power is the
600-megawatt BN-600
reactor
at the Beloyarsk Nuclear Power Station
in Russia
.
Russia has planned to build another unit, BN-800, at the Beloyarsk
nuclear power plant.
Also, Japan
's Monju
breeder
reactor is planned for a re-start, having been shut down since
1995, and both China
and India
have
announced intentions to build breeder reactors.
The breeder reactor as its name implies creates even larger
quantities of plutonium-239 than the fission nuclear reactor.
The
Clean And
Environmentally Safe Advanced Reactor (CAESAR), a nuclear
reactor concept that would use steam as a moderator to control
delayed neutrons, will potentially
be able to burn uranium-238 as fuel once the reactor is started
with
LEU fuel. This design is still
in the early stages of development.
Radiation shielding
Uranium-238 is also used as a
radiation shield — its
alpha radiation is easily stopped by the
non-
radioactive casing of the shielding
and the uranium's high
atomic weight
and high number of
electrons is highly
effective in absorbing
gamma rays and
x-rays. However, it is not as effective as
ordinary water for stopping
fast
neutrons. Both metallic depleted uranium and depleted
uranium dioxide are being used as materials
for radiation shielding. Uranium is about five times better as a
gamma ray shield than
lead, so a shield with
the same effectivity can be packed into a thinner layer.
DUCRETE, a concrete made with uranium
dioxide
aggregate instead of
gravel, is being investigated as a material for
dry cask storage systems to store
radioactive waste.
Downblending
The opposite of enriching is
downblending. Surplus
highly-enriched uranium can be downblended with depleted uranium or
natural uranium to turn it into low enriched uranium suitable for
use in commercial
nuclear fuel.
Uranium-238 from depleted uranium and natural uranium is also used
with recycled plutonium from weapons stockpiles for making
mixed oxide fuel (MOX) which is now being
redirected to become reactor fuel. This dilution, also called
downblending, means that any nation or group that acquired the
finished fuel would have to repeat the very expensive and complex
enrichment and separation processes before assembling a
weapon.
Nuclear weapons
Most modern
nuclear weapons utilize
uranium-238 as a "tamper" material (see
nuclear weapon design). A tamper which
surrounds a fissile core works to
reflect neutrons and add
inertia to the compression of the
plutonium charge. As such, it increases the
efficiency of the weapon and reduces the amount of
critical mass required. In the case
of a thermonuclear weapon uranium-238can be used to encase the
fusion fuel, the high flux of very energetic
neutrons from the resulting
fusion reaction causes the uranium-238 to
fission and adds
energy to the yield of the
weapon. Such weapons are referred to as
fission-fusion-fission weapons
after the three consecutive stages of the
explosion.
An example of such a weapon is Castle Bravo
although the fission of the unenriched uranium
tamper was not intended.
The larger portion of the total explosive yield in this design
comes from the final fission stage fueled by uranium-238, producing
enormous amounts of radioactive
fission
products.
For example, 77% of the 10.4-megaton yield of
the Ivy
Mike
thermonuclear test in 1952 came from fast fission of the depleted uranium tamper.
Because depleted uranium has no critical mass, it can be added to
thermonuclear bombs in almost unlimited quantity.
The 1961 Soviet test
of Tsar
Bomba
produced "only" 50 megatons, over 90% from fusion,
because the uranium-238 final stage was replaced with lead.
Had uranium-238 been used, the yield could have been as much as 100
megatons, and would have produced
fallout equivalent to one third of the
global total at that time.
Other weapons that have been produced by nuclear atoms are Nuclear
Submarine machine guns, Aircraft cariers and Nuclear Powered
Aircraft which are still under production.
Radioactivity and decay
Uranium-238 radiates
alpha-particles and
decays (by way of
thorium-234 and
protactinium-234) into
uranium-234. U-234 has a
half-life of 246,000 years. The relation between
U-238 and U-234 gives an indication of the age of
sediments that are between 100,000 years and
1,200,000 years in age.
U-238 decays by
spontaneous
fission and
double beta decay
with probabilities of 5 × 10
−5 and
2 × 10
−10 per 100 alpha decays,
respectively.
The
mean lifetime of uranium-238 is
1.41 × 10
17 seconds divided by 0.693 (or
multiplied by 1.443), i.e. ca. 2 × 10
17
seconds, so 1
mole of uranium-238 emits
3 × 10
6 alpha particles per second, producing
the same number of thorium-234 (Th-234)
atoms.
In a closed system an equilibrium would be reached, with all
amounts except lead-206 and uranium-238 in fixed ratios, in slowly
decreasing amounts. The amount of Pb-206 will increase accordingly
while U-238 decreases; all steps in the decay chain have this same
rate of 3 × 10
6 decayed particles per second
per mole uranium-238.
Thorium-234 has a mean lifetime of 3 × 10
6
seconds, so there is equilibrium if 1 mole of uranium-238 contains
9 × 10
12 atoms of thorium-234, which is
1.5 × 10
−11 mole (the ratio of the two
half-lives). Similarly, in an equilibrium in a closed system the
amount of each decay product, except the end product lead, is
proportional to its half-life.
As already touched upon above, when starting with pure uranium-238,
within a human timescale the equilibrium applies for the first
three steps in the decay chain only. Thus, per mole of uranium-238,
3 × 10
6 times per second one alpha and two
beta particles and gamma ray are produced, together 6.7 MeV, a rate
of 3 µW. Extrapolated over 2 × 10
17
seconds this is 600 GJ, the total energy released in the first
three steps in the decay chain
References
- ^ Table of Radioactive Isotopes
External links
See also
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