Reactor-grade
plutonium is found in
spent nuclear fuel that a
nuclear reactor has irradiated
(
burnup) for years instead of weeks or
months, leading to
transmutation of much of the
fissile, relatively long
half-life isotope
239Pu into other
isotopes of plutonium that are less
fissile or more radioactive.
Thermal-neutron reactors
(today's
nuclear power plants)
can reuse reactor-grade plutonium only to a limited degree as
MOX fuel, and only for a second cycle;
fast-neutron reactors, which
are uncommon today, can use this or any other
actinide material indefinitely.
The degree to which reactor-grade plutonium is less useful than
weapons-grade plutonium for
building
nuclear weapons is debated,
with many sources saying it is difficult or impossible, and others
saying it is relatively easy with modern technologies like
fusion boosting to overcome predetonation,
remote manipulation for assembly
of highly radioactive components, and cooling of the
pit during storage to
offset accumulation of
decay heat.
Classification by isotopic composition
|
<1976 |
>1976 |
| <7% |
Weapons grade |
| 7-19% |
Reactor grade |
Fuel grade |
| >19% |
Reactor grade |
The difference is important in assessing significance in the
context of
nuclear
proliferation.
Reprocessing
of
LWR (
PWR or
BWR)
spent fuel recovers
reactor grade plutonium (as defined since 1976), not
fuel grade.
The
DOE
definition of
reactor grade plutonium changed in 1976. Up until 1976, two
grades were recognised:
Weapons
grade, less than 7%
Pu-240 and
Reactor grade, 7%
or more Pu-240.From 1976, three grades were
recognised:
Weapons grade, less than 7%
Pu-240,
Fuel grade, 7% to 19% Pu-240
and
Reactor grade, more than 19% Pu-240.
Reactor-grade plutonium nuclear test
The
reactor grade plutonium nuclear test was a
low-yield (under 20 kilotons) underground nuclear test using non-weapons-grade plutonium, conducted at the US Nevada Test Site
in
1962.
Some information regarding this test was declassified in July 1977
under instructions from President
Jimmy
Carter as background to his decision to prohibit the
nuclear reprocessing of
spent nuclear fuel.
The plutonium used was manufactured in a
Magnox reactor in the United Kingdom, and
provided to the US under the
1958 US-UK Mutual Defence
Agreement. Its isotopic composition has not been disclosed,
other than the description
reactor grade and it has not
been disclosed which definition was used in describing the material
for this test as
reactor grade.
To overcome the ambiguity of the term
reactor grade, some
authorities describe the test as being of
non-weapons-grade plutonium.
Reuse in reactors
Fast neutron reactors can use
plutonium of any isotopic composition.
Reprocessing was planned in the 1960s when planners expected the
uranium market to become tight and
fast breeder reactors to be
needed to efficiently use uranium supplies. This became less urgent
with reduced demand forecasts and increased uranium supplies, and
commercial deployment of fast reactors was postponed.
Today's
thermal reactors can reuse
plutonium to a limited degree as
MOX fuel.
Some reactors limit MOX fuel to a fraction of the total fuel load
for nuclear stability reasons. Only the odd-mass
isotopes of plutonium are
fissile with
thermal
neutrons, and the even-mass isotopes accumulate.
Plutonium-240 is a
fertile material like
uranium-238, becoming
plutonium-241 on neutron capture, but
plutonium-242 both has a low
neutron capture cross section, and would require 3
neutron captures before becoming a
fissile nuclide.
External links
References
-
http://www.fissilematerials.org/ipfm/pages_us_en/fissile/fissile/fissile.php/
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