Reactor-grade plutonium is found in spent nuclear fuel that a nuclear reactor has irradiated (burnup/burnt up) for years before removal from the reactor, in contrast to the low burnup of weeks or months that is commonly required to produce weapons-grade plutonium, with the high time in the reactor(high burnup) of reactor-grade plutonium leading to transmutation of much of the fissile, relatively long half-life isotope 239Pu into a number of other isotopes of plutonium that are less fissile or more radioactive.
Thermal-neutron reactors (today's nuclear power stations) can reuse reactor-grade plutonium only to a limited degree as MOX fuel, and only for a second cycle; fast-neutron reactors, of which there is less than a handful operating today, can use reactor-grade plutonium fuel as a means to reduce the transuranium content of spent nuclear fuel/nuclear waste.
The degree to which typical Generation II reactor high burn-up produced reactor-grade plutonium is less useful than weapons-grade plutonium for building nuclear weapons is somewhat debated, with many sources arguing that the maximum probable theoretical yield would be bordering on a fizzle explosion of the range 0.1 to 2 kiloton in a Fat Man type device, that is, assuming the non-trivial issue of dealing with the heat generation from the higher content of non-weapons usable Pu-238, that is present, could be overcome, as the premature initiation from the spontaneous fission of Pu-240 would ensure a low explosive yield in such a device, with the surmounting of both issues being described as "daunting" hurdles for a Fat Man era implosion design and the possibility of terrorists achieving this fizzle yield being regarded as an "overblown" apprehension with the safeguards that are in place.
While others disagree on theoretical grounds and state that dependable high, non-fizzle level yields, can be achieved, arguing that it would be "relatively easy" for a well funded entity with access to fusion boosting tritium and expertise to overcome the problem of predetonation created by Pu-240, and that a remote manipulation facility could be utilized in the assembly of the highly radioactive gamma ray emitting bomb components, coupled with a means of cooling the weapon pit during storage to prevent the plutonium charge contained in the pit from melting, and a design that kept the implosion mechanisms high explosives from being degraded by the pits heat. However with all these major design considerations included, this fusion boosted reactor grade plutonium primary will still fizzle if the fission component of the primary does not deliver more than 0.2 kilotons of yield, which is regarded as the minimum energy necessary to start a fusion burn. The probability that such a device would fail to achieve this threshold yield increases as the burnup value of the fuel increases.
No information, available in the public domain, suggests that any well funded entity has ever achieved, or seriously pursued creating, a nuclear weapon with the same isotopic composition of modern, high burn up, reactor grade plutonium. All nuclear weapon states have taken the more conventional path to nuclear weapons by either uranium enrichment or producing low burn up, "fuel-grade" and weapons-grade plutonium, in reactors capable of operating as production reactors. While the isotopic content of reactor-grade plutonium, created by the most common commercial power reactor design, the pressurized water reactor, never directly being considered for weapons use.
As of April 2012, there were thirty-one countries that have civil nuclear power plants, of which nine have nuclear weapons, and almost every nuclear weapons state began producing weapons first instead of commercial nuclear power plants. Moreover, the re-purposing of civilian nuclear industries for military purposes would be a breach of the Non-proliferation treaty.
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. Before this, three grades were recognised, the change in definition for reactor grade, from describing plutonium with greater than 7% Pu-240 content prior to 1976, to reactor grade, being defined as containing 19% or more Pu-240; coincides with the 1977 release of information about a 1962 "reactor grade nuclear test".
- Super weapons grade, less than 3% Pu-240,
- Weapons grade, less than 7% Pu-240 and
- Reactor grade, 7% or more Pu-240.
From 1976, four grades were recognised:
- Super weapons grade, less than 3% Pu-240
- Weapons grade, less than 7% Pu-240,
- Fuel grade, 7% to 19% Pu-240 and
- Reactor grade, more than 19% Pu-240.
The physical mixture of isotopes in reactor-grade plutonium make it extremely difficult to handle and form and therefore explain its unsuitability as a weapon-making substance, in contrast to weapons grade plutonium, which can be handled relatively safely with thick gloves.
To produce weapons grade plutonium, the uranium nuclear fuel must spend no longer than around several weeks in the reactor core before being removed, creating a low fuel burnup. For this to be carried out in for example, a pressurized water reactor - the most common reactor design for electricity generation - the reactor would have to prematurely reach cold shut down after only recently being fueled, meaning that the reactor would need to cool decay heat and then have its reactor pressure vessel be depressurized, followed by a fuel rod defueling. If such an operation were to be conducted, it would be easily detectable, and require prohibitively costly reactor modifications.
One such example of how this process would be detected in PWRs, is that during these periods, there would be a considerable amount of down time, that is, large stretches of time that the reactor is not producing electricity to the grid. On the other hand, the modern definition of "reactor grade" plutonium is produced only when the reactor is run at high burnups and therefore producing a high electricity generating capacity factor. According to the US Energy Information Administration (EIA), in 2009 the capacity factor of US nuclear power stations was higher than all other forms of energy generation, with nuclear reactors producing power approximately 90.3% of the time and Coal thermal power plants at 63.8%, with down times being for simple routine maintenance and refuelling.
"Reactor-grade" plutonium nuclear tests and typical burnup
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 nuclear reprocessing in the USA.
The plutonium used for the US-UK 1962 device was apparently sourced from the military Magnox reactors at Calder Hall or Chapelcross in the United Kingdom, and provided to the US under the 1958 US-UK Mutual Defence Agreement. Only two US-UK underground nuclear tests occurred in 1962, the first being test shot Pampas of Operation Nougat which produced a yield of 9.5 kilotons and the second being test shot Tendrac of Operation Storax, which produced a yield cited as being "low"(under 20 kilotons). Although not necessarily of the same design, a point of comparison is the 2006 North Korean nuclear test for example, with the test devices plutonium again being produced in a Magnox reactor, this time located at Yongbyon Nuclear Scientific Research Center, and which resulted in the creation of a low yield fizzle explosion, producing an estimated yield of approximately 0.48 kilotons.
The isotopic composition of the 1962 US-UK test 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. The isotopic composition of the plutonium in the material used in the US-UK 1962 test is estimated to have been at least 85% plutonium-239, much higher than typical spent fuel from currently operating reactors.
For example, a generic Pressurized water reactor's spent nuclear fuel isotopic composition, following a typical Generation II reactor 45 GWd/tU of burnup, is 1.11% plutonium of which 0.56% is Pu-239, and 0.28% is Pu-240, which corresponds to a Pu-239 content of 50.5% and a Pu-240 content of 25.2%. For a lower generic burn-up rate of 43,000 MWd/t, as published in 1989, the plutonium-239 content was 53% of all plutonium isotopes in the reactor spent nuclear fuel.
As the above two examples display, the odd numbered fissile plutonium isotopes present in spent nuclear fuel, such as Pu-239, decrease significantly as a percentage of the total composition of all plutonium isotopes(which was 1.11% in the first example above) as higher and higher burnups take place, while the even numbered non fissile plutonium isotopes all increase in percentage - e.g. Pu-238, Pu-240 and Pu-242.