Reactor-grade plutonium (RGPu)The categorisation of nuclear material in the context of integrated safeguardsPlutonium Utilization in several MSR Designs. 2016 is the isotopic grade of plutonium that is found in spent nuclear fuel after the uranium-235 primary fuel that a nuclear power reactor uses has burnt up. The uranium-238 from which most of the plutonium isotopes derive by neutron capture is found along with the U-235 in the low enriched uranium fuel of civilian reactors.
In contrast to the low burnup of weeks or months that is commonly required to produce weapons-grade plutonium (WGPu/239Pu), the long time in the reactor that produces reactor-grade plutonium leads 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. When absorbs a neutron, it does not always undergo nuclear fission. Sometimes neutron absorption will instead produce at the neutron temperatures and fuel compositions present in typical light water reactors, with the concentration of steadily rising with longer irradiation, producing lower and lower grade plutonium as time goes on.
Generation II thermal-neutron reactors (today's most numerous 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 are a handful operating today with a half dozen under construction, can use reactor-grade plutonium fuel as a means to reduce the transuranium content of spent nuclear fuel/nuclear waste. Russia has also produced a new type of Remix fuel that directly recycles reactor grade plutonium at 1% or less concentration into fresh or re-enriched uranium fuel imitating the 1% plutonium level of high-burnup fuel.
Classification by isotopic composition
<1976 >1976<7% Weapons grade7-19%Reactor gradeFuel grade>19% Reactor grade
At the beginning of the industrial scale production of plutonium-239 in war era production reactors, trace contamination or co-production with plutonium-240 was initially observed, with these trace amounts resulting in the dropping of the Thin Man weapon-design as unworkable. The difference in purity, of how much, continues to be important in assessing significance in the context of nuclear proliferation and weapons-usability.
The DOE definition of reactor grade plutonium changed in 1976. Before this, three grades were recognised. The change in the 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". The question of which definition or designation applies, that of the old or new scheme, to the 1962 "reactor-grade" test, has not been officially disclosed.
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.
Reprocessing or recycling of the spent fuel from the most common class of civilian-electricity-generating or power reactor design, the LWR, (with examples being the PWR or BWR) recovers reactor grade plutonium (as defined since 1976), not fuel grade.http://info.ornl.gov/sites/publications/Files/Pub37993.pdf Categorization of Used Nuclear Fuel Inventory in Support of a Comprehensive National Nuclear Fuel Cycle Strategy. page 34 figure 20. Discharge isotopic composition of a WE 17×17 assembly with initial enrichment of 4.5 wt % that has accumulated 45 GWd/tU burnup/https://fas.org/nuke/intro/nuke/plutonium.htm Source: Plutonium Fuel - OECD Report, 1989
The physical mixture of isotopes in reactor-grade plutonium make it extremely difficult to handle and form and therefore explains its undesirability 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 several weeks in the reactor core before being removed, creating a low fuel burnup. For this to be carried out in 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 example of how this process could 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.Electric Power Annual 2009 Table 5.2 April 2011
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. As computations state that the energy yield of a nuclear explosive decreases by one and two orders of magnitude if the 240 Pu content increases from 5% (nearly weapons-grade plutonium) to 15%( 2 kt) and 25%,(0.2 kt) respectively. These computations are theoretical and assume the non-trivial issue of dealing with the heat generation from the higher content of non-weapons usable Pu-238 could be overcome. As the premature initiation from the spontaneous fission of Pu-240 would ensure a low explosive yield in such a device, the surmounting of both issues in the construction of an Improvised nuclear device is described as presenting "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.http://www.aps.org/units/fps/newsletters/2006/april/article2.html American Physical Society Bombs, Reprocessing, and Reactor Grade Plutonium
Gerald E. Marsh and George S. StanfordNUCLEAR WEAPONS PROLIFERATION: Outspoken Opponents of Plutonium Demilitarization Delays and Missteps in Nuclear Demilitarization: Part 4. Alexander DeVolpi, physicist (retired, Argonne National Laboratory); formerly manager of nuclear diagnostics and technical manager of arms control and nonproliferation program; author of Proliferation, Plutonium and Policy.http://www.phyast.pitt.edu/~blc/book/chapter13.html#1 Cohen. Chapter 13—THE NUCLEAR ENERGY OPTION next=> PLUTONIUM AND BOMBS
Others disagree on theoretical grounds and state that while they would not be suitable for stockpiling or being emplaced on a missile for long periods of time, dependably high non-fizzle level yields can be achieved,International Panel on Fissile Materials, Global Fissile Material Report 2011: Nuclear Weapon and Fissile Material Stockpiles and Production (see Appendix 1), retrieved on October 1, 2012.https://fas.org/rlg/980826-pu.htm Richard Lawrence Garwin, Senior Fellow for Science and Technology, Council on Foreign Relations, New York, Draft of August 26, 1998Explosive Properties of Reactor-Grade Plutonium, J. Carson Mark, Frank Von Hippel, Edward Lyman. Science and Global Security, 2009 DOI: 10.1080/08929880903368690Victor Gilinsky, Marvin Miller and Harmon Hubbard, “A Fresh Examination of the Proliferation Dangers of Light Water Reactors,” The Nonproliferation Policy Education Center pg 40 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 pre-detonation created by the presence of 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 pit's 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 a fission 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 seriously pursued creating a nuclear weapon with an isotopic composition similar to modern, high burnup, reactor grade plutonium. All nuclear weapon states have taken the more conventional path to nuclear weapons by either uranium enrichment or producing low burnup, "fuel-grade" and weapons-grade plutonium, in reactors capable of operating as production reactors, 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.States have been willing to make large investments in some cases to acquire weapon-grade rather than reactor-grade plutonium: the United States, for example, in the 1980s, considered spending billions of dollars on the Special Isotope Separation facility to enrich reactor-grade plutonium to weapon-grade.Virtual instruments -- A Los Alamos experiment.Molecular laser isotope separation (MLIS) process in a gaseous plutonium hexafluoride
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. The re-purposing of civilian nuclear industries for military purposes would be a breach of the Non-proliferation treaty.
As nuclear reactor designs come in a wide variety and are sometimes improved over time, the isotopic ratio of what is deemed "reactor grade plutonium" in one design, as it compares to another, can differ substantially. For example, the British Magnox reactor, a Generation I gas cooled reactor(GCR) design, can rarely produce a fuel burnup of more than 2-5 GWd/tU. Therefore, the "reactor grade plutonium" and the purity of Pu-239 from discharged magnox reactors is approximately 80%, depending on the burn up value.page 19, table 1 In contrast, the generic civilian Pressurized water reactor, routinely does (typical for 2015 Generation II reactor) 45 GWd/tU of burnup, resulting in the purity of Pu-239 being 50.5%, alongside a Pu-240 content of 25.2%, The remaining portion includes much more of the heat generating Pu-238 and Pu-241 isotopes than are to be found in the "reactor grade plutonium" from a Magnox reactor.
"Reactor-grade" plutonium nuclear tests