Reactor-grade plutonium (RGPu)[1][2] is the isotopic grade of plutonium that is found inspent nuclear fuel after theuranium-235 primary fuel that a nuclearpower reactor uses hasburnt up. Theuranium-238 from which most of theplutonium isotopes derive byneutron capture is found along with the U-235 in thelow enriched uranium fuel of civilian reactors.
In contrast to the low burnup of weeks or months that is commonly required to produceweapons-grade plutonium (WGPu/239Pu), the long time in the reactor that produces reactor-grade plutonium leads totransmutation of much of thefissile, relatively longhalf-life isotope239Pu into a number of otherisotopes of plutonium that are less fissile or more radioactive. When239
Pu absorbs a neutron, it does not always undergonuclear fission. Sometimesneutron absorption will instead produce240
Pu at theneutron temperatures and fuel compositions present in typicallight water reactors, with the concentration of240
Pu steadily rising with longer irradiation, producing lower and lower grade plutonium as time goes on.
Generation IIthermal-neutron reactors (today's most numerousnuclear power stations) can reuse reactor-grade plutonium only to a limited degree asMOX 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 thetransuranium content ofspent nuclear fuel/nuclear waste. Russia has also produced a new type ofRemix 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.
| <1976 | >1976 | |
|---|---|---|
| <7% | Weapons grade | |
| 7-19% | Reactor grade | Fuel grade |
| >19% | Reactor grade | |
At the beginning of the industrial scale production of plutonium-239 in war eraproduction reactors, trace contamination or co-production withplutonium-240 was initially observed, with these trace amounts resulting in the dropping of theThin Man weapon-design as unworkable.[3] The difference in purity, of how much, continues to be important in assessing significance in the context ofnuclear proliferation and weapons-usability.
TheDOE definition ofreactor gradeplutonium changed in 1976. Before this, three grades were recognised. The change in the definition forreactor grade, from describing plutonium with greater than 7%Pu-240 content prior to 1976, toreactor 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.
From 1976, four grades were recognised:
Reprocessing or recycling of thespent fuel from the most common class of civilian-electricity-generating orpower reactor design, theLWR, (with examples being thePWR orBWR) recoversreactor grade plutonium (as defined since 1976), notfuel grade.[5][6]
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.[4]
To produceweapons grade plutonium, the uranium nuclear fuel must spend no longer than several weeks in the reactor core before being removed, creating a low fuelburnup. For this to be carried out in apressurized water reactor - the most common reactor design for electricity generation - the reactor would have to prematurely reachcold shut down after only recently being fueled, meaning that the reactor would need to cooldecay heat and then have itsreactor pressure vessel be depressurized, followed by afuel rod defueling. If such an operation were to be conducted, it would be easily detectable,[4][1] and require prohibitively costly reactor modifications.[7]
One example of how this process could be detected inPWRs, 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.[8] On the other hand, the modern definition of "reactor grade" plutonium is produced only when the reactor is run at highburnups and therefore producing a high electricity generatingcapacity factor. According to the US Energy Information Administration (EIA), in 2009 thecapacity 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 Coalthermal power plants at 63.8%, with down times being for simple routine maintenance and refuelling.[9]
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The degree to which typicalGeneration II reactor highburn-up produced reactor-grade plutonium is less useful thanweapons-grade plutonium for buildingnuclear weapons is somewhat debated, with many sources arguing that the maximum probable theoretical yield would be bordering on afizzle explosion of the range 0.1 to 2kiloton in aFat Man type device. As computations state that the energy yield of a nuclear explosive decreases by one and twoorders of magnitude if the 240 Pu content increases from 5% (nearly weapons-grade plutonium) to 15%( 2 kt) and 25%,(0.2 kt) respectively.[12] These computations are theoretical and assume the non-trivial issue of dealing with the heat generation from the higher content of non-weapons usablePu-238 could be overcome. As the premature initiation from thespontaneous fission ofPu-240 would ensure a low explosive yield in such a device, the surmounting of both issues in the construction of anImprovised nuclear device is described as presenting "daunting" hurdles for aFat Man-era implosion design, and the possibility of terrorists achieving thisfizzle yield being regarded as an "overblown" apprehension with the safeguards that are in place.[13][7][14][15][16][17]
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,[18][19][20][21][22][23] arguing that it would be "relatively easy" for a well funded entity with access tofusion boostingtritium and expertise to overcome the problem of pre-detonation created by the presence of Pu-240, and that aremote manipulation facility could be utilized in the assembly of the highly radioactivegamma ray emitting bomb components, coupled with a means of cooling the weaponpit during storage to prevent the plutonium charge contained in the pit from melting, and a design that kept theimplosion mechanismshigh 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.[24] The probability that a fission device would fail to achieve this threshold yield increases as theburnup value of the fuel increases.[18]
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. Allnuclear weapon states have taken the more conventional path to nuclear weapons by eitheruranium enrichment or producing low burnup, "fuel-grade" and weapons-grade plutonium, in reactors capable of operating asproduction reactors, the isotopic content of reactor-grade plutonium, created by the most common commercial power reactor design, thepressurized water reactor, never directly being considered for weapons use.[25][26]
As of April 2012, there werethirty-one countries that have civil nuclear power plants,[27] of whichnine have nuclear weapons, and almost everynuclear 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 theNon-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 BritishMagnox reactor, a Generation Igas cooled reactor(GCR) design, can rarely produce a fuelburnup of more than 2-5 GWd/tU.[28][29] Therefore, the "reactor grade plutonium" and the purity of Pu-239 from discharged magnox reactors is approximately 80%, depending on the burn up value.[30] In contrast, the generic civilianPressurized water reactor, routinely does (typical for 2015Generation II reactor) 45 GWd/tU ofburnup, resulting in the purity of Pu-239 being 50.5%, alongside a Pu-240 content of 25.2%,[5][6] The remaining portion includes much more of the heat generatingPu-238 andPu-241 isotopes than are to be found in the "reactor grade plutonium" from a Magnox reactor.
Thereactor grade plutonium nuclear test was a "low-yield (under 20 kilotons)"undergroundnuclear test using non-weapons-gradeplutonium conducted at the USNevada Test Site in 1962.[31][32] Some information regarding this test was declassified in July 1977, under instructions from PresidentJimmy Carter, as background to his decision to prohibitnuclear reprocessing in the US.
The plutonium used for the 1962 test device was produced by the United Kingdom, and provided to the US under the1958 US-UK Mutual Defence Agreement.[31]
The initial codename for the Magnox reactor design amongst the government agency which mandated it, theUKAEA, was thePressurised Pile Producing Power and Plutonium (PIPPA) and as this codename suggests, the reactor was designed as both a power plant and, when operated with low fuel "burn-up"; as a producer of plutonium-239 for the nascent nuclear weapons program in Britain.[33] This intentional dual-use approach to building electric power-reactors that could operate as production reactors in the earlyCold War era, was typical of many nations'Generation I reactors.[34] With these being designs all focused on giving access to fuel after a short burn-up, which is known asOnline refuelling.
The2006 North Korean nuclear test, the first by the DPRK, is also said to have had a Magnox reactor as the root source of its plutonium, operated inYongbyon Nuclear Scientific Research Center in North Korea. This test detonation resulted in the creation of a low-yield fizzle explosion, producing an estimated yield of approximately 0.48 kilotons,[35] from an undisclosed isotopic composition. The2009 North Korean nuclear test likewise was based on plutonium.[36] Both produced a yield of 0.48 to 2.3 kiloton of TNT equivalent respectively and both were described as fizzle events due to their low yield, with some commentators even speculating whether, at the lower yield estimates for the 2006 test, the blast may have been the equivalent of US$100,000 worth ofammonium nitrate.[37][38]
The isotopic composition of the 1962 US-UK test has similarly not been disclosed, other than the descriptionreactor grade, and it has not been disclosed which definition was used in describing the material for this test asreactor grade.[31] According to Alexander DeVolpi, the isotopic composition of the plutonium used in the US-UK 1962 test could not have been what we now consider to be reactor-grade, and theDOE now implies, but doesn't assert, that the plutonium was fuel grade.[14] Likewise, theWorld Nuclear Association suggests that the US-UK 1962 test had at least 85%plutonium-239, a much higher isotopic concentration than what is typically present in the spent fuel from the majority of operating civilian reactors.[39]
In 2002 former Deputy Director General of the IAEA, Bruno Pelaud, stated that the DoE statement was misleading and that the test would have the modern definition of fuel-grade with a Pu-240 content of only 12%[40]
In 1997 political analystMatthew Bunn and presidential technology advisorJohn Holdren, both of theBelfer Center for Science and International Affairs, cited a 1990s official U.S. assessment of programmatic alternatives for plutonium disposition. While it does not specify which RGPu definition is being referred to, it nonetheless states that "reactor-grade plutonium (with an unspecified isotopic composition) can be used to produce nuclear weapons at all levels of technical sophistication," and "advanced nuclear weapon states such as the United States and Russia, using modern designs, could produce weapons from "reactor-grade plutonium" having reliable explosive yields, weight, and other characteristics generally comparable to those of weapons made from weapon-grade plutonium"[41]
In a 2008 paper, Kessler et al. used a thermal analysis to conclude that a hypothetical nuclear explosive device was "technically unfeasible" using reactor grade plutonium from a reactor that had a burn up value of 30 GWd/t using "low technology" designs akin toFat Man with spherical explosive lenses, or 55 GWd/t for "medium technology" designs.[42]
According to the Kessler et al. criteria, "high-technology" hypothetical nuclear explosive devices (HNEDs), that could be produced by the experiencednuclear weapons states (NWSs) would be technically unfeasible with reactor-grade plutonium containing more than approximately 9% of the heat generatingPu-238 isotope.[43][44]
The British Magnox reactor, a Generation Igas cooled reactor (GCR) design, can rarely produce a fuelburnup of more than 2-5 GWd/tU.[45][29] The Magnox reactor design was codenamedPIPPA (Pressurised Pile Producing Power and Plutonium) by theUKAEA to denote the plant's dual commercial (power reactor) and military (production reactor) role. The purity of Pu-239 from discharged magnox reactors is approximately 80%, depending on the burn up value.[30]
In contrast, for example, a generic civilianPressurized water reactor'sspent nuclear fuel isotopic composition, following a typicalGeneration II reactor 45 GWd/tU ofburnup, 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%.[46] For a lower generic burn-up rate of 43,000MWd/t, as published in 1989, the plutonium-239 content was 53% of all plutonium isotopes in the reactorspent nuclear fuel.[6] The USNRC has stated that the commercial fleet ofLWRs presently powering homes, had an averageburnup of approximately 35 GWd/MTU in 1995, while in 2015, the average had improved to 45 GWd/MTU.[47]
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 (e.g.Pu-238,Pu-240 andPu-242) increasingly accumulate in the fuel over time.[48]
As power reactor technology develops, one goal is to reduce the spent nuclear fuel volume by increasing fuel efficiency and simultaneously reducing down times as much as possible to increase the economic viability of electricity generated fromfission-electric stations. To this end, the reactors in the U.S. have doubled their average burn-up rates from 20 to 25 GWd/MTU in the 1970s to over 45 GWd/MTU in the 2000s.[29][49]Generation III reactors under construction have a designed-forburnup rate in the 60 GWd/tU range and a need to refuel once every 2 years or so. For example, theEuropean Pressurized Reactor has a designed-for 65 GWd/t,[50] and theAP1000 has a designed for average discharge burnup of 52.8 GWd/t and a maximum of 59.5 GWd/t.[50] In-designgeneration IV reactors will haveburnup rates yet higher still.
Today's moderated/thermal reactors primarily run on theonce-through fuel cycle though they can reuse once-through reactor-grade plutonium to a limited degree in the form of mixed-oxide orMOX fuel, which is a routine commercial practice in most countries outside the US as it increases the sustainability of nuclear fission and lowers the volume of high level nuclear waste.[54]
One third of the energy/fissions at the end of the practical fuel life in a thermal reactor are from plutonium, the end of cycle occurs when theU-235 percentage drops, the primary fuel that drives theneutron economy inside the reactor and the drop necessitates fresh fuel being required, so without design change, one third of the fissile fuel in a new fuel load can be fissile reactor-grade plutonium with one third less ofLow enriched uranium needing to be added to continue the chain reactions anew, thus achieving a partial recycling.[55]
A typical 5.3% reactor-grade plutonium MOX fuel bundle, istransmutated when it itself is again burnt, a practice that is typical in French thermal reactors, to a twice-through reactor-grade plutonium, with an isotopic composition of 40.8%239
Pu and 30.6%240
Pu at the end of cycle (EOC).[56][note 2] MOX grade plutonium (MGPu) is generally defined as having more than 30%240
Pu.[1]
A limitation in the number of recycles exists withinthermal reactors, as opposed to the situation in fast reactors, as in thethermal neutron spectrum only the odd-massisotopes of plutonium arefissile, the even-mass isotopes thus accumulate, in all high thermal-spectrum burnup scenarios.Plutonium-240, an even-mass isotope is, within the thermal neutron spectrum, afertile material likeuranium-238, becoming fissileplutonium-241 on neutron capture; however, the even-massplutonium-242 not only has a lowneutron capturecross section within the thermal spectrum, it also requires 3neutron captures before becoming a fissile nuclide.[55]
While most thermal neutron reactors must limit MOX fuel to less than half of the total fuel load for nuclear stability reasons, due to the reactor design operating within the limitations of a thermal spectrum of neutrons,Fast neutron reactors on the other hand can use plutonium of any isotopic composition, operate on completely recycled plutonium and in the fast "burner" mode, or fuel cycle, fission and thereby eliminate all the plutonium present in the world stockpile of once-through spent fuel.[57] The modernized IFR design, known as theS-PRISM concept and theStable salt reactor concept, are two such fast reactors that are proposed to burn-up/eliminate theplutonium stockpiles in Britain that was produced from operating its fleet of Magnox reactors generating the largest civilian stockpile of fuel-grade/"reactor-grade plutonium" in the world.[58]
In Bathke's equation on "attractiveness level" ofWeapons-grade nuclear material, the Figure of Merit(FOM) the calculation generates, returns the suggestion that Sodium Fast Breeder Reactors are unlikely to reach the desired level of proliferation resistance, while Molten Salt breeder reactors are more likely to do so.[59]
In thefast breeder reactor cycle, or fast breeder mode, as opposed to the fast-burner, the FrenchPhénix reactor uniquely demonstrated multi-recycling and reuses of its reactor grade plutonium.[60] Similar reactor concepts and fuel cycling, with the most well known being theIntegral Fast Reactor are regarded as one of the few that can realistically achieve "planetary scale sustainability", powering a world of 10 billion, whilst still retaining a small environmental footprint.[61] In breeder mode, fast reactors are therefore often proposed as a form ofrenewable or sustainable nuclear energy. Though the "[reactor-grade]plutonium economy" it would generate, presently returns social distaste and varied arguments about proliferation-potential, in the public mindset.
As is typically found in civilian European thermal reactors, a 5.3% plutonium MOX fuel-bundle, produced by conventional wet-chemical/PUREXreprocessing of an initial fuel assembly that generated 33 GWd/t before becomingspent nuclear fuel, creates, when it itself is burnt in the thermal reactor, aspent nuclear fuel with a plutonium isotopic composition of 40.8%239
Pu and 30.6%240
Pu.[56][note 2]
Computations state that the energy yield of a nuclear explosive decreases by twoorders of magnitude if the240
Pu content increases to 25%,(0.2 kt).[12]
Reprocessing, which mainly takes the form of recycling reactor-grade plutonium back into the same or a more advanced fleet of reactors, was planned in the US in the 1960s. At that time theuranium market was anticipated to become crowded and supplies tight so together with recycling fuel, the more efficientfast breeder reactors were thereby seen as immediately needed to efficiently use the limited known uranium supplies. This became less urgent as time passed, with both reduced demand forecasts and increased uranium ore discoveries, for these economic reasons, fresh fuel and the reliance on solely fresh fuel remained cheaper in commercial terms than recycled.
In 1977 the Carter administration placed a ban on reprocessing spent fuel, in an effort to set an international example, as within the US, there is the perception that it would lead to nuclear weapons proliferation.[62] This decision has remained controversial and is viewed by many US physicists and engineers as fundamentally in error, having cost the US taxpayer andthe fund generated by US reactor utility operators, with cancelled programs and the over 1 billion dollar investment into the proposed alternative, that ofYucca Mountain nuclear waste repository ending in protests, lawsuits and repeated stop-and-go decisions depending on the opinions of new incoming presidents.[63][64]
As the "undesirable" contaminant from a weapons manufacturing viewpoint,240
Pu, decays faster than the239
Pu, with half-lives of 6500 and 24,000 years respectively, the quality of the plutonium grade increases with time (although its total quantity decreases during that time as well). Thus, physicists and engineers have pointed out, as hundreds/thousands of years pass, the alternative to fast reactor "burning" or recycling of the plutonium from the world fleet of reactors until it is all burnt up, the alternative to burning most frequently proposed, that of deepgeological repository, such asOnkalo spent nuclear fuel repository, have the potential to become "plutonium mines", from which weapons-grade material for nuclear weapons could be acquired by simplePUREX extraction, in the centuries-to-millennia to come.[67][22][68]
Aum Shinrikyo, who succeeded in developingSarin andVX nerve gas is regarded to have lacked the technical expertise to develop, or steal, a nuclear weapon. Similarly,Al Qaeda was exposed to numerous scams involving the sale of radiological waste and other non-weapons-grade material. TheRAND corporation suggested that their repeated experience of failure and being scammed has possibly led to terrorists concluding that nuclear acquisition is too difficult and too costly to be worth pursuing.[69]
{{cite web}}: CS1 maint: archived copy as title (link)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 and 25%, respectively.
{{cite web}}:|author= has generic name (help){{cite journal}}:Cite journal requires|journal= (help)But there is no doubt that the reactor-grade plutonium obtained fromreprocessingLWRspent fuel can readily be used to make high-performance, high-reliability nuclear weaponry, as explained in the 1994Committee on International Security and Arms Control (CISAC) publication.
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