Uranium and plutonium are composed of several isotopes, some of which are fissile. To produce an explosive device for military purposes requires the percentage of fissile isotopes (U-235 for uranium, Pu-239 for plutonium) present in the material to be of the order of 93%. The levels reached in the nuclear power industry are, however, much lower; less than 5% for uranium and between 50 and 60% for plutonium.
Virtually any combination of plutonium isotopes -- the different forms of an element, having different numbers of neutrons in their nuclei -- can be used to make a nuclear weapon. Not all combinations, however, are equally convenient or efficient. The most common isotope, Pu-239, is produced when the most common isotope of uranium, U-238, absorbs a neutron and then quickly decays to plutonium. It is this plutonium isotope that is most useful in making nuclear weapons, and it is produced in varying quantities in virtually all operating nuclear reactors.
Plutonium containing high quantities of fissile material i.e. Pu-239 in the order of 90-95 %, is known as weapon-grade plutonium. Plutonium containing lower concentrations, in the range of 50-60 % is known as reactor-grade plutonium. The definitions of the various plutonium grades are expressed as a percentage of the isotope Pu-240 which is considered as an impurity for weapons manufacturers.
||% of Plutonium- 240
||between 7% and 18% incl.
Reactor-grade plutonium is produced in the core of a reactor when uranium-238 is irradiated with neutrons. Unlike weapon grade plutonium (which is relatively pure plutonium-239), reactor grade plutonium is a mixture of plutonium-238, 239, 240, 241 and 242. It is this mixture of isotopes which renders reactor grade plutonium less suitable as a weapon-grade material. Weapon-grade plutonium is defined as plutonium containing no more than 7 percent plutonium-240. Due to the very short 100
day irradiation periods used during the war (wartime production meant that the plutonium had to be separated as quickly as feasible after being bred), the first Plutonium bomb [Fat Man] used super-grade weapon plutonium containing only about 0.9% Pu-240.
As fuel in a nuclear reactor is exposed to longer and longer periods of neutron irradiation, higher isotopes of plutonium build up as some of the plutonium absorbs additional neutrons, creating Pu-240, Pu-241, and so on. Pu-238 also builds up from a chain of neutron absorptions and radioactive decays starting from U-235. Because of the preference for relatively pure Pu-239 for weapons purposes, when a reactor is used specifically for creating weapons plutonium, the fuel rods are removed and the plutonium is separated from them after relatively brief irradiation (at low "burnup"). The resulting "weapons-grade" plutonium is typically about 93 percent Pu-239. Such brief irradiation is quite inefficient for power production, so in power reactors the fuel is left in the reactor much longer, resulting in a mix that includes more of the higher isotopes of plutonium ("reactor grade" plutonium).
Normally for electrical power production the uranium fuel remains in the reactor for three to four years, which produces a plutonium of 60 percent or less Pu-239, 25 percent or more Pu-240, 10 percent or more Pu-241, and a few percent Pu-242. The Pu-240 has a high spontaneous rate of fission, and the amount of Pu-240 in weapons-grade plutonium generally does not exceed 6 percent, with the remaining 93 percent Pu-239.
Higher concentrations of Pu-240 can result in pre-detonation of the weapon, significantly reducing yield and reliability. For the production of weapons-grade plutonium with lower Pu-240 concentrations, the fuel rods in a reactor have to be changed frequently, about every four months or less.
Some nuclear weapons are typically designed so that a pulse of neutrons will start the nuclear chain reaction at the optimum moment for maximum yield; background neutrons from plutonium-240 can set off the reaction prematurely, and with reactor-grade plutonium the probability of such "pre-initiation" is large. Pre-initiation can substantially reduce the explosive yield, since the weapon may blow itself apart and thereby cut short the chain reaction that releases the energy.
Nevertheless, even if pre-initiation occurs at the worst possible moment (when the material first becomes compressed enough to sustain a chain reaction) the explosive yield of even a relatively simple first-generation nuclear device would be of the order of one or a few kilotons. While this yield is referred to as the "fizzle yield," a one-kiloton bomb would still have a radius of destruction roughly one-third that of the Hiroshima weapon, making it a potentially fearsome explosive. Regardless of how high the concentration of troublesome isotopes is, the yield would not be less.
The even numbered isotopes (plutonium-238, 240 and 242) fission spontaneously producing high energy neutrons and a lot of heat. Dealing with the second problem with reactor-grade plutonium, the heat generated by plutonium-238 and plutonium-240, requires careful management of the heat in the device. There are well developed means for addressing these problems and they are not considered a significant hurdle to the production of nuclear weapons, even for developing states.
Because these even numbered plutonium isotopes are more radioactive, their presence accelerates the formation of defects that occur within the metal during alpha decay of plutonium. In fact, the neutron and gamma dose from this material is significant and the heat generated in this way could melt the high-explosive material needed to compress the critical mass prior to initiation. The neutrons can also initiate a premature chain reaction thus reducing the explosive yield, typically to a few percent of the nominal yield, sometimes called the "fizzle yield". Such physical characteristics make reactor-grade plutonium difficult to manipulate and control and therefore explain its unsuitability as a bomb-making ingredient. The isotope plutonium-238 would typically consitute only 0.036 percent of weapons-grade plutonium.
While reactor-grade plutonium has a slightly larger critical mass than weapon-grade plutonium (meaning that somewhat more material would be needed for a bomb), this would not be a major impediment for design of either crude or sophisticated nuclear weapons.
A successful test was conducted in 1962, which used reactor-grade plutonium in the nuclear explosive in place of weapon-grade plutonium. The yield was less than 20 kilotons. This test was conducted to obtain nuclear design information concerning the feasibility of using reactor-grade plutonium as the nuclear explosive material. The test confirmed that reactor-grade plutonium could be used to make a nuclear explosive. This fact was declassified in July 1977. The release of additional information was deemed important to enhance public awareness of nuclear proliferation issues associated with reactor-grade plutonium that can be separated during reprocessing of spent commercial reactor fuel.
The United States maintains an extensive nuclear test data base and predictive capabilities. This information, combined with the results of this low yield test, reveals that weapons can be constructed with reactor-grade plutonium. Prior to the 1970's, there were only two terms in use to define plutonium grades: weapon-grade (no more than 7 percent Pu-240) and reactor-grade (greater than 7 percent Pu-240). In the early 1970's, the term fuel-grade (approximately 7 percent to 19 percent Pu-240) came into use, which shifted the reactor-grade definition 19 percent or greater Pu-240.
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 weapons-grade plutonium.
Reactor-grade plutonium is significantly more radioactive which complicates the design, manufacture and stockpiling of weapons. Use of reactor-grade plutonium would require large expenditures for remote manufacturing facilities to minimize radiation exposure to workers. Reactor-grade plutonium use in weapons would cause concern over radiation exposure to military service personnel.
The greater radioactivity would mean increased radiation doses to workers fabricating such weapons, and military personnel spending long periods of time in close proximity to them, and the greater heat and radiation generated from reactor-grade plutonium might result in a need to replace certain weapon components more frequently.
The odd numbered isotope, plutonium-241, is also a highly undesirable isotope as it decays to americium-241 which is an intense emitter of alpha particles, X and gamma rays. Plutonium-241 has a half-life of 13.2 years which means americium-241 accumulates quickly causing serious handling problems. Reactor-grade plutonium is significantly more radioactive which complicates the design, manufacture and stockpiling of weapons. Use of reactor-grade plutonium would require large expenditures for remote manufacturing facilities to minimize radiation exposure to workers. Reactor-grade plutonium use in weapons would cause concern over radiation exposure to military service personnel.
Weapon-grade plutonium has different characteristics. It contains mainly Pu-239 which has a half-life of 24 000 years and only very small quantities of Pu-241 (unlike reactor-grade plutonium which can contain around 15% Pu-241.) It is thus relatively stable and can be safely handled with a pair of thick gloves.
To achieve the high percentages of Pu-239 required for weapon grade plutonium, it must be produced specifically for this purpose. The uranium must spend only several weeks in the reactor core and then be removed. For this to be carried out in a LWR - the prevalent reactor design for electricity generation - the reactor would have to be shut down completely for such an operation; this is easily detectable.
The Isotopic Composition of Reactor and Weapon Grade Plutonium
|| Pu-238 (%)
|| Pu-239 (%)
(3,7% U-235, 43,000 MWd/t)1
| Weapon-Grade Plutonium
1 Source: Plutonium Fuel - OECD Report, 1989.
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