Weapon–grade Plutonium and Uranium, Tracking

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Weapon–grade Plutonium and Uranium, Tracking

Weapon-grade (or “bomb-grade”) uranium or plutonium is any alloy or oxide compound that contains enough of certain isotopes of these elements to serve as the active ingredient in a nuclear weapon. Some civilian weapon-grade materials are tracked by international organizations, especially the United Nations’ International Atomic Energy Agency (IAEA) and the European Atomic Energy Community (EURATOM), to prevent their diversion to bombs. The goal of is to prevent nuclear proliferation, that is, the possession of nuclear weapons by more and more groups.

IAEA safeguards track weapon-grade materials (or, in the case of plutonium, dilute materials that could be refined to weapons grade) in non-military nuclear fuel cycles in states that are signatories to the Non-Proliferation Treaty (NPT) of 1968. Those states that already had nuclear weapons at the time of the treaty’s creation—the United States, United Kingdom, France, Russia, and China—are not subject to IAEA safeguards. EURATOM safeguards civil plutonium and uranium in the European countries, including materials not covered by mandatory IAEA safeguards under the NPT (i.e., those in the United Kingdom and France). The IAEA and EURATOM cooperatively safeguard Europe materials to avoid redundancy.

Military nuclear materials are tracked not by the IAEA but by the governments that own them. Because the tracking techniques employed internally

by nuclear-weapons states vary from nation to nation and are always partly or wholly secret, and since EURATOM safeguards are essentially the same as IAEA safeguards, this article restricts itself to IAEA safeguards on nuclear materials in the 182 non-nuclear-weapons signatories of the NPT.

Definition of “weapon-grade”

“Weapon-grade” uranium or plutonium is sometimes defined as any alloys pure enough to be used in bombs by governments building light-weight nuclear weapons for carriage in missiles, artillery shells, or other delivery systems. The highly-enriched uranium (HEU) preferred by professional weapons designers is more than 90% uranium-235 (²³⁵ U), and the enriched plutonium used for such purposes is nearly pure metal. However, “weapons-grade” is more usefully, and more commonly, defined as any material pure enough to serve in a nuclear weapon, regardless of that weapon’s efficiency or elegance of design. Uranium enriched to only about 50%²³⁵ U, and probably less, can be used to make a crude nuclear bomb, and it is possible to make a bomb from material that is only 15– 25% plutonium (e.g., mixed-oxide fast breeder reactor fuel). A “crude” bomb would probably have an explosive force of several tens of kilotons (where one kiloton equals the explosive force of one thousand tons of trinitrotoluene, TNT), comparable to the bombs which destroyed Hiroshima and Nagasaki in 1945.

Weapon-grade fissile materials are not found in nature, but must be produced.²³⁵ U is found only in dilute form in nature. It constitutes about 0.71% of the uranium in ore, the rest being mostly the isotope²³⁸ U, which is not fissile enough to be a reactor fuel or bomb material. Reactor-grade uranium (i.e., the fuel for civilian nuclear power plants, which is about 3%²³⁵ U) and weapon-grade uranium are obtained by enriching the concentration of²³⁵ U in metal extracted from ore, a complex and expensive industrial process. The nearly pure²³⁸ U that is left over from enrichment, although useless as a fuel or explosive, can be partly transformed into plutonium by neutron bombardment in a particle accelerator or nuclear reactor. (There are several isotopes of plutonium, not all equally suitable for bombs, but because all readily available isotopic blends of plutonium are suitable for bomb-making this article refers simply to “plutonium.”)

These facts are important to the tracking or safeguarding of weapon-grade material. Since²³⁵ Uexistsin nature and only needs to be concentrated to become a bomb material, an ideal tracking system would station observers at every stage of uranium extraction and refinement, from the mine to the enrichment plant. This would cost too much, so the IAEA monitors selected industrial processes, namely enrichment plants, fuel-fabrication facilities, and reprocessing facilities. A reprocessing facility is a factory where nuclear-reactor fuel that has been isotopically altered by irradiation in a reactor core and is no longer isotopically optimal for fuel purposes (“spent” fuel) is dissolved in acid and its²³⁵ U and plutonium separated out. (What is left is high-level nuclear waste.) Reprocessing is the sole source of weapon-grade plutonium, as plutonium occurs in nature only in trace amounts; therefore, IAEA safeguards track not only separated plutonium but spent nuclear fuel.

The safeguarding

Task There are three basic stocks or inventories of weapon-grade (or pre-weapon-grade) material: military inventories, transitional inventories, and civil inventories. Military inventories consist of fissile materials (almost entirely alloys of uranium and plutonium) that are already built into weapons, are stockpiled for possible weapons use, or are stockpiled for or already used in naval reactors. (Due to size constraints, the reactors used to drive some submarines and military surface ships require weapon-grade uranium as fuel). Transitional inventories consist of materials that have been removed from weapons or declared by the states that own them to be in excess of their weapon-making needs. By far the largest transitional stockpile in the world is that of Russia, nuclear inheritor state of the Soviet Union. Civil inventories consist of materials belonging to the nuclear-power fuel cycle, including stockpiled HEU and plutonium separated from spent fuel, plutonium and HEU loaded or stockpiled as fuel for specialized reactors (e.g., fast breeders), and plutonium and HEU in spent reactor fuel of all types. The basic goal of international safeguards is to track materials in the transitional and civil inventories to prevent them from being secretly diverted to weapons by terrorists or governments. (Although the transitional inventories held by nuclear-weapons states are not required by the NPT to submit them to IAEA safeguards, some of them, notably Russia’s, are voluntarily submitted to IAEA safeguards.)

About 3,000 tons of plutonium and HEU have been produced by the civil and nuclear military facilities to date, with hundreds of kilograms of new plutonium forming constantly in the fuel rods of nuclear reactors worldwide. About 700 of these tons are in military inventories, about 1,300 tons in transitional inventories (mostly in the United States and Russia), and about 1,000 tons in civil inventories. Because the civil inventories of some of the largest nuclear-power states (i.e., the U.S., U.K., France, and Russia) are not subject to IAEA safeguards, only about 24 tons of weapon-grade plutonium and uranium—less than 1% of the world stock of these materials—is safeguarded. Though apparently small, this quantity of material could produce hundreds of nuclear weapons, and is exactly that fraction of the world’s HEU and plutonium inventory that is most vulnerable to diversion. The IAEA, like a border patrol, thus deploys its forces along a critical edge rather than spreading them over the domain of all weapons-grade materials.

Safeguards are designed to deter—by making it difficult to conceal—any attempt to concentrate non-weapon-grade nuclear material into weapon-grade material or, alternatively, to divert weapon-grade material from peaceful purposes (e.g., breeder-reactor fuel) to weapons. Safeguards are thus after-the-fact measures designed to detect a material diversion that has already occurred, quickly enough to detect the diversion before a nuclear weapon can be assembled.

Methods

the two basic methods used to track weapons-grade nuclear material are accountancy and physical inspection.

Accountancy

The NPT requires every signatory nation to “establish and maintain [its own] system of accounting for all nuclear material subject to safeguards,” that is, HEU and plutonium. In other words, the IAEA seeks to build on national accounting controls rather than building its own system from scratch. (It does so not because national controls are intrinsically better, but to control costs.) The IAEA specifies, in part, what these national accounting procedures shall be, in order to assure their adequacy and compatibility with the IAEA’s own methods.

In this context, a “system of accounting” means a system of inventorying, similar in principle to that used to run a grocery store. The IAEA defines “material balance areas,” specific physical zones which may be as large as an entire facility or as small as a single room, for which inventories must be kept. Whenever safeguarded materials are brought in or out of a material balance area, records must be made of the amounts, entering, leaving, and in the material balance area. If the totals do not add up, a diversion is suspected.

Inspection

The fact that a nation is responsible for inventorying its own nuclear materials creates an obvious opportunity for cheating: a state might simply fabricate records that show no diversions. To prevent this, the IAEA analyzes each nation’s records for inconsistency or other signs of fraud. More importantly, it conducts on-site inspections. Formerly, inspection consisted mostly of visits by IAEA personnel. Site visits by inspectors remain essential, but with the development of new detection technologies and computer systems, automatic or “unattended” safeguards have become more widely used. For example, all entry points to a material balance area might be recorded continually on sealed videotapes that are later analyzed by the IAEA for suspicious activity. Sensors that detect the presence, type, and quantity of nuclear materials by measuring neutrons, alpha particles, or gamma rays can be located at the access gate of a reprocessing plant, near a fuel-storage area, or in other key locations. (Inspections or video monitoring would assure that temporary exits are not being cut in the perimeter fence elsewhere, to escape surveillance.) The efficacy of inspection is increased by requiring that the NPT signatory state whose materials are being safeguarded submit information about the design of its nuclear facilities to the IAEA. The IAEA may also place special seals on containers of safeguarded material, then re-inspect periodically to verify that the seal has not been broken, and require that an inspector be present if the seal is to be broken. The purpose of inspections is, in short, not to directly track all flows of safeguarded material, but to keep the inventory system honest.

Measurement inaccuracy

All the measurement processes involved in tracking HEU and plutonium have built-in error. If, for example, a particular scale is only accurate to within a milligram, then there is always 1 mg of uncertainty about how much HEU or plutonium is resting on it. If a sample of HEU or plutonium that is known to have a mass of 1.00 g is broken in two and each piece measured on a scale having 1-mg inaccuracy, and if these two measurements both read 0.499 g (for a total of.998 g), it is impossible to tell whether 0.002 g of controlled material has been diverted or the numbers merely reflect random measurement error. To combat this source of uncertainty it is possible to screen for “systematic” errors, that is, errors that trend systematically in some direction; in particular, errors that always show a loss of material rather than a gain would be more suspicious. (Truly random error should sometimes show too much, rather than too little, material.) However, clever manipulation of error margins inside a complicated system could pit false gains against real losses, thus preventing the appearance of systematic error and concealing small, persistent diversions.

In reprocessing facilities, which extract HEU and plutonium from spent fuel, this problem is particularly acute, as a material balance can only be performed on each batch of liquid material processed by the plant. Cumulative material-balance inaccuracies of up to 1% are probably unavoidable in reprocessing. In a reprocessing facility such as the U.K.’s Barnwell, which was originally designed to extract 15 tons of plutonium per year from 1,500 tons of spent fuel, this would mean that 150 kg of plutonium could be diverted undetectably from the plant every year—enough for about 15 atomic bombs. The IAEA Safeguards Division seeks to track inventories in individual states to within 8 kg of plutonium and 25 kg of HEU annually—enough to easily build a single bomb. However, if a state was willing to wait a few years for each bomb, its diversions could remain within the intrinsic error limits.

Resources

BOOKS

Arlt, R., et al. “Use of CdZnTe Detectors in Hand-Held and Portable Isotope Identifiers to Detect Illicit Trafficking of Nuclear Material and Radioactive Sources.” Nuclear Science Symposium Conference Record, Vol. 1, IEEE, 2001: 4–18;4–23