What Are Isotopes?

Isotopes are forms of an element which have nearly identical chemical and physical properties but different nuclear properties. The chemical properties of elements are fixed by the number of positively charged protons in their nuclei and by the corresponding number of negatively charged electrons that they carry. The isotopes of an element have nuclei containing the same number of protons but different numbers of neutrons. Neutrons are electrically neutral, and they are important in causing the nucleus to fission, releasing a relatively large amount of energy.

Many isotopes are radioactive. They emit several main kinds of radiation, including: alpha particles, which carry positive charges and consist of two protons and two neutrons (the helium 4 nucleus); beta particles which are energetic electrons (negatively charged) or positrons (positively charged); and gamma rays, electromagnetic radiation which has no charge and are highly penetrating. Neutrons and various subatomic particles can also be released.

A key characteristic of a radioactive isotope is its half-life, which is the time taken for a quantity of an isotope to halve through radioactive decay. Half-lives can vary from fractions of seconds to hundreds of millions of years.

Fissionable Isotopes (1)

The most common isotopes in nuclear weapons are plutonium 239 and uranium 235, and each nuclear weapon in existence today uses at least several kilograms of these materials. These materials are fissile materials, which are defined technically as those isotopes that fission when irradiated with relatively low-energy, or thermal, neutrons. However, fissile materials are also commonly referred to as plutonium or highly enriched uranium (HEU).

Uranium 233, which is a fissile material, is also widely recognized as a nuclear explosive material. However, it has been used only infrequently in nuclear explosives or weapons.

The special role of these three isotopes has been recognized by the IAEA in its definition of "special fissionable materials," which is plutonium 239, uranium 233, uranium enriched in the isotopes uranium 233 and uranium 235, or any material containing one or more of the foregoing. These isotopes are subject to IAEA safeguards (see section V).

To define the verification goal of safeguards, the IAEA in conjunction with the nuclear weapon states, has developed the concept of significant quantity (SQ). This is the approximate amount of nuclear material sufficient to make a nuclear explosive, taking into account any losses during processing. For plutonium, containing less than 80 percent plutonium 238, the SQ is 8 kilograms of total plutonium. For uranium 233, the SQ is 8 kilograms. For highly enriched uranium, the SQ is 25 kilograms of contained uranium 235. For example, 90 percent enriched HEU, containing 25 kilograms of uranium 235, would have a total mass of 27.8 kilograms.

Other isotopes can be used to make nuclear explosives. Although they are not strictly speaking fissile materials, they are fissionable and can sustain a chain reaction. Attention among members of the international community is focusing increasingly on neptunium 237 and americium, leading to more controls of these materials (2). Although the IAEA's Board of Governors is considering applying more monitoring of these materials, it is unlikely to define these materials as special fissionable materials any time soon. Other isotopes can also be used in nuclear explosives, but they are too rare or radioactive to be worrisome.

Uranium 235. Uranium (U) has 92 electrons and 92 protons (the atomic number). Of the 14 isotopes in the sequence uranium 227 to uranium 240 (the mass numbers), uranium 235 and uranium 238 are the most important. With half-lives of 700 million and 4,500 million years respectively, uranium 235 and uranium 238 are relatively stable isotopes. They are not strongly radioactive and can be handled without the need for substantial protection.

Naturally occurring uranium consists of 99.283 percent (by weight) of uranium 238, 0.711 percent of uranium 235, and 0.0055 percent of uranium 234. Uranium 235 is a fissile isotope. Uranium 238 is not fissile, and no amount of it can sustain a chain reaction. It is fertile, which means it can be readily transformed into a fissile isotope by neutron irradiation.

For nuclear weapons, and for fuel burned in many types of nuclear reactors, it is necessary to increase concentrations of uranium 235. This is the process known as "enrichment" (see section II).

The following five grades of uranium are commonly recognized:

1. Depleted uranium, containing less than 0.71 percent uranium 235.
2. Natural uranium, containing 0.71 percent uranium 235.
3. Low-enriched uranium (LEU), containing more than 0.71 percent and less than 20 percent uranium 235.
4. Highly enriched uranium (HEU), containing more than 20 percent uranium 235.
5. Weapon-grade uranium, HEU containing more than 90 percent uranium 235.

LEU used to fuel commercial power reactors generally contains 2-6 percent uranium 235. Research and naval reactors use either LEU or HEU fuel.

LEU cannot be used to make nuclear explosives; HEU can be used to make nuclear explosives. For fission-type nuclear weapons, weapon-grade uranium is usually desired. However, fission-type nuclear explosives can be made with any highly enriched uranium. For example, South Africa's nuclear weapons, since dismantled, used both 80 percent enriched uranium and weapon-grade uranium. In addition, the secondary in a thermonuclear weapon may also use HEU to trigger the thermonuclear explosion.

Uranium 233. This isotope, which is created by irradiating thorium 232 with neutrons, has a half-life of 160,000 years. It is a fissile material that has been evaluated for use in nuclear weapons, although it has not become a common nuclear explosive material. It has also been evaluated as reactor fuel, via the thorium fuel cycle, but this fuel cycle has not advanced beyond the research and development stage.

Plutonium isotopes. Unlike uranium, all but trace quantities of plutonium (Pu) are manufactured material. The most common plutonium isotopes are highly radioactive, complicating their handling.

Plutonium 239 is produced in a nuclear reactor when uranium 238 is irradiated with neutrons. Its half-life is 24,000 years, and it is a fissile material. When it absorbs neutrons in a reactor, plutonium 240 is formed. Subsequent neutron captures lead to accumulations of plutonium 241 and plutonium 242. Plutonium 241 is fissile, but plutonium 240 and plutonium 242 are not. However, all of these plutonium isotopes are fissionable by fast neutrons, and thus can be used either in combination or alone in nuclear explosives. Although the weapon designer's preference is always for material with high concentrations of plutonium 239 and low fractions of other plutonium isotopes, militarily useful weapons can be made out of plutonium with low concentrations of plutonium 239 and high concentrations of plutonium 240, plutonium 241, or plutonium 242.

The plutonium used in nuclear weapons typically contains mostly plutonium 239 and relatively small fractions of other plutonium isotopes. Plutonium discharged in power reactor fuel typically contains significantly less plutonium 239 and more of other plutonium isotopes.

The following grades of plutonium are widely used:

1. Weapon-grade plutonium, containing less then 7 percent plutonium 240.
2. Fuel-grade plutonium, containing from 7 to 18 percent plutonium 240.
3. Reactor-grade plutonium, containing over 18 percent plutonium 240.

The term "super-grade plutonium" is sometimes used to describe plutonium containing less than 3 percent plutonium 240. The term "weapon-usable plutonium" is often used to describe plutonium that is in separated form and, thus able to be quickly turned into weapons components (see key terms).

Neptunium 237. Neptunium 237 (Np 237) has a half-life of over 2 million years and has no heat or radiation properties that would complicate its use in a nuclear explosive. No country is known publicly to have used neptunium to make a nuclear explosive device, although it is considered usable in nuclear weapons.

Neptunium 237 is routinely produced in nuclear reactors as a result of neutron irradiation of uranium 235 and uranium 238, the two most common constituents of nuclear fuel. It is also a decay product of americium 241. However, relatively little neptunium 237 has been extracted from irradiated fuel, unlike the case of plutonium.

Americium. The common americium (Am) isotopes are generally less suitable than neptunium 237 for making nuclear explosives, because of their higher output of radiation and heat. The three most important isotopes are americium 241, americium 242m, and americium 243.

Several americium isotopes originate as a result of neutron irradiation in reactors; americium 241 originates from the decay of plutonium 241. The total americium content of fresh spent fuel is modest, although over time considerable amounts of americium 241 accumulate.

Other isotopes. Several other isotopes, such as curium (Cm) and californium (Cf), can be used to make nuclear explosives. However, these isotopes are too rare, particularly in separated form, or too radioactive to be considered as realistic materials for nuclear explosives for at least several decades.

Tritium

Some have suggested including tritium in a treaty, although it is neither fissile nor fissionable material. Tritium is the heaviest isotope of hydrogen, containing one proton and two neutrons. It has a half-life of 12.3 years.

Although tritium is not essential to making nuclear weapons or explosives, it serves two purposes in designing nuclear weapons. In a fission weapon, tritium is used to increase the yield of the weapon in a process known as boosting. In a common form of boosting, tritium and deuterium are fused in the hollow sphere of the fissile core known as the "pit." When tritium and deuterium fuse, they produce many high-energy neutrons, which then set off additional fissions in the fissile core of the weapon. Using tritium in nuclear weapons can either lessen the amount of fissile material required or increase, or "boost," the yield of the weapon.
Tritium is also created in the secondary stage of a thermonuclear weapon and is critical to creating the fusion explosion that distinguishes thermonuclear weapons from fission weapons. After the first, or fission, stage of a thermonuclear weapon detonates, tritium is produced in lithium 6 in the secondary stage of the weapon. The tritium then fuses with deuterium in the second stage to create a fusion explosion.

In the nuclear weapon states, tritium has been commonly produced in nuclear reactors by bombarding lithium 6 with neutrons (see appendix III, figure III.A.1). Tritium can also be extracted from irradiated heavy water that has been used to moderate or cool certain types of reactors. In this case, tritium is produced by neutron irradiation of deuterium, which is a hydrogen isotope that contains one proton and one neutron and that substitutes for common hydrogen in water.

(1) This section draws heavily from David Albright, Frans Berkhout, and William Walker, Plutonium and Highly Enriched Uranium 1996: World Inventories, Capabilities, and Policies (Oxford: SIPRI and Oxford University Press, 1997), chapter 2.

(2) David Albright and Lauren Barbour, "Separated Neptumium and Americium," in The Challenges of Fissile Material Control (Washington D.C.: ISIS, 1999) chapter 5.