The isotopes commonly found in plutonium made in nuclear reactors range from plutonium-238 to plutonium-242. The amount of isotopes other than plutonium-239 produced in military or commercial reactors depends on the nature of the fuel used, the design of the reactor and to length of irradiation time.

Table 1 shows the two most common variants of plutonium isotope mixtures. The first is weapons-grade plutonium (often abbreviated WPu), which contains 93 percent or more plutonium-239. The other is a typical composition of reactor-grade plutonium (often abbreviated RPu), as produced in nuclear power reactors of the light water design. Note that reactor-grade plutonium from light water reactors usually contains more than 20 percent plutonium-240 and more than 10 percent plutonium-241. Plutonium produced in other reactors, such as graphite-moderated reactors (some of which are in operation in Britain, Russia, and elsewhere) or heavy water reactors used in Canada and elsewhere, has a composition in between that shown for weapons grade and reactor grade plutonium in the table. Table 1 also gives the half-life and specific activity of the most common plutonium isotopes. The half-life refers to the amount of time it takes for one-half of the atoms in a given sample to disintegrate. The specific activity (which is inversely related to half-life), indicates the radioactivity of a certain weight of material.

TABLE 1

Plutonium Isotope	Half-life years	Specific activity,curries per gram	Amount in weapons grade plutonium, percent	Amount in reactor grade plutonium, percent1
plutonium-238	87.74	17.3	---	1.3
plutonium-239	24,110	0.063	93.0	56.6
plutonium-240	6,537	0.23	6.5	23.2
plutonium-241	14.4	104	0.5	13.9
plutonium-242	379,00	.004	---	4.9

¹ Typical for pressurized water reactors, the most common type of reactor in operation.

Both weapons grade and reactor grade plutonium contain some plutonium-241. Plutonium-241 decays into americium-241 by emitting a beta particle. Since americium-241 has a far longer half-life (432 years) than plutonium-241 (14.4 years), it builds up as plutonium-241 decays. The gamma radiation from americium-241 decay, which is far stronger than that from plutonium-239, also builds up with the age of the plutonium sample. Therefore, the more plutonium-241 there is and the older the sample, the greater the gamma radiation from the build-up of americium-241.

Since reactor-grade plutonium contains substantial amounts of plutonium-241, the older the sample, the greater the radiation dose to workers handling it. When countries use plutonium separated from light water reactor spent fuel to make mixed oxide fuel (MOX fuel), older plutonium samples result in greater radiation doses to MOX plant workers. Older MOX plants are designed to handle reactor grade plutonium that is less than about two years old after reprocessing (which removes americium isotopes present in the spent fuel). Newer MOX plants can handle reactor-grade plutonium that is about three years old. Thus, countries like Japan and Russia that are not using reactor grade plutonium but have been separating it and stockpiling it for many years have wasted a great deal of money because the older plutonium will probably have to be reprocessed again to remove the americium-241. Therefore, it would be financially prudent, even for MOX proponents, to stop reprocessing until reasonably close to the time when the plutonium is actually fabricated into fuel.

Table 2 shows how americium-241 would build up in a 200 gram sample of reactor grade plutonium containing 1 gram of plutonium-241 at the time of reprocessing. It contains a blank column for the reader to fill in as an exercise to sharpen your technical skills. Note that the half-life of plutonium-241 is 14.4 years and that for every half-life, one-half of the plutonium-241 decays into americium-241. We are neglecting the decay of americium-241 for simplicity and since 28.8 years is a short time compared to its half life of 432 years.

TABLE 2

Isotope	Initial composition	after 2 years	after 5 years	after 14.4 years	after 28.8 years
plutonium-241	1	0.91	?	0.5	?
americium-241	0	0.09	?	0.5	?

TABLE 3

Main Branches of the Plutonium -239 Decay Series

Plutonium-239 (half-life: 24,110) alpha decay

Uranium-235 (half-life: 704,000,000) alpha decay

Thorium-231 (half-life: 25.2 hours) beta decay

Protactinium-231 (half-life: 32,700 years) alpha decay

Actinium (half-life:21.8 years) beta decay

Thorium-227 (half-life: 18.72 days) alpha decay

Radium-233 (half-life:11.43 days) alpha decay

Radon-219 (half-life: 3.96 seconds) alpha decay

Polonium-215 (half-life: 1.78 milliseconds) alpha decay

Lead-211 (half-life: 36.1 minutes) beta decay

Bismuth-211 (half-life: 2.15 minutes) alpha decay

Thallium-207 (half-life: 4.77 minutes) beta decay

Lead-207 (half-life: stable)

Half-life values from CRC Hanbook of Chemistry and Physics