Plutonium Sources

Plutonium is commonly considered a man-made element, as the vast majority of plutonium found today has been artificially generated in nuclear reactors (Table 1). Small quantities of naturally occurring plutonium isotopes, however, result from the remains of supernova explosions (Pu-244 and Pu-239) and the continuous spontaneous fission of U-238 in uranium-rich ores (Pu-239).

Based on the history of plutonium contamination in the environment since the 1940s and an evaluation of future potential releases, three important sources of plutonium contamination are: (1) the introduction into the atmosphere through nuclear weapons testing; (2) the direct accidental discharge into soils and waters from various civil and military facilities; and (3) the potential accidental losses from the nuclear fuel cycle and from underground nuclear waste depositories. The relative contribution of plutonium release has changed over time with atmospheric nuclear weapons testing predominating in the past, but constituting a smaller portion of the releases as several countries shifted to underground nuclear weapons testing in 1963. Since then, plutonium air concentrations have followed an approximate exponential decrease, with an apparent half-life of around 4 years decreasing to levels below 0.1 p,Bqm 3 (10~7 Bq = 2.7 x 10~18 Ci) in 1993.

Table 1 Plutonium isotopes in the environment

Isotope

Main sources

Half-life (years)

Principal mode of decay

Decay energy (MeV)

Remaining after 10000 yearsa (%)

Pu-238

WFb

87.7

Alpha to U-234

5.593

Pu-239

WF, NIc, Nd

24 110

Alpha to U-235

5.245

~75

Pu-240

WF, NI

6564

Alpha to U-236

5.256

~35

Pu-241

WF

14.35

Beta to Am-241

0.021

Pu-244

N

80000000

Alpha to U-240

4.666

~100

aThe US Environmental Protection Agency proposed a time-frame of 10 000 years or longer as a possible compliance period for risk assessment of the planned Yücca Moüntain, NV nüclear waste depository (http://www.epa.gov/).

cNI = nüclear indüstry releases.

Eisenbüd M (1987) Environmental Radioactivity, 3rd edn. San Diego, CA: Academic Press. http://atom.kaeri.re.kr.

aThe US Environmental Protection Agency proposed a time-frame of 10 000 years or longer as a possible compliance period for risk assessment of the planned Yücca Moüntain, NV nüclear waste depository (http://www.epa.gov/).

cNI = nüclear indüstry releases.

Eisenbüd M (1987) Environmental Radioactivity, 3rd edn. San Diego, CA: Academic Press. http://atom.kaeri.re.kr.

Atmospheric nuclear weapons testing is the primary source of plutonium on a global scale. This has resulted in worldwide deposition of plutonium from atmospheric fallout (fallout plutonium) and in localized high soil contamination levels at the test sites. It has been estimated that since 1945 atmospheric nuclear weapons testing caused the release of approximately 1.33 x 1016Bq (3.59 x 105 Ci) or 6 t of plutonium into the environment. The volatilization of both unfissioned plutonium and plutonium created by neutron irradiation of U-238 contaminates the atmosphere with several isotopes, including Pu-238, Pu-239, Pu-240, and Pu-241. Pu-239 and Pu-240 are usually reported together as 'plutonium' since they are the most abundant isotopes and cannot be easily distinguished by alpha spectrometry.

Based on the relative surface areas of land and sea, approximately two-thirds of plutonium fallout is deposited into the oceans. The measured Pu-239/Pu-240 concentrations in seawater are significantly lower than in soils, which is in part due to dilution effects. In addition, the strong affinity of plutonium for mineral surfaces results in deposition and incorporation of significant amounts of plutonium into sea bottom sediments, making this a predominant sink for plutonium (Table 2).

On a local scale, several cases of accidental plutonium release include those with the construction of plutonium-producing reactors and the extraction of plutonium from irradiated uranium at nuclear research and production centers (e.g., Hanford (Washington) and Los Alamos (New Mexico) in the United States). Later examples of plutonium releases are the nuclear reactor accidents at Windscale, UK (now Sellafield), in 1957 and at Chernobyl, Ukraine, in

Table 2 Environmental plutonium concentrations

1986, the accidental re-entry of a radioisotope-powered navigational satellite (SNAP 9A) over the Indian Ocean in 1964, the crash landing of a B52 bomber in Greenland in 1968, and the gradual release of plutonium from the processing plant at Rocky Flats, USA.

Due to the cessation of atmospheric testing and the potentially increasing utilization of nuclear power, the life cycle of nuclear fuel generation, storage, and use, and the containment of nuclear waste will become the primary (potential) source of plutonium contamination. The nuclear power industry discharged approximately 220 000 t of spent fuel containing about 1400 t of plutonium into temporary storage facilities through the year 2000. Future production is estimated to generate approximately 11 000 t of spent fuel per year. The largest contributor to the radiotoxic inventory (a measure of radiotoxic effects of spent fuel, calculated as the weighted sum of all toxic ingredients) will be Pu-239 and Pu-240, formed from U-238 by the absorption of neutrons in nuclear reactors. These isotopes will account for 80% of the inventory after 300 years, and for 90% after 500 years.

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