page_title: Plutonium in the environment
section_title: Space
hierarchical_section_title: Plutonium in the environment / Sources of plutonium / Space
caption_attribution_description: English: A cutdrawing of an GPHS-RTG that are used for Galileo, Ulysses, Cassini-Huygens and New Horizons space probes
context_page_description: Since the mid-20th century, plutonium in the environment has been primarily produced by human activity. The first plants to produce plutonium for use in cold war atomic bombs were at the Hanford nuclear site, in Washington, and Mayak nuclear plant, in Russia. Over a period of four decades, "both released more than 200 million curies of radioactive isotopes into the surrounding environment -- twice the amount expelled in the Chernobyl disaster in each instance".
  The majority of plutonium isotopes are short-lived on a geological timescale, though it has been argued that traces of the long-lived ²⁴⁴Pu isotope still exist in nature. This isotope has been found in lunar soil, meteorites, and in the Oklo natural reactor. However, one paper on marine sediments for plutonium in marine sediments, atomic bomb fallout is responsible for 66% of the ²³⁹Pu and 59% ²⁴⁰Pu found in the English Channel, while nuclear reprocessing is responsible for the majority of the ²³⁸Pu and ²⁴¹Pu present in the Earth's oceans.
context_section_description: Plutonium can also be introduced into the environment via the reentry of artificial satellites containing atomic batteries. There have been several such incidents, the most prominent being the Apollo 13 mission. The Apollo Lunar Surface Experiments Package carried on the Lunar Module re-entered the atmosphere over the South Pacific. Many atomic batteries have been of the radioisotope thermoelectric generator (RTG) type. The Plutonium-238 used in RTGs has a half-life of 88 years, as opposed to the plutonium-239 used in nuclear weapons and reactors, which has a half-life of 24,100 years. In April 1964 a SNAP-9A failed to achieve orbit and disintegrated, dispersing roughly 1 kilogram (2.2 lb) of plutonium-238 over all continents. Most plutonium fell in the southern hemisphere. Estimated 6300GBq or 2100 man-Sv of radiation was released  and led to NASA's development of solar photovoltaic energy technology.
  Chain reactions do not occur inside RTGs, so a nuclear meltdown is impossible. In fact, some RTGs are designed so that fission does not occur at all; rather, forms of radioactive decay which cannot trigger other radioactive decays are used instead. As a result, the fuel in an RTG is consumed much more slowly and much less power is produced. RTGs are still a potential source of radioactive contamination: if the container holding the fuel leaks, the radioactive material will contaminate the environment. The main concern is that if an accident were to occur during launch or a subsequent passage of a spacecraft close to Earth, harmful material could be released into the atmosphere. However, this event is extremely unlikely with current RTG cask designs.
  In order to minimise the risk of the radioactive material being released, the fuel is typically stored in individual modular units with their own heat shielding. They are surrounded by a layer of iridium metal and encased in high-strength graphite blocks. These two materials are corrosion and heat-resistant. Surrounding the graphite blocks is an aeroshell, designed to protect the entire assembly against the heat of reentering the Earth's atmosphere. The plutonium fuel is also stored in a ceramic form that is heat-resistant, minimising the risk of vaporization and aerosolization. The ceramic is also highly insoluble.
  The US Department of Energy has conducted seawater tests and determined that the graphite casing, which was designed to withstand reentry, is stable and no release of plutonium should occur. Subsequent investigations have found no increase in the natural background radiation in the area.  The Apollo 13 accident represents an extreme scenario due to the high re-entry velocities  of the craft returning from cislunar space.  This accident has served to validate the design of later-generation RTGs as highly safe.
caption: Diagram of an RTG used on the Cassini probe
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