modules as used in RTGs
Theft Radioactive materials in RTGs are dangerous and can be used for malicious purposes. They are not useful for a genuine
nuclear weapon, but still can serve in a "
dirty bomb". The
Soviet Union constructed many uncrewed lighthouses and navigation beacons powered by RTGs using
strontium-90 (Sr). They are very reliable and provide a steady source of power. Most have no protection, not even fences or warning signs, and the locations of some of these facilities are no longer known due to poor record keeping. In one instance, the radioactive compartments were opened by a thief. There are about 1,000 such RTGs in Russia, all of which have long since exceeded their designed operational lives of ten years. Most of these RTGs likely no longer function, and may need to be dismantled. Some of their metal casings have been stripped by metal hunters, despite the risk of radioactive contamination. Transforming the radioactive material into an inert form reduces the danger of theft by people unaware of the radiation hazard (such as happened in the
Goiânia accident in an abandoned Cs-137 source where the caesium was present in easily water-soluble
caesium chloride form). However, a sufficiently chemically skilled malicious actor could extract a volatile species from inert material and/or achieve a similar effect of dispersion by physically grinding the inert matrix into a fine dust.
Radioactive contamination RTGs pose a risk of
radioactive contamination: if the container holding the fuel leaks, the radioactive material may contaminate the environment. For spacecraft, 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; therefore their use in spacecraft and elsewhere has attracted controversy. However, this event is not considered likely with current RTG cask designs. For instance, the environmental impact study for the Cassini–Huygens probe launched in 1997 estimated the probability of contamination accidents at various stages in the mission. The probability of an accident occurring which caused radioactive release from one or more of its three RTGs (or from its 129
radioisotope heater units) during the first 3.5 minutes following launch was estimated at 1 in 1,400; the chances of a release later in the ascent into orbit were 1 in 476; after that the likelihood of an accidental release fell off sharply to less than 1 in a million. If an accident which had the potential to cause contamination occurred during the launch phases (such as the spacecraft failing to reach orbit), the probability of contamination actually being caused by the RTGs was estimated at 1 in 10. The launch was successful and
Cassini–Huygens reached
Saturn. To minimize the risk of the radioactive material being released, the fuel is 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
plutonium-238 used in these RTGs has a
half-life of 87.74 years, in contrast to the 24,110 year half-life of
plutonium-239 used in
nuclear weapons and
reactors. Due to the shorter half-life, plutonium-238 is about 275 times more radioactive than plutonium-239 (i.e. /
g compared to /g). For instance, 3.6
kg of plutonium-238 undergoes the same number of radioactive decays per second as 1 tonne of plutonium-239. Since the morbidity of the two isotopes in terms of absorbed radioactivity is almost exactly the same, plutonium-238 is around 275 times more toxic by weight than plutonium-239. The alpha radiation emitted by either isotope will not penetrate the skin, but it can irradiate internal organs if plutonium is inhaled or ingested. Particularly at risk is the
skeleton, the surface of which is likely to absorb the isotope, and the
liver, where the isotope will collect and become concentrated. A case of RTG-related irradiation is the
Lia radiological accident in
Georgia, December 2001.
Strontium-90 RTG cores were dumped behind, unlabelled and improperly dismantled, near the Soviet-built
Enguri Dam. Three villagers from the nearby village of
Lia were unknowingly exposed to it and injured; one of them died in May 2004 from the injuries sustained. The
International Atomic Energy Agency led recovery operations and organized medical care. Two remaining RTG cores are yet to be found as of 2022.
Accidents -27 RTG deployed by the astronauts of
Apollo 14 identical to the one lost in the reentry of
Apollo 13 There have been several known accidents involving RTG-powered spacecraft: • A launch failure on 21 April 1964 in which the U.S.
Transit-5BN-3 navigation satellite failed to achieve orbit and burned up on re-entry north of
Madagascar. The plutonium metal fuel in its
SNAP-9a RTG was ejected into the atmosphere over the Southern Hemisphere where it burned up, and traces of plutonium-238 were detected in the area a few months later. This incident resulted in the NASA Safety Committee requiring intact reentry in future RTG launches, which in turn impacted the design of RTGs in the pipeline. • The Nimbus B-1 weather satellite, whose launch vehicle was deliberately destroyed shortly after launch on 21 May 1968 because of erratic trajectory. Launched from the
Vandenberg Air Force Base, its SNAP-19 RTG containing relatively inert
plutonium dioxide was recovered intact from the seabed in the
Santa Barbara Channel five months later and no environmental contamination was detected. • In 1969 the launch of the first
Lunokhod lunar rover mission failed, spreading
polonium-210 over a large area of Russia. • The failure of the
Apollo 13 mission in April 1970 meant that the
Lunar Module reentered the atmosphere carrying an RTG and burned up over
Fiji. It carried a SNAP-27 RTG containing of plutonium dioxide in a graphite cask on the lander leg which survived reentry into the Earth's atmosphere intact, as it was designed to do, the trajectory being arranged so that it would plunge into 6–9 kilometers of water in the
Tonga trench in the
Pacific Ocean. The absence of plutonium-238 contamination in atmospheric and seawater sampling confirmed the assumption that the cask is intact on the seabed. The cask is expected to contain the fuel for at least 10 half-lives (870 years). 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 because of the high re-entry velocities of the craft returning from
cis-lunar space (the region between Earth's atmosphere and the Moon). This accident has served to validate the design of later-generation RTGs as highly safe. •
Mars 96 was launched by Russia in 1996, but failed to leave Earth orbit, and re-entered the atmosphere a few hours later. The two RTGs onboard carried in total 200 g of plutonium and are assumed to have survived reentry as they were designed to do. They are thought to now lie somewhere in a northeast–southwest running oval 320 km long by 80 km wide which is centred 32 km east of
Iquique,
Chile.
CIA Loss of RTG in India A
SNAP-19C RTG was lost near the top of
Nanda Devi mountain in India in 1965, when it was stored in a
rock formation near the top of the mountain in the face of a snowstorm. It was intended to power a CIA remote automated intelligence station collecting telemetry from the Chinese rocket testing facility at
Lop Nur. The seven capsules were probably carried down the mountain onto a glacier by a subsequent avalanche and have never been recovered. It is most likely that they melted through the glacier and were pulverized, whereupon the Pu–Zr alloy fuel oxidized soil particles that are moving in a plume under the glacier. As the glaciers from these peaks feed some of India's largest rivers, including the
Ganges, there are concerns about massive radioactive contamination originating from these RTGs. Accounts from the operatives who attempted the installation recounted that the
sherpas who aided their mission jockeyed to carry the capsules as they produced heat; referring to the capsules as
Guru Rinpoche, the warmth offered some respite from the freezing winds, but some of the operatives thought the capsules were inadequately shielded and irradiated the men around them.
Soviet use of RTGs Many
Beta-M RTGs produced by the Soviet Union to power
lighthouses and
beacons have become
orphaned sources of radiation. Its design allowed for the use of normal industrial bolts instead of intrinsically safe or safety interlocked bolts (most likely to reduce cost), and did not require the use of intrinsically safe opening mechanisms or made any use of tamper resistant shielding systems. Several of these units have also been illegally dismantled for scrap metal, or been exposed to storm conditions, freezing and water penetration, common issues in those abandoned in the harsh Russian arctic. Some have even fallen into the ocean, or have defective shielding due to poor design or physical damage. The
US Department of Defense cooperative threat reduction program has expressed concern that material from the Beta-M RTGs can be used by
terrorists to construct a
dirty bomb. However, the strontium titanate perovskite used is resistant to all likely forms of environmental degradation and cannot melt or dissolve in water.
Bioaccumulation is unlikely as SrTiO passes through the digestive tract of humans or other animals unchanged, but the animal or human who ingested it would still receive a significant radiation dose to the sensitive
intestinal lining during passage. Mechanical degradation of "pebbles" or larger objects into fine dust is more likely and could disperse the material over a wider area, however this would also reduce the risk of any single exposure event resulting in a high dose. == See also ==