What would happen if we had to face an incident involving an RTG ?
– News of October 7, 2018 –
The use of the Radioisotope Thermoelectric Generator (RTG) in space still has a bright future. In 2013, the United States of America resumed the production of plutonium-238 for space use, for the first time since the late 1980s. The US space agency has a limited stock of the precious isotope, about 35 kg . A large part of this stock would already be too degraded to be integrated on new missions. Plutonium-238 is to be used in the MARS 2020’s RTG, and probably in missions to Jupiter and Saturn. But what would happen if the launch of MARS 2020 mission goes wrong ?
Plutonium-238 is a rather special isotope because it degrades very quickly and is therefore much more active than the plutonium-239 used in nuclear weapons. It mainly generates Alpha radioactivity, which makes it relatively harmless outside the human body, but it is extremely dangerous if ingested. It must therefore be ensured that the plutonium-238 is never vaporized, so that it can not easily be inhaled or swallowed by living beings. But precisely, an rocket explosion vaporizes everything nearby.
Unfortunately, this happened in several incidents involving RTGs. In 1964, an American satellite failed to reach Earth’s orbit, spreading plutonium over Madagascar. Since this incident, NASA has been doing everything to make its RTG resistant to atmospheric reentry. Plutonium-238 is stored in the form of ceramic, which makes it resistant to heat and makes it more difficult to split. It is surrounded by a layer of iridium and graphite blocks, and an envelope acting as a shield during atmospheric reentry. In the event of a disaster, the plutonium must fall back into a single block, without dispersing into the atmosphere and the environment. At first glance, it works. Following the failure of Apollo 13, the lunar module and its RTG made a destructive atmospheric reentry. The post-incident measurements showed no evidence of plutonium-238 contamination, which means that the protective envelope worked well.
Nowadays, the risks of plutonium spreading in the atmosphere are minimal, even after the failure of a rocket launch. But if that happens, it would be necessary to monitor the dispersion zone of the cloud. Rockets are usually launched over the ocean, so it would be very unfortunate for an inhabited area to be affected. If this happens, then it will probably be necessary to evacuate people while cleaning the area. Doses of dispersed plutonium are unlikely to kill someone quickly, but the risk of bone and liver cancer may increase. Let’s hope that the MARS 2020 mission will not face this kind of problem.
Image by NASA [Public Domain], via Wikimedia Commons