Plutonium and Caesium: Nuclear-test legacy in stratosphere bigger than thought

Plutonium cycle in the atmosphere. Credit: : J.A. Corcho Alvarado

Levels of radioactive plutonium in Earth's stratosphere from nuclear tests and accidents is higher than previously thought, but probably not dangerous to humans, scientists in Switzerland said Tuesday.

It was previously thought that plutonium radionuclides—radioactive atoms which can take decades or thousands of years to degrade—were present in the stratosphere only at negligible levels.

It was also believed that levels of these pollutants were higher in the troposphere, the layer of the atmosphere that is closest to the ground, than in the stratosphere.

Both ideas turn out to be wrong, according to the new study, whose authors also found no likelihood of a hazard to health.

Radiation levels in the stratosphere are "more than three orders of magnitude higher than previously thought," study co-author Jose Corcho of the Swiss Federal Office for Civil Protection told reporters.

The team also found that volcanic eruptions may shift those pollutants from the stratosphere into the troposphere, closer to Earth.

But Corcho said there was no evidence of danger.

"The levels of plutonium and (caesium) currently found in the stratosphere are low, and comparable to the levels measured at ground level air (troposphere) at the end of the sixties and in the seventies," he explained.

The aerosol collection devices mounted on the wings of a Tiger (F5E/F), aircraft of the Swiss Air Force. 

Credit: Swiss Federal Office of Public Health

"Although I'm not a health specialist, I would say that the current levels of plutonium found in the stratosphere do not represent a risk for the population."

The study, published in the journal Nature Communication, said the radioactive particles in the stratosphere mainly came from above-ground nuclear weapons tests in the 1950s and early 1960s.

Other sources were the burn-up of an ill-fated US navigation satellite in 1964 that spilled its plutonium fuel into the atmosphere, and nuclear power plant accidents like Chernobyl and Fukushima.

The team used aerosol samples taken by Swiss military aircraft since the 1970s as part of the country's environmental surveillance programme.

More information: Paper: dx.doi.org/10.1038/ncomms4030

NASA halts work on MMRTG nuclear generator for deep space exploration

MSL’s Pu-238 fueled MMRTG in the laboratory. Credit: NASA

Another blow was dealt to deep space exploration this past weekend.

The announcement comes from Jim Green, NASA's Planetary Science Division Director.

The statement outlines some key changes in NASA's radioisotope program, and will have implications for the future exploration of the outer solar system.

We've written about the impending plutonium shortage and what it means for the future of spaceflight, as well as the recent restart of plutonium production.

NASA is the only space agency that has conducted missions to the outer planets—even the European Space Agency's Huygens lander had to hitch a ride with Cassini to get to Titan—and plutonium made this exploration possible.

Probably the most troubling aspect of the announcement is the discontinuation of procurement by NASA of flight hardware for what was to be NASA's next generation nuclear power-source for exploration, the Advanced Stirling Radioisotope Generator, (ASRG).

This was to replace the Multi-Mission Radioisotope Generator, (MMRTG) that has been in use on spacecraft for decades.

An Advanced Stirling Converter prototype in the laboratory. 

Credit: NASA

The announcement states:

"With an adequate supply of Pu-238… NASA has decided to discontinue procurement of ASRG flight hardware."

"We have given direction to the Department of Energy… to end work on the flight units."

"The hardware procured under this activity will be transferred to the Glenn Research Center to continue development and testing of the Stirling technology."

The announcement cites the current budget-constrained environment that NASA and planetary space exploration finds itself up against.

What the exact future role is of NASA Glenn beyond basic research and development isn't entirely clear, but two ASRG units that were to be flight-ready for missions in 2016 are shelved for now.

The announcement does mention that NASA will continue to utilize flight-proven MMRTG's in the near term, which provide the same approximate power output as the ASRG, albeit with less efficiency.


Plutonium is vital for outer solar system exploration. As you get farther away from the Sun, solar energy ceases to become a viable alternative power source.

Pioneers 10 & 11, Voyagers 1 & 2, Galileo, and Cassini all utilized nuclear RTGs, as does the Mars Curiosity rover and New Horizons mission headed out explore Pluto in during its July 2015 flyby.

NASA's Juno spacecraft scheduled to reach Jupiter in 2016 will be the first-ever mission without an RTG to explore the outer solar system, and it must employ three enormous solar panels to do so.

A labeled cutaway of an ASRG. Credit: 

Wikimedia Commons And although the production of the Pu-238 isotope used in space exploration was a side-benefit of the Cold War, it isn't the same stuff as its isotopic cousin Pu-239, which is used in nuclear weapons. 

Plutonium production in the United States ceased in 1989, and although the U.S. government announced earlier this year that NASA will restart the plutonium production pipeline for space exploration, production levels are only expected to reach 1 to 1.5 kilograms per year.

Read the full article here 

Joint U.S., Soviet Recycling Program of Weapons-grade Uranium

USEC Inc., a global energy company and provider of enriched uranium fuel and nuclear industry-related services for commercial nuclear power plants, announced it has completed by 90 per cent the conversion of weapons-grade uranium from dismantled former Soviet Union nuclear warheads into low enriched uranium fuel under the program called Megatons to Megawatts(TM).

Megatons to Megawatts(TM) is a 20-year, commercially financed government-industry partnership in which 500 metric tonnes of Russian weapons-grade uranium is transformed to low enriched uranium for use as commercial reactor fuel. USEC, as executive agent for the U.S. government, and JSC "Techsnabexport" (TENEX), for the Russian government, implement the programme.

Its main focus was to convert the equivalent of 20,000 nuclear warheads into nuclear fuel by end 2013. So far, the fuel generated from the conversion equals more than 193 billion gallons of gasoline, corresponding to more than 17 months of U.S. consumption

In past years, this type of fuel has helped generate 10 per cent of the United States' electricity needs.

"For nearly two decades, the Megatons to Megawatts(TM) program has provided a reliable source of fuel for commercial nuclear power plants while reducing the quantities of weapons-grade uranium remaining in the world," John K. Welch, USEC president and CEO, said in a statement.

"As we near the end of this historic nonproliferation program, USEC is preparing to transition to our new contract with TENEX, which will continue to provide a reliable supply of LEU for the company while we work to deploy the American Centrifuge technology."