A contingency plan

in check
Last Updated 24 October 2016, 18:38 IST

When plutonium was first produced and isolated on December 14, 1940, by the American scientist Glenn T Seaborg and his team, it was the isotope plutonium-238 (Pu-238) that was obtained by deuteron bombardment of uranium-238 in the 60-inch cyclotron at the University of California, Berkeley, USA. Soon after this discovery, it was Pu-239 and not Pu-238 that was sought after most, because it was easily fissionable and therefore could be used to produce nuclear weapons. In fact, the detonation of first nuclear weapon by the American scientists led by Robert Oppenheimer in 1945 (Trinity Test) and the atomic bomb that wiped out Nagasaki in the same year had each about six kg of

However, with the advent of the space programme, Pu-238 became a very important radioisotope as it is used to power  radioisotope thermoelectric generators (RTG) that produce electricity for deep space probes. The United States has used RTGs on 30 of its space missions, starting from a navy navigation satellite launched in 1961 to the recent Mars Curiosity rover in 2012.

Emitting energy
Radioisotopes emit nuclear radiations. If the emitted radiation is of high energy alpha or beta rays and the number of radiation emitted per unit time is very high, such radioactive decay is accompanied by intense heat. In RTGs, the radioactive decay heats up one of the thermocouple junctions while the other junction remains unheated and is cooled by the space environment or a planetary atmosphere there by generating electricity. RTGs have been used in some cardiac pacemakers in the early days before the advent of long life lithium batteries. Russians have used RTGs extensively for naval applications like navigation beacons mounted on vessels and buoys.

Though other radioisotopes like strontium-90 (Sr-90), americium-241 (Am-241), polonium-210 (Po-210) have been used, Pu-238 is considered the most ideal fuel material for RTGs. Pu-238 has a high heat density (0.56 W/g), which means that heat sources can be made compact and that the fuel transfers its heat effectively to power conversion devices and heater units. Its half life is 88 years and therefore, the generator can serve for several years, say about 20 to 30 years. As Pu-238 is an alpha emitter, shielding requirement is not much. Pu-238 can be made in ceramic form with very low solubility in body fluids, which renders it less hazardous in the event of an accidental release.

RTGs using Pu-238 have been deployed for space exploration safely for more than 50 years as they provide a reliable and sustained amount of electrical energy. As of now, there is no other better option. Solar power is too weak and one has to ensure that solar panels of the spacecrafts are kept facing the Sun all the time. Also, as one goes farther from the Sun, the amount of energy the solar panels can receive starts falling drastically. For instance, Jupiter is about five times further from the Sun than and the Earth and thus, receives about 25 times less energy per square metre per unit area. On the other hand, chemical batteries do not have a long life.

The Voyager 1 spacecraft left Earth in 1977 on a five-year mission to explore Jupiter and Saturn. Nearly 40 years later, the car-sized probe is still sending its data home. Voyager 1 is the most distant man-made object and is currently discovering what the edge of the solar system is like. Voyager 1 is expected to keep working until 2025 when it will finally run out of power. That is the power derived from Pu-238 which keeps the spacecraft ticking for more than 40 years.

The design of RTGs will depend on the duration of space mission as well as the kind of task a spacecraft has been assigned. The Cassini spacecraft carried three RTGs with 33 kg of plutonium oxide providing 870 watts of power as it orbits around the Saturn. Over the years, USA has also vastly improved the design of these power packs with higher fuel efficiency.

In fact, the recent versions known as multi-mission radioisotope thermoelectric generators (MMRTG), with a pack of eight RTG units, having in all 4.8 kg of plutonium oxide, can be used to generate some 125 watts of electric power. This was used in the large mobile Mars Science Laboratory and the Curiosity rover, which landed on Mars in August 2012.

When uranium fuel gets bombarded by neutrons in a nuclear reactor, neptunium-237 is produced as an activation byproduct. During the chemical processing of the irradiated fuel for recovery of plutonium, neptunium-237 can also be extracted. This has a very long half life (two million years). When the extracted neptunium-237 is converted to a target material and bombarded by neutrons in a reactor, it gets converted to neptunium-238, which decays quickly to produce Pu-238. Therefore, to get Pu-238, one has to have nuclear reactors and remote hot cell facilities for chemical and metallurgical processing.

Vanishing sources
At the end of the cold war, USA stopped producing plutonium in its nuclear facilities. In fact, the production of Pu-238 came to a complete halt in 1988. In 1993, cash-starved Russia offered USA 40 kg of plutonium-238 at a cost of about 60 million dollars. After supplying about 16 kg, Russia decided to stop the supply in 2009 citing its own depleting stock of the strategic material. This sudden stoppage of supply of Pu-238 has forced NASA to review its strategy for future space missions. The current civilian stock of 35 kg of Pu-238 will not last beyond 2020.

In an effort to avert an outer space energy crisis, the US Department of Energy (DOE) has embarked on a project to re-establish after almost 30 years, a Pu-238 production capability with an outlay of 200 million dollars over the next nine years. DOE has designated Oak Ridge National Laboratory (ORNL) as the lead laboratory for the project. ORNL has access to high flux reactors and also a series of shielded hot cells and special labs to separate and purify the intensely radioactive material.

Fuelled by the recent spectacular successes in its space exploration, if the Indian Space Research Organisation (ISRO) plans to embark on long duration space missions beyond moon and Mars, it will have no option but to secure RTGs to meet the energy needs of its satellites in space. The Department of Atomic Energy (DAE) has a well-established indigenous plutonium technology with the state-of- the-art remote hot cell facilities for chemical and metallurgical processing of highly radioactive materials.

ISRO and DAE cannot wait too long to initiate a collaborative plan to make in India the RTGs, which no country will be willing to give in view of their high strategic value.

(The author is a retired scientist, Department of Atomic Energy)

(Published 24 October 2016, 17:17 IST)

Follow us on