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Friday, December 25, 2015

Plutonium 238

Good news!  Oak Ridge National Laboratory has begun manufacturing a new supply of Plutonium 238.

Below, a pellet of Pu-238 glowing orange from internal heating due to alpha decay.
"Plutonium pellet" by Department of Energy (via Wikipedia)

Yeah, yeah I know what you are probably thinking... big effing deal.  Well, actually it IS a big deal.

Pu-238 is unstable (radioactive) and decays by emitting alpha particles.  The alpha particles have an energy of 5.6 MeV, which is a pretty hot decay energy.  Pu-238 has a half-life of just 88 years, so it is putting out these high energy particles at a very high rate.

When most radioactive isotopes decay, they emit penetrating radiation, but Pu-238 only decays by emitting an alpha.  This single decay mode makes it simple to handle, as it doesn't emit lethal penetrating radiation (neutron, beta, gamma).  For the above reasons, Pu-238 is the king of self-heating isotopes.

One gram of Pu-238 puts out 1/2 watt of heat.  One gram!!!  Five pounds would put off 1135 watts of heat, enough to make a nice room heater that would pour out relentless heat for decades.  But that's not how Pu-238 is used.

Plutonium 238 (Pu-238 from now on) is the best heat source for making Radioisotope Thermoelectric Generators, or RTGs.  RTGs are electrical power supplies with no moving parts.  They are used for deep space probes and for other devices needing long-term zero maintenance electrical power.  Russia used these (with Cesium-137 cores) as power supplies for unmanned lighthouses.  The self-heating of Pu-238 also makes it an excellent Radioisotope heater.

Below, a couple of RTGs.  RTGs use a difference of temperature at a thermoelectric junction to generate electricity.  The larger the temperature difference, the greater the power.  Thus the cooling fins.

Below, a cutaway of a RTG (courtesy of Wikipedia)

The last US facility for producing Pu-238 shut down in 1988, and since 1993, all the Pu-238 used for space missions has been purchased from Russia.  Russia has also stopped producing Pu-238, and so supplies are dwindling - partly due to spaceflight usage, and partly due to the short half life of this isotope.  NASA is approaching the point where it will have to curtail deep space missions due to lack of power supplies for the spacecraft...  Fortunately we will soon be making 1.5 Kg per year, as the process scales up.

So how do we manufacture Plutonium 238?  Glad you asked!

Plutonium 238 is created as a waste product in a nuclear reactor, but it is mingled with Pu-239 and 240.  It is not practical to separate this Pu-238 from the spent fuel.  It would first require a chemical separation of the Plutonium, followed by a very expensive isotopic separation.  Reactor-grade Plutonium is only about 2% Pu-238, so it would require a large number of stages of gaseous diffusion in a dedicated factory.  Rather than separate the desired isotope from its isomers, pure Pu-238 is can be easily made by transmuting it from Neptunium 237.

Neptunium 237 (half life 30 million years) is itself an artificial element, which can be created by two different mechanisms inside a typical nuclear reactor.  The feed stock for Neptunium 237 is either U-235 or U-238, using two different nuclear reactions, both normally occurring inside a reactor.

The first mechanism for creating Np-237 is for a fissile U-235 atom to absorb a neutron, but fail to fission - instead becoming U-236.  This U-236 then may absorb a second neutron and become U-237.  The U-237 has a half-life of 6.4 days, after which it beta decays to our feed stock, Neptunium 237.

The second mechanism for creating Np-237 is for a fast neutron to strike a U-238 atom, ejecting two other neutrons.  As in the above case, the product is U-237, which decays to Np-237.

Spent reactor fuel is about 0.4% Np-237, and with millions tons of spent fuel from commercial reactors looking for a permanent waste disposal site, there is certainly no shortage of Np-237 feed stock for the production of Pu-238.  All we need to do now is to chemically separate our Np-237 from the spent fuel, and then get a source of neutrons to do the dirty work of transmutation.

The Np-237 is exposed to a strong neutron source, captures a neutron and becomes Np-238.  Np-238 has a half life of 2.1 days, beta decaying to our desired product, Pu-238.  Simple!

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