Plutonium-238 is an isotope of plutonium. It is a radioactive substance extensively used as a longstanding fuel source in space probes.

Plutonium-238 Identification

The CAS Registry Number for this radioactive isotope is 13981-16-3.

Plutonium-238 History

Among the different isotopes of plutonium, Pu-238 was the first to be discovered. In 1941, Glenn Seaborg and associates bombarded uranium-238 using deuterons to synthesize Plutonium-238. The intermediate product Neptunium-238 goes through decomposition to form Plutonium-238.

Plutonium-238 Nucleus

There are 144 Neutrons and 94 Protons in the Plutonium-238 isotope.

Plutonium-238 Symbol

The symbol denoting Plutonium-238 is written as ²³⁸Pu.

Picture 1 – Plutonium-238

Plutonium-238 Production

Plutonium-238 can be produced by hitting uranium-238 with deuterons. This method is expressed in the following reaction.

^238U₉₂ + ²D₁ → ²³⁸Np₉₃ + 2¹n₀

²³⁸Np₉₃ → ²³⁸Pu₉₄

In the above reaction, Uranium-238 is hit by a deuteron that produces Neptunium-238 and two neutrons. Neptunium-238 then undergoes spontaneous decomposition through the emission of negatively charged beta particles and forms Plutonium-238 in the process.

Pure Plutonium-238 can be generated by irradiating americium in a nuclear reactor or by irradiating neptunium-237 which is a minor actinide derived while reprocessing spent nuclear fuel. In each case, the derived components are then subjected to a chemical reaction. This involves dissolving the components in nitric acid for extracting Pu-238 from them.

Reactor-grade plutonium obtained from spent nuclear fuel consists of several plutonium isotopes, out of which only 1% or 2% is Plutonium-238. However, due to its brief half-life, this small percentage is probably responsible for most of the short-term decay heat produced from the spent nuclear fuel. This is not an effective way of manufacturing Plutonium-238 for Radioisotope Thermoelectric Generators (RTGs) as the spent nuclear fuel would have to go through a difficult process of isotopic separation.

Plutonium-238 Properties

The various Plutonium-238 properties have been discussed below:

Appearance

It is a solid, bright silvery metal.

Critical Mass

The critical mass of Plutonium-238 for a bare sphere is 10 kg.

Critical Diameter

The critical diameter of Plutonium-238 for a uniform sphere is 9.7 cm.

Isotope Mass

The Isotope mass of Plutonium-238 is 238.049553 u.

Decay Mode

Plutonium-238 decay modes include alpha emission, spontaneous fission, ³²Si cluster emission, and ²⁸Mg + ³⁰Mg double cluster emission.

Decay Energy

The energy released by Plutonium-238 during decomposition is 5.593 MeV.

Branching Ratio

The branching ratio for Plutonium-238 while decaying through alpha emission is 100%.

The branching ratio for Plutonium-238 while decaying through spontaneous fission is 1.9 x 10^-9.

The branching ratio for Plutonium-238 while decaying through ³²Si cluster emission is 1.4 x 10^-16.

The branching ratio for Plutonium-238 while decaying through ²⁸Mg + ³⁰Mg double cluster emission is 6 x 10^-17.

Daughter Nuclide

Uranium-234 is produced as a daughter nuclide when Pu-238 undergoes decomposition.

Magnetic Dipole Moment

The magnetic dipole moment of Plutonium-238 is 0 μN.

Spin Parity

Spin parity of Pu-238 is represented as J^π = 0⁺ (atomic boson).

Binding Energy

The binding energy per nucleon of Pu-238 is 7.568354 MeV.

Specific Activity

The specific activity SA of Plutonium-238 is 634 GBq/g.

Plutonium-238 Half-Life

In radioactivity, the half-life is the time taken by a specific amount of a radioactive substance that undergoes decomposition to be decreased by half. The half-life of Plutonium-238 is 87.7 years.

Plutonium-238 Radioactive Decay

The unstable atomic nucleus of Plutonium-238 loses energy in order to reach a stable stage. This reaction is defined as radioactive decay. Plutonium-238 releases around 5.593 MeV of energy through radioactive decay.

Plutonium-238 Decay Equation

The alpha decomposition of Plutonium-238 is shown in the equation below:

²³⁸Pu₉₄ → ²³⁴U₉₂ + ⁴He₂

The helium nucleus ₂⁴He in this reaction equation has got atomic number 2 and mass number 4. Helium here indicates the alpha particle. This reaction can also be represented in the following way:

²³⁸Pu₉₄ → ²³⁴U₉₂ + α

Plutonium-238 Decay Chain

Plutonium-238 goes through decomposition to produce Uranium-234 as the daughter nuclide. At the end of this Alpha decay chain, the stable element that is produced is Lead-206. The complete decay chain is expressed in the following reaction:

Plutonium-238 → Uranium-234 → Radium-226 → Radon-222 → Polonium-218 → Lead-214 → Bismuth-214 → Polonium-214 → Lead-210 → Bismuth-210 → Polonium-210 → Lead-206

Plutonium-238 Fission

Plutonium-238 is not a fissile material and so it is incapable of sustaining chain reactions. However, this substance is fissionable when struck by high-energy neutrons.

Plutonium-238 Uses

The various uses of Plutonium-238 are described below:

• Plutonium-238 is a very strong alpha particle emitter. Since the chances of getting other forms of more powerful and penetrating radiation are minimal, this isotope of Plutonium is used in radioisotope heater units and Radioisotope Thermoelectric Generators (RTGs).
• This substance produces almost 0.5 watts of heat per gram. It is thus used as an important source of power for fuelling interplanetary probes and unmanned spacecraft. Nuclear batteries using Pu-238 have been used in the Voyager and Pioneer space probes.
• Trace amounts of Plutonium-238 were also used in manufacturing early pacemaker batteries.
• A combination of Pu-238 and beryllium produces neutrons that are used for research purposes.

Plutonium-238 Contamination and Health Risks

Plutonium-238 is a dangerous carcinogenic substance. Like Pu-239, Pu-238 is hard to locate once it enters the body and has been absorbed by it.

The main health hazards come from the alpha (α) radiation emitted by Pu-238. These particles are much heavier than the beta and gamma radiation particles and so when they are within the body, they constantly bombard a particular area thereby causing cancer.

Traces of Plutonium-238 mostly get lodged in soft tissues, like in the bone marrow, the liver, on bone surfaces, and other non-calcified bony structures. The major threat to human health comes from inhaling this radioactive substance. This can damage the cells and tissues of the lungs and the bronchial tubes. The substance can also enter the body through cuts and abrasions and be absorbed into the bloodstream.

Plutonium-238 is one of the most indispensable of all the radioactive isotopes. Without it, the cause of space research would have been much more difficult to pursue.

References
1. http://www.chemicool.com/elements/plutonium.html
2. http://www.wolframalpha.com/input/?i=Pu-238
3. http://www.ieer.org/fctsheet/pu-props.html
4. http://www.ieer.org/ensec/no-3/puhealth.html