Problem 58 - Chemistry and Chemical Reactivity

Question

If a shortage in worldwide supplies of fissionable uranium arose, it would be possible to use other fissionable nuclei. Plutonium, one such fuel, can be made in "breeder" reactors that manufacture more fuel than they consume. The sequence of reactions by which plutonium is made is as follows: (a) \(\mathrm{A}^{238} \mathrm{U}\) nucleus undergoes an \((\mathrm{n}, \gamma)\) to produce \(^{239} \mathrm{U}\) (b) \(^{239} \mathrm{U}\) decays by \(\beta\) emission \(\left(t_{1 / 2}=23.5 \mathrm{min}\right)\) to give an isotope of neptunium. (c) This neptunium isotope decays by \(\beta\) emission to give a plutonium isotope. (d) The plutonium isotope is fissionable. On collision of one of these plutonium isotopes with a neutron, fission occurs, with at least two neutrons and two other nuclei as products. Write an equation for each of the nuclear reactions.

Step-by-Step Solution

Answer

The reactions are: \( \mathrm{^{238}U} + \mathrm{n} \rightarrow \mathrm{^{239}U} \), \( \mathrm{^{239}U} \rightarrow \mathrm{^{239}Np} + \beta^- \), \( \mathrm{^{239}Np} \rightarrow \mathrm{^{239}Pu} + \beta^- \), and \( \mathrm{^{239}Pu} + \mathrm{n} \rightarrow \mathrm{fission} \, \text{products} + 2-3 \, \mathrm{n} \).

Step 1: Uranium-238 to Uranium-239

The first reaction involves Uranium-238 capturing a neutron to become Uranium-239. This is represented by \( \mathrm{^{238}U} + \mathrm{n} \rightarrow \mathrm{^{239}U} \).

Step 2: Uranium-239 to Neptunium-239

The Uranium-239 isotope is unstable and undergoes beta decay, where it emits a beta particle (\(\beta^-\)) and transforms into Neptunium-239. This can be written as \( \mathrm{^{239}U} \rightarrow \mathrm{^{239}Np} + \beta^- \).

Step 3: Neptunium-239 to Plutonium-239

Neptunium-239 also undergoes beta emission, leading to the formation of Plutonium-239. This is represented by \( \mathrm{^{239}Np} \rightarrow \mathrm{^{239}Pu} + \beta^- \).

Step 4: Plutonium-239 Fission

Finally, the fission reaction of Plutonium-239 occurs when it collides with a neutron. This process produces two or more neutrons and lighter nuclei. A generalized equation can be \( \mathrm{^{239}Pu} + \mathrm{n} \rightarrow \mathrm{fission} \text{ products} + 2-3\,\mathrm{n} \).

Key Concepts

Nuclear Fission

Nuclear fission is a process where a heavy nucleus splits into two or more smaller nuclei, along with the release of energy. This chain reaction is fueled by the collision of atomic nuclei with neutrons, leading to an energetic explosion.
Some key points about nuclear fission:

• It releases a significant amount of energy, which is harnessed for power generation in nuclear reactors.
• The process begins when a neutron strikes a heavy nucleus, like Uranium-235 or Plutonium-239.
• Once the nucleus absorbs the neutron, it becomes unstable and splits into two lighter nuclei, releasing additional neutrons and energy in the form of radiation.

These additional neutrons can then go on to strike other nuclei, potentially causing a chain reaction. This is exactly how nuclear reactors work, where the controlled fission reaction produces steam that drives turbines to generate electricity.
Fission products are usually highly radioactive, and their safe disposal is a key challenge in the nuclear industry.

Beta Decay

Beta decay is a type of radioactive decay where a beta particle is emitted. A beta particle can either be an electron ( \( \beta^- \) ) or a positron ( \( \beta^+ \) ). In our context, we focus on \( \beta^- \) decay since it's the primary decay type in the production of fissionable material like Plutonium-239.
Important details about beta decay include:

• In \( \beta^- \) decay, a neutron in the nucleus converts to a proton, emitting an electron (beta particle) and an antineutrino.
• This transformation increases the atomic number of the element by one, while its atomic mass remains unchanged.
• Beta decay transforms one chemical element into another, like Uranium-239 ( \( \mathrm{^{239}U} \) ) decaying into Neptunium-239 ( \( \mathrm{^{239}Np} \) ).

Beta decay plays a crucial role in nuclear transmutation, allowing the conversion of one element into a valuable or desired isotope.

Breeder Reactors

Breeder reactors are a particular type of nuclear reactor that generate more fissile material than they consume. This makes them an essential asset in addressing potential shortages in nuclear fuel.
Here are some aspects of breeder reactors:

• They convert abundant fertile isotopes like Uranium-238 and Thorium-232 into fissile isotopes such as Plutonium-239 and Uranium-233.
• The breeder process is facilitated by neutron capture, followed by beta decay, turning non-fissionable elements into usable nuclear fuel.
• This process effectively extends the nuclear fuel supply, making nuclear power a more sustainable option.
• Plutonium produced in breeder reactors can be used in traditional nuclear reactors or in weapons, necessitating stringent controls and regulations.

Through the use of breeder reactors, the nuclear industry can better sustain itself in terms of resources, helping to alleviate pressures on natural uranium supplies.