Question

A concern in the nuclear power industry is that, if nuclear power becomes more widely used, there may be serious shortages in worldwide supplies of fissionable uranium. One solution is to build breeder reactors that manufacture more fuel than they consume. One such cycle works as follows: (i) \(\mathrm{A}^{238} \mathrm{U}\) nucleus collides with a neutron to produce \({ }^{239} \mathrm{U}\). (ii) \({ }^{239} \mathrm{U}\) decays by \(\beta\) emission \(\left(t_{1 / 2}=24\right.\) minutes \()\) to give an isotope of neptunium. (iii) The neptunium isotope decays by \(\beta\) emission to give a plutonium isotope. (iv) The plutonium isotope is fissionable. On its collision with a neutron, fission occurs and gives energy, at least two neutrons, and other nuclei as products. Write an equation for each of these steps, and explain how this process can be used to breed more fuel than the reactor originally contained and still produce energy.

Short answer

Breeder reactors convert uranium-238 to plutonium-239, which fissions to yield energy and neutrons, breeding more plutonium fuel.

Step-by-step solution

Initial Collision

In the first reaction, a uranium-238 (^{238}U) nucleus absorbs a neutron to turn into uranium-239 (^{239}U). This can be represented with the nuclear equation:\[ \mathrm{^{238}U} + \mathrm{n} \rightarrow \mathrm{^{239}U} \] This reaction is important because it starts the process by increasing the mass number of uranium, making it unstable.

First Beta Decay

Uranium-239 (^{239}U) is radioactive and undergoes beta decay in which a neutron is converted into a proton with the emission of an electron (\beta^-) and an antineutrino. This leads to the formation of neptunium-239 (^{239}Np).\[ \mathrm{^{239}U} \rightarrow \mathrm{^{239}Np} + \beta^- + \bar{u}_e \] This step changes the element from uranium to neptunium.

Second Beta Decay

The neptunium-239 (^{239}Np) undergoes another beta decay, where it again converts one neutron into a proton, resulting in plutonium-239 (^{239}Pu).\[ \mathrm{^{239}Np} \rightarrow \mathrm{^{239}Pu} + \beta^- + \bar{u}_e \] This decay changes the element from neptunium to plutonium, which is fissionable.

Fission of Plutonium

Plutonium-239 (^{239}Pu) is a fissionable material. When it captures an additional neutron, it becomes unstable and splits (fissions), releasing energy, additional neutrons, and smaller atomic fragments (fission products):\[ \mathrm{^{239}Pu} + \mathrm{n} \rightarrow \text{[Fission products]} + E + 2 \text{ or more neutrons} \] These neutrons can go on to trigger further fission reactions in other plutonium or uranium nuclei, forming a chain reaction.

Breeding More Fuel

The neutrons from plutonium fission are key to breeding more fuel. Some of these neutrons can be absorbed by more ^{238}U to restart the cycle, converting it to more ^{239}Pu through the steps outlined previously. This process effectively breeds more plutonium fuel than was originally present, while also continuing to produce energy.

Key concepts

Fissionable Uranium

Fissionable uranium refers to isotopes of uranium that are capable of sustaining a nuclear fission chain reaction.
The most common fissionable isotope of uranium used in nuclear reactors is uranium-235 (\(^{235}\text{U}\)), although uranium-238 (\(^{238}\text{U}\)) plays a crucial role in breeder reactors. In its natural state, uranium is composed of about 99.3% \(^{238}\text{U}\), which is not directly fissionable but can be transformed into fissionable plutonium-239 (\(^{239}\text{Pu}\)) through a series of nuclear reactions.
Uranium-238 captures a neutron to become uranium-239 (\(^{239}\text{U}\)), which then undergoes beta decay to form neptunium-239 and eventually plutonium-239. Through this conversion process, breeder reactors essentially "breed" fissionable material from uranium-238, compensating for shortages of uranium-235.
Understanding the transformation process is essential for students learning about nuclear power generation and the strategies used to supply the necessary fuel sustainably.

Beta Decay

Beta decay is a type of nuclear decay where a neutron in the nucleus of an atom is transformed into a proton, releasing a beta particle and an antineutrino. This process alters the identity of the element by increasing its atomic number by one.
In the context of breeder reactors, beta decay plays a critical role in transforming uranium-239 into other useful substances. Uranium-239 undergoes beta decay to become neptunium-239 (\(^{239}\text{Np}\)), which further undergoes beta decay to produce plutonium-239 (\(^{239}\text{Pu}\)).
The equation representing the decay of uranium-239 is:\[ \mathrm{^{239}U} \rightarrow \mathrm{^{239}Np} + \beta^- + \bar{v}_e\]A similar equation describes the decay of neptunium-239:\[ \mathrm{^{239}Np} \rightarrow \mathrm{^{239}Pu} + \beta^- + \bar{v}_e\]This sequence of transformations is critical for converting non-fissionable material into a form that can sustain nuclear fission, which highlights the importance of beta decay in nuclear energy production.

Nuclear Fission

Nuclear fission is a process where the nucleus of a fissionable atom, such as plutonium-239 or uranium-235, splits into two or more smaller nuclei along with the release of energy and neutrons. This reaction is what powers nuclear reactors and atomic bombs.
When a fissionable atom like plutonium-239 captures a neutron, it becomes temporarily unstable and splits. This splitting releases a significant amount of energy in the form of kinetic energy of the fragments and gamma radiation. In addition, two or more neutrons are typically released, which may induce further fission reactions in nearby fissionable nuclei.
A typical fission reaction for plutonium-239 can be represented as:\[ \mathrm{^{239}Pu} + \mathrm{n} \rightarrow \text{[Fission products]} + E + 2 \text{ or more neutrons}\]The neutrons produced can trigger additional fission events, creating a self-sustaining chain reaction that is central to the operation of nuclear reactors. This dynamic process allows breeder reactors to produce more fuel than they consume, turning the reaction cycle into a means of both energy generation and fuel production.

Plutonium-239

Plutonium-239 is a man-made isotope and one of the most important fissionable elements used in nuclear reactors and weapons. It is produced from uranium-238 through a series of reactions and beta decays, as described in breeder reactor designs.
Plutonium-239 is highly valued for its ability to sustain a nuclear chain reaction, which is crucial for both generating energy and breeding new fuel. It acts as the end product of the uranium to plutonium conversion cycle in breeder reactors, making these systems efficient in creating more plutonium than they originally consumed.
The fission of plutonium-239 is a highly efficient energy release mechanism due to the production of a significant number of neutrons and energy during fission. Its formula when undergoing fission is:\[ \mathrm{^{239}Pu} + \mathrm{n} \rightarrow \text{[Fission products]} + E + n\]This reaction is fundamental to maintaining the ongoing production of energy and the self-sustaining cycle of fuel in a breeder reactor, illustrating the vital role of plutonium-239 in advanced nuclear technology.