Question

The most common reaction used in breeder reactors involves the bombardment of uranium- 238 with neutrons: \({ }^{238} \mathrm{U}\) is converted by this bombardment to \({ }^{239} \mathrm{U}\). The uranium- 239 then undergoes two beta decays, first to \({ }^{239} \mathrm{~Np},\) and then to \({ }^{239} \mathrm{Pu},\) which is a fissionable material and the desired product. Write balanced nuclear equations for the bombardment reaction and the two beta-decay reactions.

Short answer

The balanced nuclear equations for the bombardment reaction and two beta-decay reactions in the breeder reactor process are: 1. \( {}^{238}\text{U} + {}^1\text{n} \rightarrow {}^{239}\text{U} \) 2. \( {}^{239}\text{U} \rightarrow {}^{239}\text{Np} + {}^0\text{e}^{-} \) 3. \( {}^{239}\text{Np} \rightarrow {}^{239}\text{Pu} + {}^0\text{e}^{-} \)

Step-by-step solution

First Reaction: Bombardment of U-238 with Neutrons

For the U-238 bombardment reaction, Uranium (U) is struck by an extra neutron (n). On the left side, we have 238 protons and neutrons in U, and 1 neutron. On the right side, we have U-239 which has 239 protons and neutrons. The balanced nuclear equation can be written as follows: \[ {}^{238}\text{U} + {}^1\text{n} \rightarrow {}^{239}\text{U} \]

Second Reaction: Beta Decay of U-239 to Np-239

In beta decay, a neutron is converted into a proton and an electron. The electron is ejected from the nucleus as a beta particle. Since a neutron has changed to a proton, the neutron count stays the same, but the element changes to the next higher atomic number (here, from uranium (Z=92) to neptunium (Z=93)). The balanced nuclear reaction for this beta decay is: \[ {}^{239}\text{U} \rightarrow {}^{239}\text{Np} + {}^0\text{e}^{-} \]

Third Reaction: Beta Decay of Np-239 to Pu-239

In the second beta decay, a neutron in Np-239 is converted into a proton, and an electron is ejected as a beta particle. Neptunium (Z=93) changes to plutonium (Z=94) as the atomic number increases by one due to the new proton. The balanced nuclear reaction for this beta decay is: \[ {}^{239}\text{Np} \rightarrow {}^{239}\text{Pu} + {}^0\text{e}^{-} \] So, the balanced nuclear equations for the bombardment reaction and two beta-decay reactions are: \[ {}^{238}\text{U} + {}^1\text{n} \rightarrow {}^{239}\text{U} \] \[ {}^{239}\text{U} \rightarrow {}^{239}\text{Np} + {}^0\text{e}^{-} \] \[ {}^{239}\text{Np} \rightarrow {}^{239}\text{Pu} + {}^0\text{e}^{-} \]

Key concepts

Beta Decay

Beta decay is a type of radioactive decay in which a neutron is transformed into a proton within an atom's nucleus. This transformation also produces an electron, which is emitted from the nucleus as a beta particle. Such changes increase the atomic number by one, while the mass number remains the same.
In the case of our exercise, after the uranium-239 nucleus undergoes beta decay, it turns into neptunium-239. Here, the atomic number increases from 92 to 93.
This process continues with neptunium-239, which undergoes a second beta decay to form plutonium-239, again increasing the atomic number, from 93 to 94.

Uranium-238

Uranium-238 ( {}^{238} ext{U} ) is a common isotope of uranium found in nature. It is non-fissionable, meaning it cannot easily undergo fission to release energy. Despite this, it plays a crucial role in nuclear reactors, particularly breeder reactors, due to its ability to absorb neutrons.
When uranium-238 is bombarded with neutrons, it becomes uranium-239, setting the stage for successive transformations through beta decay. This change is critical as it converts uranium-238 into a form that eventually turns into more useful, fissionable materials like plutonium-239.

Breeder Reactors

Breeder reactors are a special type of nuclear reactor designed to generate more fissile material than they consume. Unlike conventional reactors, breeders maximize the utility of uranium-238 by transforming it into plutonium-239, a highly fissile material.
This process enhances the sustainability of nuclear energy. Breeder reactors operate by bombarding uranium-238 with slow-moving neutrons, which initiates a series of reactions. These include neutron absorption and subsequent beta decays, converting non-fissionable materials into fissionable ones. By effectively "breeding" fuel, these reactors promise a more efficient nuclear fuel cycle.

Neptunium-239

Neptunium-239 ( {}^{239} ext{Np} ) is an intermediate isotope formed during the nuclear reactions in breeder reactors. When uranium-239 undergoes beta decay, it transforms into neptunium-239.
This isotope is short-lived in the nuclear process, typically existing just long enough to undergo another beta decay into plutonium-239. Although neptunium-239 is not used as fuel directly, its role as a stepping stone from uranium-239 to plutonium-239 is vital in the production of usable nuclear fuel.

Plutonium-239

Plutonium-239 ( {}^{239} ext{Pu} ) is a significant product of the nuclear reactions in breeder reactors. It is a fissionable material, similar to uranium-235, and is widely used as reactor fuel. Plutonium-239 is produced through the beta decay sequence that begins with uranium-238 capturing a neutron.
Its high fissionability makes it an important asset in nuclear technology, capable of sustaining chain reactions in nuclear reactors. By converting uranium-238 into plutonium-239, breeder reactors effectively double the useable energy derived from natural uranium, underscoring the importance of this nuclear transformation.