Problem 62 - Chemistry

Question

Breeder reactors are used to convert the nonfissionable nuclide 238 \(\mathrm{U}\) to a fissionable product. Neutron capture of the 238 \(\mathrm{U}\) is followed by two successive beta decays. What is the final fissionable product?

Step-by-Step Solution

Answer

The final fissionable product after 238U undergoes neutron capture and two successive beta decays in a breeder reactor is 239Pu (plutonium). The process can be described as follows: Neutron capture: \(_{92}^{238}\textrm{U} + _0^1n \rightarrow _{92}^{239}\textrm{U}\) First beta decay: \(_{92}^{239}\textrm{U} \rightarrow _{93}^{239}\textrm{Np} + _{-1}^{0}e\) Second beta decay: \(_{93}^{239}\textrm{Np} \rightarrow_{94}^{239}\textrm{Pu} + _{-1}^{0}e\)

Step 1: Understand neutron capture and beta decay

A breeder reactor works by converting a non-fissionable isotope, in this case, 238U, into a fissionable product. The process starts with neutron capture by 238U, which will increase the atomic mass but retain the same atomic number. After the neutron capture, the isotope undergoes two successive beta decays. In a beta decay, a neutron transforms into a proton, and an electron (beta particle) is emitted. This causes the atomic number to increase by 1 and the atomic mass to remain the same.

Step 2: Neutron capture by 238U

First, we will analyze the neutron capture by 238U. When a neutron is captured by 238U, the atomic number remains the same but the atomic mass increases by 1: Neutron capture: \(_{92}^{238}\textrm{U} + _0^1n \rightarrow _{92}^{239}\textrm{U}\) The resulting isotope after neutron capture is 239U.

Step 3: First beta decay

Now, let's examine the first beta decay. In beta decay, one neutron is converted into a proton and emits an electron. So, the atomic number increases by 1 and the atomic mass remains the same: First beta decay: \(_{92}^{239}\textrm{U} \rightarrow _{93}^{239}\textrm{Np} + _{-1}^{0}e\) The resulting isotope after the first beta decay is 239Np (neptunium).

Step 4: Second beta decay

Finally, we will analyze the second beta decay. Again, a neutron is converted into a proton, with the emission of a beta particle: Second beta decay: \(_{93}^{239}\textrm{Np} \rightarrow_{94}^{239}\textrm{Pu} + _{-1}^{0}e\) The resulting isotope after the second beta decay is 239Pu (plutonium).

Step 5: Identify the final fissionable product

After two successive beta decays following the neutron capture of 238U, the final fissionable product is 239Pu (plutonium).

Key Concepts

Breeder Reactors

Breeder reactors are a fascinating type of nuclear reactor with a unique capability. They are designed to extend the nuclear fuel supply by transforming non-fissionable isotopes into fissionable ones. In simpler terms, breeder reactors "breed" fuel, creating more fissile material than they consume.

This is particularly important for sustainability in nuclear energy. The primary mechanism involved in breeder reactors is the conversion of uranium-238 ( U extsubscript{92} extsuperscript{238} ) into plutonium-239 ( Pu extsubscript{94} extsuperscript{239} ), a fissionable product. This is achieved through neutron capture and subsequent beta decays. Key benefits include:

• Efficiency: Breeder reactors make better use of uranium resources by utilizing isotopes not used in traditional reactors.
• Sustainability: They help in managing resource scarcity by producing fuel from otherwise non-usable isotopes.

As remarkable as they are, breeder reactors require careful engineering and operational oversight to manage the processes and materials involved.

Neutron Capture

Neutron capture is a critical process in nuclear chemistry, especially within breeder reactors. It occurs when a nucleus captures a free neutron, leading to an increase in its atomic mass while keeping its atomic number the same. In the context of the exercise, uranium-238 captures a neutron to become uranium-239 ( U extsubscript{92} extsuperscript{239} ).

This process initiates the transformation of a stable isotope into ones that are further transformed through radioactive decay processes. Neutron capture is essential because:

• It lays the foundation for creating new isotopes that can undergo further reactions to become fissionable.
• It is a spontaneous and neutral process, not requiring additional energy input.

In breeder reactors, this step is crucial to converting stable isotopes into useful ones for energy production.

Beta Decay

Beta decay is a fundamental radioactive process that helps in the transformation of isotopes into more useful forms. In this process, a neutron in the nucleus is converted into a proton, releasing an electron, known as a beta particle. This results in an increase in the atomic number by one, while the mass number remains unchanged.

For example, uranium-239 undergoes beta decay to form neptunium-239 ( Np extsubscript{93} extsuperscript{239} ), and later neptunium-239 undergoes another beta decay to transform into plutonium-239 ( Pu extsubscript{94} extsuperscript{239} ).

• Beta decay increases the atomic number, effectively changing the element to the next one in the periodic table.
• It allows breeder reactors to transmute isotopes into fissionable materials efficiently, such as the conversion of uranium into plutonium.

Beta decay is crucial for nuclear chemistry, allowing the shift from less useful isotopes to valuable ones in energy production.

Fissionable Isotopes

Fissionable isotopes are nuclear fuel variants that can undergo fission to release energy for power generation. Unlike fissile isotopes, which readily fission at lower energy neutron impacts, fissionable isotopes require high-energy neutrons to initiate the process. Breeder reactors play a significant role in creating more of these isotopes from non-fissionable ones.

Plutonium-239 ( Pu extsubscript{94} extsuperscript{239} ) is one such fissionable isotope generated from uranium-238 through neutron capture and beta decay processes. Some characteristics of fissionable isotopes include:

• They are crucial for sustaining nuclear chain reactions in reactors.
• Offer a reliable alternative to uranium-235, which is naturally fissile and commonly used in nuclear reactors.

Utilizing fissionable isotopes efficiently ensures the long-term viability of nuclear energy as a resource-rich and low-emission power source.