Problem 58 - Chemistry: The Central Science in SI Units, Global Edition

Question

Which of the following statements about the uranium used in nuclear reactors is or are true? (i) Natural uranium has too little \({ }^{235} \mathrm{U}\) to be used as a fuel. (ii) \({ }^{238} \mathrm{U}\) cannot be used as a fuel because it forms a supercritical mass too easily. (iii) To be used as fuel, uranium must be enriched so that it is more than \(50 \%^{235} \mathrm{U}\) in composition. (iv) The neutron-induced fission of \({ }^{235} \mathrm{U}\) releases more neutrons per nucleus than fission of \({ }^{238} \mathrm{U}\)

Step-by-Step Solution

Answer

The true statements about uranium used in nuclear reactors are (i) Natural uranium has too little \({ }^{235} \mathrm{U}\) to be used as a fuel, and (iv) The neutron-induced fission of \({ }^{235} \mathrm{U}\) releases more neutrons per nucleus than fission of \({ }^{238} \mathrm{U}\).

Step 1: Statement (i)

Natural uranium has too little \({ }^{235} \mathrm{U}\) to be used as a fuel. This statement is true. Natural uranium consists of around 0.72% of the isotope \({ }^{235} \mathrm{U}\) and about 99.3% of the isotope \({ }^{238} \mathrm{U}\). The \({ }^{235} \mathrm{U}\) isotope is significantly more fissionable than \({ }^{238} \mathrm{U}\), so its small proportion in natural uranium makes it insufficient for use as fuel in nuclear reactors without further processing.

Step 2: Statement (ii)

\({ }^{238} \mathrm{U}\) cannot be used as a fuel because it forms a supercritical mass too easily. This statement is false. \({ }^{238} \mathrm{U}\) is not as fissionable as \({ }^{235} \mathrm{U}\), and it does not easily form a supercritical mass. In fact, \({ }^{238} \mathrm{U}\) requires a significant amount of energy input (such as fast-moving neutrons) to initiate fission, while \({ }^{235} \mathrm{U}\) can undergo fission with slow-moving (thermal) neutrons.

Step 3: Statement (iii)

To be used as fuel, uranium must be enriched so that it is more than \(50 \%^{235} \mathrm{U}\) in composition. This statement is false. The percentage of \({ }^{235} \mathrm{U}\) required in uranium for nuclear reactors varies depending on the type of reactor and the desired power output, but it is typically between 3% and 5%. Some reactors can even use natural uranium. It is important to note that weapons-grade uranium, used for nuclear weapons, is typically enriched to around 90% \({ }^{235} \mathrm{U}\).

Step 4: Statement (iv)

The neutron-induced fission of \({ }^{235} \mathrm{U}\) releases more neutrons per nucleus than fission of \({ }^{238} \mathrm{U}\). This statement is true. The fission of \({ }^{235} \mathrm{U}\) releases more neutrons per nucleus (averaging around 2 to 3) compared to the fission of \({ }^{238} \mathrm{U}\) (which usually releases around 1 to 2 neutrons per nucleus). The additional neutrons released from \({ }^{235} \mathrm{U}\) fission contribute to sustaining a nuclear chain reaction more effectively. In conclusion, the true statements about uranium used in nuclear reactors are (i) and (iv).

Key Concepts

Uranium Isotopes

Uranium is a heavy metal with several isotopes, but the most significant ones for nuclear applications are \(^{235}U\) and \(^{238}U\). An isotope refers to variants of a particular chemical element that differ in neutron number. For uranium, the number of protons remains the same, but the two common isotopes have different numbers of neutrons, affecting their nuclear properties.

• \(^{235}U\) is the rarer isotope, making up about 0.72% of natural uranium. It is highly valued in nuclear physics for its ability to sustain a nuclear fission chain reaction.
• \(^{238}U\), comprising about 99.3% of naturally occurring uranium, is less useful for direct energy production through fission due to its lower reactivity but plays a role in breeding fissile material in reactors.

Understanding these isotopes' properties is essential for their utilization in nuclear power generation. Their differences influence how uranium is processed and used in nuclear reactors.

Nuclear Fission

Nuclear fission is a process where a heavy nucleus, such as that of \(^{235}U\), splits into smaller nuclei upon absorbing a neutron. This splitting releases a significant amount of energy, which is harnessed in nuclear power plants to generate electricity. The process also releases additional neutrons, which can induce further fission, creating a chain reaction that is crucial for sustained energy production.

• During fission, \(^{235}U\) releases 2 to 3 neutrons, allowing it to efficiently maintain a chain reaction.
• Meanwhile, \(^{238}U\) mostly captures neutrons without fission but can become \(^{239}Pu\), a fissile material that can sustain fission reactions.

The controlled fission chain reactions in reactors provide a stable heat output that can be converted to steam and drive turbines for electricity production.

Nuclear Reactor Fuel

Nuclear reactors rely on fuel composed primarily of uranium isotopes to produce energy via fission. The effectiveness of the fuel depends on the proportion of \(^{235}U\), as it is the isotope that easily undergoes fission with thermal neutrons. To meet safety and efficiency standards, the uranium used in power reactors typically requires enrichment, meaning the percentage of \(^{235}U\) is increased.

• Typically, uranium fuel used in commercial nuclear reactors is enriched to contain about 3% to 5% \(^{235}U\).
• In some designs, like Canada's CANDU reactors, natural uranium with minimal enrichment is used, due to their ability to utilize heavy water as a neutron moderator.

Nuclear fuel generates heat continuously over months or even years, making it a potent and lasting energy source.

Enriched Uranium

Enriched uranium refers to uranium that has undergone a process to increase the proportion of \(^{235}U\) relative to \(^{238}U\). Enrichment is essential for most nuclear reactors because natural uranium's content of \(^{235}U\) is too low for efficient energy production. The enrichment process typically results in fuel with \(^{235}U\) concentrations between 3% and 5% for commercial reactors.

• Enriched uranium allows for a sustainable chain reaction by providing a greater ability to capture neutrons and undergo fission.
• In contrast, weapons-grade uranium is enriched to about 90% \(^{235}U\), highlighting the difference between fuel for energy production and material for nuclear weapons.

The need for enriched uranium underscores the demand for advanced technologies and careful handling of nuclear materials to prevent proliferation and ensure safe energy use.