Problem 58 - Chemistry The Central Science In Si Units

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

Statements (i) and (iv) are true.

Step 1: Evaluate Statement (i)

Statement (i) claims that natural uranium contains too little \(^{235}\text{U}\) to be used as a fuel. Naturally occurring uranium is composed of about 99.3% \(^{238}\text{U}\) and only about 0.7% \(^{235}\text{U}\), which is not sufficient for most nuclear reactors and requires enrichment. Therefore, statement (i) is true.

Step 2: Evaluate Statement (ii)

Statement (ii) suggests that \(^{238}\text{U}\) cannot be used as fuel because it forms a supercritical mass too easily. \(^{238}\text{U}\) is not fissile, meaning it does not easily sustain a chain reaction and actually cannot form a supercritical mass on its own. Therefore, statement (ii) is false.

Step 3: Evaluate Statement (iii)

Statement (iii) claims that uranium must be enriched to more than 50% \(^{235}\text{U}\) to be used as fuel. For most commercial nuclear reactors, uranium needs to be enriched to only 3-5% \(^{235}\text{U}\). Consequently, statement (iii) is false.

Step 4: Evaluate Statement (iv)

Statement (iv) asserts that the neutron-induced fission of \(^{235}\text{U}\) releases more neutrons per nucleus than the fission of \(^{238}\text{U}\). This is generally correct as \(^{235}\text{U}\) is a better neutron source, releasing 2 to 3 neutrons per fission event compared to \(^{238}\text{U}\). Therefore, statement (iv) is true.

Key Concepts

Uranium Enrichment

Uranium enrichment is a process that increases the percentage of the fissile isotope \(^{235}\text{U}\) in uranium. Natural uranium is mostly composed of \(99.3\%\) of \(^{238}\text{U}\) and only about \(0.7\%\) of \(^{235}\text{U}\). This level of \(^{235}\text{U}\) is too low to efficiently sustain a nuclear chain reaction in reactors designed for commercial energy production.

To solve this, uranium needs to be enriched. The enrichment process modifies the composition so that the proportion of \(^{235}\text{U}\) is increased to between 3\% and 5\% for most commercial reactors. Methods such as gas diffusion, gas centrifuge, and laser enrichment are typically used to separate the isotopes and increase the concentration of \(^{235}\text{U}\).

• Gas diffusion method: relies on the slight difference in molecular weight between isotopes.
• Gas centrifuge method: utilizes centrifugal force to separate isotopes based on mass.
• Laser enrichment methods: use lasers to selectively excite \(^{235}\text{U}\) isotopes.

Nuclear Reactors

Nuclear reactors are installations used to initiate, regulate, and control nuclear fission reactions. They facilitate a controlled environment where nuclear fission can occur safely to generate energy. The core, which is filled with fuel (often enriched uranium), is where the fission reaction takes place.

Nuclear reactors work by using the heat produced by fission to heat water, creating steam that drives turbines to produce electricity. The entire process involves several components:

• The fuel: typically enriched uranium or plutonium.
• The moderator: often water or graphite, slows down fast-moving neutrons.
• The control rods: made of materials like cadmium or boron, which absorb excess neutrons to control the fission rate.
• The coolant: a fluid, often water or a gas, that transfers heat away from the reactor core.

By controlling the fission reaction using these components, nuclear reactors can provide a steady and reliable source of energy.

Fission of Uranium

The fission of uranium, particularly \(^{235}\text{U}\), is a nuclear reaction where the nucleus of an atom splits into smaller parts. This process releases a significant amount of energy, primarily due to the conversion of mass into energy as described by Einstein’s famous equation, \(E=mc^2\).

During fission, one neutron strikes the nucleus, causing it to become unstable and split. This split releases energy in the form of heat, along with more neutrons. These emitted neutrons may continue to initiate further fission reactions, creating a chain reaction.

The released neutrons per fission are crucial for sustaining the chain reaction:

• \(^{235}\text{U}\) typically releases 2 to 3 neutrons, promoting the continuation of the chain reaction.
• In contrast, \(^{238}\text{U}\) is less naturally fissile and does not sustain a chain reaction without additional help, such as becoming plutonium-239.

This chain reaction is the fundamental process behind nuclear energy generation.

Natural Uranium Composition

Natural uranium is an element found in the Earth's crust with a composition of primarily two isotopes: \(^{238}\text{U}\) and \(^{235}\text{U}\).

In its natural state, uranium is roughly composed as follows:

• \(^{238}\text{U}\) makes up about \(99.3\%\) of the material.
• \(^{235}\text{U}\) only constitutes about \(0.7\%\) of naturally occurring uranium.

Though present in minimal amounts, \(^{235}\text{U}\) is the vital isotope for sustaining nuclear reactions, which is why enrichment processes are necessary to increase its percentage.

Despite being less prevalent, \(^{235}\text{U}\) is highly significant in nuclear chemistry because of its ability to easily undergo fission, making it crucial for nuclear reactors and energy production.