Problem 57 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) Which of the following are required characteristics of an isotope to be used as a fuel in a nuclear power reactor? (i) It must emit gamma radiation. (ii) On decay, it must release two or more neutrons. (iii) It must have a half-life less than one hour. (iv) It must undergo fission upon the absorption of a neutron. (b) What is the most common fissionable isotope in a commercial nuclear power reactor?

Step-by-Step Solution

Answer

The required characteristics for an isotope to be used as a fuel in a nuclear power reactor are (ii) on decay, it must release two or more neutrons, and (iv) it must undergo fission upon the absorption of a neutron. The most common fissionable isotope in a commercial nuclear power reactor is Uranium-235 (\(^{235}\textrm{U}\)).

Step 1: Part (a): Analyzing the required characteristics for nuclear fuel isotope

To determine which characteristics are needed for an isotope to be used as fuel in a nuclear power reactor, we will analyze each listed characteristic one by one. (i) It must emit gamma radiation: Gamma radiation is not a mandatory requirement for fuel isotopes in a nuclear reactor. Gamma radiation is emitted during the decay of some radioactive isotopes, but it is not a factor in determining whether an isotope can be used as fuel in a reactor. (ii) On decay, it must release two or more neutrons: This is an important characteristic for a nuclear fuel isotope. When an isotope undergoes fission, the release of two or more neutrons ensures that a chain reaction can be sustained. One of these neutrons will cause the fission of another nucleus, while the rest will be absorbed by other materials or escape the reactor. (iii) It must have a half-life less than one hour: This characteristic is not necessary for a nuclear fuel isotope. Having a short half-life means that the isotope will decay rapidly, which may not be ideal for maintaining a sustained chain reaction in a nuclear reactor. In fact, most nuclear fuels have half-lives on the order of years or even millennia, allowing them to provide energy for long periods. (iv) It must undergo fission upon the absorption of a neutron: This is a crucial characteristic for nuclear fuel isotopes. To sustain a chain reaction in a nuclear reactor, the fuel isotope must undergo fission upon the absorption of a neutron. This process releases energy which is then used to heat water, produce steam, and ultimately generate electricity.

Step 2: Part (b): Identifying the most common fissionable isotope in commercial nuclear power reactors

Now let's move on to the second part of the problem, which requires us to identify the most common fissionable isotope used in commercial nuclear power reactors. The most common fissionable isotope used in commercial nuclear power reactors is Uranium-235 (\(^{235}\textrm{U}\)). This isotope has an abundant supply on Earth, and its fission releases a considerable amount of energy, making it an ideal fuel for nuclear reactors. Other isotopes, such as Plutonium-239 (\(^{239}\textrm{Pu}\)) and Thorium-232 (\(^{232}\textrm{Th}\)), are also used in some reactors but are less common than Uranium-235. So, to summarize: Characteristics (ii) and (iv) are required for an isotope to be used as a fuel in a nuclear power reactor. The most common fissionable isotope in a commercial nuclear power reactor is Uranium-235 (\(^{235}\textrm{U}\)).

Key Concepts

Isotope Characteristics

In nuclear physics, isotopes are different forms of the same element with varying numbers of neutrons. For an isotope to be used as a fuel in a nuclear reactor, certain characteristics are essential. Firstly, it must undergo fission when it absorbs a neutron. This means the isotope splits into smaller nuclei, releasing a significant amount of energy and neutrons.

Another key characteristic is the emission of neutrons during fission. The emitted neutrons can then propagate the chain reaction necessary in nuclear reactors. However, other characteristics, such as emitting gamma radiation or having a short half-life, are not as crucial for fuel isotopes.

• Emitting gamma radiation is not necessary for a nuclear reactor fuel.
• A short half-life is generally not preferred as it leads to rapid decay of the fuel.
• Releasing two or more neutrons during fission fosters a stable chain reaction, which is very important.

Nuclear Fission

Nuclear fission is the process where the nucleus of an atom splits into two or more smaller nuclei, along with the release of protons, neutrons, and energy. This splitting releases a large amount of energy, which is harnessed in nuclear power plants to generate electricity. When a nuclear reactor uses fissionable isotopes, like Uranium-235, it kicks off a chain reaction of splitting atoms.

Crucial to this is the absorption of neutrons by a fissionable atom's nucleus. Upon absorption, the nucleus becomes unstable and splits, dynamically releasing additional neutrons. These newly freed neutrons strike other nearby atomic nuclei, perpetuating the fission process. The vast amount of energy released during each fission event is converted into heat, eventually used to produce steam and power turbines:

• Energy Release: Large energy bursts released due to atomic splits are concerted efforts to produce electricity.
• Chain Reaction: Multiple fission events set off a series of reactions, maintaining the energy production cycle.

Uranium-235

Uranium-235 is the most common isotope used in nuclear reactors due to its favorable fission characteristics.

Naturally occurring in only about 0.7% of uranium, it is nonetheless a potent fuel option. Once a Uranium-235 nucleus absorbs a neutron, it becomes unstable and undergoes fission, releasing substantial energy and additional neutrons that fuel other reactions. This isotope is well-suited for sustaining chain reactions over long periods.

• Abundant but Enriched: Requires enriching processes to increase its concentration for use in reactors.
• Efficient Energy Production: Produces large amounts of energy from relatively small isotopic quantities.
• Chain Reaction Compatible: Easily supports sustained chain reactions due to neutron release upon fission.

Chain Reaction Sustainability

Sustaining a chain reaction is key to the ongoing production of energy in a nuclear reactor. When a fuel isotope such as Uranium-235 undergoes fission, it releases neutrons that then prompt further fission events. This continuation of fission events is what we refer to as a chain reaction. Properly managing this reaction ensures a steady supply of energy:

• Neutron Economy: Enough neutrons must be available to sustain ongoing fission reactions, making neutron efficiency critical.
• Control Mechanisms: Reactors use control rods to absorb excess neutrons and prevent an uncontrolled, potential reactor meltdown.
• Continuous Process: Maintaining a balanced, self-sustaining reaction is essential for long-term energy production.

Chain reaction sustainability is the crucial process that lies at the core of a reactor's ability to produce energy reliably. With effective control, this allows for efficient, long-lasting energy production that meets electricity needs.