Problem 57 - Chemistry The Central Science In Si Units

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

(a) The required characteristic is to undergo fission upon neutron absorption. (b) Uranium-235 (U-235) is the most common fissionable isotope used.

Step 1: Analyzing the characteristics of isotopes for nuclear fuel

In nuclear power reactors, the key requirement for an isotope to be used as fuel is its ability to undergo fission upon the absorption of a neutron. This process releases a substantial amount of energy. Thus, option (iv) "It must undergo fission upon the absorption of a neutron" is a required characteristic. Other options like emitting gamma radiation or releasing multiple neutrons upon decay are not mandatory.

Step 2: Recognizing the excess conditions

Option (i) 'It must emit gamma radiation' is not a required characteristic for nuclear fuel. Fissionable isotopes often do emit radiation, but it's not a prerequisite for their selection as a fuel. Option (ii) 'On decay, it must release two or more neutrons' is not essential either; however, releasing neutrons is important once the fission process starts. Option (iii) 'It must have a half-life less than one hour' is incorrect because a usable isotope must have a long enough half-life to remain effective over time in a reactor.

Step 3: Identifying the common fissionable isotope

The most common isotope used in commercial nuclear reactors is Uranium-235 (U-235). It is a fissionable material known for its ability to sustain a nuclear chain reaction when neutrons are absorbed.

Key Concepts

Isotopes

Isotopes are different forms of the same element. They have the same number of protons in the nucleus but different numbers of neutrons. This means that while isotopes of an element have similar chemical properties, they can have very different nuclear properties. For example, some isotopes are stable, while others are radioactive. This makes them useful in various applications, such as medical imaging, carbon dating, and nuclear reactors.

When it comes to nuclear fuel, not all isotopes are suited to fuel a reactor. For instance, in a nuclear reactor, we look for isotopes that can easily undergo fission—a process where the nucleus splits into smaller, more stable nuclei, releasing energy in the process. This characteristic allows certain isotopes to be used effectively as fuel. One such isotope that is widely used due to its ability to sustain a fission chain reaction is Uranium-235 (U-235).

Fission

Fission refers to the splitting of an atomic nucleus into two or more smaller nuclei, along with several free neutrons and a significant amount of energy. This process is fundamental in nuclear reactors because it is the source of the vast amounts of energy that reactors produce.

For fission to occur, a neutron must first hit the nucleus of a fissionable material, like Uranium-235. When this happens, the compound nucleus is briefly formed, which is unstable and quickly splits, releasing energy and more neutrons. These newly released neutrons can further induce fission in adjacent nuclei, creating a self-sustaining chain reaction.

The considerable amount of energy released by fission is used to heat water, producing steam that spins turbines to generate electricity. Therefore, the ability of an isotope to undergo fission is the primary criterion when selecting nuclear fuel. This is why certain characteristics, like emitting gamma radiation, are not critical for the choice of nuclear fuel.

Nuclear Reactors

Nuclear reactors are sophisticated systems designed to control nuclear fission reactions. Essentially, they convert the nuclear energy released in fission into electrical energy. Understanding how reactors operate is key to appreciating the importance of selecting the right isotopes for fuel.

In a reactor, Uranium-235 is commonly used as fuel. The reactor core contains fuel rods packed with this isotope. When U-235 nuclei absorb neutrons, they become unstable, undergo fission, and release energy and additional neutrons. This set-up ensures a continuous fission chain reaction, which is carefully moderated to control the rate of the reaction and maintain safety.

• The control rods in reactors can absorb excess neutrons and regulate the chain reaction.
• A coolant, which is often water, removes heat from the reactor core.
• The reactor is encased in a containment structure to prevent the escape of radiation.

Nuclear reactors are engineered to make efficient and safe use of the energy from fission, showcasing the necessity of having a comprehensive understanding of nuclear processes.

Uranium-235

Uranium-235 is a specific isotope of uranium that is particularly significant in the field of nuclear power. It is one of the few naturally occurring isotopes capable of sustaining a nuclear chain reaction, making it a crucial component of nuclear fuel.

• Uranium-235 has 92 protons and 143 neutrons.
• It constitutes about 0.7% of natural uranium, with the majority being Uranium-238.
• Uranium-235 is favored for its ability to readily undergo fission upon neutron absorption.

In commercial nuclear reactors, U-235 is essential because of its optimal characteristics for maintaining a controlled chain reaction. When U-235 atoms split, they release energy that is harnessed to generate electricity. Additionally, this process produces more neutrons, which perpetuate the reaction by causing more fissile atoms to split.

U-235's capability to release energy efficiently and its rarity in nature are among the reasons why it is one of the most sought-after isotopes for nuclear power generation.