Question

(a) Which of the following statements about the uranium used in nuclear reactors is or are true? (i) Natural uranium has too little \({ }^{295} \mathrm{U}\) to be used as a fuel. (ii) \({ }^{24} \mathrm{U}\) cannot be used as a fucl because it forms a supereritical mass too casily. (iii) To be used as fuel, uranium must be enriched so that it is more than \(50 \%{ }^{2.35} \mathrm{U}\) in composition. (iv) The neutron-induced fission of \({ }^{235} \mathrm{U}\) releases more neutrons per nucleus than fission of \({ }^{2.85} \mathrm{U}\). (b) Which of the following statements about the plutonium shown in the chapter-opening photograph explains why it cannot be used for nuclear power plants or nuclear weapons? (i) None of the isotopes of Pu possess the characteristics needed to support nuclear fission chain reactions. (ii) The orange glow indicates that the only radioactive decay products are heat and visible light. (iii) The particular isotope of plutonium used for RTGs is incapable of sustaining a chain reaction. (iv) Plutonium can be used as a fuel, but only atter it decays to uranium.

Short answer

The correct statements are: (a.i) Natural uranium has too little \(^{235}U\) to be used as a fuel, and (a.iv) The neutron-induced fission of \(^{235}U\) releases more neutrons per nucleus than fission of \(^{238}U\). The correct explanation for why the plutonium in the chapter-opening photograph cannot be used for nuclear power plants or nuclear weapons is (b.iii) The particular isotope of plutonium used for RTGs is \(^{238}Pu\), which is incapable of sustaining a chain reaction.

Step-by-step solution

(a.i) Analyzing Statement (i) about natural uranium:

Natural uranium contains only a small amount of the isotope \({ }^{235}\mathrm{U}\) (about 0.7%). This isotope is fissile and is the primary fuel for nuclear reactors. However, since the isotope is not abundant enough in natural uranium, the statement is true – natural uranium has too little \({ }^{235}\mathrm{U}\) to be used as a fuel.

(a.ii) Analyzing Statement (ii) about \({ }^{24}\mathrm{U}\):

The notation "\({ }^{24}\mathrm{U}\)" is not correct, as the number "24" is supposed to represent the mass number of the isotope. One might assume that the statement refers to \({ }^{234}\mathrm{U}\), which is a non-fissile isotope. The statement itself is incorrect, so we will not consider it true.

(a.iii) Analyzing Statement (iii) about uranium enrichment:

To be used as fuel in a nuclear reactor, uranium must be enriched, i.e., the percentage of the \({ }^{235}\mathrm{U}\) isotope must be increased. However, the statement that it must be enriched to "more than \(50 \%{ }^{235}\mathrm{U}\)" is not true. Typical enrichment levels for reactor fuel are between 3% and 5% \({ }^{235}\mathrm{U}\), not more than 50%. The statement is false.

(a.iv) Analyzing Statement (iv) about neutron-induced fission:

The neutron-induced fission of \({ }^{235}\mathrm{U}\) does indeed release more neutrons per nucleus than the fission of \({ }^{238}\mathrm{U}\). The given isotope "\({ }^{2.85}\mathrm{U}\)" again seems to be a typo or transcription error, presumably referring to \({ }^{238}\mathrm{U}\). Therefore, the statement is true.

(b) Analyzing statements about plutonium:

We have four statements about plutonium and we need to determine which one explains why it cannot be used for nuclear power plants or nuclear weapons: (b.i) Some isotopes of plutonium, like \({ }^{239}\mathrm{Pu}\), do possess the characteristics needed to support nuclear fission chain reactions. Therefore, this statement is false. (b.ii) The orange glow in the chapter-opening photograph is due to the decay of radioactive isotopes and the resulting heat production, but it does not exclude the possibility of nuclear fission reactions. This statement is false as well. (b.iii) The particular isotope of plutonium used for Radioisotope Thermoelectric Generators (RTGs) is \({ }^{238}\mathrm{Pu}\). This isotope cannot sustain a nuclear chain reaction, which makes this statement true and the correct answer for this question. (b.iv) Plutonium does not need to decay to uranium before being used as a fuel. Some plutonium isotopes can be used as nuclear fuel directly, so this statement is false.

Key concepts

Uranium Enrichment

Uranium enrichment is a crucial process in making uranium suitable for use in nuclear reactors. Natural uranium consists mostly of \(^{238}\text{U}\), a non-fissile isotope, and only about 0.7% of \(^{235}\text{U}\), which is fissile and essential for nuclear reactions. However, for nuclear reactors, this percentage is insufficient.

The enrichment process increases the proportion of \(^{235}\text{U}\) to about 3-5%, making it viable for power generation. Here's an overview of how this process works:

• Centrifugation: A common method where uranium hexafluoride gas is spun at high speeds to separate isotopes based on mass differences.
• Diffusion: A method that uses barriers to separate isotopes by molecular effusion. Although less common now, it was historically significant.

Understanding enrichment helps explain why natural uranium can't be directly used as nuclear fuel; it must be processed to increase its \(^{235}\text{U}\) content.

Plutonium Isotopes

Plutonium isotopes are significant in the context of nuclear reactions. Plutonium, like uranium, has several isotopes, but not all are suitable for the same purposes. The most notable isotopes include:

• \(^{239}\text{Pu}\): This isotope is fissile, meaning it can sustain a nuclear fission chain reaction. It's used in nuclear reactors and nuclear weapons.
• \(^{238}\text{Pu}\): Often used in RTGs (Radioisotope Thermoelectric Generators) for its ability to produce heat through radioactive decay, but it cannot sustain a chain reaction.

The variety of plutonium isotopes means they have diverse applications, but only some like \(^{239}\text{Pu}\) are relevant to power generation and weaponry. This distinction is vital in nuclear science, affecting their utilization in different fields.

Uranium Isotopes

Uranium isotopes include several forms, with the most prevalent being \(^{238}\text{U}\) and \(^{235}\text{U}\). Here's what you need to know about them:

• \(^{235}\text{U}\): A fissile variety, critical for nuclear reactors due to its ability to sustain chain reactions. However, it is relatively scarce, leading to the need for enrichment.
• \(^{238}\text{U}\): The most abundant form, it is not fissile, but it plays a role in breeding more fissile materials. In reactors, \(^{238}\text{U}\) can capture neutrons to eventually form \(^{239}\text{Pu}\).

Understanding these isotopes explains much about nuclear energy production. Natural uranium's low \(^{235}\text{U}\) content is why enrichment is necessary, transforming inert material into a potent energy source. Each isotope's characteristics determine their roles and applications in nuclear technology.