Question

which of the following isotopes normally fissionable (A) \({ }_{92} \mathrm{U}^{233}\) (B) \({ }_{92} \mathrm{U}^{238}\) (C) \({ }_{92} \mathrm{U}^{235}\) (D) \({ }_{93} \mathrm{~Np}^{239}\)

Short answer

The normally fissionable isotope among the given options is (C) \({ }_{92} \mathrm{U}^{235}\).

Step-by-step solution

Option A: \({ }_{92} \mathrm{U}^{233}\)

The isotope \({ }_{92} \mathrm{U}^{233}\) is fissile, which means it can sustain a chain reaction when bombarded with thermal neutrons. Therefore, \({ }_{92} \mathrm{U}^{233}\) can be considered as a fissionable isotope.

Option B: \({ }_{92} \mathrm{U}^{238}\)

The isotope \({ }_{92} \mathrm{U}^{238}\) is fertile, meaning it can be converted into a fissile isotope (\({ }_{92} \mathrm{U}^{239}\)) when it captures a neutron. However, it doesn't usually undergo fission when bombarded with thermal neutrons. So, \({ }_{92} \mathrm{U}^{238}\) is not considered as normally fissionable.

Option C: \({ }_{92} \mathrm{U}^{235}\)

The isotope \({ }_{92} \mathrm{U}^{235}\) is fissile and can sustain a chain reaction when bombarded with thermal neutrons. Therefore, \({ }_{92} \mathrm{U}^{235}\) is considered as a normally fissionable isotope.

Option D: \({ }_{93} \mathrm{~Np}^{239}\)

The isotope \({ }_{93} \mathrm{~Np}^{239}\) is a transuranic element that can undergo fission when bombarded with fast neutrons, but not thermal neutrons. Therefore, it is not normally considered as fissionable.

Final Answer

Based on the given options, the normally fissionable isotope is: (C) \({ }_{92} \mathrm{U}^{235}\)

Key concepts

Nuclear Fission

Nuclear fission is a process where the nucleus of a heavy atom, like uranium or plutonium, splits into two smaller nuclei. This splitting happens when the atom absorbs a neutron and becomes unstable.

• The fission process releases a large amount of energy, which can be harnessed for power generation.
• During fission, additional neutrons are released, which can strike other nuclei, leading to a chain reaction.

In nuclear reactors, controlling the chain reaction is crucial to ensure safety. By using control rods that absorb excess neutrons, the reaction can be regulated. Fission can occur either spontaneously or be induced by neutrons. Most power plants use induced fission, where isotopes like { }_{92} {U}^{235} are bombarded intentionally with thermal neutrons.

Fissile Materials

Fissile materials are isotopes capable of sustaining a nuclear fission chain reaction with thermal neutrons. The key attribute of fissile materials is that they can sustain a long sequence of fission events.

• Uranium-233, Uranium-235, and Plutonium-239 are common fissile materials.
• They can capture slow-moving (thermal) neutrons and undergo fission.

The ability of fissile materials to maintain a chain reaction is what makes them crucial for both nuclear power plants and nuclear weapons. For example, uranium-235, which occurs naturally, is often used in reactors due to its ability to sustain a chain reaction easily. This is why uranium enrichment is often necessary. Enrichment increases the percentage of uranium-235 in a sample to ensure that the reaction can be sustained.

Thermal Neutrons

Thermal neutrons are slow-moving neutrons with lower kinetic energy than fast neutrons. Their lower energy makes them particularly effective at inducing fission in certain fissile materials.

• These neutrons have energies similar to room temperature, about 0.025 electron volts (eV).
• They interact more easily with certain isotopes due to their longer wavelength.

Thermal neutrons are vital in nuclear reactors because they are more likely to cause fission in isotopes like uranium-235 and plutonium-239. Reactors are designed to slow down fast neutrons so they become thermal neutrons. This moderation process is often achieved using substances like heavy water or graphite, which do not absorb the neutrons but slow them down efficiently.