Which is not a fissionable material? — Nuclear Chemistry and Radioactivity Chemistry Question

💡 Solution & Explanation

Step 1 - Definitions - **Fissile material**: Can be fissioned by **thermal (slow) neutrons** — the most important type for nuclear reactors. - **Fertile material**: Cannot be fissioned by slow neutrons itself, but can be converted (transmuted) into a fissile material. Step 2 - Evaluate Each Option **(A) $\ce{^{238}_{92}U}$**: Uranium-238 is a **fertile** material. It does NOT undergo fission with slow/thermal neutrons. It requires fast neutrons (above ~1 MeV) for fission. Instead, when it absorbs a slow neutron, it is transmuted to $\ce{^{239}_{94}Pu}$, which is fissile. → **Not fissionable by slow neutrons.** This is the answer. **(B) $\ce{^{235}_{92}U}$**: Uranium-235 is **fissile** — it readily undergoes fission when it absorbs a thermal neutron. It is the primary fuel in most nuclear reactors. → Fissionable. Incorrect. **(C) $\ce{^{233}_{92}U}$**: Uranium-233 is produced from Thorium-232 and is an excellent fissile material. → Fissionable. Incorrect. **(D) $\ce{^{239}_{94}Pu}$**: Plutonium-239 is highly fissile and is used in nuclear weapons and reactors. → Fissionable. Incorrect. $$\boxed{\text{Answer: A — }\ \ce{^{238}_{92}U}\ \text{ is not fissionable by thermal neutrons}}$$