Which of the following is a naturally occurring radioactive material that can be found in ore deposits and used as a fuel source in some reactors?

• A. Smoke Detectors
• B. Natural Uranium
• C. High Level Radiation
• D. Plutonium

Answer: (B)

The concept tested is recognizing a substance that occurs naturally in nature and can serve as a nuclear fuel in some reactor designs. Natural uranium fits this: it is found in ore deposits around the world and, in its natural form, contains about 0.7% fissile uranium-235 with the remainder mostly uranium-238. Some reactor designs, notably heavy-water–moderated ones like CANDU reactors, can use natural uranium as fuel without enrichment. In most other reactors, the uranium must be enriched to raise the U-235 fraction to about 3–5% to sustain fission reliably.

Other options aren’t used as natural reactor fuels in the same way. Smokemeasurements of americium-241 are common in smoke detectors but aren’t a natural fuel and aren’t mined as ore. High level radiation describes intensity, not a material. Plutonium is primarily manufactured in reactors from uranium and isn’t found in useful quantities as a natural fuel. So, natural uranium is the correct choice.