Context: In nature, uranium is found as uranium-238 (99.2742%) and uranium-235 (0.7204%). Isotope separation concentrates (enriches) the fissionable uranium-235 for nuclear weapons and most nuclear power plants, except for gas cooled reactors and pressurised heavy water reactors. Most neutrons released by a fissioning atom of uranium-235 must impact other uranium-235 atoms to sustain the nuclear chain reaction. The concentration and amount of uranium-235 needed to achieve this is called a 'critical mass'.

Question: What percentage of uranium in nature is uranium-235?
Answer: 0.7204%

Question: What is the most prevalent natural isotope of uranium?
Answer: uranium-238

Question: What is the term for the amount of uranium-235 needed to sustain a nuclear chain reaction?
Answer: critical mass

Question: What percentage of natural uranium is isotope 238?
Answer: 99.2742%

Question: Along with gas cooled reactors, what type of reactor doesn't use uranium-235?
Answer: pressurised heavy water

Question: What percentage of uranium in nature is uranium-335?
Answer: Unanswerable

Question: What is the most prevalent unnatural isotope of uranium?
Answer: Unanswerable

Question: What is the term for the amount of uranium-335 needed to sustain a nuclear chain reaction?
Answer: Unanswerable

Question: What percentage of unnatural uranium is isotope 238?
Answer: Unanswerable

Question: Along with gas heated reactors, what type of reactor doesn't use uranium-235?
Answer: Unanswerable