Which of the following statements about the uranium used in nuclear reactors is or are true? (i) Natural uranium has too little 235U to be used as a fuel. (ii) 238U cannot be used as a fuel because it forms a supercritical mass too easily. (iii) To be used as fuel, uranium must be enriched so that it is more than 50% 235U in composition. (iv) The neutron-induced fission of 235U releases more neutrons per nucleus than the fission of 238U.

Verified step by step guidance

1. Step 1: Understand the composition of natural uranium. Natural uranium consists mostly of two isotopes: 238U and 235U. The isotope 235U is fissile, meaning it can sustain a nuclear chain reaction, but it is only about 0.7% of natural uranium.
2. Step 2: Evaluate statement (i). Natural uranium has too little 235U to be used as a fuel. This statement is true because the concentration of 235U in natural uranium is too low to sustain a chain reaction in most nuclear reactors without enrichment.
3. Step 3: Evaluate statement (ii). 238U cannot be used as a fuel because it forms a supercritical mass too easily. This statement is false. 238U is not fissile and does not form a supercritical mass easily. Instead, it is fertile, meaning it can be converted into a fissile material (239Pu) in a reactor.
4. Step 4: Evaluate statement (iii). To be used as fuel, uranium must be enriched so that it is more than 50% 235U in composition. This statement is false. For most nuclear reactors, uranium is enriched to about 3-5% 235U, not more than 50%.
5. Step 5: Evaluate statement (iv). The neutron-induced fission of 235U releases more neutrons per nucleus than the fission of 238U. This statement is true. The fission of 235U typically releases more neutrons, which is crucial for sustaining a chain reaction.