With reference to radioisotope thermoelectric generators (RTGs), consider the following statements: 1. RTGs are miniature fission reactors. 2. RTGs are used for powering the onboard systems of spacecrafts. 3. RTGs can use Plutonium-238, which is a by-product of weapons development. Which of the statements given above are correct?

1. RTGs are miniature fission reactors. — Incorrect. RTGs do not use a controlled fission chain reaction; they generate electricity from the heat released by the natural radioactive decay of isotopes.

2. RTGs are used for powering the onboard systems of spacecrafts. — Correct. They are widely used in spacecraft, especially deep-space missions where solar power is unreliable.

3. RTGs can use Plutonium-238, which is a by-product of weapons development. — Correct. Pu-238 is a common RTG fuel, and it can be obtained during nuclear weapons/nuclear fuel production processes, though it is not itself weapons-grade material.

Therefore, statements 2 and 3 only are correct.