Chain Reaction of Uranium

Q. U-235 requires a neutron to be bombarded into it to undergo fission because

1. The energy of U-235 increases due to collision with the incoming neutron
2. The U-235 nucleus absorbs the neutron and becomes U-236 which is highly unstable
3. The neutron due to its high energy breaks apart the nucleus
4. None of the above.

Q. Which of the following is the major reason of a self-sustained chain reaction of U-235 fission?

1. High instability of U-235 isotope.
2. High instability of U-236 isotope
3. A neutron is required to trigger the fission of U-235 and after each fission neutrons are created, which trigger another reaction.
4. A neutron is required to trigger the fission of U-235 and we continuously bombard the neutron on the fuel sample.