What is meant by nueclar reactor

Q. (a) What is a nuclear reactor? What is the fuel used in a nuclear reactor?
(b) What type of nuclear reaction is carried out in a nuclear reactor?
(c) How is the working nuclear reactor of a power plant shut down in an emergency?
​(d) Name five places in India where nuclear power plants are located.

Q. (i) What is meant by Radioactivity?
(ii) What is meant by nuclear waste?
(iii) Suggest one effective way for the safe disposal of nuclear waste.

Q. What is use of helium in nuclear reactors?

Q. When $U-235$ is fissioned in a nuclear reactor, the nuclear reaction is maintained by