nuclear reactor : the device giving large amount of energy (nuclear) through fission process at controlled rate is called nuclear reactor . see ,in this atomic pile,first fussion rxn helps in production of fast neutrons.if fast neutrons pass through moderator they become thermal neutrons . these thermal neutrons r utilizied to fission rxn to produce large amount of energy................

purpose of moderator to reduce the enrgy of neutron...

nuclear fusion : it is a process in which two lighter nuclei fuse together to form a heavy nucleus and energy will be released . the mass of the nuceus that is formed will always be less thansum of the individual nucleii

 hydrogen bond based on nuclear fusion.

nuclear fission : the process of breaking of nucleus in to two or more nucleii is known as fission process.

 nudge me if u have any doubts !!!!!!!