nuclear fission is the splitting of a heavy nucleus with atomic no gr8err than 82 into two or more nuclei along with emission of particles like nuetrons ( u fission for instance) with emission of of a lot  of energy .

nuclear fusion is the reaction in which two or more smaller  nuclei fuse or combine to form a heavy nucleus along with the emission of a lot of energy than fission itself.but it cannot b controlled easily like fission but has the advantage of leaving no radioactive waste.this reaction is called the thermo nuclear reaction coz it can take place only under higj temmp of  order 10⁷ K ..and under hi pressure

critical size is the minimum  size fissile material that can have atleast one nuetron to produce fission

the device where  controlled nuclear reactons take place is a reactor

radiation hazrda are those radiation diseases a person suffers from when he is exposed to radioactive radiation u may reacll the death of a russian spy who was allegedly killed by exposing him to radiation gradully