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.

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what are critical size and radiation hazards??

the critical size of a fissionable substance is that smallest size which is necessary to maintain the chain reaction in the substance!!!

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radiation hazards

since nuclear reactions produce very high temperature it may cause ultimate death. when the radiations emitted due to nuclear reaction fall on our body , it initiate improper cell division which causes death

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