This might a little tricky question, but without doubt 2 will open the paragraph. The resultant ionising radiation in 4 is a consequence of large scale nuclear fission mentioned in 2. Thus 24 form a pair. 4 and 1 are contrasting ideas, something that justifies the presence of the conjunction "but" in 1 . The resultant ionising radiation has become the most serious agent of pollution... but the attention of the layman has been captured by the atom bomb.... 3 comes as the appropriate conclusion. 3 can't come after 4 because both 4 and 3 talk nuclear fission, which does not justify the presence of the word "however" in 3. We use however to contrast two ideas, but both 4 and 3 talk of peaceful uses of atomic energy. Nuclear fission is used for purposes other than atomic bomb. Thus 2413 is the right sequence.