Chemical energy can either be exothermic or endothermic....but nuclear reactions produce large amount of heat and for the same reason reactors are constructed near water bodies just to ensure the availability of water to cool the reactors. Chemical reactions can result in mild, moderate or vigorous results, but nuclear reactions are vigorous ones even though they are made to control in reactors. Chemical reactions allow the reactant molecules just to react and to form new compound. It is during this process that energy is released. But in nuclear fission large molecule breaks down in to small ones releasing heavy amount of energy. In fusion it is the combination of them that results in energy. A chemical reaction can even take place in a small test tube. But for a nuclear reaction to take place , we must provide preferable environment along with high precautions to ensure the safety.