Nuclear reactions can be used in the following except

Nuclear reactions release huge amounts of energy and radiation. Atomic bombs use uncontrolled fission. Nuclear power stations use controlled fission to heat water into steam and generate electricity. Radiation from cobalt-60 is used in radiotherapy to destroy cancer cells.

Purifying water is done with filtration, chlorination and distillation. These are chemical or physical methods, not nuclear ones. So purifying water is the exception.