Hello!
The form of radiation that is used to directly INCREASE the temperature of water in a nuclear reactor is Gamma radiation.
In a nuclear reactor, the nuclear fission produces gamma radiation, this kind of radiation is one of the most energetic and is composed of high-energy photons. When getting in contact with water, gamma radiation ionizes it, and that ionization will increase the kinetic energy of the molecules, increasing the temperature of water.
Have a nice day!