In a water-cooled power reactor (or in a water-filled spent fuel pool, SFP), if a power surge occurs as a result of a reactivity initiated accident, an understanding of the transfer of heat from the surface of the cladding to the water is very useful. In a French study, metal pipe immersed in water (both under typical PWR and SFP conditions), was electrically heated to simulate the generation of heat within a fuel pin by nuclear processes. The temperature of the pipe was monitored by thermocouples and for the tests conducted under PWR conditions the water entering the larger pipe (14.2 mm diameter) holding the test metal pipe (9.5 mm outside diameter and 600 mm long) was at 280 °C and 15 MPa. The water was flowing past the inner pipe at circa 4 ms and the cladding was subjected to heating at 2200 to 4900 °C s to simulate an RIA. It was found that as the temperature of the cladding increased the rate of heat transfer from the surface of the cladding increased at first as the water boiled at nucleation sites. When the heat flux is greater than the critical heat flux a boiling crisis occurs. This occurs as the temperature of the fuel cladding surface increases so that the surface of the metal was too hot (surface dries out) for nucleation boiling. When the surface dries out the rate of heat transfer decreases, after a further increase in the temperature of the metal surface the boiling resumes but it is now film boiling.