[en] The output from the UKAEA Prototype Fast Reactor at Dounreay has been limited because of small steam leaks in the sodium-heated evaporator units. There have been 33 leak events in the three evaporator units in which a total of about 75 small leaks have been found. All the leaks have been in the tube-to-tubeplate welds resulting in steam entering the low pressure gas space above the sodium. The background to these leak events is discussed together with the phenomena which have caused them. Remedial measures including heat treatment of welds, shot peening of welds and provision of a second barrier by sleeving are described. (U.K.)