[en] This paper presents the results of an experimental program to determine the consequences of a guillotine break in the feedline of a CANDU PHWR steam generator with integral preheater. A model preheater assembly simulating the important features of the preheater in its early response to a feedline rupture was used. The experiments were carried out at feedwater inlet conditions corresponding to normal operating and 5% power conditions respectively. The effect of tube-to-thermal plate clearance was also investigated. In general, depressurization occurs in stages within the feedline, with the pressure dropping to ambient in 7 to 11 ms, whereas it is more uniform and slower in the preheater. Pressure loading on the preheater was within acceptable limits for all tests. The zero tube-to-thermal plate clearance resulted in higher pressure loading on the thermal plate than with the nominal clearance. Also blowdown from the 5% power condition resulted in greater pressure loading on the preheater than the normal operating power condition