Nuclear steam generator tube orifice for primary temperature reduction

In a nuclear steam generator having an array of U-shaped steam generator tubes, each of the tubes carrying a flow of water therethrough for the transfer of heat therefrom to water to be heated surrounding the steam generator tubes, an orifice insert for placement in an inlet opening of the steam generator tubes, the orifice insert is described comprising: a sleeve having an outlet end, and inlet end located in use in the vicinity of the inlet opening of the steam generator tube, and a longitudinal axis, at least a portion of the outer diameter of the sleeve being substantially the same as the inner diameter of the steam generator tube, and an orifice disposed in and extending the entire length of the sleeve for directing the flow of water from the inlet end to the outlet end of the sleeve. The orifice comprises a first cylindrical orifice portion having an inlet end and an outlet end, the inlet end being in fluid communication with the inlet and of the sleeve; and a second cylindrical orifice portion adjacent the first cylindrical orifice portion and having an inlet end and an outlet end; wherein the diameter of the second cylindrical orifice portion is smaller than the diameter of the first cylindrical orifice portion to present a transition to the fluid flow, whereby the pressure and thus the temperature of the flow of water past the transition considering the direction of the flow, is reduced and the corrosive attack of the flow of water on the interior surface of the steam generator tubes is likewise reduced