Patent Number: 050009087
Section: abstract

The present invention relates to intermittently introducing, under carefully controlled conditions, an inert gas, into at least the channel head primary coolant water outlet side of steam generators during the drain-down of a nuclear-powered steam generating system. Predetermined quantities of such gas are introduced into open ends of the inverted U-tubes which terminate at the tubesheet in the channel heads to thereby alleviate the propensity for formation of vacuum pockets at the tops thereof, and at the same time to provide that water columns defined severely near their tops by the water within each such tube and collectively near their bottoms by the water extending downward through the steam generator channel head, are not disrupted by the inadvertent formation of a gas/water interface formed in the general vicinity of the tubesheet and resulting from the flooding of the underside thereof with said gas. There exists a dependent and inversely proportional relationship between the pressure of gas introduction and the ratio of pulse time-on to pulse time-off in the instant gas introduction procedure, to thereby effect an optimum ratio of about 1.1 between the volume of nitrogen introduced into and the volume of water removed from such inverted U-tubes.