Patent Number: 
Section: claims

1. A nuclear reactor plant which includesa helium cooled nuclear reactor positioned in a cavity defined by a shell; anda closed loop liquid cooling system which includesat least one coolant chamber defined by a wall having ends which are sealed off respectively by a top and a bottom and positioned in the cavity adjacent to the nuclear reactor;a pump having an inlet side and an outlet side;a coolant inlet pipe which is connected in flow communication with the outlet side of the pump and which enters the coolant chamber through the top and extends downwardly through the coolant chamber to a discharge end positioned within the coolant chamber; an outlet leading from the coolant chamber which is connected in flow communication with the inlet side of the pump; andat least one anti-siphon bleed opening provided in that portion of the coolant inlet pipe positioned in the coolant chamber at a position spaced from the discharge end whereby the coolant inlet pipe and the coolant chamber are connected or connectable in flow communication. 2. A nuclear reactor plant as claimed in claim 1, which includes an anti-siphon valve mounted in the inlet pipe. 3. A nuclear reactor plant as claimed in claim 1, in which a plurality of anti-siphon bleed openings are provided in that portion of the coolant inlet pipe which is positioned at the highest level within the coolant chamber. 4. A nuclear reactor plant as claimed in claim 3, in which the anti-siphon bleed openings are in the form of holes in the coolant inlet pipe, said holes having a combined area of between 1% and 10% of the cross-sectional area of the coolant inlet pipe. 5. A nuclear reactor plant as claimed in claim 3, in which the inlet pipe has a nominal diameter of 100 mm and between four and eight anti-siphon bleed openings are provided therein. 6. A nuclear reactor plant as claimed in claim 3, in which the bleed openings are circular and have a diameter of between 5 mm and 10 mm.