Abstract: A nuclear reactor, preferably of the fast type, has a core surrounded by a breeding blanket which is composed of elements extending side by side with gaps therebetween so as to allow in certain conditions a leakage flow to adjust itself between the breeding blanket and an upper plenum. Mixing of the coolant takes place in at least one jet apparatus which operates with the flow of coolant supplied to the reactor vessel as the thrust medium, in such a way that coolant is drawn from the breeding blanket.

Abstract: The invention relates to the detection of very small gas leakages through liquid seals around a passage, which seal may be situated around a shaft.Before it was not easy to detect gas leakages to the environment through liquid seals from radio actively contaminated spaces.Said disadvantage can be avoided, by determining the gas leakage to the environment through a liquid seal by applying at one side of the seal a gas activable by neutrons and subsequently determining the concentration of the said gas in the sealing liquid as function of the time.