1. A NUCLEAR REACTOR INCLUDING A CLOSED TANK CONTAINING A REACTOR CORE, A VOLUME OF LIQUID LESS THAN THE VOLUME OF THE INTERNAL SPACE OF THE SAID TANK AND MEANS PROVIDING FORCED CIRCULATION OF SUCH LIQUID IN A LIQUID FLOW CIRCUIT PASSING THE SAID REACTOR CORE, SUCH CIRCULATION PROVIDING MEANS INCLUDING A CONDUIT COMPRISING A COOLING SYSTEM AND A PUMP AND EXTENDING EXTERNALLY OF THE SAID TANK FROM AN UPSTREAM END TO A DOWNSTREAM END OF SAID CONDUIT, THE SAID UPSTREAM END EXTENDING IN FLUID TIGHT MANNER THROUGH THE WALL OF SAID TANK INTO THE UPPER PORTION OF SAID TANK AND THE SAID DOWNSTREAM AND EXTENDING IN FLUID TIGHT MANNER THROUGH THE WALL OF SAID TANK INTO THE LOWER PORTION OF THE SAID TANK, THE SAID MEANS FURTHER INCLUDING AN ELONGATED HOLLOW MEMBER SITUATED INSIDE THE SAID TANK IN THE SAID VOLUME OF LIQUID AND IN COMMUNICATION WITH THE SAID DOWNSTREAM END OF THE SAID EXTERNALLY EXTENDING CONDUIT, THE SAID HOLLOW MEMBER DEFINING A CHAMBER THEREIN AND BEING PROVIDED, AT ONE END, WITH AN INLET MOUTH A RESTRICTED AREA IN COMMUNICATION WITH THE SAID VOLUME OF LIQUID AT A POINT OF THE SAID LIQUID FLOW CIRCIRCUIT DOWNSTREAM OF THE SAID REACTOR CORE, SAID INLET MOUTH GEING FOLLOWED BY SAID COMMUNICATING WITH A PORTION HAVING A DIVERGING CROSS SECTIONAL AREA, SAID PORTION EXTENDING INTO AND COMMUNICATING WITH SAID CHAMBER, SAID CHAMBER RECEIVING THE SAID DOWNSTREAM END OF THE SAID EXTERNALLY EXTENDING CONDUIT THROUGH AN OPENING OF RESTRICTED CROSS SECTIONAL AREA, COMMUNICATING WITH THE DOWNSTREAM END OF SAID EXTERNALLY EXTENDING CONDUIT, THE SAID CHAMBER BEING DEFINED BY CONVERGING WALLS AND BEING PROVIDED WITH AN OUTLET MOUTH AT THE CONCERGING END OF SAID WALLS, SAID CONVERGING END OF SAID WALLS COMMUNICATING WITH SAID LIQUID FLOW CIRCUIT AT A LOCATION UPSTREAM OF THE SAID REACTOR CORE, THE SAID MEANS FURTHER INCLUDING A PASSAGE OF RESTRICTED CROSS SECTIONAL AREA INCLUDED IN THE SAID LIQUID FLOW CIRCUIT AND ENCIRCLING THE SAID CONVERGING END OF SAID WALLS.