1. IN A THERMAL ENERGY GENERATOR OF A POWER PLANT UTILIZING CONTROLLED NUCLEAR FISSION OF A REACTOR AND LOCATED WITHIN THE HULL OF A MOBILE EQUIPMENT, THE COMBINATION COMPRISING, FIRST CLOSED CYCLE APPARATUS HAVING CIRCULATED FLUID ARRANGED TO ABSTRACT HEAT FROM THE REACTOR, SECOND CLOSED CYCLE APPARATUS CONTAINING LIQUID AND VAPOR GENERATED FROM THE LIQUID WITH A PORTION OF THE TWO CYCLES OVERLAPPING IN HEAT EXCHANGE RELATION, A LIQUID-VAPOR INTERFACE OCCURING IN THE OVERLAP, A COMMERICALLY GAS TIGHT CONTAINER FOR THE REACTOR, THE FIRST CYCLE APPARATUS, AND AT LEAST THE OVERLAP OF THE SECOND CYCLE, EXHAUSTING MEANS CONNECTED TO THE INTERIOR OF THE CONTAINER, AND DISCHARGING AT A SUBSTANTIAL DISTANCE THEREFROM, A BELLOWS SENSITIVE TO THE DIFFERENTIAL OF THE PRESSURE WITHIN THE CONTAINER AND THE PRESSURE WITHIN THE HULL, AND A PILOT VALVE ACTUATED BY SAID BELLOWS TO PRODUCE A FLUID LOADING PRESSURE TO CONTROL SAID EXHAUSTING MEANS TO MAINTAIN A DESIRED DIFFERENTIAL PRESSURE BETWEEN THE CONTAINER AND THE HULL.