BWR critical-power-enhancing water rod (85-EN-3)

A fuel assembly for a boiling water reactor comprising a group of vertical, parallel, elongated fueld tubes containing a fissionable material, said assembly having a critical heat transfer zone in its upper portion;

• at least one water tube parallel to said fuel tubes and located among them, said water tube having an inlet adjacent its lower end;
• means closing the upper end of said water tube;
• an inner tube within the upper portion of said water tube having a lower end in sealing relationship with the inner surface of said water tube, forming an annulus with said water tube and having at least one opening adjacent its upper end;
• at least one upper exit opening in said water tube above the lower end of said inner tube and located within the critical heat transfer zone of said assembly, whereby water will flow upwardly through said water tube, upwardly through said inner tube, then downwardly through said water tube annulus and out through said upper exit opening, into the critical heat transfer zone of said assembly.