Patent Number: 052308588
Section: abstract

A fuel bundle for a boiling water nuclear reactor having an improved water rod is disclosed, this water rod having two discrete compartments upper and lower for containing water. The bottom compartment flows moderator from the bottom of the fuel bundle through the water rod compartment to discharge points located through the water rod side walls between the upper most spacers; the top compartment opens upwardly and is naturally filled with liquid moderator settling out of the upwardly flowing moderator during reactor operation. There results the requisite presence of liquid moderator within the water rod for supplying the upper two phase region of the fuel rod with nearly full moderator density. At the same time, water flow through of the water rod from the bottom of the fuel bundle to the upper two phase region of the fuel bundle improves the thermal hydraulic performance. Specifically, water is discharged in the upper two phase region of the fuel bundle at the center of the fuel bundle where the density of the upwardly flowing moderator has the largest vapor fraction. As a result, liquid moderator is injected where it has the maximum beneficial effect on moderator density and the beneficial formation of a liquid film immediately upstream of the spacers. Improved thermal hydraulic performance of the fuel bundle results.