Abstract: A radial neutron reflector is disclosed for a light water nuclear reactor. The irregular space between the core periphery and the core barrel surrounding the core is filled with a structure comprising material other than hydrogen such as zirconia or stainless steel so as to displace the reactor water coolant in this region. The structure is provided with appropriately sized flow holes to pass reactor coolant therethrough and to balance the pressure within the structure to substantially coincide with the axial pressure profile at the core periphery.

Abstract: In a pressurized water reactor in which a dropped control rod is detected by a negative rate of change of neutron flux which exceeds a preset negative limit, the reactor is only shutdown if the axial offset exceeds a preselected limit, so that it is possible to maintain power, and in many cases full power, despite the presence of a dropped rod. Where part length control rods are used during power changes, separate limiting values on axial offset are used when these rods are fully inserted and when they are fully retracted.

Abstract: A nuclear fuel rod contains nuclear fuel pellets that have incorporated therein a hybrid burnable absorber that affects a moderation of the burn-out rate of the system containing the rod. The nuclear fuel pellets contain 1 to 20 percent by weight gadolinium oxide and 0.02 to 1.0 percent by weight of boron carbide particles of a size between 20 to 100 microns in diameter, the particles coated with a 0.5 to 10 micron thick coating of a helium gas-impervious coating.

Abstract: Elongated tubular elements containing burnable absorber material are clustered in an array to provide a burnable absorber assembly for insertion within one or more guide thimbles of a fuel assembly for controlling reactivity. The burnable absorber assembly, in alternative embodiments, include three tubular elements in triangular relationship and four tubular elements in rectangular relationship. The use of the burnable absorber assembly within the guide thimbles of a fuel assembly results in increased core thermal margin, minimization of end of cycle reactivity penalty, increase in operating cycle length before required refueling, and the ability to utilize burnable absorber assemblies having different strengths at different locations within the fuel assembly for controlling fuel assemblies having significant power gradients, as well as finer control of radial powered distribution and thus achieving lower peaking factors.