Fuel array for an undermoderated nuclear reactor

Claim 1

In an undermoderated light water moderated and cooled nuclear reactor comprising a core which consists of a plurality of fuel arrays of identical cross-section arranged side by side and comprising first fuel arrays which are located in a central portion of said core and which contain fissile enriched fuel material and second fuel arrays which are located at the periphery of said core and which contain fertile material only, each of said arrays having: a prism shaped casing of regular polygonal cross section having lateral walls whose external surface is planar; and a plurality of long cylindrical rods containing fissile enriched material in said first fuel arrays and containing fertile material only in said second arrays, said rods being arranged parallel to an axis of said casing and in a uniform triangular lattice within said casing, with a spacing between said rods which is so selected that the neutron flux in said core during operation of said reactor is epithermal; the improvement consisting in that a plurality of parallel channels are formed out of an internal initially flat surface of said lateral walls, each channel being part cylindrical and coaxial to an associated one of those of said rods which are placed in an outside row of the array, each of said rods having a spacing wire helically wound thereof for determining the spacing between adjacent ones of said rods and between the rods in the external row and the respective channels, said channels being so sized that the moderation ratio is substantially constant throughout a cross-sectional area of said array.