Nuclear reactor core assembly

In a nuclear reactor having a core assembly and a fluid coolant which is circulated at a first flow rate during operation and a second flow rate during shutdown, said first flow rate being substantially higher than said second flow rate, a plurality of fuel elements and a plurality of blanket elements arranged in the core assembly, said fuel elements including first conduit means for directing fluid coolant therethrough, said blanket elements having second conduit means for directing fluid coolant therethrough, said second conduit means having flow restrictor means therein, the improvement wherein said flow restrictor means comprise a plurality of surface roughened elements for limiting flow therethrough, said elements being arranged in a predetermined pitch and having an initial predetermined surface roughness and having a predetermined size, said flow restrictor means having a static configuration wherein the number, pitch, size and roughness of said elements are selected to initially provide a relative roughness of at least about 0.05 and a Reynolds number (N.sub.Re) at said second flow rate which is less than and immediately adjacent the Reynolds criterion (N.sub.Re, C), and the Reynolds number at said first flow rate being greater than the Reynolds criterion, whereby the friction factor is substantially lower in said second conduit means at shutdown than at full power operation, whereby at the second flow rate, the percentage of the total coolant flow which flows through said second conduit means is substantially greater than the percentage of the total coolant flow which flows through said second conduit means at said first flow rate.