Patent Number: 054229227
Section: claims

1. A fuel assembly comprising a plurality of fuel rods which are composed of cladding tubes wherein a plurality of fuel pellets including fissile material are inserted, and  at least a moderating rod which is filled with moderator for moderating neutrons generated by nuclear fissions,  characterized in that  a ratio of a sum of transversal cross section area of portions of said moderating rods which are filled with moderator to a sum of transversal cross section area of said fuel pellets averaged in an axial direction of the fuel assembly is at least 0.4.  the transversal cross section area of portions filled with moderator per moderating rod is in a range of 14-50 cm.sup.2.  a ratio of a sum of transversal cross section area of said moderator in a transversal cross section surrounded with imaginary planes which are extended downwards from outer edges of an upper tie plate which bundles said plurality of fuel rods to a sum of transversal cross section area of said fuel pellets is a value. in a range 2.7-3.4.  at least one of said moderating rods is a double wall water rod wherein water level moves up or down depending on a flow rate of the moderator.  a ratio of a sum of transversal cross section area of said moderator filled in a water gap region around said fuel assemblies to the sum of transversal cross section area of said fuel pellets is utmost 0.7.  a control rod which is composed of a plurality of absorbing rods including neutron absorber bundled in a form having a cruciform cross section, and is inserted into a water gap region around said fuel assemblies, wherein  a ratio of a sum of surface area of said absorbing rods to a sum of surface area of said fuel rods is at least 0.2.  a control rod which is composed of a plurality of absorbing rods including neutron absorber bundled in a form having a cruciform cross section, and is inserted into a water gap region around said fuel assemblies, wherein  a ratio of a sum of transversal cross section area of said absorbing rods to a sum of transversal cross section area of said water gap region is at least 0.4.  a ratio of a sum of transversal cross section area of said moderator to the sum of transversal cross section area of said fuel pellets is a value in a range 3.0-3.5. 2. A fuel assembly claimed in claim 1, wherein 3. A fuel assembly claimed in claim 1, wherein 4. A fuel assembly claimed in claim 1, wherein 5. A reactor core comprising any one of the fuel assemblies claimed in any of claims from claim 1 to claim 4. 6. A reactor core claimed in claim 5, wherein 7. A reactor core claimed in claim 5, comprising 8. A reactor core claimed in claim 5, comprising 9. A reactor core claimed in claim 5, wherein