Nuclear reactor arrangement and method of operating safe effective to increase the thermal amargin in high power density regions

Claim 1

A nuclear reactor cooled by a moderating fluid comprising a plurality of longitudinally extending fuel element assemblies comprising the reactor core, one end of said fuel element assemblies in said core constituting a fluid inlet end, each of said fuel element assemblies having a predetermined cross section and having a plurality of longitudinally extending fuel elements therein, said fuel elements arranged in said fuel element assemblies in a regular predetermined lattice array with each fuel element occupying one lattice position, said fuel element assemblies further including a plurality of guide tubes extending longitudinally through said fuel element assemblies, the cross sectional area of said guide tubes being at least equivalent to the portion of the cross sectional area of said fuel element assemblies occupied by two of said fuel elements, and thus occupying portions of at least two lattice positions, said guide tubes having an open end communicating with said fluid inlet end and having lateral openings therethrough spaced well from said fluid inlet end whereby cooler fluid from said fluid inlet end may be conducted through said guide tubes past a first portion of the core and introduced through said openings into desired other portions of the core, flow restricting means of a first size in at least some of said guide tubes and flow restricting means of a second size in others of said guide tubes, said flow restricting means located at the fluid inlet end of said guide tubes whereby the amount of fluid flow through different guide tubes varies, and control elements comprising fingers each having a cross sectional area only slightly smaller than the cross sectional area of said guide tubes, said control element fingers adapted to fit into selected ones of said guide tubes and mounted so as to be movable longitudinally therein.