Nuclear reactor shutdown control rod assembly

In a nuclear reactor having a reactor core and a reactor coolant flowing therethrough, a temperature responsive, self-actuated nuclear reactor shutdown control rod assembly, comprising:

• an upper drive line terminating at its lower end with a substantially cylindrical wall member having inner and outer surfaces, said wall member having a plurality of circumferentially disposed openings adjacent a lower end thereof;
• a lower drive line having a lower end adapted to be attached to a neutron absorber, said lower drive line having a substantially cylindrical upper end, an upper portion of the upper end having a diameter greater than a lower portion of the upper end of said lower drive line, said upper portion having a diameter less than that of the inner surface of said wall member of said upper drive line, said upper portion fitting within said wall member in the vicinity of the openings, said lower drive line having a recess formed on an outer surface thereof and interconnecting said upper portion and said lower portion thereof;
• a ring movable disposed about the outer surface of the wall member of said upper drive line, said ring having a first portion with an inside diameter slightly larger than the outside diameter of said wall member, and a second portion with an inside diameter greater than that of the first portion so as to form a space between said wall member and said ring;
• thermal actuation means adapted to be in heat exchange relationship with coolant in an associated reactor core and in contact with said ring, said actuation means being mounted externally of said lower drive line and secured to said wall member of said upper drive line, said actuator means providing for upward movement of said ring when reactor coolant temperature is increased;
• and
• a plurality of balls located within the openings in said upper drive line, each of said balls having a diameter greater than the thickness of the wall member of said upper drive line, the balls being normally retained radially inwardly by the first portion of said ring so that a portion of each ball protrudes radially inwardly beyond the inner surface of said upper drive line into the lower portion of the upper end of said lower drive line and adjacent the recess formed on the outer surface of said lower drive line to provide vertical support for said lower drive line at the recess;
• whereby when reactor coolant approaches a predetermined design temperature said actuation means moves said ring sufficiently so that the balls move radially out from said recess and into said space formed by the second portion of said ring thereby removing said vertical support for said lower drive line such that the lower drive line moves downwardly and inserts an associated neutron absorber into an associated reactor core resulting in automatic reduction of reactor power.