Patent Number: 
Section: claims

1. A system comprising:a nuclear reactor reactivity control system, the nuclear reactor reactivity control system constructed to:control a rate of reactivity change resulting from power changes through all ranges of power levels as required by NRC regulations in effect on Nov. 1, 2015; andhold a core of a nuclear reactor controllable by the nuclear reactor reactivity control system in a subcritical state under cold conditions solely via movement of a single pair of blocks neutron reflector/moderator blocks; andwherein, the nuclear reactor reactivity control system comprises the single pair of neutron reflector/moderator blocks, the pair of neutron reflector/moderator blocks external to and partially surrounding the nuclear reactor, each of the pair of neutron reflector/moderator blocks comprising neutron reflective material with moderation properties. 2. The system of claim 1, further comprising:the nuclear reactor. 3. The system of claim 1, further comprising:a reactor containment structure that substantially surrounds the nuclear reactor. 4. The system of claim 1, further comprising:a core power detector coupled to a control system of the nuclear reactor, the control system constructed to, responsive to a signal from the core power detector, move the pair of neutron reflector/moderator blocks relative to an activated portion of the core of the nuclear reactor. 5. The system of claim 1, wherein:the pair of neutron reflector/moderator blocks is, via at least one motor, movable both vertically and horizontally relative to a core of the nuclear reactor. 6. The system of claim 1, wherein:suspension rod braces limit movement of the pair of neutron reflector/moderator blocks relative to a core of the nuclear reactor. 7. The system of claim 1, wherein:the pair of neutron reflector/moderator blocks comprises carbon black. 8. The system of claim 1, wherein:a position of the pair of neutron reflector/moderator blocks relative to a core of the nuclear reactor is manually controllable. 9. The system of claim 1, wherein:the pair of neutron reflector/moderator blocks is coupled to a vertical gearing rod, the vertical gearing rod, via a vertical gearing rod motor, constructed to move the pair of neutron reflector/moderator blocks vertically relative to the nuclear reactor. 10. The system of claim 1, wherein:the pair of neutron reflector/moderator blocks is coupled to a carriage, the carriage, via a carriage brace motor, constructed to cause the pair of neutron reflector/moderator blocks to reduce neutron reflection relative to the nuclear reactor by laterally traversing the pair of neutron reflector/moderator blocks. 11. The system of claim 1, wherein:the pair of neutron reflector/moderator blocks is coupled to a carriage, the carriage, via a carriage brace motor, constructed to cause the pair of neutron reflector/moderator blocks move away from the nuclear reactor. 12. The system of claim 1, wherein:the pair of neutron reflector/moderator blocks is suspended from a carriage brace by suspension rods, the carriage brace comprising two separate halves that are coupled together via center gearing, the pair of neutron reflector/moderator blocks positioned with a vertical gearing rod substantially through a center of each block and coupled to a suspension carriage, the suspension carriage engaged with the carriage brace, a carriage brace motor causing teeth of the carriage brace to move the carriage and thereby move the pair of neutron reflector/moderator blocks relative to a core of the nuclear reactor. 13. The system of claim 1, wherein:the pair of neutron reflector/moderator blocks is electrically controlled by a control logic system of the nuclear reactor. 14. The system of claim 1, wherein:the pair of neutron reflector/moderator blocks are suspended on suspension rods at core height of the nuclear reactor. 15. The system of claim 1, wherein:vertical gearing rods are substantially in the center of the pair of neutron reflector/moderator blocks and allow for vertical positioning of the pair of neutron reflector/moderator blocks. 16. The system of claim 1, wherein:the pair of neutron reflector/moderator blocks comprises petals like a flower. 17. The system of claim 1, wherein:the pair of neutron reflector/moderator blocks shaped in a configuration like an iris of a camera. 18. The system of claim 1, wherein:a shutdown feature of the nuclear reactor reactivity control system uses a separation of release locks to a vertical gearing rod, which allows the pair of neutron reflector/moderator blocks to drop down and away from the core to cause a shutdown condition for the core due to the removal of the pair of neutron reflector/moderator blocks, which increases fast and thermal neutron leakage from the core such that the core is no longer coupled neutronically and is shutdown.