Patent Number: 
Section: claims

1. An apparatus comprising:a nuclear reactor including a reactor pressure vessel and a nuclear reactor core comprising fissile material disposed in a lower portion of the reactor pressure vessel;a reactor cavity inside of which is disposed the lower portion of the reactor pressure vessel, the reactor cavity and a wall of the pressure vessel defining an annular gap therebetween; andan annular neutron stop located at an elevation above the uppermost elevation of the nuclear reactor core, the annular neutron stop including an outer neutron stop ring including an outermost neutron stop ring attached to the wall of the reactor cavity and a middle neutron stop ring attached to the outermost neutron stop ring, an inner neutron stop ring attached to the reactor pressure vessel and comprising a high temperature neutron absorbing material that is stable at an exterior temperature of the reactor pressure vessel during nuclear reactor operation, and comprising neutron absorbing material filling, the annular neutron stop being disposed in the annular gap between the reactor pressure vessel and a wall of the reactor cavity,wherein the outer neutron stop ring defines an annular cutout portion in which the inner stop ring is disposed, the middle neutron stop ring comprises the high temperature neutron absorbing material, and the outermost neutron stop ring comprises a neutron absorbing material that is different from the high temperature neutron absorbing material. 2. The apparatus of claim 1 wherein an interface between the outer neutron stop ring and the inner neutron stop ring is stair-stepped or staggered to block neutrons from streaming through the interface. 3. The apparatus of claim 1 wherein the high temperature neutron absorbing material comprises a composition including boron carbide (B4C). 4. The apparatus of claim 1 wherein the neutron absorbing material that is different from the high temperature neutron absorbing material comprises borated concrete. 5. The apparatus of claim 1 wherein the annular neutron stop comprises a boron containing neutron absorber component in a thermally insulating matrix material. 6. The apparatus of claim 5 wherein the thermally insulating matrix material comprises vermiculite. 7. An apparatus comprising:a nuclear reactor including a reactor pressure vessel and a nuclear reactor core comprising fissile material disposed in a lower portion of the reactor pressure vessel;a reactor cavity inside of which is disposed the lower portion of the reactor pressure vessel, the reactor cavity and a wall of the pressure vessel defining an annular gap therebetween;an annular neutron stop located at an elevation above the uppermost elevation of the nuclear reactor core, the annular neutron stop including an outer neutron stop ring defining an annular cutout portion, an inner neutron stop ring disposed in the annular cutout portion, and comprising neutron absorbing material filling, the annular neutron stop being disposed in the annular gap between the reactor pressure vessel and a wall of the reactor cavity;a tube penetrating through the annular neutron stop; anda neutron plug disposed in the tube at the penetration of the tube through the neutron stop, the neutron plug comprising neutron absorbing material. 8. The apparatus of claim 7 wherein the neutron plug comprises a boron containing neutron absorbing material. 9. The apparatus of claim 7 wherein the tube comprises an excore instrument guide tube and the apparatus further comprises:an excore instrument suspended from the neutron plug by a cable. 10. The apparatus of claim 9 wherein the neutron plug includes a tortuous path and the apparatus further includes:one or more wires passing through the tortuous path and operatively connecting with the excore instrument. 11. An apparatus comprising:a nuclear reactor including a reactor pressure vessel and a nuclear reactor core comprising fissile material disposed in a lower portion of the reactor pressure vessel;a reactor cavity inside of which is disposed the lower portion of the reactor pressure vessel;an annular neutron stop located at an elevation above the uppermost elevation of the nuclear reactor core, the annular neutron stop comprising neutron absorbing material filling an annular gap between the reactor pressure vessel and the wall of the reactor cavity;an excore instrument guide tube penetrating through the annular neutron stop;a neutron plug including a tortuous path disposed in the excore instrument guide tube at the penetration of the excore instrument guide tube through the neutron stop, the neutron plug comprising neutron absorbing material;an excore instrument suspended from the neutron plug by a cable; andone or more wires passing through the tortuous path and operatively connecting with the excore instrument. 12. The apparatus of claim 11 wherein the annular neutron stop comprises:an outer neutron stop ring attached to the wall of the reactor cavity; andan inner neutron stop ring attached to the reactor pressure vessel. 13. The apparatus of claim 12 wherein an interface between the outer neutron stop ring and the inner neutron stop ring is stair-stepped or staggered to block neutrons from streaming through the interface. 14. The apparatus of claim 12 wherein the inner neutron stop ring comprises a high temperature neutron absorbing material that is stable at an exterior temperature of the reactor pressure vessel during nuclear reactor operation and the outer neutron stop ring comprises a neutron absorbing material that is different from the high temperature neutron absorbing material. 15. The apparatus of claim 11 wherein the annular neutron stop comprises a boron containing neutron absorber component in a thermally insulating matrix material.