Patent Number: 042736137
Section: claims

1. A method of operating a heavy-water-moderated nuclear reactor having an active core comprising a pressure tube including a cluster of fuel rods of natural-uranium-containing rod segments, some of which rods occupy exterior positions in the cluster and the remainder of which rods occupy interior positions in the cluster, said method comprising the Phases: A. providing interior rod positions at one end of the cluster with thorium-containing rod segments to irradiate the thorium until its multiplication factor is built up to about that of natural uranium; and  B. utilizing said thorium-containing rod segments irradiated in the interior rod positions of the cluster to refuel exterior rod positions at the charge end of the cluster.  C. removing depleted thorium-containing rod segments from the discharge end of exterior rod positions of the cluster and utilizing them to refuel interior rod positions at the charge end of the cluster. 2. The method according to claim 1, wherein, during Phase A, the thorium-containing rod segments are refueled from the charge end of the cluster, are advanced along the length of the cluster during each refueling step, and are removed from the dischage end of the cluster for use in refueling the exterior rod positions in Phase B. 3. The method according to claim 1, wherein, during Phase A, the thorium-containing rod segments are refueled from one end of the cluster, are retained in the same position in the cluster during each refueling step, and are removed from the said one end of the cluster for use in refueling the exterior rod positions in Phase B. 4. The method according to claim 3, wherein each thorium-containing rod segment is irradiated only during two refueling steps in Phase A. 5. The method according to claim 1, including after Phase B, the further Phase: 6. The method according to claim 5, wherein after all the interior and exterior rod positions have been refueled with thorium-containing segments during Phase C, fresh thorium-containing rod segments are periodically introduced into the interior rod positions of the cluster to replace rod segments whose thorium has reached its irradiation limit. 7. The method according to claim 5, wherein said Phase B is initiated only after a sufficient number of thorium-containing rod segments have been irradiated to refuel the rod segments of all the exterior rod positions at each refueling during Phase B. 8. The method according to claim 7, wherein Phase A includes a number of subphases in each of which only a fraction, less than the total number, of interior rod positions are refueled with said thorium-containing rod segments, the number of subphases in Phase A being sufficient to irradiate enough thorium-containing rod segments to refuel the rod segments of all the exterior rod positions of the cluster at each refueling during the subsequent Phase B. 9. The method according to claim 8, wherein said cluster includes 37 fuel rods, 18 of which occupy interior positions in the cluster and 19 of which occupy exterior positions in the cluster; and wherein Phase A includes 10 subphases in each of which 2 interior rod positions are refueled with said thorium-containing rod segments. 10. The method according to claim 8, wherein, in addition to the number of subphases in Phase A sufficient to irradiate enough thorium-containing rod segments to refuel the rod segments of all the exterior rod positions of the cluster at each refueling during the subsequent Phase B, Phase A includes one additional refueling step to introduce a delay for storing the irradiated segments long enough for the protactinium to decay into Uranium-233 before being utilized to refuel the rod segments of the exterior rod positions in the cluster. 11. The method according to claim 1, wherein the core includes a plurality of said pressurized tubes disposed in a vessel containing a moderator of heavy water, the rods of all of said pressurized tubes being refueled as set forth. 12. The method according to claim 11, including the further step of providing in Phase A booster rods of enriched uranium in the moderator in order to increase the reactivity of the core, thereby permitting an increase in the number of thorium-containing rod segments irradiated in Phase A to shorten the transition to Phase B. 13. The method according to claim 12, wherein said booster rods are subsequently removed. 14. The method according to claim 1, wherein the refueling of the interior positions at one end of the cluster also includes natural uranium-containing rod segments in addition to thorium-containing rod segments. 15. The method according to claim 1, wherein said thorium-containing rod segments used for refueling the interior positions in Phase A consist of pure natural thorium. 16. The method according to claim 1, wherein at least some of said thorium-containing rod segments used for refueling the interior positions in Phase A include thorium diluted with beryllium. 17. The method according to claim 16, wherein said beryllium-diluted rod segments include thorium-oxide pellets interspersed with a larger number of beryllium-oxide pellets such that the resonance integral of the thorium is close to that of the infinite dilute value. 18. The method according to claim 17, wherein the thorium-oxide pellets in the beryllium-diluted rod segments constitute about 10%-20% of the total number of pellets in such segments. 19. The method according to claim 1, wherein said thorium-containing rod segments include centers of natural uranium surrounded by thorium. 20. The method according to claim 19, wherein said thorium-containing rod segments each includes a sleeve having pellets of pressed particles, each of said pellets having a center of natural uranium surrounded by thorium. 21. The method according to claim 20, wherein the natural uranium constitutes about 30% of the total volume of the thorium-containing rod segments. 22. A nuclear reactor adaped to be operated in accordance with the method of claim 1, said nuclear reactor having an active core comprising a pressure tube including a cluster of fuel rods some of which occupy exterior positions and the remainder of which occupy interior positions in the cluster, said cluster of rods being constituted of a plurality of cluster segments each including a pair of end plates and a plurality of rod segments attached thereto to extend therebetween, said rod segments and said end plates including cooperable mechanical fastener devices permitting the quick attachment and detachment of the rod segments to the end plates to enable refueling the reactor as set forth. 23. A nuclear reactor according to claim 22, wherein said cooperable mechanical fastener devices include bayonet pins carried by said rod segments receivable in bayonet slots formed in said end plates. 24. A nuclear reactor according to claim 22, wherein there are a plurality of pressure tubes each including a cluster of fuel rods, said plurality of tubes being disposed in a vessel containing a moderator of heavy water. 25. The nuclear reactor according to claim 24, wherein said vessel further includes booster rods of natural-uranium-containing elements.