Patent Number: 042736137
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a method of operating a nuclear reactor, and also to a nuclear reactor construction enabling it to be operated in accordance with the novel method. The invention is particularly applicable to heavy-water moderated power reactors, and is therefore described below with respect to such application. Nuclear reactors moderated by heavy water provide a number of advantages as compared to those using ordinary water as a moderator. One advantage is that it is possible to use natural uranium as well as slightly enriched uranium as the fuel. Another advantage is that the burn-up is high. Such reactors, however, are subject to a number of disadvantages, besides the high price of heavy water. Thus, they exhibit a "positive void coefficient," which means that if heavy water is lost, the reactivity rises thereby increasing the danger of a "run-away." In addition, even though the burn-up rate is high, such reactors still require large amounts of natural uranium, the present supplies of which are extremely limited. OBJECTS AND SUMMARY OF THE INVENTION An object of the present invention is to provide a nuclear reactor, and a method of operating a reactor, providing advantages in the above respects. More particularly, an object of the present invention is to provide a method of operating a nuclear reactor, and a reactor construction enabling such operation, in which a significant portion of the total core power is derived from burning natural thorium, rather than uranium. Thorium-232 (which constitutes nearly 100% of natural thorium) is converted into fissile uranium-233 by the capture of neutrons, the conversion being effected after the lapse of a few days to permit decay of the protactinium produced upon the exposure of the thorium. Because of the relatively high thermal absorption and low resonance capture of irradiated thorium, as compared to natural uranium, applying the invention to heavy-water moderated reactors would remove the present "positive void coefficient" of such reactors, which would thereby reduce the danger of a "run-away" by loss of moderator. In addition, such a reactor construction and operation would have the further important advantage of producing substantial savings in the requirements for uranium. The invention is applicable to reactors, particularly of the heavy-water moderated type, having an active core comprising a pressure tube including a cluster of fuel rods of fissile-material (particularly natural uranium) rod segments, some of which rods occupy exterior positions in the cluster and the remainder of which rods occupy interior positions in the cluster. According to a broad aspect of the present invention, there is provided a method of operating a nuclear reactor of the above type, 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 the 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. In the preferred embodiment of the invention described below, the method includes, after Phase B, the further Phase C of 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. 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. The method may include 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. According to another aspect of the present invention, there is provided a nuclear reactor adapted to be operated in accordance with the above described method, the 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. The cluster of rods is 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. The rod segments and the end plates include 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. In the preferred embodiment described below, the cooperable mechanical fastener devices include bayonet pins carried by the rod segments receivable in bayonet slots formed in the end plates.