Patent Number: 039714446
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the drawing a reaction vessel, such as a nuclear reactor, is shown containing a charge or core made up of two groups of spherical elements 2, 3 arranged in uniform mixture. In each of the groups, the spherical elements have the same diameter, however, the spherical elements 2 in one group have a different diameter from the spherical elements 3 in the other group. Means, not shown, are arranged to circulate the uniformly mixed charge of spherical elements 2, 3 during the operation of the reaction vessel. The elements are introduced in through the top of the reaction vessel 1, passed downwardly through the vessel, and are removed through the discharge pipe located at the lower end of the vessel. Control rods 4, only one control rod is shown in the drawing, are positioned within the reaction vessel extending into the body of the uniformly mixed fuel elements for regulating the reaction. From the reaction vessel 1, each of the control rods extend upwardly into a cylinder 5. Each cylinder has a double-acting piston 6 at its upper end within the cylinder. By selectively supplying hydraulic or pneumatic fluid or pressurized gas (helium) into the cylinder, the control rods can be moved upwardly and downwardly, as required, within the reaction vessel 1. The diameter of the elements in each group is selected in such a way that the circulation of the elements 2, 3 within the reaction vessel does not cause any substantial segregation of the elements into separate groups. If, considering the reaction vessel, its parameters and the conditions to be experienced during operation, a standard size element could be selected as 60mm. With such a standard element forming the diameter for one of the groups, the diameter for the other group could be selected in the range of 5 to 35 % smaller or greater than 60 mm and preferably in the range of 5 to 20% smaller or greater. The arrangement of least two groups of fuel elements could be used in a reactor such as the type presently under construction in Germany known as the THTR (thorium high temperature reactor). The THTR is a 300 MWe nuclear power plant and uses helium as a coolant gas in the primary circuit for transferring the heat removed from the fission reaction to the steam generators in the secondary circuit. The reactor core is located within a prestressed concrete pressure vessel and has a diameter of 5.6m and a height of 6m. The core is made up of 675,000 fuel element spheres providing a pebble bed volume of 125m.sup.3. Each fuel element is 60mm in diameter, that is, about the size of a tennis ball, with an outer fuel-free graphite shell having a wall thickness of 5mm. Within the shell, the fuel is contained in the form of 35,000 small coated particles of 0.3 to 0.4mm diameter coated with pyro-graphite embedded in a graphite matrix. The heavy metal contents of each fuel element is 0.96g U235 (93 % enriched) and 962g Th 232. The core is enclosed within a graphite reflector with fuel element loading pipes passing through the upper end of the reflector for adding fuel element spheres to the core and with a centrally arranged fuel element discharge pipe located in the bottom of the reflector. To control and shut down reactor operation, 42 absorber or shutdown rods are arranged for insertion into the core in direct contact with the fuel elements with 36 control rods located laterally of the core in boreholes formed in the reflector. The incore rods are preferably designed for normal shut-down of the reactor as well as for scram in the case of fault conditions. The reflector rods are primarily used for temperature and partial load control. Continuous fueling of the core is practiced to ensure uniform and high burn-up of the fuel elements, the fuel elements move slowly downwardly through the core with the fuel elements in the central regions moving faster than those in the outer annular regions between the central region and the reflector. On the average a fuel element resides within the core a period of 36 months and usually is passed through the core five to seven times, preferably six times. As fuel elements are removed from the bottom of the core, they pass through the discharge pipe into a singulizer and are then led to a damaged sphere separator where fuel elements whose shape and dimensions have significantly changed are eliminated from the recirculation cycle. The other fuel elements, after having passed a buffer line, are led into a distinguishing and burn-up measurement device. According to sphere burn-up, and depending on the fueling program, a computer decides whether the element is discharged from the circulating cycle or recycled back into the core at its top. Continuous circulation means that during operation a certain number of elements per day at full power are added to the top of the core and a similar number are removed from the bottom. It is possible that a period of time may pass between each charging and discharging step, even up to several days in time, however, such circulation is still considered continuous. By comparison, it is known to leave fuel elements within a core for an extended period of time and then during a period of shutdown to replace at least a portion of the elements. However, such operation is not a continuous circulation of the elements moving downwardly through the core during reactor operation. The helium gas coolant is forced downwardly through the reactor core, removing heat from the fuel elements and exiting through boreholes in the bottom reflector. If, instead of a single size fuel element, the present invention is employed and two groups of differently sized elements are utilized, it would be preferable to take the 60mm diameter of the THTR fuel elements as a standard. To assure a random rather than ordered orientation of the fuel elements within the core, as might develop when a single size element is used, at least one additional group of fuel elements is selected for use along with a group of the 60mm elements. The additional group would be in the range of 5 to 35 %, and preferably in the range of 5 to 20 %, greater or less than the standard 60mm diameter. In other words, the diameter of one group would be 60mm while the diameter of the other group would be selected from the range of 48 to 57mm or 63 to 72mm, depending on whether, in accordance with the various conditions involved, it is decided to employ a fuel element size smaller or larger than the standard size. To determine the difference in pressure acting on control rod fuel elements when one size fuel element is employed in the core as compared with two groups of fuel elements each of a different diametral size, testing has indicated that a very significant reduction in fuel element stress is obtained where two different sized diameter spherical elements are used and are uniformly mixed together. It had been thought that the use of two different size elements would result in segregation due to continuous recycling, however, by employing the two different diametral sized groups, it is possible to maintain a random and non-segregated arrangement which assures continuous recirculating operation with adequate control without adverse stress effects on the fuel elements and the control rods. While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.