The present invention relates to a method of operating nuclear reactors, in particular, to a method of operating a boiling water reactors adapted to use cross-shaped control rod.
In the boiling water reactors, the power distribution is skewed to the lower portion of the core of reactor due to the axial void distribution. It has therefore been attempted to dispose a burnable poison, typically gadolinea, at the lower portion of the core where the peaking of the power exists, or to insert control rods from the bottom of the core to a small depth (referred to as shallow control rods), thereby to flatten the axial power distribution. At the same time, in order to obtain a uniform radial distribution of the power, control rods of large insertion depth (referred to as deep control rods) are dispersed over the cross-section of the core.
Since the control rods are used for a multiplicity of purposes including the flattening of axial and radial power distributions, as well as the control of reactivity, the arrangement of the control rods is rendered highly complicated as shown in FIG. 1 showing a cross-section of the entire reactor core.
In FIG. 1, each square or box represents a unit consisting of a control rod and four fuel assemblies disposed around the control rod. The numeral appearing in each box represents the number of section as counted from the bottom to which the associated control rod is inserted from the bottom of the core, on an assumption that the whole core height is divided into 24 (twenty four) sections. The control rods in the blank boxes are fully withdrawn. The planning of such a complicated control rod pattern as that shown in FIG. 1 requires a number of steps of calculations. In addition, this operating method involves various problems, such as a steep change of power level observed in the areas around the ends of the shallow control rods for controlling the axial power distribution.
In order to overcome these problems, Japanese Patent Application Nos. 115268/76 and 115269/76 propose methods of operation in which the core is divided at its mid portion into two regions in the axial direction, and the upper region is operated at a larger infinite multiplication factor than the lower region, so as to reduce the skewing of the power distribution attributable to the presence of the void distribution, thereby to flatten the power distribution. In the reactors operated in accordance with these proposed methods, the shallow control rods are not required since it is not necessary to flatten the axial power distribution by the control rods. This affords a larger degree of freedom for the selection of positions of the deep control rods, which has been restricted due to the presence of the shallow control rods, so that the flattening of the radial power distribution pattern has become easier to attain.
Incidentally, in the operating method using deep and shallow control rods in combination, the control rods are divided into two groups A and B, as shown in FIGS. 2a and 2b. Further, the group A can assume two patterns: an A.sub.1 pattern in which deep and shallow control rods are disposed at boxes A.sub.1 and A.sub.2, respectively, and an A.sub.2 pattern in which deep and shallow control rods are disposed at boxes A.sub.2 and A.sub.1, respectively. Similarly, the group B can assume two patterns: pattern B.sub.1 and pattern B.sub.2.
In practical operation, the four patterns are cyclically changed from one to another in the order of, for example, A.sub.1 to B.sub.1, to A.sub.2, to B.sub.2 and then again to A.sub.1, to each burn-up of 1000Wd/t. At the same time, the operation of control rods of B group has to be checked by a locking mechanism, which is referred to as a control rod value minimizer, when A.sub.1 or A.sub.2 pattern is selected.
FIG. 3 shows an example of control rod pattern planned in accordance with the above-explained operating method for the aforementioned reactor which can be managed only by the deep control rods. The core of this example is identical to that of a 800 MWe class boiling water reactor which will be mentioned later in the description of the preferred embodiments of the present invention. That is, this core is designed in accordance with the teachings of the aforementioned Japanese Patent Application Nos. 115268/76 and 115269/76. In this core, as explained before, the axial power distribution is conveniently flattened, so that the required operation standard can be fulfilled even by the conventional operating method. The maximum linear heat generating rate and minimum critical power ratio MCPR of this core are 10.42 KW/ft and 1.40, respectively. The grouping of the control rods, however, restricts degree of freedom of selection of inserting positions of the control rods, because it relies upon the simultaneous use of the deep and shallow control rods. Thus, this operating method exhibits a power peaking which is larger by 4% than that realized in the operating method of the invention in which, as will be detailed later, the control rod pattern in the form of concentric circles is adopted.