| 摘要 |
In the field of commercial nuclear reactors, there is an increasing demand for long-life control blades in order to meet the requirements of higher economy and reduction in the disposal of radioactive wastes. A control blade proposed by the invention stands a long use by virtue of the use of a long-life neutron absorber which is typically made of hafnium. Despite the use of hafnium which has a large specific weight (13.3 g/cm3), the size, shape and weight of the control blade are substantially the same as those of convertional control blades which employ boron carbides B4C as the neutron absorber, so that the control blade can be back-fitted in existing boiling water reactors without difficulty. The control blade of this invention is a flux-trap-type control blade in which long-life neutron absorber plates or sheets are arranged to oppose each other in the thicknesswise direction of the wing within the sheath plate such that a water gap is preserved between the opposing neutron absorber plates, so that the weight of the hafnium, which occupies most part of the total weight of the control blade, is reduced by an amount corresponding to the volume of the water gap. The thickness of the neutron absorber plates or sheets is increased at the upper portion of the control blade where the neutron exposure is specifically high and where a large neutron absorption power or capacity is required for ensuring a sufficient reactor shut-down margin, while the thickness is reduced in other portions so as to increase the size of the water gap. In consequence, the weight of the control blade incorporating heavy hafnium as the neutron absorber is reduced almost to the same level as that of ordinary control blades, without being accompanied by any reduction in the reactivity worth. |
