Patent Number: 060289066
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a control rod installed in a boiling water reactor (BWR), and particularly to a control rod using metal hafnium. In a conventional boiling water reactor, boron carbide type control rods and hafnium type control rods have been used. With a boron carbide type control rod, a plurality of neutron absorber tubes filled with boron carbide powder are covered by U-shaped sheaths, and these are formed in a cruciform manner. Stainless steel (SUS) is used in the neutron absorber tubes and the U-shaped sheaths. In hafnium type control rods, metal hafnium (Hf) rods, plates or oval tubes are arranged in a cruciform manner. Boron carbide and Hf are used as neutron absorber material. Hf does not have a large thermal neutron absorption cross section, but has a lot of peaks in the resonance energy region. This means that reactivity control ability can be maintained over a long period of time, so it is effective as a reactor control material. As conventional examples using hafnium, for example, a structure using hafnium in an upper portion and side portion of a BWR control rod is disclosed in Japanese patent publication No. Sho. 58-44237. Hf has excellent corrosion resistance in high temperature water, which is why it is suitable as a material for reactors. However, since it is expected to maintain reactivity control ability for a long period of time, and also to be used for a long time, there here is a need to maintain high corrosion resistance over a long period of time in an in-pile oxidizing environment. Proposals relating to improving corrosion resistance of Hf are made with this type of situation in mind. For example, Japanese patent laid-open publication No. Sho. 59-208044, discloses technology for improving nodular corrosion resistance by alloying niobium and zirconium (Zr) in specific proportions. Japanese patent laid-open publication No. Sho. 61-66188 discloses technology for prolonging the in-pile usage time by forming a diffuse cementation-coat of steel, chrome, nickel, niobium etc. on the outer surface of an Hf alloy including a small quantity of Zr. With these technologies of the related art, improvements in corrosion resistance can be expected, but they presume a large-scale manufacturing process or processing method. On the other hand, as means for forming an oxidation cover layer on the surface of Hf, an autoclaving oxidation process has been considered. This method is generally used in zircalloy type fuel cladding pipes, and an oxidation cover layer is formed by processing in high temperature and high pressure vapor for a fixed time. However, with this method, processing facilities become large in size and the process time is long, which has a significant influence on the manufacturing process. With the method disclosed in Japanese patent laid-open publication No. Sho. 61-66188 as well, if a permeation layer is formed by ion coating, there is need for an annealing process taking a number of hours after that. Accordingly, the influence in the manufacturing process can not be ignored in this case either. SUMMARY OF THE INVENTION An object of the present invention is to provide a manufacturing method of a control rod for a boiling water reactor, having excellent corrosion resistance in high temperature water and excellent wear resistance at the time of manufacture, and in which the influence of the manufacturing process is slight. In order to achieve the above described object, a first aspect of the present invention provides a manufacturing method of a control rod for a boiling water reactor comprising a tie rod having a cruciform cross section, sheaths having a U-shaped cross section attached to each end of the tie rod and hafnium rods, plates or tubes contained inside the sheaths as neutron absorber material, comprising the steps of: providing an anodic oxidation film on a surface of the neutron absorber material; and assembling the neutron absorber material in the structure of the control rod. In a second aspect of the present invention, said film providing step in the first aspect is carried out in an electrolyte of ammonium borate or oxalic acid, with a conductive material as cathode and the neutron absorber material as anode. In a third aspect of the present invention, said film providing step in the first aspect is carried out in an electrolyte of ammonium borate, with a direct power source of 10-300V, with platinum or stainless steel as cathode and the neutron absorber material as anode. In a fourth aspect of the present invention, said film providing step in the first aspect is carried out by soaking gradually said neutron absorber material in an electrolyte for forming gradually said anodic oxidation film on the surface of said neutron absorber material. In a fifth aspect of the present invention, said film providing step in the first aspect is carried out in an electrolyte, with a first cathode material inserted into the hafnium tube in order to provide an anodic oxidation film on inside surface of the hafnium tube, a second cathode material provided outside the hafnium tube in order to provide an anodic oxidation film on an outer surface of the hafnium tube and said neutron absorber material as anode. By using the anodizing treatment, since only electrolytic bath facilities need to be prepared, the processing can be completed in small-scale facilities. Also, since the processing time is reduced, it is possible to obtain the same effects as with other oxidation processes without affecting the manufacturing process. The principle thing about anodizing is that it uses electrolysis of water, and it is common practice to use the object to be treated as the anode, and a conductive material such as platinum or stainless steel etc., as the cathode. If there is a conduction between the two electrodes, the oxygen component of the water forms oxide at the surface of the object material, constituting oxide film processing. If the conduction is direct current, resistance varies due to such factors as the object material, film thickness, and properties of impurities mixed in the film, which means that the extent of the process can be regulated by using a fixed voltage holding method. The electrolyte can be any substance as long as it is conductive, but if the corrosion resistance of the oxide film is considered, it is preferable to use a material including ions such that only a few impurities are exchanged for interstitials of a crystal lattice or a base material components. In the present invention, there is provided a manufacturing process for a control rod constructed of a neutron absorber tube of metal hafnium rods, plates or oval tubes, having an oxide film treatment step in an electrolytic bath filled with the electrolyte, before a step of assembling the hafnium member into a control rod frame. The metal hafnium forming a surface protection film resulting from the oxide film treatment prevents scratches and wear in subsequent processes, and can be assembled in a more uniform surface condition inside the control rod. Also, during use of an actual reactor, it is expected that reactivity control ability will be maintained over a long period of time, and it is possible to ensure high corrosion resistance over a long period of time under an in-pile oxidizing environment, so that the reactor will be used for longer than at the current time.