Patent Number: 058928057
Section: summary

BACKGROUND OF THE INVENTION This invention is related to a method of operation of a nuclear power plant, and particularly to a method suitable for operation of a boiling water reactor power plant. Generally, corrosion products, such as metallic ion components and insoluble components (clad) and the like, are dissolved little by little into water from structural materials, such as piping, pumps, heat exchangers, etc., of a boiling water reactor power plant (hereinafter referred to as BWR power plant). Most of the corrosion products formed in a turbine system on an upper side of a clean-up system for condensate water are removed by the clean-up system. But, the corrosion products formed in a feed water system at a downstream side of the clean-up system for condensate water are not removed. These corrosion products enter into the nuclear reactor together with the feed water. Most of the corrosion products which enter into the nuclear reactor deposit on the surface of the fuel rods. The corrosion products which are deposited on the surface of the fuel rods are transformed into radioactive nuclides by irradiation with neutrons. For example, Ni and Co contained in the corrosion products are transformed into .sup.58 Co and .sup.60 Co, which have a long half life, respectively, by neutron irradiation. A part of the corrosion products, being a radioactive substance, dissolves from the surface of the fuel rods into the reactor water, or exfoliates from the surfaces of the fuel rods. The dissolved or the exfoliated corrosion products deposit on the inner surfaces of equipment and pipings of a primary loop recirculation system or the clean-up system for the reactor water. Therefore, the dosage rate of the equipment and the pipings of the primary loop recirculation system, or of the reactor water cleanup system, increases. In the case where NiFe.sub.2 O.sub.4 and CoFe.sub.2 O.sub.4 are composite oxides of the corrosion products of Ni and Co that deposit on the surfaces of the fuel rods, the speed of re-dissolution of the composite oxides into the reactor water, or the speed of their exfoliation from the surfaces of the fuel rods, becomes very slow. The composite oxides stay on the surfaces of the fuel rods for a long time after they have deposited. As a result, re-deposition of the corrosion products, which are radioactive substances, on the equipment and pipings is suppressed. Ni and Co in the reactor water form stable composite oxides by setting a weight ratio of Fe/Ni in the feed water to 2 or more (actually, about 3). In order to obtain the Fe/Ni weight ratio, a technique for controlling the iron concentration in the feed water has already been adopted in the BWR. However, even when the Fe concentration is controlled, as mentioned above, a new phenomenon was observed wherein the concentration of the radioactive substances, such as .sup.60 Co, in the reactor water changes. The reason is that the concentration of the radioactive substances in the reactor water does not stabilize due to an increase of the quantity of Cr dissolving from stainless steel pipes used for heat transfer pipes of the feed water heater. The increase in the dissolution quantity of Cr accelerates re-dissolution of clad deposited on the fuel rods. Several measures are described in Japanese patent Laid-open Print No. 5-288893. These measures suppress a fluctuation in the concentration of radioactive substances in the reactor water, to control the Cr quantity in the deposit on the fuel rods in addition to controlling the Fe quantity. The Laid-open Print describes a method for reducing the Cr content in clad deposited on the fuel rods by methods such as a method for properly managing the concentration of an oxidizing agent in the feed water, a method for substituting a material having a high Cr content for a low Cr content material, a method for making alkali ions coexistent with Cr ions in the reactor water, and a method for efficiently operating the clean-up equipment for cleaning the reactor water. Japanese Patent Publication No. 68914 describes a technique that controls the pH of the reactor water to a weak alkali state. This is the technique that suppresses an uptake of radiation to the main pipe and is applied only at the time of the starting operation of the nuclear reactor when the uptake speed is fast. But, Japanese Patent Laid-open Print No. 5-288893 does not pay attention to the injection time of an alkali metal. The technique disclosed in Japanese Patent Publication No. 6-8914 is applied only to controlling the weak alkali state at the time of starting, but this technique cannot suppress Cr loading on the fuel rods in new fuel assemblies to be loaded during every operating cycle. SUMMARY OF THE INVENTION An object of this invention is to provide a method of operating a BWR power plant that can reduce Cr loading on fuel rod surfaces over all operating cycles. A first feature of this invention for achieving the above object is to inject an alkali metal or alkaline earth metal into the reactor water during the term from a starting operation of a preoperating test or of each fuel of the cycle of the nuclear reactor to the 2000 EFPH, and to stop the injection of the alkali metal or the alkaline earth metal during the period between the 2000 EFPH and the stopping of the BWR power plant. EFPH stands for an effective full power hour, which represents a percentage of an integral value of the actual thermal output per a rated thermal output within a time period of the actual thermal output. Cr deposited on the surfaces of the fuel rods dissolves into the reactor water thereby to locally acidify the reactor water on the surfaces of the fuel rods. This accelerates the dissolution of the deposit into the reactor water. As will be described later, the deposit of Cr on the surfaces of the fuel rods occurs by taking Cr into an oxide film during the term of forming the oxide film on the fuel rod surfaces. The above term is from the starting operation of the preoperating test or of each fuel cycle of the nuclear reactor to the 2000 EFPH. By the injection of the alkali metal or the alkaline earth metal into the reactor water during the above term, Cr easily exists in the reactor water as Cr oxide ions. Therefore, the amount of Cr that deposits on the surfaces of the fuel rods as a Cr oxide decreases. This phenomenon lowers the local acidification of the reactor water, which is caused by dissolution of Cr deposited on the fuel rod surfaces so that the dissolution of the radioactive substances into the reactor water is suppressed. The concentration of the radioactive substances contained in the reactor water decreases, resulting in a decrease in the surface dose rate of the pipes and the devices for the primary loop recirculation system, the clean-up system of reactor water and so on. A second feature of this invention is to provide a method for operating a BWR power plant comprising pipings of a clean-up system, both ends of which are connected to a nuclear reactor, wherein the clean-up system for the reactor water comprises the pipings of the clean-up system, a first clean-up unit having ion exchange resin to which an alkali metal or alkaline earth metal is added, the first clean-up unit being installed in the pipings of the clean-up system, and a second clean-up unit having an ion exchange resin to which no alkali metal or the alkaline earth metal is added and which is connected in parallel with the first clean-up unit, the improvement of which comprises the following steps: supplying the reactor water in the nuclear reactor to the piping of the clean-up system, returning the reactor water to the nuclear reactor after the reactor water is purified by the first clean-up unit during the term from the starting test of said nuclear reactor or the starting operation of each fuel cycle to the 2000 EFPH, and returning the reactor water to said reactor after purifying it using the second clean-up unit during the period between the 2000 EFPH and the stopping of the nuclear reactor. By the second feature, the same effect as in the first feature is achieved. According to the second feature, the alkali metal or the alkaline earth metal can be easily shifted from the ion exchange resin in the first clean-up unit to the reactor water during the above term, thereby cleaning the reactor water with the first clean-up unit. A third feature of this invention is to inject a material, which changes the pH of the feed water to the acid side, into the feed water from the feed water system or the condensate system during the period between the starting operation and the 2000 EFPH and to stop the injection of the material after the above period lapses. By injecting the material for changing the pH of the feed water to the acid side into the feed water from the feed water system or the condensate system during the above term, nonradioactive Cr which exists in the interior of the structure of the feed water system or the condensate system becomes a stable Cr oxide. Therefore, the dissolution of the Cr into the cooling water of the primary system is suppressed, and the Cr quantity included in the reactor water is decreased more than in the case of the first feature. A fourth feature of this invention is to inject hydrogen of a molar concentration which is 2.about.3 times the molar concentration of dissolved oxygen into the feed water during the above term. The injection of the hydrogen at the above concentration reduces the corrosion potential of the feed water in the above term. Therefore, the Cr on the interior of the structure of the feed water system or of the condensate system becomes a stable Cr oxide, and the dissolution of Cr into the feed water is suppressed. A fifth feature of this invention is to use cladding tubes which have no oxide film on the surfaces thereof for fuel rods of fuel assemblies of 0(zero) GW.multidot.day/t burn-up loaded in the nuclear reactor. Because of the amount of Cr which is taken into an oxide film from the reactor water in forming the oxide film on the surfaces of the cladding tubes in the nuclear reactor, there is little dissolution of the Cr on the surfaces of the cladding tubes. Therefore, even when the fuel assemblies of 0(zero) GW.multidot.day/t burn-up comprising fuel rods containing cladding tubes not formed with an oxide film on the surfaces are loaded in the nuclear reactor, the concentration of the radioactive substances contained in the reactor water is small. A sixth feature of this invention is to control the quantity of oxygen which is injected into feed water from a condensate system or a feed water system so as to obtain a dissolved oxygen concentration of 10 ppb to 30 ppb in the feed water during the above term, whereby corrosion of the condensate system or of the feed water system in the term is remarkably decreased. Consequently, the quantity of the Cr brought into the nuclear reactor together with the feed water remarkably decreases. The quantity of the Cr taken into the surfaces of fuel rods decreases, and the concentration of the radioactive substances contained in the reactor water decreases. A seventh feature of this invention is to increase the quantity of reactor water supplied to a clean-up system for the reactor water for purifying the reactor water during the above term. In the term, the amount of reactor water to be supplied to the reactor clean-up system is increased so that the quantity of the radioactive substances in the reactor water can be decreased. An eighth feature of this invention is to use a material of low Cr dissolution for at least one part of the primary system pipings, which are connected to the nuclear reactor, in which cooling water flows, so that the dissolution quantity of the Cr from the low Cr dissolution material absolutely decreases. Therefore, the cr quantity itself which is taken into the nuclear reactor is reduced, and the Cr quantity deposited on the fuel rods is reduced.