Patent Number: 050154362
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a water-cooled direct cycle nuclear power plant, and to a method for controlling iron concentration in cooling water in order to further decrease radioactive corrosion product concentration in reactor water. A control method of a prior art for controlling corrosion product concentration in feed water is disclosed in Japanese Patent Laid-Open No. 61-79194, where .sup.58 Co..sup.60 Co ion concentration in reactor water is controlled at low level by maintaining Fe/Ni concentration ratio in a range of 2 to 10. However, in this control method, a sufficient consideration is not paid to the case where the nickel concentration is further decreased. In the above prior art, the control is carried out only in dependence on Fe/Ni concentration ratio in feed water. In this control method, .sup.60 Co ion concentration was frequently higher than an expected value, although .sup.58 Co ion concentration may be maintained at low level. Namely, the Fe/Ni concentration ratio is not a sufficient control index. SUMMARY OF THE INVENTION The object of the present invention is to provide a water-cooled direct cycle nuclear power plant having a control means which controls iron concentration in feed water at a suitable level in order to maintain .sup.60 Co ion concentration in reactor water at low level even when nickel concentration becomes relatively low in feed water. The above-mentioned object of the present invention is achieved by a water-cooled direct cycle nuclear power plant, which includes a nuclear reactor, a turbine, a condenser, a purifying means and a feed water heater successively arranged as main constituent elements, and further comprises means for measuring iron concentration in cooling water and means for injecting iron into cooling water for controlling iron amount in cooling water at optimum level. The iron-injecting means includes a processing unit which calculates iron concentration increment to be added on the basis of the data obtained by the above-mentioned iron concentration measuring means in order to supply the iron amount into cooling water for realizing an iron accumulation rate not less than 0.5 mg/m.sup.2 /hr on fuel rod surface, and a control unit for supplying the calculated amount of iron to feed water. In the present invention, iron concentration in cooling water is measured, iron accumulation rate on fuel rod surface is calculated based on the measured data, and iron amount to be injected from iron-injecting means is controlled so as to maintain the calculated iron accumulation rate to be not less than 0.5 mg/m.sup.2 /hr. To maintain Fe/Ni ratio in feed water in a range of 2 to 10 is effective for changing nickel and cobalt adhered to fuel rod surface into chemically stable and almost insoluble ferrite oxide (NiFe.sub.2 O.sub.4, CoFe.sub.2 O.sub.4, etc.), and decreasing the amount of activated .sup.58 Co or .sup.60 Co dissolving into reactor water in comparison with a case where nickel and cobalt are adhered on fuel rod as a monooxide. However, as shown in FIG. 2-A, .sup.60 Co is considered to dissolve into reactor water not from the whole portion of corrosion product adhered on fuel rod surface, but only from a portion of a certain thickness of the product which serves as a dissolvable layer. In consequence, in case iron accumulation layer is thin as shown in FIG. 2-B, the most portion of .sup.60 Co accumulated on fuel rod surface contributes to the dissolution into reactor water, while in case iron accumulation layer is thick as shown in FIG. 2-C, only a part of accumulated .sup.60 Co contributes to the dissolution. Since the half life of .sup.60 Co is about five years, which is relatively long in comparison with an usual plant operation cycle of one year, the specific activity of Co adhered to fuel rod is regarded as monotonously increasing along time lapse. Actually, the .sup.60 Co concentration increasing rate in reactor water caused by plant operation is usually rather small in comparison with the increase of the specific activity. This is for the reason that, as mentioned above, only a part of .sup.60 Co adhered to fuel rod contributes to the dissolution. In other words, it may be understood that an iron layer newly adhered to fuel rod has a function to shield a dissolution of .sup.60 Co from an oldly adhered layer having a higher specific activity into reactor water. With this view point, a relation between iron accumulation rate on fuel rod and .sup.60 Co concentration increase rate in reactor water is calculated, result of which is shown in FIG. 3, showing a tendency that the latter becomes smaller as the former becomes greater. From this figure, it is found that the .sup.60 Co concentration increase rate in reactor water becomes small in a case where the iron accumulation rate on fuel rod is not less than 0.5 mg/m.sup.2 / hr. Therefore, iron concentration can be controlled at an optimum level by using iron accumulation rate as a control index. However, if iron amount accumulated on fuel rod become unnecessarily great, heat transfer from fuel rod to reactor water is deteriorated with a risk of causing failures of fuel cladding tubes; and .sup.54 Mn and .sup.59 Fe, which are generated by iron activation, increases, causing a increase of total radioactive concentration. Therefore, it is required to limit the amount of iron to be accumulated. In this text, the nuclear reactor is of a type where water is used as a coolant and boiled on fuel rod surfaces such as seen in a boiling water reactor or advanced thermal reactor. The term of iron concentration means a concentration of all irons existing in cooling water including ionized Fe, not-ionized iron hydroxide, iron oxide etc. regardless of their chemical states. The iron concentration measuring means is a device which can measure the iron amount included in a sampled specimen for atomic absorption or ion chromatograph in a dissolved and ionized state of iron. The control means for controlling iron amount in cooling water at an optimum level is a device comprising a memory and a calculator required for deducing an iron accumulation rate on fuel rod surface from the measured iron concentration, and a control unit constituted by a flow rate control valve or a pump of variable flow rate required for controlling iron-injecting rate according to the calculated results of the calculator. The iron-injecting means is a device comprising an iron generating means by virtue of ionizing or cladding through electrolytic analysis, or a water tank containing water including iron constituent, and a pump for injecting the water including iron constituent into the cooling water.