Patent Number: 050154362
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiment 1 In FIG. 1, steam generated in a reactor 10 actuates a turbine 1 and is then condensed in a condenser 2; when condensed water including corrosion products is passed from the condenser 2 through condensed water pre-filter 4 and condensed water desalter 5 by means of condensed water pump 3, most portion of the corrosion products are removed. The purified water is flowed through feed water pump 6, low pressure feed water heater 7, pressurizing pump 8 and high pressure feed water heater 9 into nuclear reactor pressure vessel 10. Corrosion products carried into pressure vessel 10 were composed of corrosion product which had not been removed in condensed water desalter 5 and, in addition, Ni or the like which was mainly generated due to corrosion of high pressure feed water heater 9. The amount of the corrosion products carried into pressure vessel was measured by analysing a specimen picked through a sampling line 11 by means of a concentration measuring means 12. The sampling of specimen and the measuring of iron concentration existing in the specimen were effected in the manner explained below. A specimen holder having a sheet of millipore filter of 0.45 .mu.m in pore size and two or three sheets of positive iron-exchanging papers was attached to a sampling line 11. Then, in order to catch iron clad contained in the reactor-cooling water, the cooling water was made to flow through the specimen holder disposed in the sampling line 11 at a flow rate of about 100 ml so that an accumulation flow amount of the cooling water flowing therethrough was in a range of 100 to 150 litters. The iron clad caught by the millipore filter was dissolved in a heated hydrochloric acid of 200 ml in volume and 6N in concentration and then distilled water was added thereto to provide a constant volume. The positive ion-exchanging paper for catching iron ion was immersed in hydrochloric acid of 15 ml in volume and 2N in concentration for a period of not less than 5 minutes, this treatment being repeated two or three times, and finally an iron ion-containing solution of a constant volume was prepared by use of hydrochloric acid of 2N in concentration. The resultant specimen solution having been thus prepared was measured by use of a usually used atomic absorption spectro photometer to thereby obtain an iron concentration, the specific process of the atomic absorption spectro photometry being prescribed in JIS K 0121. In the present invention, the measured iron concentration was converted into iron accumulation rate on fuel rod through calculator 13 while referring to plant parameters stored in a data memory 14 by use of the following equation. ##EQU1## where, .alpha.: iron accumulation rate on fuel rod (mg/m.sup.2 /hr) C: iron concentration in feed water (ppb) PA1 F: nominal feed water flow rate (t/hr) PA1 S: fuel rod surface area (m.sup.2) PA1 P: plant output power at measurement time (MW) PA1 P.sub.max : rated output power (MW) When iron accumulation rate .alpha. is smaller than 0.5, the iron concentration in cooling water should be increased correspondingly to the amount (0.5-.alpha.), namely, iron concentration to be added was calculated by the following equation 2: ##EQU2## where, .delta.C: lower limit of iron concentration in feed water to be added Iron amount to be injected from an iron-injecting means 15 correspondingly to the above lower limit value .delta.C was obtained as .delta.C.times.F. However, actually, the iron amount to be poured is required to be further added by an iron amount which compensates the iron adhering to structures and tubings of reactor. Since experiences in the past showed that 80-90% of iron carried into reactor adhered to fuel rod, the lower limit of the iron amount to be further added may be determined as 10-20% of (C+.delta.C). A suitable amount of iron was injected from iron pouring means 15 under control of flow rate control valve 16. The ion-injecting device 15 was provided with a device for generating electrolytic iron disclosed in Japanese Utility Model Unexamined Publication No. 63-135200. The construction of the electrolytic iron-generating device used in the embodiment is shown in FIG. 8, which device comprises a water tank 21 operating as a source of water containing carbonic ion, an electrolytic cell 22 provided with an iron sheet electrode 25 operatively connected to the water tank 21 so as to receive the CO.sub.2 gas, and a device 23 for discharging cabonic ion contained in iron ion-including water generated in the electrolytic cell 22. By using the electrolytic iron-generating device, water containing iron ion of about 100 ppm was obtained under such conditions that CO.sub.2 gas was fed to the water reservoir 21 through a nozzle 24 at a rate of 50 litter/hour to generate the carbonic ion in water, N.sub.2 gas being fed to the electrolytic iron-generating device 23 at a rate of 100 litter/hour through a nozzle 26 for agitating water in the vicinity of the iron sheet electrode, N.sub.2 gas being fed in the carbonic ion-removing device 23 at a rate of 200 litter/hour through a nozzle 27 so as to remove carbonic ion contained in the iron ion-including water, electrolytic current being 20 A at 100 V, degased pure water being fed to the water reservoir 21 at a rate of 60 litter/hour. By controlling the flow rate of the resultant ion ion-including water through a flow control valve, it was possible to increase ion concentration in feed water by 0.3 to 1.0 ppb in a case of a nuclear power plant of 1100 MWe and 6400 t/h in feed water flow rate. On the other hand, when iron accumulation rate .alpha. obtained from equation (1) is greater than 3, preferably greater than 2 (mg/m.sup.2 /hr), iron concentration in cooling water should be decreased correspondingly to the value (.alpha.-2.0), namely as follows: ##EQU3## where, .delta.C': iron concentration to be decreased in feed water (ppb) The iron accumulation rate on fuel rod can be controlled smaller than 2.0 (mg/m.sup.2 /hr) by decreasing the iron amount to be poured by an amount (.delta.C'.times.F) corresponding to the above .delta.C' by means of flow rate control valve 16, or by stopping the iron injection. In a case of injecting water containing iron ion of 100 ppm in concentration in a nuclear power plant of 1100 MWe class, amount of change in feed flow rate corresponding to .delta.C'.times.F becomes 64 .delta.C' (l/h) (, that is, .delta.C'.times.6400/100). By changing the flow rate by this value, the value of .alpha. becomes in a preferred range of not more than 2.0. However, in a case of relatively high iron concentration, there occurs such unfavorable phenomenon as radioactivity increases due to the increment of iron concentration in feed water. Thus, after the lapse of operating time of 5000 hours, it is preferred to control the injecting rate of the iron ion-containing water so that the .alpha. value may be in a range of 0.5 to 1.0. But, if concentration of iron ion contained in the feed water exceeds a value corresponding to 0.5 regarding the .alpha. value even in a case where the injecting rate of the iron ion-containing water fed into the feed water is zero, it is unnecessary to effect the injecting of the iron ion-containing water into the feed water. Since in this case it becomes impossible to effect the control of ion concentration in the feed water, it is preferred to improve the ability of condensate-purification means. By controlling the iron concentration in feed water between 0.5 and 2.0 (mg/m.sup.2 /hr) as described above, the increase rate of .sup.60 Co concentration can be maintained at a low level. Although the output value (P) of the plant at the time of measuring iron concentration is used for calculating iron accumulation rate .alpha. in equations (1), (2) and (3), it can be replaced by feed water flow rate at the measuring time, as follows: ##EQU4## wherein F' is a feed water flow rate at measuring time (t/hr). MODIFIED EXAMPLE For the above-mentioned embodiment, a modification is possible as described below. In an initial operational stage of first cycle of a new nuclear power plant, the probability of contacting of iron with nickel or with cobalt on fuel rod surface is considered low, because corrosion product is still of a small quantity. Considering this matter, it may be preferable to set the lower limit of iron accumulation rate on fuel rod to be not less than 0.5 until corrosion product is adhered on the whole surface of fuel rod in the initial operational stage of the first cycle. It is effective for maintaining dose rate of primary system at a low level to positively form an iron adhesion layer on fuel rod surface at an early stage of operation, which iron adhesion layer reduces the concentration of radioactive corrosion product in reactor water in a period of time when a rather great quantity of radioactive corrosion product is being adhered to structures or tubings of plant. EMBODIMENT 2 In the nuclear power plant of the second embodiment of the invention in which the accumuration rate of iron accumurated on fuel rods is to be controlled, a relation between a concentration of iron contained in feed water and an accumuration rate of iron on the fuel rod was previously obtained in accordance with parameters inherent in the power plant. Then, as shown in FIG. 4, there was determined a target value for controlling the iron concentration of the feed water with respect to the operating time of the nuclear power plant. In compliance with the target value, an actual iron concentration of the feed water was controlled during the operation of the plant so that the accumulation rate or iron accumulated on the fuel rods was not less than 0.5 mg/m.sup.2 /hr. Specific example is explained below with respect to a nuclear power plant of 1100 MWe. The parameters of the plant were a rated heat output (P.sub.max) of 3300 MWt, a feed water flow rate of 6400 t/h, and a fuel rod surface area of 7000 m.sup.2. Under an assumption of the rated output operating, the control target lower values of .alpha. was determined to be 0.7 till 5000 hours from the commencement of the operating of the plant and to be 0.5 after the lapse of 5000 hours therefrom. Thus, an iron concentration of the feed water was 0.77 ppb till 5000 hours from the commencement of the operating of the plant by calculating from the formula (1) and was 0.55 ppb after 5000 hours therefrom, as shown in FIG. 4. In FIG. 9 there is shown a result of analysis of a simulation with respect to .sup.60 Co ion concentration of feed water in a case of effecting an operating of the plant in accordance with the iron concentration-controlling pattern of FIG. 4. For comparison, in a case where the iron concentration was made to be a constant value of 0.3 ppb during the whole operating period with the result that the iron accumulation rate of the fuel rod (.alpha.) was a constant value of 0.27, there was also effected another simulation to thereby obtain an analysis result shown in FIG. 10. In comparing the results shown in FIGS. 9 and 10, it was found that in the second embodiment of the invention the .sup.60 Co concentration of the feed water was reduced by 20% to 40% in comparison with the case of FIG. 10 where no control of iron concentration was effected. Thus, it was deemed that, after the lapse of 10000 hours from the commencement of the operating of the plant shown above, a dose rate of the surface of piping was about 25 mR/h, while the dose rate was about 35 mR/h in the case of FIG. 10 (, that is, the dose rate in the case of FIG. 10 is higher by about 20% than the case of FIG. 9 of the invention). This difference further increases in a case where iron concentration in the feed water is less than the case of FIG. 10. In a case where a pattern for controlling the concentration of iron contained in the feed water is previously determined, it is unnecessary to calculate the accumulation rate of iron accumurated on the fuel rod after measuring the iron concentration of the feed water, so that the control thereof can be effected in a simple manner while control in response to variation in output can not be effected. As shown in FIG. 4, the iron concentration of the feed water is made to be in a high level during an initial period of the operating cycle of the plant while the iron concentration is made to be in a low level after the lapse of the initial period thereof, so that the fuel rod surface is covered by iron in an early stage in the initial period of the operating cycle to thereby adhere nickel and cobalt (both contained in the feed water) on the fuel rod while changing the nickel and cobalt into chemically stable states such as NiFe.sub.2 O.sub.4 and CoFeO.sub.4. EMBODIMENT 3 Although in the first and second embodiments, a rather rough estimation for .sup.60 Co concentration increase rate is used for controlling the iron concentration in feed water, the relation between iron accumulation rate on fuel rod and .sup.60 Co concentration increase rate in reactor water shown in FIG. 3 has actually a wide variation range. Therefore, in the third embodiment, control method of which is shown in FIG. 5, .sup.60 Co concentration in reactor water, feed water flow rate, operation date as well as iron concentration in feed water are input into the calculator and stored in the memory, and then iron accumulation rate on fuel rod and .sup.60 Co concentration increase rate in reactor water are calculated based on the above data. When the .sup.60 Co concentration increase rate is lower than a predetermined value, the newestly measured iron accumulation rate is registered as a new lower limit of the suitable iron accumulation rate. On the other hand, when the .sup.60 Co concentration increase rate is higher than the predetermined value, iron accumulation rate greater than the newestly measured iron concentration rate by 10-20% is registered as a new lower limit of the suitable iron accumulation rate. Thus, by varying the range of the optimum iron accumulation rate, the control of iron accumulation rate can be made the most optimum one for a peculiar plant and for a peculiar date. When the lower limit has been defined, other processes are identical to those in the first embodiment. MODIFIED EMBODIMENT Here, .sup.60 Co concentration in reactor water and .sup.54 Mn concentration in reactor water are used as control indexes for controlling iron concentration in feed water. As shown in FIG. 6, when .sup.60 Co concentration in reactor water become higher, iron concentration in feed water is controlled at a higher level, while when .sup.52 Mn concentration in reactor water become higher, iron concentration in feed water is controlled at a lower level. In this case, since the radioactive corrosion products to be decreased are directly designated as control indexes, the control method is easy to understand, and can be carried out even if the detail of the reaction behavior is not clearly known. However, this method is not so optimum, because there is a considerable time delay regarding both the phenomenon of the activation and a time when numerical data showing the effective control by use of iron concentration is obtained. EMBODIMENT 4 In a plant where nickel is so rich that Fe/Ni ratio is smaller than 2, nickel concentration measured in the feed water is required to be incorporated into control indexes for calculating iron accumulation rate, even if iron concentration rate is maintained to be not less than 0.5 (mg/m.sup.2 /hr). Nickel accumulation rate on fuel rod can be calculated through equation (1) by replacing iron concentration in the equation (1) by nickel concentration. Then, iron concentration in the feed water is controlled so that the ratio of iron accumulation rate to nickel accumulation rate become greater than 2. By virtue of this control method, nickel or cobalt adheres on fuel rod in a chemically stable state (NiFe.sub.2 O.sub.4, CoFe.sub.2 O.sub.4, etc.), thereby minimizing dissolution of radioactive cobalt. It should be noticed that the amount of nickel dissolved in the feed water from plant structures is not negligible for calculating nickel accumulation rate differently from iron case, and that only 60-70% of this nickel adheres to fuel rod surface, which value is small in comparison with iron case. For dealing with this matter in a simple manner, the nickel generation in reactor may be neglected by estimating the accumulation rate on fuel rod to be somewhat higher. This simplification causes little error. Further, in an operation suffering great output power variation such as in a starting test, the above-mentioned processes are not necessarily required, because, in a state of output power variation, corrosion products are liable to separate from fuel rod, and a sufficient control effect can not be expected. In the invention, Fe/Ni ratio is preferred to be in a range of 2-5. EMBODIMENT 5 In the embodiment 1 the device for measuring the concentration of iron contained in the feed water was disposed between the high pressure feed-water heater and the pressure vessel of the nuclear reactor as shown in FIG. 1. In the embodiment 5, two devices for measuring the concentration of iron of the feed water were provided, the first device thereof being disposed with the same position as in the embodiment 1 and the second device was disposed at a position which was downstream side of the ion-injecting point and was at upstream side of the feed-water pump as shown in FIG. 7. By comparing two values measured through these upstream and downstream devices, it was possible to detect a loss of injected iron which loss occurred due to the adhesion of the iron to the feed-water heater and piping position between these two devices. In a case where this loss is varied in a large degree, it is deemed that a chemical stage of the injected iron is changed, with the result that it becomes possible to detect an abnormality of the iron ion-injecting device. According to the present invention, .sup.60 Co concentration increase rate in reactor water can be maintained at a low level through whole period of operation substantially without increasing the concentration of radioactive corrosion product in reactor water which product adheres to plant structure such as piping. By virtue of the lowered .sup.60 Co concentration in reactor water, .sup.60 Co amount adhering to tubings of primary system of plant is decreased, thereby decreasing dose rate of the primary system which dose rate must be taken into consideration at the time of periodic inspections.