JP2821167B2 - Water-cooled direct cycle nuclear power plant - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a water-cooled direct cycle nuclear power plant, and more particularly to a nuclear print and a cooling water suitable for reducing the concentration of radioactive corrosion products in reactor water. It relates to a method for controlling iron concentration.

[Conventional technology]

A conventional method for controlling the concentration of corrosion products in feed water is disclosed in
Fe / Ni in feed water as described in JP 61-79194
⁵⁸ Co · ⁶⁰ Co of'll go-between furnace water to keep the concentration ratio of 2 to 10
It was intended to control so as to lower the ion concentration. However, sufficient consideration has not been given to the optimal control when the nickel concentration decreases.

[Problems to be solved by the invention]

Although the above-mentioned conventional technology focuses on only the Fe / Ni concentration ratio in the feed water to perform the optimal control, even when the ⁵⁸ Co ion concentration in the reactor water achieves a low level by satisfying this condition, may be higher than the ^{level 60} Co ion concentration is expected, a problem as an index of control is Zantsu.

An object of the present invention is to provide a water-cooled direct cycle nuclear power plant capable of maintaining a low ⁶⁰ Co ion concentration in reactor water even when the nickel concentration in feedwater is low, and an operation method thereof.

[Means for solving the problem]

The above-mentioned object is to measure the iron concentration in the cooling water and measure the iron concentration on the fuel rod surface in a water-cooled direct cycle nuclear power plant that includes a nuclear reactor, a turbine, a condenser, a purification device, and a feedwater heater as main components sequentially. 0.5mg / m ² / hr accumulation rate
In order to supply the above iron content to the cooling water, it can be achieved by calculating the excess or deficiency of the iron concentration from the iron concentration measurement data and controlling the iron content in the cooling water to an optimal amount.

[Action]

The present invention measures the iron concentration in the cooling water, calculates the accumulation rate of iron on the fuel rod surface from the measured data, and cools the calculated accumulation rate to be 0.5 mg / m ² / hr or more. The iron concentration in the water is controlled by the amount of iron injected from the iron injection device. Maintaining the Fe / Ni ratio in the feedwater at 2 to 10 is to reduce the chemical form of nickel and cobalt adhering to the fuel rod surface to ferrite oxide (NiFe ₂ O ₄ , CoFe ₂ O ₄ ) The amount of ⁵⁸ Co and ⁶⁰ Co produced as a result of the conversion into the reactor water can be reduced as compared with when nickel or cobalt adheres in the form of monooxide. However, instead of ⁶⁰ Co from all parts of the corrosion products adhering to the fuel rods surface as shown in a) of FIG. 2 is eluted into the reactor water, ⁶⁰ in the thickness that can contribute to an elution Co Is eluted in the reactor water. Therefore, as shown in b), even when the chemical form of the corrosion product is ferrite oxide, when the accumulation of iron is small, most of the ⁶⁰ Co accumulated on the fuel rod surface elutes into the reactor water. Will contribute. On the other hand, when the amount of accumulated iron is large as in c), only a part of ⁶⁰ Co accumulated on the fuel rod surface contributes to elution into the reactor water.

The half-life of ⁶⁰ Co is about 5 years, which is longer than the normal operation cycle of 1 year, and the specific activity of Co attached to the fuel rods can be considered to increase monotonically with time. The rate of increase of the ⁶⁰ Co concentration is small but normal compared to the increase in specific activity. This corresponds to the fact that ⁶⁰ Co accumulated on all fuel rod surfaces did not contribute to elution as described above. That is, it can be explained that the iron layer newly adhering to the fuel rod has an effect of shielding the elution of ⁶⁰ Co into the reactor water from the old adhering matter whose specific activity becomes higher. From this point of view, when the accumulation rate of iron on fuel rods and the rate of increase of ⁶⁰ Co concentration in reactor water in the operating plant are calculated, the accumulation rate of iron on fuel rods as shown in FIG. It was found that the rate of increase of the ⁶⁰ Co concentration in the reactor water became smaller as it became larger. From this figure, the accumulation rate of iron on the fuel rod is 0.
It can be seen that in the range of 5 mg / m ² / hr or more, the increase rate of the ⁶⁰ Co concentration in the reactor water becomes small. Therefore, the iron concentration in the cooling water can be optimized using the accumulation rate of iron in the fuel rod as an index.

However, if the amount of iron accumulated on the fuel rod surface is increased more than necessary, heat transfer from the fuel rods to the reactor water is hindered, which may cause damage to the fuel cladding tube or ⁵⁴ Mn, Since there is an adverse effect of increasing the total radioactivity concentration due to an increase in ⁵⁶ Fe, it is necessary to avoid bringing in unnecessary iron.

The term "reactor" as used herein refers to a type of reactor in which water is used as a coolant and boiling occurs on the surface of a fuel rod, such as a boiling water reactor or a new type of converter. The iron concentration in the cooling water refers to the concentration of all iron contained in the cooling water, regardless of chemical form, such as ionized Fe, non-ionized iron hydroxide, iron oxide, and the like. An apparatus for measuring iron concentration is a state in which all iron contained in a sampled sample such as atomic absorption or ion chromatography can be dissolved and ionized and the amount thereof can be quantified. The device that controls the iron content in the cooling water to the optimum amount is a memory and a calculation device required to convert the iron accumulation rate on the fuel rod surface with the measured data of the iron concentration as input, and the injection amount based on the calculation result. Is a control device composed of a flow control valve for changing the pressure or a pump capable of controlling the flow. The iron injection device has iron ions for ionizing or further forming iron by anodic electrolysis, a clad generator or a tank for storing water containing iron components, and a pump for injecting water containing iron components into cooling water. Device.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a diagram showing a system of a boiling water nuclear power plant. After the turbine 1 is operated, the condensate containing the corrosion product that has exited the condenser 2 is condensed by a condensate pre-filter 4 by a condensate pump 3. Most of the corrosion products are removed when passing through the condensate desalter 5. The purified water is led to a reactor pressure vessel 10 through a feedwater pump 6, a low-pressure feedwater heater 7, a booster pump 8, and a high-pressure feedwater heater 9. Corrosion products brought into the reactor pressure vessel 10 are not removed by the condensate demineralizer 5, but also include Ni and the like generated mainly by corrosion of the high-pressure feed water heater 9. The carry-in amount can be known by measuring the sample taken through the sampling line 11 by the concentration measuring device 12.

Specific examples of sample sampling and iron concentration measurement are shown below. A sampling line 11 is equipped with a sample holder in which one 0.45 μm Millipore filter and two or three cation exchange papers are set, and an integrated flow rate of 100 to 150 is passed at a flow rate of about 100 ml / min. The iron clad collected in the Millipore filter is heated to dissolve by adding 200 ml of 6N hydrochloric acid, and then made up to a constant volume with distilled water. The iron ions collected on the cation exchange paper are immersed in 15 ml of 2N hydrochloric acid for 5 minutes or more to elute, and the volume is adjusted to 2 to 3 times with 2N hydrochloric acid.
The sample water thus adjusted is measured using an atomic absorption spectrophotometer, and the iron concentration is calculated. JIS K 0121 (general rules of atomic absorption analysis method) is referred to for general matters common to the atomic absorption method.

In the present invention, this measured value is input to the arithmetic unit 13, and the arithmetic unit converts the iron accumulation rate into the fuel rods using the plant parameters stored in the knowledge base 14 according to the following equation. .

Where: α: accumulation rate of iron in fuel rod (mg / m ² / hr) C: iron concentration in feed water (ppb) F: rated feed water flow rate (t / hr) S: fuel rod surface area (m ² ) P: Output during measurement (MW) Pmax: Rated output (MW) The accumulation rate α of iron in the fuel rod obtained by equation (1) is 0.
When it is smaller than 5, control is performed so as to increase the iron concentration in the cooling water by an amount equal to or more than (0.5-α). That is, (1)
The increased iron concentration in the feed water is determined using the following equation obtained by modifying the equation.

Here, δC: the lower limit of the iron concentration in the feed water to be increased (ppb) The iron injection amount from the iron injector 15 corresponding to the lower limit δC of the iron concentration in the feed water to be increased is δC × F ( mg / h
r). The amount of iron actually injected is δC ×
It is necessary to make F excessive enough to compensate for the amount of F that adheres to the structural material inside the furnace, the recirculation pipe, and the like. From the past experience, the ratio of iron brought into the furnace from the water supply system to the fuel rods is almost 80 to 90
%, The excess is (C + δC) 10 to
The lower limit may be 20%. The amount of iron injected into the water supply from the iron injection device 15 can be adjusted using the flow control valve 16 so that the iron injection rate determined in this manner is obtained.

As a specific example of the iron injection device 15, there is one disclosed in Japanese Utility Model Application Laid-Open No. 63-135200 as an electrolytic iron generator for generating iron-containing water, and its configuration is shown in FIG. The electrolytic iron generator includes a CO ₂ injection raw water tank 17, an electrolytic tank 18, and a CO ₂ diffuser 19. In the CO ₂ injection raw water tank, C is injected through the gas injection port 20.
O ₂ gas was flowed into the electrolytic cell at a rate of 50 / h, and in the electrolytic cell, N ₂ gas was further introduced at a rate of 100 / h through the gas injection port 22, and C
In the O ₂ gas dissipator, N ₂ gas is supplied at 200 /
h at a rate of 100 V2
Assuming that the deaerated pure water to be supplied is 0 A and the amount of the supplied deaerated water is 60 / h, water containing about 100 ppm of iron ions can be obtained. Using a pump containing iron ions obtained in this way and a flow control valve,
By adjusting the flow rate between 20 / h and 60 / h, it is possible to increase the iron concentration in the feedwater by 0.3 to 1.0 ppb in a 1100 MWe class plant with a feedwater flow rate of 6400 t / h.

Conversely, the accumulation rate α of iron in the fuel rod calculated by equation (1)
Is greater than 3.0, preferably 2.0 (mg / m ² / hr), (α-
Control to reduce the iron concentration in the cooling water equivalent to 2.0). That is, the iron concentration in the supply water to be reduced is obtained using the following equation obtained by modifying the equation (1).

Here, δC ′: iron concentration in the feed water to be reduced (ppb) The flow control valve by the injection amount (δC ′ × F) corresponding to the iron concentration δC ′ in the feed water to be reduced obtained by the equation (3).
By reducing the amount by using 16 or by stopping the injection, the accumulation rate α of iron in the fuel rod can be controlled to 2.0 (mg / m ² / hr) or less.

When water containing 100 ppm of iron ions is injected into a 1100 MWe class plant as described above, the amount of change in the injection flow rate corresponding to δC ′ × F is Becomes As a result, the α value is preferably 2.0 or less,
If the iron concentration is too high, there may be complications that cause an increase in radioactivity caused by iron.
It is preferable to suppress the injection amount to 1.0.

In any case, it is not necessary to inject iron when the water supply concentration exceeds 0.5, even if the injection amount is 0, but iron concentration cannot be controlled, so the performance of the condensate purification system should be improved. Is preferred.

By controlling the iron concentration in the feedwater as described above,
Since the accumulation rate α of iron in the fuel rod can be kept in the range of 0.5 to 2.0 (mg / m ² / hr), the rate of increase of the ⁶⁰ Co concentration in the reactor water can be reduced.

In Equations (1) to (3) of the above embodiment, the input of the output P when the iron concentration is measured is also used to calculate the accumulation rate α in the fuel rod, but the feedwater flow rate at the time of measurement is used instead. You may. At this time, the expressions (1) to (3) are rewritten as follows.

Here, F ': water supply flow rate at the time of measurement (t / hr) Modification In the above embodiment, the iron concentration is controlled by focusing only on the accumulation rate of iron in the fuel rod. It is estimated that in the early operation of the first cycle of the plant, the probability of iron and nickel or cobalt coming into contact on the fuel rod surface is reduced due to less corrosion products on the fuel rod surface. For this reason, in the initial operation of the first cycle, it is preferable that the lower limit of the accumulation rate of iron in the fuel rod be set to 0.5 or more until the corrosion product adheres to the entire fuel rod. In this way, it is possible to form an adhesion layer of iron early on the fuel rod surface and reduce the concentration of radioactive corrosion products in the reactor water at a time when the rate of incorporation of radioactive corrosion products into structural materials such as pipes is high. It is effective in keeping the primary system dose rate low.

Embodiment 2 In a second embodiment of the present invention, in a plant in which the accumulation rate of iron in the fuel rod is to be controlled, the iron concentration in the feed water and the iron concentration in the fuel rod are determined in advance in accordance with a plant-specific parameter. By calculating the relationship between the accumulation rates and setting a control target value of the iron concentration in the feed water with respect to the operation time as shown in FIG. 4, the iron concentration in the feed water during operation is controlled based on this. 0.5 (mg / m ² / hr) iron accumulation rate on fuel rod
Or more.

A specific example is shown below for a 1100 MWe plant. The approximate values of the plant parameters are a rated heat output Pmax of 3300 MWt, a feedwater flow rate of 6400 t / h, and a fuel rod surface area of 7000 m ² . Therefore, assuming that the rated output operation is performed, α is set to 0.
7, After that, if the control target is set to the lower limit of 0.5, the iron concentration in the water supply will be 0.7 from equation (1) until 5000 hours.
7, it becomes 0.55ppb after that. After rounding up the second decimal place with some allowance, the iron concentration control target pattern in the water supply shown in FIG. 4 is obtained.

FIG. 9 shows the results of a simulation analysis of the ⁶⁰ Co ion concentration in the reactor water when the operation was performed according to the iron concentration control pattern. For comparison, the analysis results for the case where the iron concentration in the feedwater was kept at 0.3 ppb and α at 0.27 for all operating hours were compared.
Figure 10 shows. Analytical results show that between 2000 and 6000 hours, values are as low as 20% to 40%. As a result,
The dose rate on the piping surface after 10,000 hours is about 25 mR /
h, but about 35 mR /
h and is expected to be 40% higher. This difference is even greater when the amount of iron is smaller.

If the iron concentration control pattern in the feedwater is set in advance, the control is simplified because the process of calculating the accumulation rate of iron in the fuel rods after measuring the iron concentration in the feedwater is not required, The consideration for the output fluctuation is omitted.

In Fig. 4, the iron concentration in the feedwater is set high in the first half of the operation cycle and low in the second half of the operation cycle. As mentioned earlier, the fuel rod surface is covered with iron early in the first half and nickel and other cobalt are stabilized. This is because it is deposited on the fuel rod in a simple chemical form.

Example 3 In the previous example, the range of the increase rate of the ⁶⁰ Co concentration in the reactor water was broadly defined, and the iron concentration in the feed water was to be optimally controlled based on this. The relationship between the rate of accumulation of iron in the fuel rods and the rate of increase of the ⁶⁰ Co concentration in the reactor water actually varies considerably, so a method for optimizing each plant is shown in Fig. 5. Other control methods can be used. That is, in addition to the iron concentration in the feed water, the ⁶⁰ Co concentration in the reactor water, the feed water flow rate, the date, etc. are input, and these values are stored in a database, and the iron accumulation rate in the fuel rods and the ⁶⁰ Co concentration in the reactor water Is calculated. ^{60 in the} reactor water
If the increase rate of the Co concentration is equal to or less than the set increase rate, the latest iron accumulation rate in the fuel rod obtained is registered as the lower limit value of the optimal iron accumulation rate. Conversely, if the increase rate of the ⁶⁰ Co concentration in the reactor water is higher than the set increase rate, it indicates that the lower limit of the accumulation rate of iron on the fuel rod used in the previous control was too small. Then, a value obtained by increasing the accumulation rate of iron in the fuel rod used in the previous control by 10 to 20% is registered as the lower limit value of the optimal accumulation rate of iron. By changing the optimum value of the accumulation rate of iron in the fuel rods in this manner, it becomes possible to control the iron concentration in the feedwater optimally for each plant and each operation time. The method of controlling the iron concentration in the water supply after the optimum lower limit value of the iron accumulation rate on the fuel rods is determined may be the same as in the first embodiment.

Modification As a method for controlling the iron concentration in the feed water, the concentration of ⁶⁰ Co and the concentration of ⁵⁴ Mn in the reactor water can be used as indices. That is,
As shown in Fig. 6, when the ⁶⁰ Co concentration in the reactor water increases, the iron concentration in the feedwater increases, and when the concentration of ⁵⁴ Mn in the reactor water increases, the iron concentration in the feedwater increases. Is a method of controlling the temperature to be low. In this case, since the radioactive corrosion product to be reduced is directly used as an index, it is easy to understand, and it is possible to control even if the phenomenon is unknown. However, the activation process and the control effect have a time delay to appear, which is insufficient from the viewpoint of optimization.

Example 4 Even in the case where the accumulation rate of iron on the fuel rod surface is maintained at 0.5 (mg / m ² / hr) or more, in a plant where the nickel is so large that the Fe / Ni ratio on the fuel rod surface becomes 2 or less. The nickel concentration in the water supply must also be taken into the process of calculating the optimum iron concentration in the water supply, with the measured values also being optimal. That is, the nickel accumulation rate is calculated from the equation (1) in which iron is replaced with Ni in the first equation that calculates the accumulation rate of iron on the fuel rod surface, and the ratio of the iron accumulation rate to the nickel accumulation rate on the fuel rod surface is calculated. The iron concentration in the feed water is controlled so that the value does not become 2 or less. In this way, nickel and cobalt have stable chemical forms (NiFe ₂ O
₄ , COFe ₂ O ₄ etc.) adheres to the fuel rods and suppresses elution of radioactive cobalt. However, when calculating the accumulation rate of nickel on fuel rods, unlike the case of iron, the amount of nickel generated from structural materials in the furnace cannot be ignored, and the rate of adhesion to fuel rods is smaller than that of iron. It should be noted that it is about 60 to 70%, but it is not a big error to estimate the rate of sticking to the fuel rods rather than simply ignoring the generation in the furnace. When the output fluctuation is large, such as during a start-up test, it is not always necessary to satisfy the above conditions. This is because, when the output fluctuates, the corrosion products are easily separated from the fuel rods, and the control effect cannot be expected. In the present invention, a particularly preferred Fe / Ni ratio is 2 to 5.

Embodiment 5 In Embodiment 1, the position for measuring the concentration of iron carried into the reactor was only between the high pressure feed water heater and the reactor pressure vessel as shown in FIG. As shown in the figure, two measurement points can be added downstream of the iron injection point and also upstream of the feedwater pump. In this case, by comparing the two measured values on the upstream side and the downstream side, it is possible to know the loss of the injected iron due to the adhesion of the iron to the feed water heater and the piping on the way. When this loss fluctuates extremely, it is considered that the chemical form of the injected iron has changed, and it is possible to detect an abnormality of the iron injection device.

〔The invention's effect〕

According to the present invention, without significantly increasing the concentration of the precipitation of radioactive corrosion products in the reactor water, it is possible to keep the increase rate of ⁶⁰ Co concentration in the reactor water low, ⁶⁰ Co low reactor water through operating cycles The concentration can be maintained. If the concentration of ⁶⁰ Co in the reactor water is low, the amount of ⁶⁰ Co adhering to the primary cooling system piping is also reduced, so that the dose rate of the primary system at the time of the periodic inspection can be reduced, which has the effect of reducing the exposure dose.

[Brief description of the drawings]

FIG. 1 is a diagram showing a water supply system of a boiling water reactor for illustrating one embodiment of the present invention, and FIG. 2 is a diagram schematically showing the relationship between ⁶⁰ Co elution from the fuel rod surface and iron accumulation, Figure 3 is a diagram showing the relationship between the increase rate of ⁶⁰ Co concentration of accumulation rate and reactor water of iron into the fuel rod surface, Figure 4 shows the relationship between the control target value of iron concentration in the feedwater and operation time and Fig, Fig. 5 control process flow diagram of the iron concentration in the feedwater when the accumulation rate of iron into the fuel rod is variable, Figure 6 is a ⁶⁰ Co concentration and ⁵⁴ Mn concentration in the reactor water as an index FIG. 7 is a system diagram showing an example in which a device for measuring the iron concentration is provided at two positions on the upstream side and the downstream side of the feed water heater, and FIG. 9 is a schematic cross-sectional view of an electrolytic iron generator, and FIG. 9 and FIG.
⁶⁰ Co furnace and the concentration change of water is a graph showing the operation time and ⁶⁰ Co concentration changes in the reactor water in the case of not changing the iron concentration. 1 ... turbine, 2 ... condenser, 3 ... condenser pump, 4
………………………………………………………………………………………………………………………………… …………………………………………… ·····
High pressure feed water heater, 10 Reactor pressure vessel, 11 Sampling line, 12 Concentration measuring device, 13 Diagnostic device, 14 Knowledge base, 15 Iron injection device, 16 Iron injection quantity control valve, 17… CO ₂ press-in raw water tank, 18 …… Electrolysis tank, 19
…… CO ₂ diffuser, 20 …… CO ₂ gas injection port, 21 …… Iron electrode, 22,23 …… N ₂ gas injection port.

Continuing from the front page (72) Inventor Shunsuke Uchida 1168 Moriyama-cho, Hitachi City, Ibaraki Pref. Energy Laboratory, Hitachi, Ltd. (56) References JP-A-61-213693 (JP, A) JP-A-62-233796 (JP) , A) JP-A-63-11898 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G21D 3/00-3/18

Claims (6)

(57) [Claims] In a water-cooled direct cycle nuclear power plant which includes a nuclear reactor, a turbine, a condenser, a purification device, and a feedwater heater as main components in sequence, a device for measuring iron concentration in cooling water, In order to supply the amount of iron at which the accumulation rate of iron becomes 0.5 mg / m ² / hr or more to the cooling water, an insufficient amount of the iron concentration is calculated from the iron concentration measurement data, and the iron content in the cooling water is controlled to an optimal amount. A water-cooled direct cycle nuclear power plant, comprising an iron injection device provided with a device for performing the above. 2. The accumulation rate of iron on the fuel rod surface is 1 to 2 mg / m ² / h.
2. The water-cooled direct cycle nuclear power plant according to claim 1, wherein the iron content in the cooling water is controlled to an optimum amount so as to be r. 3. The water-cooled direct cycle nuclear power plant according to claim 2, wherein the position where the iron component is injected into the cooling water is located downstream of the condensate purification device. 4. The method according to claim 1, wherein the iron concentration in the cooling water is measured on the downstream side of the condensate purifier and upstream of the feed water heater, or between the reactor pressure vessel and the feed water heater, or both. 2. A water-cooled direct cycle nuclear power plant according to claim 1. 5. A method for operating a water-cooled direct cycle nuclear power plant, which includes a reactor, a turbine, a condenser, a purifier, and a feedwater heater as main components sequentially, comprising: measuring an iron concentration in cooling water; In order to supply the amount of iron at which the accumulation rate of iron on the rod surface is 0.5 mg / m ² / hr or more to the cooling water, the excess or deficiency of the iron concentration is calculated from the measured data of the iron concentration, and is optimal for the cooling water. A method for operating a water-cooled direct-cycle nuclear power plant, which operates while injecting an amount of iron. 6. A method of operating a water-cooled direct cycle nuclear power plant, which includes a reactor, a turbine, a condenser, a purifier, and a feedwater heater as main constituents in a sequential manner. In addition to calculating the rate of accumulation of iron in the cooling water, the nickel concentration in the cooling water is measured, and the iron / nickel concentration ratio in the cooling water is calculated from the measured nickel concentration and the iron concentration. Even if the iron accumulation rate is 0.5 mg / m ² / hr or more, if the concentration ratio of iron and nickel in the cooling water is 2 or less, the optimal amount of iron in the cooling water is adjusted so that the concentration ratio becomes 2 or more. A method for operating a water-cooled direct cycle nuclear power plant, characterized by operating while pouring water.