JPH0214114B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a mixed bed condensate demineralizer containing a granular cation exchange resin and an anion exchange resin, and is particularly suitable for use in condensate desalination in nuclear power generation. Concerning vessels. Conventional nuclear reactor systems, especially boiling water reactors (BWR)
The system consists of the structure shown in Figure 1,
Steam flowing out from the reactor pressure vessel 1 equipped with the reactor purification system 10 flows into the condenser 5 via the high pressure turbine 2, the moisture separator 3 and the low pressure turbine 4, and is condensed in the condenser 5 to form condensate. becomes. The low pressure turbine 4 communicates via a bleed air system 11 with a feed water heater 9, which in turn communicates with a condenser via a heater drain system 12. Therefore, corrosion products (mainly iron oxides, hereinafter referred to as crud) generated in the condenser 5, extraction system 11, feed water heater 9, etc. become impurities in the condensate, and these impurities are removed from the condensate demineralizer. It is removed in 7. The condensate from which impurities have been removed flows into the reactor pressure vessel 1 via the feed water heater 9, and thereafter the condensate is repeatedly circulated in the same manner. In the reactor configured as above, the condensate desalination device 7
The problem is that the crud that was not removed during the process flows into the reactor vessel 1 and becomes activated, and through this activation, it adheres to the equipment and piping, resulting in an increase in the plant dose rate (exposure rate). It's summery. However, recently, it has become clear that reducing the amount of iron brought into the water supply can reduce the rate of increase in plant doses, so the target for iron brought into the water supply has been set at 1 ppb or less. As shown in FIG. 2, the condensate demineralizer 7 includes a container 13 connected to a water supply system 18 and a condensate system 19, an ion exchange resin 15 and a Johnson screen 17 provided on a water sprinkling plate 14 and a batten 16. A condensate system 19 consisting of a structure containing a cladding
The water flows down onto the water sprinkling plate 14 in the container 13, is then sprayed onto the ion exchange resin 15 to remove impurities, and flows out through the Johnson screen 17 into the water supply system 18. The ion exchange resin 15
In BWR, H-type cation exchange resin and OH-type anion exchange resin are used, and the particle size is approximately 300 ~
It is well known that this is a mixed bed that combines a cation exchange resin and an anion exchange resin with approximately the same diameter of 1200 μm. Here, the ion exchange resin whose desalination performance has deteriorated due to the removal of impurity ions and crud caused by leakage of cooling water (mainly seawater) in the condenser, etc. is reversed by air scrubbing etc. in a separately installed resin regeneration system. It is reused after cleaning to remove the adhering crud and recovering the desalting performance by regenerating chemicals. In chemical regeneration, cation exchange resin is regenerated with acid and anion exchange resin with alkali, so the mixed bed type ion exchange resin must be completely separated into cation exchange resin and anion exchange resin before regeneration. It won't happen. Most of the existing plants treat condensate using a single condensate demineralizer using ion exchange resin as mentioned above, which is sufficient to remove impurity ions, but the crud removal ability is low and condensate demineralization is insufficient. The condensate concentration at the outlet of the salter is 5 to 10 ppb. Therefore, the amount of crud (mainly composed of iron oxide) brought into the reactor is 200 ~
It can reach up to 400Kg. As a countermeasure to the above, by lowering the linear flow velocity in the condensate demineralizer, the time that condensate remains in the condensate demineralizer increases the probability of collision between the crud and the ion exchange resin, and the ion exchange resin There is a way to improve the probability of catching crud. However, as a result of testing using this method, the normal linear flow rate of 110 m/
Even if the speed was reduced to 70 m/h, it was not possible to reduce the crud concentration to below 1 ppb.
In this method, it is conceivable that the iron removal performance of the demineralizer will improve if the linear flow velocity is further reduced;
The condensate flow rate is regulated by the plant output, and operating the demineralizer at a low linear flow rate means an increase in the amount of resin, which is undesirable because it increases operating costs and increases the amount of radioactive waste. In view of the above, the present invention aims to significantly reduce the radiation dose rate of the plant by significantly increasing the iron removal ability of the condensate demineralizer. In a mixed bed condensate demineralizer with built-in resin, 297~
a cation exchange resin with a particle size distribution of 1190 μm;
It is characterized in that the particle size distribution of the ion exchange resin mixed with an anion exchange resin having a particle size distribution of 297 to 840 μm is 500 or more in terms of screen index. Embodiments of the present invention will be described below with reference to the drawings. The structure of this example is almost the same as that of the conventional example shown in FIG. The difference is that the proportion of small particle size resin is increased and the particle size distribution of the cation exchange resin is minimized. The relationship between the resin particle size distribution and the iron removal performance of the demineralizer is shown in Figure 3 based on the experimental results. (The experimental conditions were the condensate treatment conditions of a normal demineralizer, and the positive and anion exchange resins with the same particle size distribution were used in the resin amount ratio.
The mixture was filled at a ratio of 1.6/1 and processed at a linear flow rate of 110 m/h. The iron clad concentration of raw water is 15
It was ~20ppb. ) Here, the screen index (hereinafter referred to as SI) is an index that expresses the state of particle size distribution, and a coefficient (screen factor) normalized from the reciprocal of the square of the resin particle size is set, and the percentage of the distribution is Multiply the above coefficient to obtain the SI for the particle size distribution of the entire resin.
It can be obtained by finding and summing them all up.
The relationship between resin particle size distribution, screen, and factor is shown in Table 1.

【table】

[Table] It is clear from Figure 3 that the iron removal performance of the condensate demineralizer can be improved by increasing the SI of the ion exchange resin. Here, the particle size of the ion exchange resin used in the condensate demineralizer is usually 297 to 1190 μm, taking into consideration the pressure loss of the demineralizer and the backwash separation efficiency of the resin.
is within the range of Resins in this particle size range are in the middle of 750
It has an average particle size of ~800μm, and the particle size distribution state has a uniformity coefficient of about 1.5, with an almost symmetrical distribution with a peak near the average particle size, and the distribution shown in Table 2 is common. be. Uniformity coefficient = Diameter of sieve mesh corresponding to cumulative residual content of 40% (
μm)/Diameter of sieve mesh corresponding to 90% cumulative residual content (μm)
)

[Table] As a result, the SI of the resin filled in a normal condensate demineralizer is at most 400, since the proportion of small particles is small.
It can be seen that the degree of Therefore, it can be seen that iron removal performance can be improved by increasing the proportion of small particle size resin and increasing SI. The iron removal performance is improved by increasing the SI, that is, by increasing the proportion of small particle size resin.The reason why the iron removal performance is improved by increasing the proportion of small particle size resin is because the gap in the resin layer is narrowed due to the small particle size of the resin, which has the effect of trapping iron cladding particles. This is thought to be due to the fact that the surface area of the entire resin layer increases due to the smaller resin particle size, which increases the frequency of collisions between the cladding and the resin, and increases the efficiency with which the cladding adheres to the resin surface. FIG. 4 shows the relationship between the ratio of cation exchange resin to anion exchange resin and iron removal performance. (The experimental conditions were the condensate treatment conditions of a normal demineralizer, in which positive and anion exchange resins with the same particle size distribution were mixed and filled, SI = 388, and the linear flow rate was 108 m/h. ) From this, it can be seen that the iron removal performance of the demineralizer is influenced by the ratio of cation exchange resin to anion exchange resin, and that the iron removal performance improves as the ratio of cation exchange resin increases. As shown in Figure 5, this is due to the electrical potential 2 of the cation exchange resin in the neutral region.
0 is negative, whereas the electrical potential 21 of the anion exchange resin is positive and the electrical potential 22 of the iron cladding is positive. Therefore, it is thought that the rate at which clades are attached to and captured by the ion exchange resin is due to the fact that the cation exchange resin has a higher electrostatic attractive force. This shows that in order to improve the iron removal performance of the demineralizer, it is sufficient to increase the proportion of cation exchange resin. However, the role of desalination in condensate purification is not only to remove iron crud, but also to remove impurity ions when seawater leaks from the condenser, allowing the plant to operate for a certain period of time. Therefore, in order to effectively remove impurity ions in a condensate demineralizer, it is necessary to adjust the amount ratio of positive and anion exchange resin in the demineralizer because the impurity ions usually exist in approximately equivalent amounts of cations and anions. Approximately the same amount of cation exchange resin is suitable, and in order to effectively remove iron cladding, the proportion of cation exchange resin is significantly increased.
Decreasing the proportion of anion exchange resin is not preferable because impurity anions tend to leak from the demineralization tower. Therefore, in order to improve the iron removal performance of a demineralizer, the particle size of the cation exchange resin should be reduced to increase the surface area of the cation exchange resin in the resin layer (as a result, the ratio of the amount of cation exchange resin should be reduced). This method is effective and avoids the problem of leakage of impurity anions. This is because the diffusion rate of ions in the liquid is much faster than that of cladding, which is a fine particle, and the ion removal ability is hardly affected by the resin particle size and is determined only by the amount of resin. Even if is large, leakage of impurity anions will not occur. From the above results, to improve the iron removal performance of the demineralizer,
It has become clear that it is effective to increase the SI and decrease the particle size of the cation exchange resin. On the other hand, as a measure to improve desalination performance, there are problems caused by reducing the particle size of the resin as described above.
The following two matters are possible. () Problems with the separability of positive and anion exchange resins during resin cleaning () Problems with increased pressure loss in the demineralizer due to small resin particle size distribution Understanding the backwash separability of ion exchange resins regarding item (1) above As a means to do this, backwash separation can be confirmed by calculating the terminal velocity of the positive and anion exchange resin during sedimentation and examining the relationship between this calculated value and the particle size at that time. In other words, the backwash separability deteriorates in a region where there is no difference in the terminal velocity of the positive and anion exchange resins. Generally, the terminal velocity U of the resin is calculated by the following formula. U = (4g ² (ρ _p - ρ) D ³ /225μρ) ^1/3 ... (1) where ρ _p : resin density, ρ: liquid specific gravity, g: gravity,
D: particle size, μ: liquid viscosity The density of the ion exchange resin is the cation exchange resin
1.28 g/cm ³ , anion exchange resin 1.07 g/cm ³ , and when the terminal velocity U is calculated from the above equation (1) and illustrated, it becomes as shown in Fig. 6, where the solid line 23 is the cation exchange resin, and the broken line is the cation exchange resin. 24 shows the case of anion exchange resin. In backwash separation of cation and anion exchange resins, the terminal velocity must be anion exchange resin < cation exchange resin. In the particle size range of normal ion exchange resins from 297 to 1,190 μm, for example, if the anion exchange resin is 1,190 μm, the terminal velocity of the cation exchange resin with a particle size of 400 μm or less is smaller than that of the anion exchange resin, and the terminal velocity of the anion exchange resin is smaller than that of the anion exchange resin. cannot be separated. Here, the particle size of the anion exchange resin is 840 μm.
If it is below, the cation exchange resin is in the range of 297 to 1190 μm, and the terminal velocity is always cation exchange resin > anion exchange resin, and backwash separation of cation and anion exchange resins can be performed reliably. Regarding the above item (), the relationship between the resin particle size and the differential force loss is directly proportional to SI because the pressure loss of the resin-filled layer is proportional to the reciprocal of the square of the resin particle size. Since the pressure drop is directly proportional to the SI, the upper limit of the SI is determined by the pressure drop allowed in the demineralizer in the condensate purification system. However, as mentioned above, the ion exchange resin SI filled in current demineralizers is about 400, and the pressure loss in the resin layer is about 0.8 Kg/ ^cm2 , so even if the SI is increased to 500, the pressure loss is an increase of 0.1 Kg/cm ² at most, which is within the allowable differential pressure margin and is not an increase in differential pressure that would cause a problem. Here, pressure loss is determined by SI, but since cation exchange resin has a greater contribution to iron removal performance than anion exchange resin even with the same SI, anion exchange resin should be used as much as possible within the allowable SI range. By increasing the particle size of the resin and decreasing the particle size of the cation exchange resin, the effective iron removal performance of the demineralizer can be improved.
In other words, if the particle size of the anion exchange resin is set to 840 μm or less and the particle size of the cation exchange resin is reduced within the allowable SI range, taking into account the separability of the positive and anion exchange resins, effective iron removal performance can be achieved. It is possible to improve this, suppress the rise in differential pressure in the demineralizer, and completely backwash and separate the positive and anion exchange resins. As described above, the present invention increases the proportion of small particle size ion exchange resin and also makes it possible to reduce the particle size of cation exchange resin, which has a large contribution to iron removal performance. It is possible to suppress the increase in pressure loss in the process, improve iron removal performance, and reduce the iron concentration in condensate demineralizer treated water to 1 ppb or less. Therefore, according to the present invention, the radiation dose rate of the plant can be significantly reduced by significantly increasing the iron removal performance of the condensate demineralizer.

[Brief explanation of drawings]

Figures 1 and 2 are a system diagram of a nuclear power plant and a cross-sectional view of its condensate demineralizer, Figure 3 is the screen index and iron removal performance, and Figure 4 is the positive and anion exchange resin amount ratio and removal. Iron performance, Figure 5 shows condensate
FIG. 6 is a diagram showing the relationship between PH and the surface potential of the cladding and resin, and the resin particle size and terminal velocity. 1... Reactor pressure vessel, 2, 4... Turbine,
7...Condensate demineralizer.

Claims (1)

[Scope of Claims] 1. A mixed-bed condensate demineralizer containing a particulate cation exchange resin and an anion exchange resin, a cation exchange resin having a particle size distribution of 297 to 1190 μm and a particle size distribution of 297 to 840 μm. A mixed bed condensate demineralizer characterized in that the particle size distribution of the ion exchange resin mixed with an anion exchange resin having a particle size distribution is 500 or more in terms of screen index.