Patent Number: 047643382
Section: description

PREFERRED EMBODIMENTS OF THE INVENTION One embodiment of the present invention will be described below, referring to FIG. 3. Corrosion products brought into nuclear reactor pressure vessel 7 through feed water system 11 deposit on the surfaces of fuel rods and are radioactivated into radioactive corrosion products such as cobalt-60, etc. thereon. A portion of the radioactive corrosion products is again dissolved into core water to migrate through the core water, and deposit and accumulate on the machine member and piping in recycle system 13 such as recycle pump 2, etc. in nuclear reactor housing 12, causing an increase in the dose rate. The corrosion products on the surfaces of fuel rods are so-called cruds containing iron oxide as the major component, which mainly takes a chemical form of .alpha.-Fe.sub.2 O.sub.3, i.e. hematite. Minor components such as nickel, cobalt, etc. are adsorbed onto the hematite to form nickel ferrite, cobalt ferrite, etc. FIG. 4 shows dependency on pH of dissolution rate of cobalt from cobalt ferrite having an average particle size of 1 .mu.m (.mu.g/g.hr). When the pH is lowered, that is, shifted to the acidic side, it can be seen therefrom that the cobalt dissolution rate is abruptly increased. FIG. 5 shows cobalt-60 concentration in core water calculated on the basis on pH of dissolution rate of cobalt from cobalt ferrite shown in FIG. 4. The main source for cobalt-60 is the surfaces of fuel rods, as described above, and in spite of remarkable change in the amount of dissolved cobalt, the cobalt dissolution rate itself is too low to change the total amount of cobalt-60 retained on the fuel rods, and thus the total amount of cobalt-60 is kept substantially constant. The cobalt-60 concentration in core water is proportional to the amount of cobalt-60 dissolved from fuel rods, that is, to the amount of cobalt from cobalt ferrite. When the pH in core water is shifted from the neutral to the acidic side, that is, when it is less than pH 7, the cobalt-60 concentration in core water is abruptly increased, whereas, when the pH is shifted to the alkaline side, that is, when it is more than 7, the cobalt-60 concentration is considerably reduced, as shown in FIG. 5. In FIG. 3, the pH in core water is adjusted by adding an involatile alkali such as NaOH to the core water through alkaline chemical injection line 14, where the pH in core water depends on the amount of injected alkali. A portion of the injected alkali is removed in reactor-purifying unit 8, another portion thereof is carried over by steam to turbine 3 and removed from the core water, and further portion thereof is adsorbed onto the machine members and piping of the primary cooling system. Generally, the involatile alkali is removed mainly in the reactor-purifying unit among said three removing means. Let the amount of injected alkali be S (moles/hr), the alkali concentration in core water be C (moles/l), the density of core water be .gamma. (kg/l), the flow rate in the reactor-purifying unit be G.sub.C (kg/hr), the percent alkali removal in the reactor-purifying unit be .epsilon., the flow rate of main steam be Gs (kg/hr), the percent alkali carry-over by steam be .alpha., and the total deposition rate to the machine members and piping be .beta. (l/hr). The alkali concentration in core water can be obtained according to the following formula (1): ##EQU1## In the case of involatile alkali, for example, NaOH, EQU .epsilon./.gamma.G.sub.C &gt;&gt;.alpha./.gamma.G.sub.s, .beta. On the other hand, in the case of volatile alkali, for example, NH.sub.4 OH, EQU .alpha./.gamma.G.sub.s &gt;&gt;.epsilon./.gamma.G.sub.C, .beta. Generally, .epsilon.G.sub.C depends upon the desired degree of removing metallic impurities from core water in the plant. In the case of adding a volatile alkali, the alkali is carried over by the steam, and thus to keep pH in the core water, that is, the alkali concentration, constant, a larger amount of the alkali must be added than in the case of adding the involatile alkali. Thus, it is preferable to add an involatile alkali rather than a volatile alkali. The involatile alkali for this purpose includes alkali metal hydroxides such as NaOH and LiOH, alkaline earth metal hydroxides such as Ca(OH).sub.2, and organic alkali compounds. The organic alkali compounds are liable to disappear through radiolysis in the nuclear reactor, and the alkaline earth metal hydroxides are liable to form insoluble impurities, and are readily depositable mainly on fuel rods. On the other hand, the alkali metal hydroxides are stable at a high temperaure even under irradiation of radioactive rays, and are easiest to handle. Correlation between the amount of NaOH added as an alkali and pH is given below. The NaOH concentration C in core water and pH value H are given according to the following equation (2): EQU H=log {(C+10.sup.-7)10.sup.14 }=14+log (C+10.sup.-7) (2) In the standard type BWR (MWe), the flow rate G.sub.C through the reactor-purifying unit is about 100 tons/hr, and when the percent NaOH removal is presumed to be 100% in the reactor-purifying unit, correlation between the amount of injected NaOH and pH as given in FIG. 6 will be obtained. The carry-over of NaOH by the main steam is negligible in view of the NaOH material balance, but is not always preferable from the viewpoint of turbine side, particularly because Na is radioactivated in the reactor to form .sup.24 Na having a half-life of 15 hours. The carry-over rate is increased proportionally to an increasing NaOH concentration in the core water. Correlation between the .sup.24 Na concentration in condensed water and pH on the basis of the upper limit value of the carry-over rate of involatile component is shown in FIG. 7. When pH is higher than 8.5 in the case of adding NaOH, the radioactivity of condensed water reaches even 10.sup.-4 .mu.Ci/ml, and thus more than the necessary pH is not preferable for the control of condensed water radioactivity. Thus, it is important from the viewpoint of controlling an increase in the dose rate in the primary cooling system and maintenance of the entire system to keep the pH of core water at 7-8.5. It is possible to control dissolution of cobalt-60 from fuel rods without any increase in the radioactivity level of condensed water by keeping the pH of core water in said range, and consequently the cobalt-60 concentration in the core water and furthermore the surface dose rate in the machine members and piping of the primary cooling system can be reduced. Particularly preferable pH range is 7.5-8.0. There is a correlation between the .sup.24 Na concentrations in core water and pH in core water in the case of adding NaOH as given in FIG. 7. The correlation can be given according to the following equation (3) EQU A=2.times.10.sup.16 .delta..phi.(.lambda.Vc/Gc)C (3) where .phi.: average thermal neutron flux in nuclear reactor (n/cm.sup.2.sec.) PA1 .delta.: thermal neutron cross-section of .sup.23 Na (cm.sup.2) PA1 Vc: cooling water holdup in nuclear reactor (kg) PA1 .lambda.: decay constant of .sup.24 Na (hr.sup.-1) PA1 A: .sup.24 Na concentration in core water (.mu.Ci/ml) PA1 C.sub.Fe : concentration of Fe.sup.+2 as typical cation in condensed water (ppb) PA1 .epsilon.: probability of formed NaOH passable through cation exchange resin layer without trapping in the resin layer By monitoring .sup.24 Na concentration from the correlation between .sup.24 Na concentration and pH in core water given by equations (2) and (3), the core water pH can be determined. Another embodiment of adjusting pH of core water by adding an alkali is shown in FIG. 8, where cation exchange resin represented by R--(SO.sub.3 H).sub.2 is filled in condensed water desalter 6 as a filter, and a portion of the cation exchange resin is substituted with Na as a typical alkali metal. When the cation exchange resin undergoes ion exchange with Fe.sup.+2 as a typical cation, an alkali is released according to the following reactions: In the ordinary cation exchange resin, EQU R--(SO.sub.3 H).sub.2 +Fe(OH).sub.2 .fwdarw.R--(SO.sub.3).sub.2 Fe+2H.sub.2 O In the Na-substituted form, cation exchange resin, EQU R--(SO.sub.3 Na).sub.2 +Fe(OH).sub.2 .fwdarw.R--(SO.sub.3).sub.2 Fe+2NaOH If a mixing ratio of Na-substituted form cation resin to the total cation exchange resin is x, the concentration C.sub.Na of NaOH leaking from the outlet of condensed water desalter according to the present embodiment can be obtained according to the following equation (4): EQU C.sub.Na =4.times.10.sup.-8 .epsilon..multidot.xC.sub.Fe (4) where .epsilon. depends on the properties of cation exchange resin and is estimated to have a value of up to 0.1. When the cation concentration is constant, pH of core water can be controlled to any desired value by controlling the mixing ratio x of Na-substituted form cation exchange resin from the following correlation equations (5), (6) and (7): EQU S=Gf C.sub.Na (5) EQU C=S/.epsilon.Gc (6) EQU H=14+log (C+10.sup.-7) (7) where Gc, C, S and H are the same meanings as defined in the equations (1) and (2), and Gf is a feed water rate. Under typical BWR conditions, Gc/Gf is a value of up to 0.02, C.sub.Fe is a value of up to 1.0, and .epsilon. is a value of up to 0.1, where correlation between x and H is shown in FIG. 9. The desired pH of 7.0 to 8.5 can be continuously maintained by setting the mixing ratio x of Na-substituted form cation exchange resin to 0.1 to 0.5 according to equation (8) as will be given later without providing a special means for injecting an alkali. Against any fluctuation in the cation concentration C.sub.Fe in condensed water the pH can be kept in said desired range by changing the mixing ratio of the Na-substituted form cation exchange resin. Furthermore, the pH can be kept in said desired range by keeping the .sup.24 Na level in core water constant as shown above. A preferable embodiment for replacing a portion of cation exchange resin with the Na-substituted form cation exchange resin is shown below. Usually, a condensed water desalter uses a mixture of cation exchange resin and anion exchange resin, and regenerating treatment for recovering the ion exchanging capacity is carried out by separating the cation exchange resin and the anion exchange resin from each other by difference in specific gravity and chemically regenerating the cation exchange resin with H.sub.2 SO.sub.4 and the anion exchange resin with NaOH. FIG. 10 schematically shows the present embodiment. The mixture of ion exchange resins is transferred from condensed water desalter 21 to anion-cation separating column 22, where the anion exchange resin and the cation exchange resin are separated from each other by the difference in specific gravity. The separated anion exchange resin is led to anion exchange resin-regenerating column 23, while the cation exchange resin to cation exchange resin-regenerating column 24, where the former is regenerated with NaOH, and the latter with H.sub.2 SO.sub.4. The regeneration reactions proceed as follows: In the cation exchange resin-regenerating column, EQU R--(SO.sub.3).sub.2 Fe+H.sub.2 SO.sub.4 .revreaction.R--(SO.sub.3 H).sub.2 +FeSO.sub.2 The reaction usually proceeds from the right side to the left side, but the reversed proceding, that is, from the left side to the right side, is possible by using about 1N H.sub.2 SO.sub.4, whereby the cations trapped on the resin can be released as sulfate. Separation of anion exchange resin from cation exchange resin in the anion exchange resin-cation exchange resin separating column is carried out by the difference in specific gravity between the two resins, and the cation exchange resin having a larger specific gravity is withdrawn from the bottom of the separating column, whereas the anion exchange resin having a smaller specific gravity is withdrawn through nozzle 26 on separation level 25 at an intermediate height of the separating column. Separation level 25 differs from one plant to another and can be provided near the bottom or the top upon proper selection in view of the predetermined mixing ratio of anion exchange resin to cation exchange resin. In the present embodiment, the anion exchange resin and the cation exchange resin are mixed at an mixing ratio y' by volume in excess of the predetermined mixing ratio y of the anion exchange resin to the cation exchange resin. As a result, 100% cation exchange resin can be withdrawn from the bottom and a mixture of the anion exchange resin and the cation exchange resin from the separating level. In the anion exchange resin-regenerating column, regeneration is carried out with NaOH, where regeneration of cation exchange resin proceeds as follows: EQU R--(SO.sub.3).sub.2 Fe+NaOH.revreaction.R--(SO.sub.3 Na).sub.2 +Fe(OH).sub.2 EQU R--(SO.sub.3 H).sub.2 +NaOH.revreaction.R--(SO.sub.3 Na).sub.2 +2H.sub.2 O By returning the regenerated anion exchange resin and cation exchange resin to the condensed water desalter, the mixing ratio x of the Na-substituted form cation exchange resin in the condensed water desalter will be as follows: ##EQU2## To meet a fluctuation in the cation concentration in the condensed water or to meet the pH of core water to be predetermined, the mixing ratio x can be adjusted by replacing a portion of the resin mixture with anion exchange resin in the anion exchange resin-regenerating column before the regeneration, thereby reducing the ratio x, or by replacing it with cation exchange resin, thereby increasing the ratio x. By replacing the regenerating solution for the anion exchange resin-regenerating column with a solution of LiOH, or others, leakage of any desired alkali species is made possible. According to further embodiment of the present invention, a portion of cation exchange resin is replaced with Na-substituted form cation exchange resin in the same manner as shown in FIG. 10 at the regeneration of reactor-purifying desalter 8 in place of the condensed water desalter shown in FIG. 8. It is also possible to add NaOH to core water by mixing Na-substituted form cation exchange resin with the cation exchange resin for both condensed water desalter and reactor-purifying desalter. According to still further embodiment of the present invention, a portion of cation exchange resin as powdery resin used in the condensed water desalter or reactor-purifying desalter can be replaced with Na-substituted form cation exchange resin. The non-regenerative use of powdery resin is usual, and thus a portion of Na-substituted form cation exchange resin is made ready before precoating and can be used in mixture with the ordinary H-form cation exchange resin. According to the present invention, it is possible to suppress any increase in the concentration of radioactive corrosion products in core water in a direct cycle type, light water-cooled nuclear reactor without any substantial change in the plant hardware even if there are such disturbances as a resin leakage or lowering of pH in the core water. Particularly, the present invention can be readily applied to the existing plants without any substantial change in the plant hardware. This is a remarkable advantage of the present invention.