JP2523696B2 - Direct cycle nuclear power plant - Google Patents

Description

TECHNICAL FIELD The present invention relates to a direct cycle nuclear power plant, a method of operating the same, and a fuel rod used therefor. A main steam system resulting from carryover of radioactive nitrogen atoms ¹⁶ N. And turbine system dose rate reduction.

[Prior Art] In a direct cycle reactor such as a boiling water reactor or an advanced converter (ATR), the turbine is directly driven by the steam generated in the reactor due to the nuclear fission reaction heat.
The carry-over of radioactive nitrogen atoms ¹⁶ N generated as a result of activation of oxygen atoms in water molecules of reactor water (hereinafter referred to as reactor water) in the reactor core by neutron irradiation causes turbine system dose rate. . The dose rate of the turbine system with ¹⁶ N is several R / h in terms of the surface dose rate of the main steam pipe.
Reduction is necessary in one respect.

(1) Reduction of radiation exposure during inspection work during reactor operation.

(2) Dose rate regulation at the boundary of the nuclear power plant site (skyshine regulation).

For the above-mentioned purpose, conventionally, as shown in FIG. 2, both sides and the upper part of the main steam pipe 9 have been shielded by the iron plate 14,
Measures from the viewpoint of suppressing the generation and release of ¹⁶ N in the reactor have not been implemented because the understanding of the phenomenon itself has not progressed.

In plants where stress and sensitized stainless steel stress corrosion cracking prevention measures, such as those used in Japan's nuclear reactors, are not implemented, hydrogen injection is widely used to reduce the oxygen concentration in reactor water. It is being applied, but in that case, as shown in Fig. 3, the problem that the concentration of ¹⁶ N in the main steam increases as the hydrogen injection amount increases has been found. Cannot be implemented in some plants. The mechanism of ¹⁶ N increase by hydrogen injection has not been clarified. For this purpose, for example,
The nuclear power plants described in JP-A-57-194399 or JP-A-62-151797 have been proposed.

[Problems to be Solved by the Invention] Among the above-mentioned conventional techniques, the one disclosed in JP-A-57-194399 is provided with a device for removing N ₂ . However, substantially ¹⁶ N does not exist in the chemical form of N ₂ , but is entrained in water vapor in the form of NO _x . Therefore, it is extremely impossible to effectively remove the ¹⁶ N with the N ₂ removing device.
Further, the N ₂ gas injection apparatus described in JP-A-62-151797 has a drawback that the isotope exchange reaction efficiency of ¹⁴ NH ₃ and ¹⁶ NH ₃ is poor.

Meanwhile, ¹⁶ N of the turbine system is in this, but shielding by the iron plate was the only measures conventionally have the following problems.

(1) The energy of gamma rays emitted from ¹⁶ N is 6 to 7
Since it is as high as MeV, the thickness of the iron plate required for shielding is about 150 mm. Therefore, the total weight of the iron plate covering the main steam pipe and the turbine main body is several hundred tons, and it is necessary to make the turbine building itself capable of supporting such heavy objects. You will be constrained.

(2) During regular inspection, the main steam piping and the turbine are dismantled and repaired. At this time, the heavier the shield iron plate, the worse the workability.

(3) When hydrogen is injected, the dose rate can be increased to about 5 times the normal rate as seen in FIG. When the dose rate is to be reduced by strengthening the shielding, it is necessary to include even a major remodeling of the turbine building itself in some cases due to the above-mentioned restrictions such as weight.

As shown above, the ¹⁶ N dose rate reduction measure by strengthening shielding cannot be an essential measure in various aspects.

It is an object of the present invention to prevent stress corrosion cracking and reduce the amount of ¹⁶ N generated in reactor water transferred to the main steam system.

[Means for solving problems]

The purpose is to inject a substance containing nitrite ions or a substance that produces nitrite ions into the primary cooling water, a measuring unit for measuring the nitrite ion concentration in the primary cooling water, and the measuring unit And a control means for controlling the injection amount of the substance based on the nitrite ion concentration.

Further, it is achieved by providing a suppressing means for suppressing the transfer of radioactive nitrogen oxides in the reactor water into the water vapor by injecting PbO ₂ into the primary cooling water.

[Action]

 The operation of the present invention will be described below.

The present invention firstly uses the property that ¹⁶ N converts to a non-radioactive oxygen atom with a half-life of 7.2 seconds. Ie ¹⁶ N
Can be held in the liquid phase or solid phase in the pressure vessel for a time corresponding to the half-life or longer,
By itself, ¹⁶ N in main steam can be reduced to a concentration of less than half. The means for holding the liquid phase or the solid phase is based on the following findings. That is, (1) The present inventors manufactured an experimental device as shown in FIG. 4 in order to clarify the mechanism of the main steam system of ¹⁶ N. That is, nitrogen and hydrogen were dissolved in the autoclave 17 in advance and after reaching a predetermined temperature, irradiation with gamma rays was performed, and the concentration of ammonia produced as a result of the reaction of hydrogen and nitrogen with the radiolysis product of water was measured. The result is 1
5 to 60 for each temperature of 5, 60, 100, 150, 200 ℃
This is as indicated by the circles in the figure.

(2) On the basis of the experimental results obtained here, the present inventors found a reaction scheme consisting of the main components shown in FIG.
Numerical simulation was performed by computer using the reaction rate constants of 75 reactions. The results are shown in Figs.
It is shown by the solid line in the figure. The simulation result and the measured value agreed with a factor of 2 over the entire temperature range. Also, there is a good agreement that the increase in the concentration of ammonia at low temperature tends to be saturated, but that at 200 ° C it tends to increase linearly.

(3) Figure 11 shows the results of simulating the concentration increase rate of the turbine system ¹⁶ N during hydrogen injection experiments in European and American plants using the same reaction scheme. The calculated values are in good agreement with the measured values for the rate of rise and the hydrogen concentration at the core entrance where the rise begins, compared to the measured values for ordinary plants, and are sufficient as a dose rate evaluation model for the reaction scheme ¹⁶ N given here. Can be said to be realistic.

(4) The findings newly obtained from the results shown in FIG.
The chemical form of ¹⁶ N transferred to the turbine system is that it is mostly nitric oxide and not ammonia as previously thought.

This will be described with reference to FIG. 5 showing the mutual relation of the components.

The ¹⁶ N formed from oxygen atoms in water molecules in the core reacts with hydrogen atoms in the radiolysis products of water to NH and is further reduced to ammonia, which is the rate of this pathway. Since the constant is relatively slow, NO formed by the reaction of N and OH is the main component initially formed from ¹⁶ N. Since NO is relatively stable and is also a gas, NO is released in water vapor.

On the other hand, when the atmosphere is oxidative, NO is oxidized by OH radicals or the like and converted into nonvolatile nitrous acid, nitric acid or the like. During normal operation without hydrogen injection, about 80% of ¹⁶ N is retained in water as nitric acid. Hydrogen injected with implanting hydrogen, nitrate with hydrogen atoms formed by _{H 2 + OH → H + H} 2 O ...... (1) is converted to nitrite, NO concentration is increased in equilibrium with this, therefore, ¹⁶ The concentration of N in main steam also increases. However, the hydrogen atom concentration does not immediately increase due to hydrogen injection, and until a certain concentration is reached, H + O ₂ → HO ₂ (2) H + H ₂ O ₂ → H ₂ O + OH (3) Since it is consumed in the reaction with hydrogen peroxide, it does not increase until the oxygen and hydrogen peroxide concentrations become sufficiently low. This is also shown in FIG. It can be seen that the dose rate begins to rise from the point where the concentration of hydrogen atoms begins to increase significantly. To summarize the above, (i) ¹⁶ N is released in the main steam in the chemical form of ¹⁶ NO.

(Ii) As the concentration of hydrogen atoms increases, the amount of ¹⁶ NO released increases.

Therefore, the reduction of ¹⁶ N is achieved by the following means. That is, decrease the hydrogen atom concentration or increase the OH radical concentration.
¹⁶ Reduce the equilibrium concentration of NO in water.

¹⁶ NO is converted to a non-molecular component by reacting with some additive component.

¹⁶ NO is trapped in the adsorbent for a certain time or more in water.

Increase the ¹⁴ NO concentration, which has the same chemical properties, in advance,
Suppresses the transfer (release) of ¹⁶ NO from the liquid phase to the water vapor phase due to the gas-liquid equilibrium relationship.

The above measures may be effective only in the region between the core portion and the separator outlet. That is, the two-phase flow section is up to the separator outlet, and ¹⁶ N retained in the liquid phase at the separator outlet becomes stable ¹⁶ O due to radioactive decay while circulating in the primary system regardless of its chemical form. Therefore, there is no effect on the dose rate of the main steam system. That is, if the transfer of ¹⁶ N from the liquid phase to the gas phase in the flow path to the separator is delayed on average by the means of (i) and (iii) above, ¹⁶ N reduction of the main steam system can be achieved.

Below, the effectiveness of the measures that can be considered especially from the viewpoint of reducing the hydrogen atom concentration or increasing the OH concentration is shown. here,
NO, NO ₂ , and NO ₂ ⁻ are selected as examples, and the ¹⁶ NO reduction effect is evaluated by analysis.

Figures 12 to 14 show ¹⁶ NO concentration in water vapor, oxygen in reactor water, hydrogen peroxide,
The calculated values of nitric acid and nitrite concentration are shown. In either case, it can be seen that if the core inlet concentration is kept at 10 ^-6 to 10 ^-5 mol / l, the ¹⁶ NO concentration can be reduced by about one digit.
Furthermore, it can be seen that the effect of reducing oxygen and hydrogen peroxide can be expected. On the other hand, if it is injected up to about 10 ^-4 mol / l, nitric acid and nitrous acid will become high in concentration, and oxygen concentration will also increase, so if too much is injected, there is a problem with the stress corrosion rate of the material. I understand. The above results can be interpreted as follows. That is, in the process in which the concentration of NO increases as a result of the injected component reacting with the radiation product of water, for example, NO ₂ ⁻ + H → NO + OH ⁻ (4) NO ₂ + H → NO ₂ ⁻ + H ⁺ … (5 ) And the like reduce the equilibrium concentration of hydrogen atoms, and the reactions such as H + OH → H ₂ O (6) are less likely to proceed, and the OH radical concentration relatively increases. Therefore even ¹⁶ N generated by the reactor core becomes ^{16 NO, 16 NO + OH →} H + + 16 NO 2 - ... easily take the form of nitrous acid by (7), Once in nitrite, ¹⁶ NO ₂ ^- + H → ¹⁶ NO + OH ^- ... reaction hardly proceeds because of the low concentration of hydrogen atoms (8). As described above, the equilibrium concentration of ¹⁶ NO is lowered, so the concentration of ¹⁶ NO in main steam is also reduced. On the other hand, the decrease in hydrogen peroxide and oxygen concentrations is considered to be due to reactions such as NO ₂ ⁻ + H ₂ O ₂ → NO ₃ ⁻ + H ₂ O (9).

Next, Fig. 15 shows the solution when N ₂ is injected. In the case of N ₂ , it is not as significant as the above three compounds. It can be considered that this is because N ₂ partially changes to NO, NO _2, etc., but is not efficient and has a high rate of leaving N ₂ as it is to steam. If there is no efficient of N _2, readily available and inexpensive.

In addition to these, there are components such as silver ions, chromic acid, and copper that have a fast reaction rate with hydrogen atoms, and similar ¹⁶ NO reduction effects can be expected by adding these components in reactor water.

From the above results, in the present invention, the most effective method for reducing hydrogen atoms is the method of injecting NO ₂ or NO ₂ ⁻ .

Even increasing the OH radical concentration instead of reducing the hydrogen atom ^{concentration, 16 NO + OH → 16 NO} 2 - + H + It is also possible to reduce the ¹⁶ NO concentration by reaction ...... (10). As an example, Fig. 16 shows the analysis results when N ₂ O and H ₂ O _{2 were} injected.
It is shown in FIG. Each injected component has the effect of increasing the OH radical concentration in water by the reaction of N ₂ O + e ⁻ _aq + H ₂ O → N ₂ + OH + OH ⁻ (11) H ₂ O ₂ → 2OH (12). The figure shows that increasing the OH radical concentration also has a certain effect on reducing ¹⁶ NO. H ₂ O ₂ injection does not appear as a large effect, but it is because it is thermally decomposed into oxygen by the time it reaches the core, and it is released into the gas phase in the core. Therefore, hydrogen peroxide should be injected as close to the core as possible. In the above equation, e ^- _aq is called a hydrated electron and is always present under irradiation due to radiolysis of water. Since hydrogen peroxide is also always present under irradiation, it is possible to increase the OH radical concentration by promoting the decomposition reaction (12) of hydrogen peroxide using a catalyst such as platinum or a transition metal.

FIG. 18 shows the ¹⁶ NO concentration when the decomposition rate of ordinary hydrogen peroxide is multiplied by the numerical value shown on the horizontal axis, and although it is not remarkable, the reducing effect is shown.

The above-mentioned method of decreasing the hydrogen atom concentration or increasing the OH radical concentration is not preferable under the condition that a large amount of hydrogen is present. This is because the added component reacts with hydrogen atoms and the effect of reducing the dissolved oxygen concentration due to hydrogen injection is impaired. Hydrogen is generated in the nuclear reactor by radiolysis of water, but in the present invention, it is particularly preferable to carry out under conditions where at least such naturally occurring amount of hydrogen is suppressed.

The present invention can also achieve ¹⁶ N suppression by the following method. That is, since it is known that ¹⁶ N is released in the form of ¹⁶ NO, a component that chemically reacts with NO to convert it into a non-volatile substance, such as FeSO ₄ which forms a complex with NO.
It is also possible to reduce ¹⁶ N by adding such substances to reactor water. In addition, absorption of nitrogen oxides such as PbO ₂ , aluminum silicate, and activated carbon, addition of adsorbent materials to reactor water, or coating of the material surface around the core with ¹⁶ N can be achieved.

[Embodiment] An embodiment of the present invention will be described below with reference to FIG. FIG. 1 shows a schematic configuration of a direct nuclear power plant at the time of injection. The nitrous acid injection device 1 is a water supply system 7, a reactor cleaning system 6, a control rod cooling device 12, an in-core instrumentation pipe 11, a core. It is connected to the emergency cooling system 8 or the like. In principle, these injection points have the same effect regardless of which one is selected, and any one of them may be used, but as mentioned above, it depends on some factors such as hydrogen peroxide before reaching the core 4. If the shape is likely to change, the injection point is preferably close to the core 4, such as the control rod cooling device 12 and the in-core instrumentation pipe 11. Since the injection component can be expected to accumulate in the primary cooling system, it is not necessary to inject a large amount from the beginning, and the monitor device 13 measures the monitor amount corresponding to the injection component such as nitric acid, nitrous acid, and hydrogen peroxide, The optimum injection rate and injection amount may be set based on changes in the surface dose rate of the main steam pipe 9. When nitrogen oxides such as NO and N ₂ O are injected, NO _x is released to the off-gas system, so it may be better to provide a NO _x treatment device.

FIG. 19 is a block diagram showing the outline of the liquid injection device, in which water containing nitric acid, nitrous acid, hydrogen peroxide, etc. is stored in the storage tank 26 and injected into the reactor primary system piping 27 by the water injection pump 25. To be done. When nitric acid or nitrous acid is used, nitric acid or nitrous acid is formed in water in the corresponding nitrogen oxide gas and then injected. When nitric acid, nitrous acid, etc. are injected,
0.3-1 Nm ³ / h of NO is released every hour. Therefore, the injection amount may be an amount commensurate with this loss, and the maximum daily dose is 20 to ³⁰ Nm ³ . The injection rate can be controlled by changing the opening of the valve 24.

FIG. 20 shows a monitor system for measuring the concentration of nitrous acid. The water sampled from the reactor primary system piping 22 is partially bypassed while the rest is a high-pressure constant-flow pump.
The mixture is passed through an ion chromatograph consisting of 30, a separation column 31, and a conductivity meter 32 to separate and quantify nitric acid and nitrous acid.

FIG. 21 shows the structure of an injection device for injecting an NO adsorbent or an absorbent, etc.
The absorbent material is stored in the container 36 and appropriately supplied to the storage tank 37. In the storage tank 37, the water is stirred by the stirrer 38 in order to prevent the additive components from precipitating. This suspension water is heated and injected into the reactor primary system piping 22 similarly to the solution injection device shown in FIG.

FIG. 22 shows a method for accelerating the decomposition of hydrogen peroxide generated as a result of radiolysis of water. The outer surface of the fuel rod cladding tube 40 has a catalyst for decomposing hydrogen peroxide such as platinum or transition metal. The layer 41 is formed. As a method of using a catalyst for decomposing hydrogen peroxide, apart from this, a decomposition catalyst powder may be dispersed in reactor water.

According to the above embodiment, the dose rate can be reduced to about 1/10. Therefore, there are advantages that bring about the following beneficial results.

(1) It is possible to reduce radiation exposure during inspection work during operation of the Turbine building reactor.

(2) It is possible to improve the work efficiency and reduce the design strength of the turbine building by reducing the weight or disposing of the shielding iron plate around the turbine.

(3) The efficiency of regular inspection work is improved by making the turbine building an uncontrolled area.

(4) To reduce the dose rate at the boundary of the nuclear power plant site, it is possible to construct 4 to 5 plants on the site of the existing plant.

In addition, the addition of nitrogen oxides has the effect of reducing oxygen and hydrogen peroxide in the reactor water, which is also advantageous in terms of corrosion resistance of the material.

〔The invention's effect〕

According to the present invention, during the operation of the direct cycle nuclear power plant, the oxygen concentration in the reactor water can be suppressed to such an extent that it does not cause a problem with stress corrosion cracking of the material, and ¹⁶ N is transferred from the reactor water to the main steam system. Since the amount of ¹⁶ NO, which is the main component of the, can be reliably reduced, carryover of ¹⁶ N to the turbine system can be significantly suppressed and the dose rate can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing an arrangement of a shielding iron plate around a turbine, and FIG. 3 is a dose rate change of a turbine system during hydrogen injection in a boiling water reactor. Fig. 4 is a block diagram of an experimental apparatus prepared for evaluating radiolysis of nitrogen-dissolved water, Fig. 5 is a diagram showing the relationship between nitrogen compounds, and Figs. It shows the measured value of ammonia concentration generated in water when nitrogen and hydrogen are added together with the result of numerical simulation by computer. Water temperature is 15, 60, 100, 1 in order from FIG.
Fig. 11 shows the results at 50 and 200 ° C, and Fig. 11 shows the calculated values of the increase rate of ¹⁶ NO in the turbine system during hydrogen injection experiments in boiling water reactors and the calculated values of the concentration of hydrogen atoms in the reactor water. Figures shown in comparison with Fig. 12 to Fig. 16 show various nitrogen oxides, ¹⁶ NO concentration in water vapor when nitrogen molecules are injected, oxygen in reactor water,
FIG. 17 is a diagram showing changes in the concentrations of hydrogen peroxide, nitric acid, and nitrous acid, and injecting components in order from FIG. 12, NO, NO ₂ , NO ₂ ⁻ , N ₂ , and N ₂ O, FIG. 17 Figure ¹⁶ shows the ¹⁶ NO concentration in main steam when hydrogen peroxide was injected, and the changes in oxygen and hydrogen peroxide concentrations in the reactor water. Figure 18 shows the concentration of main steam in the main steam when the decomposition reaction of hydrogen peroxide was accelerated. ¹⁶ NO concentration, a diagram showing changes in oxygen and hydrogen peroxide concentrations in reactor water, FIG. 19 is a diagram showing a solution injection device, FIG. 20 is a diagram showing a nitrite concentration monitoring device, FIG.
Figure 21 shows a device for adding powdered ingredients,
FIG. 22 is a diagram showing a method for promoting decomposition of hydrogen peroxide. 1 ... Injection device, 2 ... Pressure vessel, 3 ... NO _x treatment device, 4 ...
Core, 5 ... Recirculation pipe, 6 ... Reactor purification system, 7 ... Water supply pipe,
8 ... Emergency core cooling system, 9 ... Main steam pipe, 10 ... Turbine, 11 ... Reactor instrumentation pipe, 12 ... Control rod cooling system, 13 ... Water quality monitoring device, 14 ... Shield iron plate, 15 ... Gamma ray source, 16 ... Heater, 17 ... Autoclave, 18 ... Water quality measuring device, 19 ... Liquid feed pump, 20 ... Solution storage tank, 21 ... Gamma ray irradiation chamber, 22
… Primary reactor system piping, 23… Heat exchanger, 24… Flow control valve, 25…
Liquid feed pump, 26 ... Solution storage tank, 27 ... Nitrogen oxide gas cylinder, 28 ... Valve, 29 ... Heat exchanger, 30 ... High pressure pump, 31 ... Separation column, 32 ... Conductivity meter, 33 ... Ion chromatograph, 34
... Heat exchanger, 35 ... Liquid feed pump, 36 ... Catalyst, adsorbent filling tank, 37 ... Solution storage tank, 38 ... Stirrer, 39 ... Fuel pellet, 40 ... Fuel rod cladding tube, 41 ... Catalyst layer.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shoji Sakagami 1168 Moriyama-cho, Hitachi, Ibaraki Hitachi Energy Research Institute Co., Ltd. (72) Shunsuke Uchida 1168 Moriyama-cho, Hitachi, Ibaraki Hitachi Energy Research Co., Ltd. In-house (72) Inventor Minoru Miki 3-1-1, Saiwaicho, Hitachi, Ibaraki Inside Hitachi factory, Hitachi Ltd. (56) References JP 62-151797 (JP, A) JP 60-201298 ( JP, A) JP 59-27295 (JP, A) JP 60-41755 (JP, B2)

Claims (3)

(57) [Claims] 1. An injection means for injecting a substance containing nitrite ion or a substance producing nitrite ion into primary cooling water,
A measuring means for measuring the nitrite ion concentration in the primary cooling water, and a control means for controlling the injection amount of the substance based on the nitrite ion concentration measured by the measuring means. Cycle type nuclear power plant. 2. The direct cycle nuclear power plant according to claim 1, wherein the substance is nitrogen oxide. 3. A direct cycle nuclear power plant equipped with a suppressing means for suppressing transfer of radioactive nitrogen oxides in reactor water into steam by injecting PbO ₂ into primary cooling water. plant.