SE503114C2 - Method of operation of nuclear power plants - Google Patents

Abstract

The operation of a power station having a boiling water reactor (1) with fission zone(s) (2) in a reactor vessel, in which steam is generated; a steam turbine (7), in which the energy in the steam is (partly) utilised; a condenser (8), in which the steam is condensed after passing through the turbine; a filter (9) for purifying the condensate; pumps (10,14) for recycling the condensate to the reactor vessel; and cycle(s) (4) for circulating the reactor water and cooling the fission zone. The water (15) in the reactor vessel is treated with an additive (I) which increases the pH. (I) contains an alkalising agent (II) which is not volatile under the conditions prevailing in the reactor. (II) is dosed in an amt. low enough to keep the pH below 7 at the prevailing temp. (Tp), and is enriched in the water close to the fuel rods, so that the pH of this water is more than 7 at Tp.

Description

10 15 20 25 30 35 503114 2 ducts in said circuits. Among other things, arises corrosion products in the form of oxides containing one or several of the metals included in construction materials such as occurs in the circulatory system, such as mainly iron but also nickel, cobalt, chromium, copper, titanium, molybdenum, tungsten, zinc, manganese and zirconium. In addition, contaminants can come into the circulation circuits through leakage. In the event that one reactor tank with nuclear fuels in the circulation circuit becomes the corrosion products are radioactive. To in connection with the reactor reduce the solubility of corrosion products and thereby suppressing the dissolution of corrosion products which deposited or formed on, for example, fuel elements in reactor tank and suppressing continued corrosion raises the pH the value of the water present in the reactor tank, the reactor water.

Such a pH increase is preferably obtained by adding an alkalizing agent. Due to the high water temperature ration which resides in a boiling water reactor and the narrowing which occurs in and in connection with heat generated in this is it is an advantage to use non-volatile alkalizing agents.

Thus, it must enrich a non-volatile alkalizing agent. remedies arising from the evaporation in and in connection to the reactor is taken into account so that not the alkalization of the reactor water there becomes so strong that alkalizing agent precipitates in the reactor or that the environment in the reactor otherwise way is made corrosive. Too strong alkalization of the water due to the enrichment of the alkalizing agent in connection with the elongation can thus result in corrosion damage occurs on the fuel enclosure as well as in that an increased dissolution occurs by corrosion products which have deposited on or formed in or adjacent to the reactor. In addition there is a risk that if a minor leak occurs in the pressure-bearing systems, the hot water, which is preferred moreover, a temperature in excess of 100 ° C has to be extended upon exit from the system due to the alkalization increased and that the alkalizing agent precipitates as a result of the cooking. 10 15 20 25 30 35 1143% 114 3 In a circulating circuit comprising a nuclear reactor. where one fuel core in the form of one or more in a water-filled tank arranged fuel elements generate heat that is used for production of steam, leads to excessive alkalization in in connection with the narrowing of water in or adjacent to the fuel core to radioactive corrosion products which formed and / or deposited on the fuel elements dissolves and with the water is discharged into the circulation circuit, whereby the radiation level in connection with the circulation circuit increases.

In addition to evaporation at the fuel core, alkalizing agents are enriched in the reactor water, for example by evaporation due to gamma radiation being absorbed by water in gaps in the core region. Furthermore, enrichment can take place as a result of leakage of water with a temperature exceeding 100 ° C from pressure-bearing systems, by evaporating the water at the exit to atmospheric pressure. For example, such leaks occur as a result of leaky joints, closed but leaky valves and through corrosion or thermal fatigue caused cracks in the pressure-bearing material.

An object of the invention is to provide a method for raising and controlling the pH level, by adding one alkalizing agents, in circulating circuits forming part of a heat-generating power plants where water evaporates and in the steam to maintain a low solubility in the water of the corrosion products formed and the contained energy is recovered, deposited on the design element included in the circulation circuit without risking an enrichment of alkalizing agents in connection with the boiling of water in the fuel core too excessive alkalization of the water in or adjacent to the water evaporating in the reactor or in connection with a minor leakage from pressure equipment.

A further object of the invention is that when it was used to control the pH level in a nuclear power plant where a fuel core in the form of fuel elements is arranged in a tank filled with water, prescribe a method to lower the solubility which are formed and / or deposited on or in connection with fuel in the radioactive corrosion products. 10 15 20 25 30 35 505114 4 menten. thereby reducing the build-up of radiation fields in or in connection with the circulation circuit without, when a non-volatile alkalizing agent is used, cause one to heavy alkalization of the water in or adjacent to that the water evaporates in the reactor or in connection with a minor leakage in the circulation circuit.

THE INVENTION When operating a heat-generating nuclear power plant with a boiling water ten reactor, water in or adjacent to a fuel core in the form of nuclear fuel elements immersed in a reaction tortank. The meadow contains energy extracted before the meadow condensed, purified and returned to said reactor by a circulation circuit. To reduce the solubility of on the nuclear fuel formed and deposited corrosion products and thereby reducing the build-up of radiating fields in the tank connected to the circulation circuits, is added to an i existing bulk volume of water in the reactor tank, according to the invention an additive with a pH-raising effect. This additive contains at least one, under the pressures and temperatures as rows in the reactor, non-volatile alkalizing agent, wherein the additive is dosed so that the content of the additive the agent in bulk volume, that is, the volume of water that located in the reactor tank outside the fuel core, is poured sufficiently low so that the pH of the water does not exceed the value 7 at that temperature and under the other conditions as rows in this volume of water. In one embodiment of the invention the content of added alkalizing agent is poured lower than the contribution to the hydrogen ion concentration obtained from water autoprotolysis. The added non-volatile alkalizing agent enriched in the water immediately present in the was connected to the fuel elements in connection with water is evaporated, raising the pH of this water through the concentration of the alkalizing agent.

Preferably, the pH of the water in the bulk volume is maintained in one reactor tank at a value up to weakly alkaline pH 7, measured at saving temperature, at the water temperatures that 10 15 20 25 30 35 503114 5 lines when operating a boiling water reactor, that is temperatures up to about 285 ° C, while the pH of the water immediately adjacent to the fuel elements is allowed to rise to values significantly higher than 7, measured at rescue temperature, in connection with the alkalisation let anrikas.

The alkalizing agent can be added to the reactor tank either by adding it to the purified condensate or feed the water before it is returned to the reactor tank or through that the alkalizing agent is added to the water in or in directly connection to the reactor tank or one adjacent to cooling and / or circulation circuit arranged in the reactor tank.

As the alkalizing agent, the water is preferably added one solution comprising either at least one aqueous solution dissolved, hydroxide of an alkali metal such as lithium, sodium or potassium or a solution of ammonia or a solution of one or more organic amides, or combinations of these.

The addition of the alkalizing agent is preferably controlled as follows that its contribution to the content of hydroxide ions in the water is less than the contribution to the hydrogen ion content of the water which arises at saving temperature due to the water autoprotolysis.

In one embodiment of the invention is added to the reactor water a mixture of compounds, which together forms a pH buffer system. The buffering ability can obtained or elevated; - of the added compounds themselves, of the appointed associations together with i reactor water existing compounds. of added compounds which, when in the reactor water rescue temperature and level of radioactive radiation, decomposing divided and form substances which in themselves or together with in The existing compounds of the reactor water increase the number of the reactor water buffering ability. 10 15 20 25 30 35 503114 6 Preferably the buffer is added in such a small amount that the pH i bulk volume is below pH 7 measured at saving water temperature ratur, while in connection with the buffering system enriched during boiling in connection with the fuel core or elsewhere, the pH at saving water temperature is buffered at one value exceeding pH 7.

As buffering additives, systems based on one were used or more polyprotonic inorganic acids, such as phosphorus the acid system, the sulfuric acid system, the carbon dioxide system or the silicic acid system or systems based on organic acids and / or bases.

FIGURES The invention is explained in more detail below with reference to to the accompanying figures and examples. Figure 1 shows the change in pH of neutral water as a function of temperature, Figure 2 shows the pH at an elevated temperature, 285 ° C, as a function of pH at room temperature, and Figure 3 shows circuits for water and meadow in one nuclear power plant with a boiling water reactor.

DESCRIPTION OF FIGURES Figure 3 shows heat-generating power plant, illustrated in shape of a boiling water reactor, a reactor vessel has 1 fuel element ment 2, control rods 3 and main circuits 4, of which one is shown in the figure. Each main circulation circuit has a pump 5. The main circulation pumps 5 ensure that the reactor core get adequate cooling. The reactor vessel 1 is part of a or several circuits for water and meadow 6, in which in addition, a steam turbine 7, capacitors 8, condensate purification filter 9, pumps 10 and 14 and preheater ll. IN the steam formed in the reactor core emits its energy to the turbine 7 rotor and condensed after the passage of the turbine 7 in the condenser 8, which comprises cooling water circuits 12 with pumps 13. The condensate pump 14 pumps condensate from the condensate 10 15 20 25 30 35 503114 7 8, through the condensate cleaning filter 9 and after preheating preheater ll back into the reactor vessel l with feed the pump 10. When operating this power plant in accordance with it According to the present method, a pH-raising solution is added, which at least one alkalizing agent, the one in the reactor vessel 6 containing the bulk volume 15 of reactor water.

Preferably, the alkalizing agent is added to the bulk volume 15 by adding the purified condensate in circulation steam / feed water circuit 6 before the condensate returned to the bulk volume of reactor water in the reactor tank or by adding the alkalizing agent the water in or directly adjacent to the reactor tank 1 or a main device arranged in connection with the reactor tank circulation circuit 4 or other connected to the reactor tank circulation circuit. Preferably the alkali is added to the water. dispersant in the form of an aqueous solution comprising at least one dissolved hydroxide of an alkali metal such as lithium, sodium or potassium. Alternatively, ammonia or ammonia dissolved in water as well as organic amides are added.

The addition of the alkalizing agent is controlled so that the pH in the bulk water does not exceed 7. In an embodiment of the invention is adapted to the additive so that its contribution to the content of hydroxide ions in the bulk water is less than that contribution to wet ion content in the water that arises when prevailing temperature due to the autoproteolysis of the water.

The pH raising additive comprises in one embodiment of the invention a system comprising compounds which together form a buffer system. The buffering ability obtained or increased; - of the added compounds themselves, of the appointed associations together with i existing reactor water compounds, of added compounds which, when in the reactor water prevailing temperature and level of radioactive radiation, divided and form substances which in themselves or together with in The existing compounds of the reactor water increase the number of the reactor water buffering ability. 10 15 20 25 30 35 503114 8 Preferably, it is poured upon addition of the buffering system pH in bulk volume 15 at a value below pH 7 at saving water temperature while in connection with the component enriched in the buffer system in connection with fuel core 2 pH at saving water temperature buffered at a value exceeding pH 7.

As buffering systems, systems based on one or several polyprotonic inorganic acids such as phosphoric acid systems the sulfuric acid system, the carbon dioxide system or the silicic acid systems and / or systems based on organic acids and / or baser.

Said buffer solution is dosed to a content in the bulk volume 15 so that the pH of the bulk volume 15 does not exceed 7 during rescue temperature while in connection with the fuel core 2 to as a result of the narrowing, the buffer solution is enriched.

The pH of the water is adjusted by the added buffer solution also here in a range from neutral to weakly alkaline which at the water temperature prevailing in or in connection to the fuel core. that is, temperatures up to about 285 ° C and slightly alkaline pH 7.0, measured at prevailing temperature, while at the enrichment adjacent to the hearth water is buffered to a pH exceeding 7.

The fact that non-volatile additives to reactor water net is enriched in connection with the core 2 by boiling in connection with the meadow formation is used in alkalization according to the invention by making the alkali feed, which is made the reactor water, is poured so low that the pH at the saving temperature in the bulk volume 15 is only moderately increased. when enriched next to fuel 2, the alkali content will increase sharply in the water immediately adjacent to the fuel 2, The result is that the pH increases to values above 7 next to fuel 2, while remaining lower than 7 in bulk volume 15. 10 15 20 25 30 503114 At operating temperature, about 28.9 ° C, the alkali additive can be added the reactor water to be significantly greater than at room temperature, without significantly affecting the pH. This is because of the water autoprotolysis increases with increasing temperature up to about 250 ° C, ie the equilibrium H20 = H ++ OH ' shifted to the right. Which means that the pH value for neutral water decreases as the hydrogen ion content increases.

The change in the pH of neutral water with increasing temperature shown in Figure 1.

A slight alkalization of pure water with lye. for example with a pH unit from pH 7.0 to 8.0 at room temperature, will only to raise the pH at high temperature marginally. This is because, that the elevated content of hydrogen ions and hydroxide ions at high temperature increases the buffer capacity of the water, and thereby dampening the effect of the lye additive. This is shown in Figure 2, where the pH at elevated temperature is shown as a function of pH at room temperature.

The degree of enrichment of non-volatile substances in the fuel strongly influenced by various factors. For example, enrichment increases at increasing heat flux density. Thick coatings are considered to increase the enrichment. In particular, thick porous coatings are considered to give high degree of enrichment. For nuclear fuel that is not abnormal soiled by coatings, it is realistic to assume that the degree of enrichment varies in the range of about 10-1000.

Example 1 Sodium hydroxide is metered into the reactor water so that the content Na * becomes 1 pmol / l. The pH measured at 25 ° C then increases from 7.0 to ca 8.0. At the operating temperature 286 ° C, the pH increases according to Figure 2 from 5.6 to 5.7. be 100, which means that the content of Na * ions becomes 100 umol / l, and the pH measured at 25 ° C becomes 10.0.

The enrichment at the fuel is assumed This gives at 286 ° C the pH value about 7.0, which is an increase at operating temperature with about 1.5 pH units. 10 15 20 25 30 35 505114 10 If the enrichment is assumed to be 1000, the content of Na * ions will increase to be 1 mmol / l and the pH at operating temperature to be about 8.5.

A significant increase in pH is thus achieved next door the fuel without appreciably affecting the pH of the bulk water.

In case of extreme enrichment, for example due to abnormally contaminated fuel le or due to narrowing of reactor water in case of leakage, can pH even with moderate dosing rise to undesirably high levels.

To limit the pH during enrichment, the alkalization can be done with a suitable buffer solution. The invention means, that the buffer is dosed at such low levels that the pH of the reactor the water at operating temperature is not allowed to exceed 7. At gradually increasing enrichment, the content of the buffer will be dominate over the ions from water autoprotolysis, and pH the value will increase asymptotically towards a constant value determined by the composition of the buffer.

Example 2 A buffer solution of equal parts H2SiO3 and HSiO3 'is dosed to the reactor water. At the operating temperature of the reactor water is the pka value for silicic acid approx. 9. The dosage is made so that the content of H2SiO3 and HSiO3 'in the reactor water is 1 μmol / l each. This is lower than the hydrogen ion content in pure water at the current temperature, which is about 3 umol / l. pH in bulk the water will therefore not increase significantly.

The enrichment at the fuel is assumed to be 100, which gives the content 100 pmol / l of the buffer next to the fuel. At this level the pH is completely controlled by the buffer, and the pH becomes approx. 9. If the enrichment increases sharply, eg to 10,000, the pH will still to be 9. The result is that the pH is limited to 9 next to the fuel at normal and high concentrations, while it remains virtually neutral in the bulk water.Often the reactor water contains small amounts of pollutants such as affects the pH value. Silicic acid, for example, always occurs in levels that vary from a few ug / l up to the level several hundred ng / l. Because the pH of the water at operating temperature is lower than the pka value of the silicic acid, the silicic acid is present in its acid form. 10 15 20 25 30 35 503114 11 By adding a suitable amount of alkali, one can neutralize part of the acid and thereby create a buffer system of H2SiO3 + HSiO3 '.

Example 3 The reactor water is assumed to contain 1 μmol / l silicon as silicic acid, which corresponds to 28 pg / l measured as silicon. No other topics assumed to be present in the water. To the reactor water Dose KOH to a content of 0.5 pmol / l. This leads to, that half of the silicic acid is transferred to its base form, and we receives 0.5 μmol / l each of H2SiO3 + HSiO3'_ bulk water will not be affected by the lye additive, because of the water content of hydrogen ions due to autoprotolysis, about 3 umol / l at operating temperature, is significantly higher than the KOH content. At enrichment by a factor of 100 increases the content of H2SiO3 + HSiO3 'to 50 μmol / l each, which means that the silicic acid system buffers the pH to the value about 9 in the same way as in Example 2.

The result of the adjusted lye dosage is that the pH limited to pH 9 next to the fuel at normal and high enrichments, while remaining virtually unaffected in bulk water.

In a boiling water reactor, the silicon content in the reactor water varies usually in a regular way. The content is controlled by the boiling of the reactor water and of the amount of silicon bound in the reactor. the ion exchange of peat water purification. A common process is, that the silicon content increases from the level of about 10 ug / l to the specified the limit of the reactor. The limit value is different in different reactors, but is usually in the range 100-500 ug / l measured as Si. When the content approaches the limit value, you usually change ion exchange mass, whereby the content of food drops to the level of about 10 pg / l.

Example 4 The reactor water is assumed to have a silicon content, which varies linearly from 1.0 to 9.0 pmol / l measured at operating temperature as a function of operating time. The filter cycle is assumed to be 200 days. 10 15 20 25 503114 12 The reactor water also contains sulfuric acid from decomposition the sulfonic acid groups of leaked cation exchange mass with it constant content 0.5 μmol / l, Na * with content 0.2 μmol / l and Cl ' with a content of 0.1 pmol / l.

The excess acid without silicon can be calculated as follows: The acid surplus = ztngsou- (Ima fl- [cl-J> = 1.o- When silicic acid is included, the excess acid becomes [H2SiO3] + 0.9 umol / l.

At the start of the filter cycle, the silicon content is 1.0 μmol / l. Acid- umol / l. KOH is now added in one amount so that half of the silicic acid is transferred to base form.

For this, eat 0.9 umol / l to the excess acid, the excess without silicon is 0.9 neutralize it and 0.5 μmol / l to neutralize lyse half of the silicic acid, ie together 1.4 umol / l.

With this lye addition, the pH value when enriched comes next the fuel to be buffered to about 9 measured at operating temperature.

After 100 operating days, the silicic acid content is l.O + 0.5 (9.0-l.0) = 5.0 umol / l. To buffer the pH to 9, a lye additive is now required corresponding to the content 0.9 + 5.0 / 2 = 3.4 μmol / l.

The result of adapting the lye dosage to variations in the silicon content is also here, that the pH is limited to 9 adjacent the fuel at normal and high enrichments, while remaining virtually unaffected in the bulk water. Even without adaptation of the alkalization, the presence occurs silicon to give a certain buffering effect in combination with the alkalizing agent.

Claims (16)

A method of operating a nuclear power plant with a boiling water reactor (1) comprising at least one core of nuclear fuel element (2) placed in a reactor tank and in which steam is generated, a steam turbine (1). 7) in which the energy contained in the steam is at least partially recovered, condenser (8) in filter (9) for recirculation of which the steam below the steam turbine is condensed, (10, 14) the condensate to the reactor and at least one circuit (4) for purification of the condensate, pumps for circulating reactor water and cooling the fuel core, characterized in that an additive with at least a pH-raising effect and which contains at least one, under non-volatile alkali, arranged in said reactor tank, the conditions prevailing in the reactor, , bulk volume (15) of water, the agent being dosed so that; that the content of the agent in the bulk volume is kept low enough to maintain a pH value below pH 7 measured at the prevailing temperature while the added non-volatile alkalizing agent is enriched in the water immediately adjacent to the fuel elements, the pH of this water through the alkalizing agent concentration is raised to a pH value exceeding 7 at the prevailing temperature Method according to claim 1, characterized in that the alkalizing agent is added to the purified condensate before it is returned to the reactor tank. Method according to claim 1, characterized in that the alkalizing agent was added to the water in or in direct connection one in connection with the reactor tank (4). to the reactor tank or arranged cooling purification or circulation circuit Method according to one of the preceding claims, characterized in that a solution comprising at least one water-soluble hydroxide of an alkali metal such as lithium, sodium or potassium was added as alkalizing agent. 10 15 20 25 30 35 503114 14 Method according to one of the preceding claims, characterized in that ammonia or ammonia dissolved in water was added as alkalizing agent. Method according to one of the preceding claims, characterized in that an organic amide is added as the alkalizing agent. Method according to the preceding claim, characterized in that a mixture of compounds according to some or all of claims 4-6 is added as alkalizing agent. Method according to one of the preceding claims, characterized in that alkalizing agent is added in such an amount that its contribution to the content of hydroxide ions in the water is less than the contribution to the hydrogen ion content in the water which arises at the prevailing temperature due to the autoprotolysis of the water. Method according to one of the preceding claims, characterized in that a mixture of compounds is added to the reactor water which, when they are added to the reactor water, forms a pH buffer system. Method according to Claim 9, characterized in that compounds are added to the reactor water which, together with compounds present in the reactor water, form a pH buffer system. Method according to claim 9 or claim 10, characterized in that compounds are added to the reactor water which, at the prevailing temperature and level of radioactive radiation in the reactor water, decompose and form substances which in themselves or together with compounds present in the reactor water form a pH buffer system. Method according to claim 9, claim 10 or claim 11, characterized in that when adding the buffering system the pH of the bulk volume at the prevailing water temperature does not exceed pH 7 while, in connection with the buffering system being enriched in connection with the industry hardness, the pH at radiating water temperature is buffered at a value exceeding pH 7. Method according to one of Claims 9 to Claim 12, characterized in that a system based on one or more polyprotonic inorganic sulfuric acid systems, acids such as the phosphoric acid system, was added as a buffering system. the carbon dioxide system or the silicic acid system. A method according to any one of claims 9 to claim 12, characterized in that as a buffering system a system based on the ammonia system was added. Method according to one of Claims 9 to Claim 12, characterized in that a system based on organic acids and / or bases was added as a buffering system. Method according to one of Claims 9 to Claim 12, characterized in that a system based on organic amides was added as a buffering system.