JP5985232B2 - Criticality prevention device, nuclear power plant and criticality prevention method - Google Patents

Description

  The present invention relates to a criticality prevention device for preventing recriticality after fuel melting in a furnace has occurred due to severe accident of a nuclear power plant, a nuclear power plant including the same, and a criticality prevention method.

  In an event where the core is seriously damaged (hereinafter “severe accident”) at a nuclear power plant that greatly exceeds the design standard event, it is assumed that the core will be melted due to insufficient cooling of the core by the coolant. Is done. Even when the reactor is shut down, the “recritical” event that makes the reactor critical again due to the change in the shape of the fuel due to the melting of the core is caused by the increase of fission reaction in the reactor. May lead to release of substances into the environment.

  Preventing recriticality is important from the viewpoint of confinement of radioactive materials. Since fast reactors contain more fissile material than light water reactors, many methods for preventing recriticality have been studied. For example, Patent Document 1 discloses a technique for preventing recriticality by providing a high-melting-point, high-density recritical relaxation region below the fuel assembly.

  In addition, studies on prevention of recriticality have been conducted for light water reactors. In Patent Document 2, recriticality is prevented by disposing a neutron absorber at the bottom of the reactor pressure vessel, and in Patent Document 3, A technique for maintaining subcriticality by containing neutron absorbers composed of silver, cadmium, and boron compounds in separators and dryers of boiling water nuclear power plants and dropping these absorbers during core melting is disclosed. .

  On the other hand, it has been found that the entire core does not necessarily melt and reach the bottom of the reactor pressure vessel depending on the melting state of the core. For example, even in the three-mile island accident in the United States, it has been reported that the central part of the core melts and a healthy fuel part remains in the lower part of the core (Non-Patent Document 2).

JP 2002-90489 A JP 2000-98078 A JP-A-6-160572

S. Hagen, et al., “Lessons Learned from CORA Program,” Transactions from the International Topical Meeting on Probabilistic Safety Assessment, Park City, Utah, September 1996. J. M. Broughton, et al., “A Scenario of the Three Mile Island Unit2 Accident,” Nucl. Technol., 87, 34 (1989)

  Non-Patent Document 1 clarifies how fuel, cladding tubes, and control rods change by simulating core melting through experiments. As shown here, the melting point of stainless steel is 1723K, and the melting point of the control rod made of stainless steel is lower than 3123K which is the melting point of UO2.

  Therefore, depending on the temperature conditions in the furnace, it is conceivable that the control rods are first melted, and then the fuel melting is partially stopped to form a molten core at a certain height. In this case, re-criticality may occur when a certain water level is reached by subsequent water injection.

  In such an event, it is not effective to place a neutron absorber at the bottom of the furnace as shown in Patent Document 2, and if the separator and dryer remain healthy and maintained at the top, Patent Document There is a possibility that the neutron absorber will not fall as shown in FIG.

  In addition, when the melting core has a high temperature due to decay heat and the fuel shape has collapsed, a sufficient coolant cannot be secured, and it is highly possible that water evaporates and exists as water vapor.

  In light water reactors, water has a function as a moderator for slowing down fast neutrons generated by fission into low-speed neutrons called thermal neutrons, which are prone to fission. There is an easy-to-optimize value, but there is less chance of reaching criticality due to insufficient deceleration in the water vapor state.

  On the other hand, the decay heat decreases with the passage of time, so that after several months have passed since the melting of the core, the water vapor in the furnace becomes water, and the neutron moderating effect may increase and become critical.

  In addition, boron in the control rod, which was present at the beginning, may flow out with the coolant as time passes, and may become critical due to a decrease in the neutron absorber.

  As a measure for preventing such criticality, a method in which an aqueous solution containing boron such as an aqueous borate solution or a sodium pentaborate solution is always allowed to flow as a coolant can be considered. However, an aqueous solution containing boron does not corrode metals. In order to promote, it is not desirable from the viewpoint of ensuring the long-term integrity of the reactor pressure vessel and the containment vessel.

  It is also conceivable to inject boron carbide, which is a control rod, in the form of particles and externally injected together with the coolant, but granular boron carbide cannot be injected because it may cause the feedwater pump to malfunction, or the molten core. There is a possibility that boron carbide does not penetrate into the inside.

  Therefore, in order to prevent recriticality as described above, it is desirable that it is dissolved in water like boric acid at the time of injection, solidifies in the furnace, adheres to the molten core, and does not easily dissolve in water. .

  Accordingly, an object of the present invention is to prevent recriticality after fuel melting in a furnace occurs due to severe accidents at a nuclear power plant.

In order to achieve the above-described object, a criticality prevention apparatus according to the present invention is arranged outside a nuclear reactor vessel that houses a core, and a boron solution supply device that supplies a boron solution, and the boron solution is the reactor vessel. A deposition compound supply that feeds into the reactor vessel a deposition compound solution that is a solution of a deposition compound that reacts with the boron solution to precipitate a substance containing boron into the reactor vessel when injected into the reactor vessel characterized in that it comprises apparatus and, a.

  The present invention also provides a criticality prevention method for preventing recriticality after melting of fuel in the furnace due to severe accidents at a nuclear power plant, and a boron solution supply step of supplying a boron solution containing boron to the reactor core; And a precipitation compound supply step of supplying a precipitation compound solution, which is a solution of a precipitation compound that reacts with the boron solution to precipitate a substance containing boron, to the core.

  According to the present invention, it is possible to prevent recriticality after fuel melting in the furnace has occurred due to severe accidents at a nuclear power plant.

It is a block diagram which shows the structure of embodiment of the nuclear power plant which concerns on this invention. It is a flow figure showing an embodiment of a nuclear power station concerning the present invention.

  Hereinafter, embodiments of a nuclear power plant according to the present invention will be described with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted.

[First Embodiment]
FIG. 1 is a block diagram showing a configuration of an embodiment of a nuclear power plant according to the present invention.

  The nuclear power plant includes a core 1, a reactor pressure vessel 2 that houses the core 1, a main steam pipe 3, a main turbine 6, and a criticality prevention device 20.

  The first feed water pump 9 a and the second feed water pump 9 b are components of the criticality prevention device 20. The criticality prevention device 20 has a boron solution supply device 21 connected to the suction side of the first water supply pump 9a and a precipitation compound supply device 22 connected to the suction side of the second water supply pump 9b.

  A main steam pipe 3 for transferring main steam generated in the reactor pressure vessel 2 is connected to the reactor pressure vessel 2. The main steam is guided by a main steam pipe 3 to a main turbine 6 driven by the main steam. The exhaust steam from the main turbine 6 is cooled and condensed by the condenser 7 to become condensate.

  Condensate is sent to the reactor pressure vessel 2 by a first feed water pump 9a and a second feed water pump 9b provided in parallel to each other in the feed and condensate piping 8.

  A turbine bypass pipe 5 that connects the main steam pipe 3 and the condenser 7 without the main turbine 6 is provided, and a turbine bypass valve 5 a is installed in the turbine bypass pipe 5.

  A solids removal filter is provided in the feed and condensate water pipe 8 so that the solids are not brought from the reactor pressure vessel 2 to the downstream side of the main steam pipe 3, particularly the first feed water pump 9a and the second feed water pump 9b. 23 is provided.

  Note that, in the supply and condensate water pipe 8, a portion from the suction side of the first feed water pump 9 a to which the boron solution supply device 21 is connected to the first feed water pump 9 a, the first compound feed device 22 to which the precipitation compound supply device 22 is connected. A portion from the suction side of the second water supply pump 9 b to the second water supply pump 9 b constitutes a part of the criticality prevention device 20. Further, the solid matter removal filter 23 also constitutes a part of the criticality prevention device 20.

  The boron solution supply device 21 includes a boric acid aqueous solution supply tank 21a, a boric acid aqueous solution supply valve 21b, and a first connection portion 21c. The boric acid aqueous solution supply tank 21a side and the boric acid aqueous solution supply valve 21b side can be disconnected and connected by the first connecting portion 21c.

  The precipitation compound supply device 22 includes a precipitation compound supply tank 22a, a precipitation compound supply valve 22b, and a second connection portion 22c. The deposition compound supply tank 22a side and the deposition compound supply valve 22b side are separated by a second connection portion 22c and can be connected.

  Here, the boron solution is described as boric acid water, but may be a sodium pentaborate solution.

  Moreover, as a compound for precipitation for reacting with a boron solution to precipitate a compound containing boron, a compound containing calcium can be used. As the solution of the compound containing calcium, a calcium hydroxide solution or a calcium carbonate solution can be used.

  FIG. 2 is a flowchart showing this embodiment. The criticality prevention method when a severe accident occurs at a nuclear power plant is described below.

First, the main steam stop valve 4 is closed, the turbine bypass valve 5a is opened, and a path from the reactor pressure vessel 2 to the condenser 7 is secured. Further, the first water supply pump 9a, the second water supply pump 9b, the boron solution supply device 21 and the precipitation compound supply device 22 are set in an operable state (S01).

  In the following steps, before the boric acid aqueous solution is supplied by the boron solution supply device 21, the first water supply pump 9 a is used, and before the precipitation compound solution is supplied by the precipitation compound supply device 22, The 2nd water supply pump 9b is made into an operation state.

  After step S01, the boric acid aqueous solution supply valve 21b of the boron solution supply device 21 is opened, and the boric acid aqueous solution is sent to the reactor pressure vessel 2 via the first feed water pump 9a. The molten core 1 (hereinafter referred to as “core 1”) is sufficiently filled with an aqueous boric acid solution (S02).

  After step S02, the boric acid aqueous solution supply valve 21b is closed, and the supply of the boric acid aqueous solution is stopped (S03).

  Various methods can be used to determine that the core 1 is sufficiently filled with the boric acid aqueous solution. For example, when the liquid level gauge of the reactor pressure vessel 2 can be used, the liquid level gauge may be used. If there is no leakage from the reactor pressure vessel 2, it can be estimated from the flow rate of the first feed water pump 9 a and the time when the boric acid aqueous solution is sent to the reactor pressure vessel 2.

  Under the present circumstances, if the height in the reactor pressure vessel 2 of the core 1 is assumed to be the height of the original core, for example, it is a safe side.

  After step S03, the deposition compound supply valve 22b of the deposition compound supply device 22 is opened, and the deposition compound solution is sent to the reactor pressure vessel 2 via the second feed water pump 9b. The precipitation compound solution is allowed to reach the core 1 (S04).

  After step S04, the precipitation compound supply valve 22b is closed, and the supply of the precipitation compound solution is stopped (S05).

  Various methods can be used to determine whether or not the precipitation compound solution has reached the core 1 in the reactor pressure vessel 2. For example, it can be estimated from the flow rate of the second water supply pump 9 b and the time when the deposition compound solution is sent to the reactor pressure vessel 2.

  In addition, it is possible that the flow path to the core 1 is blocked by the reaction of the boric acid aqueous solution and the precipitation compound solution in the middle when the pipes and flow paths etc. on the way to the core 1 are narrow. There are cases where there is concern about sex. In such a case, after the step S02 and before the step S03, the volume of the volume with an allowance for the volume of the narrow flow path with the boric acid aqueous solution supply valve 21b and the precipitation compound supply valve 22b closed. There may be a step of sending water.

  The effect | action of this embodiment comprised as mentioned above is demonstrated below.

  When a severe accident occurs, first, in step S01, an atom is passed from the reactor pressure vessel 2 via the condenser 7, the solid matter removal filter 23, the first water supply pump 9a, and the second water supply pump 9b. A system configuration in which operation by a flow path returning to the furnace pressure vessel 2 is possible is performed.

  Thereafter, the boron solution is sent from the boron solution supply device 21 to the reactor pressure vessel 2 via the first feed water pump 9a by the operation of step S02. By this operation, the core 1 is filled with the boron solution.

  In step S02, the deposition compound solution is sent from the deposition compound supply device 22 to the reactor pressure vessel 2 via the second feed water pump 9b, and finally the deposition compound solution reaches the core 1. To do.

  Around the core 1, the reached deposition compound solution reacts with the boron compound to precipitate a compound containing boron, and around the core 1, the precipitated compound containing boron is attached and fixed.

  As a compound containing boron to be precipitated, 2-calcium-6-borate-5-hydrate compound is known. Magnesium-borate is also known to be more insoluble than calcium-borate. Therefore, the calcium compound of this embodiment may be replaced with a magnesium compound.

  A stable state of the precipitated boron-containing compound around the core 1 prevents the critical state from being reached.

  In addition, when a narrow flow path exists in the path to the core 1, by providing a step of sending a volume of water with a margin in the volume of the narrow flow path between Step S02 and Step S03, The feed water containing the boric acid aqueous solution, the feed water, and the feed water containing the compound solution for precipitation pass through a narrow flow path in a flow close to the piston flow, so that the water flows into the core 1 through the path reaching the core 1. There is no blockage.

  According to the present embodiment as described above, it is possible to prevent the recriticality after the fuel melting in the furnace has occurred due to the severe accident of the nuclear power plant.

[Other Embodiments]
As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention.

  For example, the reactor type of the target nuclear power plant is not limited to a water-cooled nuclear reactor, and may be, for example, a gas-cooled reactor.

  Further, in the embodiment, the case of the feed water pump as the pump provided in parallel has been described, but if provided in parallel, in addition to the feed water pump, an emergency core cooling system pump or a reactor purification system It may be a pump or the like. Furthermore, you may use a gravity fall system, without using a pump. Moreover, you may combine these.

  Further, in a normal state of the nuclear power plant, when a severe accident occurs without providing the boric acid aqueous solution supply tank 21a side and the precipitation compound supply tank 22a side, the boric acid aqueous solution supply tank 21a side and the precipitation compound are provided. A supply tank 22a side is provided, the boric acid aqueous solution supply tank 21a side is connected to the boric acid aqueous solution supply valve 21b side at the first connection part 21c, and the precipitation compound supply tank 22a side is connected to the second connection part 22c. It may be connected to the deposition compound supply valve 22b side.

  For existing nuclear power plants, a plurality of storage tanks already installed for other purposes on the boric acid aqueous solution supply tank 21a side and the precipitation compound supply tank 22a side are used for supplying and depositing boric acid aqueous solution at the time of an accident. You may divert each for supply of a compound. Alternatively, an existing storage tank may be used for the boric acid aqueous solution supply tank 21a, and the precipitation compound supply tank 22a side may be additionally installed.

  Further, combinations other than boric acid aqueous solution, calcium hydroxide solution or calcium carbonate solution can be used in the present invention in a combination in which a compound containing boron is precipitated by a chemical reaction. In the case of a compound having a large decrease in solubility with respect to temperature rise, the injection location and the system configuration are carefully taken so as not to pass through the high-temperature channel before reaching the vicinity of the core 1.

  Moreover, you may combine the characteristic of each embodiment. Furthermore, these embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention.

  These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... Core, 2 ... Reactor pressure vessel (reactor vessel), 3 ... Main steam pipe, 4 ... Main steam stop valve, 5 ... Turbine bypass piping, 5a ... Turbine Bypass valve, 6 ... main turbine, 7 ... condenser, 8 ... feed condensate piping, 9 ... feed pump, 9a ... first feed pump (first pump), 9b: second feed pump (second pump), 20 ... criticality prevention device, 21 ... boron solution supply device, 21a ... boric acid aqueous solution supply tank, 21b ... boric acid aqueous solution Supply valve (first stop valve), 21c... First connection part, 22... Compound supply device for precipitation, 22a... Compound supply tank for precipitation, 22b. 2nd stop valve), 22c ... 2nd connection part, 23 ... Solid matter removal filter

Claims (9)

Placed outside the reactor vessel that houses the reactor core,
A boron solution supply device for supplying a boron solution;
When the boron solution is injected into the reactor vessel, the deposition vessel is a precipitation compound solution that reacts with the boron solution to precipitate a boron-containing substance in the reactor vessel. A precipitating compound supply device to be supplied inside ,
A criticality prevention device comprising:   The criticality prevention device according to claim 1, wherein the boron solution is stored in a boron solution supply device arranged outside the reactor vessel.   3. The criticality prevention apparatus according to claim 1, wherein the boron solution contains at least one of a sodium pentaborate solution or boric acid water. 4.   The criticality prevention device according to any one of claims 1 to 3, wherein the precipitation compound solution is a solution containing at least one of calcium and magnesium.   The criticality prevention apparatus according to claim 4, wherein the precipitation compound solution includes at least one of a calcium hydroxide solution and a calcium carbonate solution. A first pump for supplying cooling water to the reactor vessel;
A second pump installed in parallel with the first pump and supplying cooling water to the reactor vessel;
A first stop valve connected to the suction side of the first pump;
A second stop valve connected to the suction side of the second pump;
Further comprising
The boron solution is supplied to the suction side of the first pump through the first stop valve,
The precipitation compound solution is supplied to the suction side of the second pump via the second stop valve.
The criticality prevention device according to any one of claims 1 to 5, wherein the criticality prevention device is provided. A main turbine driven by main steam generated in the reactor vessel;
A main steam pipe for sending the main steam from the reactor vessel to the main turbine;
A condenser for condensing exhaust steam from the main turbine;
A turbine bypass pipe connecting the main steam pipe and the condenser without going through the main turbine;
Supply and condensate piping from the condenser to the reactor vessel;
A first feed water pump provided in the middle of the feed and condensate piping to supply cooling water to the reactor vessel;
A second water supply pump provided in parallel with the first water supply pump in the middle of the supply and condensate piping to supply cooling water to the reactor vessel;
A first stop valve connected to the suction side of the first water supply pump;
A second stop valve connected to the suction side of the second water supply pump;
Further comprising
The boron solution is supplied to the suction side of the first water supply pump through the first stop valve,
The precipitation compound solution is supplied to the suction side of the second water supply pump through the second stop valve.
The criticality prevention device according to any one of claims 1 to 5, wherein
A nuclear power plant comprising: A criticality prevention method for preventing recriticality after fuel melt in a furnace has occurred due to a severe accident at a nuclear power plant,
A boron solution supply step for supplying a boron solution containing boron to the core;
A precipitating compound supply step of supplying to the core a precipitating compound solution that is a solution of a precipitating compound that reacts with the boron solution to precipitate a substance containing boron;
The criticality prevention method characterized by having.   The criticality prevention method according to claim 8, wherein the precipitation compound supply step is performed after the boron solution supply step.

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