GB2519919A

GB2519919A - Combined active and passive reactor cavity water injection cooling system

Info

Publication number: GB2519919A
Application number: GB1504152.8A
Authority: GB
Inventors: Yong Yu; Xia Yuan; Xia Zhao; Guoqiang Zhang; Jingyan Li; Daiyong Song; Like Wan; Bin Zhao
Original assignee: China Nuclear Power Engineering Co Ltd
Current assignee: China Nuclear Power Engineering Co Ltd
Priority date: 2012-09-27
Filing date: 2013-09-24
Publication date: 2015-05-06
Anticipated expiration: 2033-09-24
Also published as: GB201504152D0; MY176719A; CN102867549B; ZA201502772B; GB2519919B; CN102867549A; WO2014048290A1

Abstract

Disclosed is a combined active and passive reactor cavity water injection cooling system. The structure of the system comprises a passive reactor cavity water injection tank (1) and a reactor cavity water injection cooling pump (3), wherein the passive reactor cavity water injection tank (1) is connected to a reactor cavity (2) via a passive injection pipeline, the reactor cavity water injection cooling pump (3) is provided outside a containment vessel (5), an inlet pipe of the reactor cavity water injection cooling pump (3) is connected to a refuelling water tank (4), and an outlet pipeline of the reactor cavity water injection cooling pump (3) passes through the containment vessel (5) and connects to the reactor cavity (2). The system serves as a severe accident countermeasure, using a combined active and passive multi-redundancy and multi-variation method such that if an accident occurs, reactor core melt is carried away, and heat is discharged from the reactor core, thus providing the safety function of guarding against melt-through.

Description

COMBINED ACTIVE AND PASSIVE REACTOR CAVITY WATER

INJECTION COOLING SYSTEM

Technical Field

The present invention relates to reactor design technique, and more particularly, to a combined active and passive reactor cavity water injecting and cooling system.

Description of Related Art

In nuclear power plants around the world, two types of measures are applied to deal with molten corium. The first type is In-Vessel Retention (IVR), such as API 000 reactors designed by United States. In the event of a severe accident when core melting is unavoidable, the integrity of the bottom head of the pressure vessel can be maintained by flooding the reactor cavity and cooling down the outer wall of the pressure vessel, thereby trapping any molten corium materials in the pressure vessel. This type of measure, which cools down the molten corium materials by passive manner, has an advantage that the structure is simpler and costs less (however, it is not applicable to high-power nuclear power plants) and be able to confine any molten materials in the pressure vessel, thus preventing leakage of any radioactive materials and maintaining integrity of the containment.

However, the cooling down and stratification phenomena of molten materials are not well appreciated till now and the failure margin thereof is difficult to determine and, therefore, there are still some risks in this respect. Furthermore, the passive system of AP1000 is only applicable to nuclear power plants equipped with a passive safety system. For nuclear power plants equipped with special active safety facilities, the said system is insufficient to handle with a station blackout.

The second type is Ex-Vessel Retention (EVR). The examples in this respect include VVER-I000, EPR. The design philosophy of EPR reactors designed by EDF is that, once the molten corium melts through the pressure vessel, they will be introduced to an extension space into which cooling water is also introduced in a passive maimer to cool down the ductile molten corium. This type of measure has the advantage of high safety and faster solidification of the molten materials.

However, it requires a large space, the solidified molten materials occupy a large area and a large pressure occurs when the molten materials are cooled down. In WWER type nuclear power units designed by Russia, a special core catcher is used to collect and cool down the molten corium in a passive manner. Depend on special molten materials catcher and cooling method, the molten materials exhibit a compact structure after solidified, which is favorable to subsequent decomposition treatment. Furthermore, as the molten materials are always confined in the heat exchanger and are in contact with the atmosphere and cooling water in the contaimnent in a less area, the leakage of fission products is reduced and the pressure inside the containment is lower. However, as the molten materials are cooled down at a slow rate, it would take a long time (usually up to several months) for the molten materials to solidify.

Brief Summary of the Invention

The objective of the present invention is to provide a combined active and passive reactor cavity water injection cooling system to accommodate the demand for safe design of nuclear power plants. In the event of severe accidents of * nuclear power plants, borated water is allowed to flow through the reactor cavity by the reactor cavity water injecting and cooling system to take away the heat released from the molten corium, thus reducing the temperature of the reactor pressure vessel and maintaining the integrity of the pressure vessel.

To achieve the objective described above, the present invention employs the technical solutions below: A combined active and passive reactor cavity water injection cooling system, comprising a passive injection tank and injection pump, the passive injection tank being connected to a reactor cavity through passive injection lines, the injection pump being disposed outside a containment, an suction pipe of the injection pump being connected to an inlet pipe of IRWST, a discharge pipe of the injection pump penetrating through the containment to be connected with the reactor cavity.

Further, the combined active and passive reactor cavity water injection cooling system, wherein the passive injection tank is disposed inside the containment, the passive injection lines connected with the passive injection tank consist of one upper injection line and one lower injection line which have different diameters, the two injection lines are combined into one header which penetrates into the reactor cavity to connect with a reactor vessel insulation.

Further, of the upper and lower injection lines which have different diameters, the upper line has a larger diameter and is used to provide a large flow to flood the reactor cavity at the early stage of the system commissioning; the lower line has a smaller diameter and is used to maintain a long-term injection flow to the reactor cavity; each of the injection lines respectively has a DC motor-operated valve powered by a storage battery and a non-return valve provided.

Further, the passive injection tank may aJso be disposed outside the containment; two injection pumps are provided, the passive injection lines connected with the passive injection tankk are respectively connected with the discharge pipes of the two injection pumps.

Further, the combined active and passive reactor cavity water injection cooling system, wherein two injection pumps are provided, the discharge pipes of the two injection pumps respectively pass through an isolation valve of the containment and then penetrate through the contaimnent, and are then combined into one header which is connected with the header of the passive injection lines.

Further, the conibined active and passive reactor cavity water injection cooling system, wherein the refueling water tank connected with the suction pipes of injection pumps is disposed at a sump inside the containment below the reactor core.

Further, the combined active and passive reactor cavity water injection cooling system, wherein the suction pipes of injection pumps are also connected with a fire protection water source outside the containment.

Further, the combined active and passive reactor cavity water injection cooling system, wherein when the refUeling water tank is connected with low-pressure safety injection pumps, the suction pipes of the reactor cavity water injecting and cooling system are connected with suction pipes of the low-pressure safety injection pumps.

Further, the combined active and passive reactor cavity water injection cooling system, wherein the passive injection tank is constructed of an enclosed reinforced concrete structure and is provided with stainless steel liner.

Further, the combined active and passive reactor cavity water injection cooling system, wherein all of the pipes and fittings of the system are constructed of austenitic stainless steel material.

The advantageous effects of the present invention are as follows: The active part of the present invention mainly forces cooling water to be injected into the reactor cavity thereby perniitting rapid forced circulation cooling of the molten cUrium for a long time under severe conditions; while the passive part can still introduce cooling water into the reactor cavity to allow long-term cooling in the event of a station blackout. By virtue of the reactor cavity water injecting and cooling system of the present invention, it is possible to prevent molten corium from burning through the pressure vessel and, as a result, failure of the containment can be avoided ultimately and LERF value can be effectively decreased. The present invention is characterized in that the system is in redundant configuration, can be implemented in different forms, occupies less space, is highly reliable and it takes less time for molten materials to solidify.

Brief Descriptioll of the Several Views of the Drawings Fig. 1 is a structural schematic diagram of an embodiment of the present invention depicting a passive injection tank of a reactor cavity water injecting and cooling system being disposed inside a containment.

Detailed Description of the Invention

The present invention provides a combination of active and passive means to inject cooling water into the reactor cavity to cool down the molten coriurn inside the pressure vessel. In particular, it is not only possible to remove the heat from the molten corium in an active and long-term circulation way, but also possible to achieve]ong-term cooling down of the molten corium in a passive way in the event of a station blackout. This can prevent molten coriums from burning through the base plate of the containment, leading to failure of the last barrier of nuclear power plants.

The combined active and passive reactor cavity water injection cooling system (US) comprises injection pumps, a passive injection tank as well as associated valves and piping facilities. Normally, there are disposed two (but not necessarily limited to two) injection pumps and one passive injection tank which can be arranged either inside or outside the containment.

The passive part of the CIS system comprises the passive injection tank H disposed inside (or outside) the containment. In order to accommodate the requirement for initial large flow flooding and later injection flow of cooling water, two injection lines having different diameters respectively at upper and lower sides are disposed inside the passive injection tank. The injection line at upper side has a larger diameter, serving to supply large flow for flooding of the reactor cavity during initial period of the system operation. The injection line at lower side has a smaller diameter and is intended to maintain an injection flow into the reactor cavity for a long time. The specific diameters of the both injection lines may be designed according to the reactor power in conjunction with the actual engineering conditions. To ensure reliability of passive water injection into the reactor cavity, four DC motor-operated valves connected in parallel and two non-return valves are provided for isolation purpose. After passing through the said valves, two passive reactor cavity water injection lines are re-combined into one header which penetrates inside the reactor cavity and is connected with the thermal insulation.

The four motor-operated valves connected in parallel are driven by a DC motor which is powered by storage batteries.

The main equipment in the active part of the CIS system is disposed outside the containment. The suction pipes of the two injection pumps are connected with the refueling water tank. In cases where the refueling water tank is connected with low pressure safety injection pumps, the suction pipes of injection pumps are respectively connected with the suction pipes of the two low pressure safety injection pumps so as to reduce the quantity of the parts penetrating through the containment. Water intake occurs at the internally (or externally) disposed refueling water tank. In a preferred solution, the refueling water tank is disposed at a sump inside the containment below the reactor core. After passing through the isolation valves of the contaimnent, two discharge lines of injection pumps penetrate through the contaimnent and are then re-combined into one header which is connected with the header of the passive part of the reactor cavity water injecting system. This design aims to reduce the quantity of openings made in the concrete structure of the reactor cavity thereby maintaining the stability of the civil structure of the reactor cavity. Alternatively, the suction pipe of each reactor cavity may be connected with a fire protection water source outside the containment.

The normal operation of the CTS system implies that, whenever a severe damage accident of the reactor core of a nuclear power plant occurs, the CIS system can be put into service. During normal operation of the nuclear power plant, the CIS system is stopped and in standby condition.

Below is a detailed description of the present invention in connection with the accompanying drawings and the preferred embodiments.

Embodiment 1: As shown in Fig. I, the combined active and passive reactor cavity water injection cooling system (CIS), comprises one passive injection tank 1 and two injection pumps 3, the passive injection tank 1 is connected with a reactor cavity 2 through a passive injection line, injection pumps 3 are disposed outside a containment 5, suction pipes of injection pumps 3 are connected with a refueling water tank 4, discharge pipes of injection pumps 3 penetrate through the containment 5 to be connected with the reactor cavity 2. The refueling water tank 4 may be disposed inside or outside the containment. Preferably, the refueling water tank is disposed at a sump below the reactor cavity. Placing the refueling water tank at the lowest point is advisable to collect the water amount resulting from the containment spraying and pipe breaking.

In this embodiment, the passive injection tank is disposed inside the containment, the passive injection lines connected with the passive injection tank consists of two lines respectively at upper and lower side which have different diameters and these two injection lines are combined into one header which penetrates inside the reactor cavity to be connected with the thermal insulation of the pressure vessel. Of the two injection lines having different diameters respectively at upper and lower side, the upper injection line has a larger diameter, serving to supply large flow for flooding of the reactor cavity during initial period of the system operation. The lower injection line has a smaller diameter and is intended to maintain an injection flow into the reactor cavity for a long time. Each of the injection lines is provided with DC motor-operated valves powered by storage batteries and a non-return valve.

The discharge pipes of the two reactor cavity water injection and cooling pumps 3 respectively pass through a containment isolation valve and then penetrate through the containment 5 and are then combined into one header which is connected with the header of the passive injection line. In cases where the reffieling water tank is connected with low pressure safety injection pumps, the suction pipes of injection pumps are respectively connected with the suction pipes of the two low pressure safety injection pumps so as to reduce the quantity of the parts penetrating through the containment. Water intake occurs at the internally (or externally) disposed refueling water tank. The suction pipes of the reactor cavity water injection and cooling pump 3 are also connected with a fire protection water source system 6 outside the containment.

The passive injection tank is constmcted of an enclosed reinforced concrete structure and is provided with stainless steel linet All of the pipes and fillings of the system are consfructed of austenitic stainless steel material.

Following a reactor core damage accident, an alarm will be issued when the reactor core outlet temperature reaches 650°C and the CIS system is required to be put into service. The active part of the CIS system is first initiated and one of the two injection pumps is started to intake water from the internally (or externally) disposed refueling water tank so as to allow continuous reactor cavity injection cooling. When the water level in the internally (or externally) disposed refueling water tank is temporarily unavailable, the CIS system is connected with the fire protection water piping inside the containment building through a temporary pipe which supplies cooling water for the CIS system. When the internally disposed refueling water tank restores operation, the CIS system may be disconnected from the fire protection piping and resume the use of the water supply in the refueling water tank.

After the active part of the CIS system is put into service, the coolant injected into the reactor cavity flows through the surface of the pressure vessel to take away the heat generated from the molten corium inside the pressure vessel and flows out through the gaps between the main piping and the reactor cavity and is ultimately collected into the refueling water tank. Injection pumps deliver the coolant back into the reactor cavity, thus establishing a continuous circulation cooling.

If the active part of the CIS system becomes unavailable (e.g., a station blackout occurs and the emergency diesel generator is also unavailable), the operator may manually open the local DC motor-operated valves powered by storage batteries in the main control room or at outside of the contaimnent. From the passive injection tank, cooling water is injected between the thenial insulation and the pressure vessel and passive injection into the reactor cavity occurs under the effects of gravity according to the change in heat release from the molten corium. Initially, the larger-diameter line at upper side and the smaller-diameter line at lower side operate together to flood the reactor cavity. Subsequently, the smaller-diameter line at lower side supplies continuous make-up water to the reactor cavity, resulting in the outer wall of the pressure vessel being flooded in the coolant all the time and, consequently, preventing molten corium from burning through the pressure vessel. As the accident is mitigated, the heat release from molten corium gradually decreases and the required amount of cooling water is gradually reduced. In this process, the water level in the passive water tank is dropping and the cooling water flow rate capable of being supplied from the passive water tank is gradually declining as well. To accommodate the requirements for maintaining the cooling water flow rate and the cooling water level in the reactor cavity, it may be necessary to provide make-up water for the water tank. Following a severe accident, the passive heat removal system (PCS) will also be initiated to remove the heat from inside the containment to the atmosphere of the ultimate heat sink where desuperheating and pressure reduction occurs. For the structure of the PCS system, refer to Chinese patent application No. 2012 10090809.9. The PCS system condenses the steam contained in the containment atmosphere and the resulting product flows downwards in a passive way along the wall surface of the heat exchanger inside the containment under the effects of gravity and is ultimately collected into the passive reactor cavity water injection tank. In this way, make-up water is supplied to the water tank in a passive way in case of a station blackout. Also, the maintenance personnel shall restore the operation of the active part of the CIS system and the safety spray system to permit long-term cooling of molten corium as well as long-tenn pressure reduction and cooling of the atmosphere in the containment. After the active part of the CIS system restores the operation, injection pump continues to inject cooling water into the reactor cavity. Even if the motor-operated valves powered by storage batteries cannot be closed, the non-return valve is cable to protect the passive reactor cavity water injection tank from being contaminated.

After the severe accident is mitigated and when the operator believes that the risk of damage to the lower head of the pressure vessel no longer exists, the operator will close and shut down the above equipment.

Embodiment 2: The present invention also provides the combined active and passive reactor cavity water injection cooling system (CIS) in another configuration. It differs from embodiment I mainly in that the passive reactor cavity water injection tank is disposed outside the containment and the passive injection lines connected with the passive reactor cavity water injection tank are respectively connected with the discharge pipes of the two injection pumps. The refueling water tank which acts as a cooling water source may alternatively be disposed outside the containment.

The start-up mode and operating process of the CIS system in embodiment 2 is similar to those in embodiment 1. However, as the passive reactor cavity water injection tank is disposed outside the containment, it becomes impossible that the steam condensate in the PCS system be utilized to supply make-up water to the passive reactor cavity water injection tank. As such, additional external make-up water lines may be provided.

The above disclosure is related to the detailed technical contents and inventive features thereof People skilled in this field may proceed with a variety of modifications and replacements based on the disclosures and suggestions of the invention as described without departing from the characteristics thereof.

Nevertheless, although such modifications and replacements are not frilly disclosed in the above descriptions, they have substantially been covered in the following claims as appended.

Claims

Claims 1. A combined active and passive reactor cavity water injection cooling system, comprising a passive injection tank (I) and injection pump (3), the passive injection tank (1) being connected to a reactor cavity (2) through passive injection lines, the injection pump (3) being disposed outside a containment (5), an suction pipe of the injection pump (3) being connected to a refueling water tank (4), a discharge pipe of the injection pump (3) penetrating through the containment (5) to be connected with the reactor cavity (2).
2. The combined active and passive reactor cavity water injection cooling system as claimed in claim 1, wherein the passive injection tank (1) is disposed inside the containment (5), the passive injection lines connected with the passive injection tank (1) consist of one upper injection line and one lower injection line which have different diameters, the two injection lines are combined into one header which penetrates inside the reactor cavity to connect with a thermal insulation of the pressure vessel.
3. The combined active and passive reactor cavity water injection cooling system as claimed in claim 1, wherein the passive injection tank may be disposed outside the containment; two injection pumps are provided, the passive injection lines connected with the passive injection tank are respectively connected with the discharge pipes of the two injection pumps.
4. The combined active and passive reactor cavity water injection cooling system as claimed in claim 2, wherein, of the upper and lower injection lines which have different diameters, the upper line has a larger diameter and is used to provide a large flow to flood the reactor cavity at early stage of system commissioning; the lower line has a smal]er diameter and is used to maintain a long-term injection flow to the reactor cavity; each of the injection lines respectively has a DC motor-operated valve powered by a storage battery and a non-return valve provided.
5. The combined active and passive reactor cavity water injection cooling system as claimed in claim 2, wherein two injection pumps (3) are provided, the discharge pipes of the two injection pumps respectively pass through an isolation valve of the contaimnent and then penetrate through the containment, and are then conThined into one header which is connected with the header of the passive injection lines.
6. The combined active and passive reactor cavity water injection cooling system as claimed in claim 1, wherein the refueling water tank connected with the suction pipes of injection pumps is disposed at a sump inside the containment below the reactor core.
7. The combined active and passive reactor cavity water injection cooling system as claimed in claim I or claim 5, wherein the suction pipes of injection pumps (3) are also connected with a fire protection water source system (6) outside the containment.
8. The combined active and passive reactor cavity water injection cooling system as claimed in claim 7, wherein when the retheling water tank is connected with low-pressure safety injection pumps, the suction pipes of injection pumps are connected with suction pipes of the low-pressure safety injection pumps.
9. The combined active and passive reactor cavity water injection cooling system as claimed in claim 1, wherein the passive injection tank is constructed of an enclosed reinforced concrete structure and is provided with stainless steel lineL
10. The combined active and passive reactor cavity water injection cooling system as claimed in claim 1, wherein all of the pipes and fittings of the system are constructed of austenitic stainless steel material.