CA3085050A1 - Method and maintenance of a vacuum building connected to a plurality of nuclear reactor units of the candur type, corresponding suppression and filtered containment discharge system - Google Patents

Abstract

The maintenance method comprises the following steps: - obtaining a suppression and filtered containment discharge system (21) ; - safely connecting the suppression and filtered containment discharge system (21) to the inner volumes of the reactor containments (7), the suppression and filtered containment discharge system (21) being designed for ensuring the following functions: * condensation of steam generated inside the inner volumes of the reactor containments (7) in case of accidents; * keeping the inner volumes of the reactor containments (7) at a slightly negative pressure; * discharging the atmosphere from the inner volumes of the reactor containments (7) to the environment after separation of most of airborne radioactive elements; - safely isolating the vacuum building (1) from the inner volumes of the reactor containments (7) ; - implementing a maintenance operation in the isolated vacuum building (1) isolated from the inner volumes of the reactor containments (7) while the at least one reactor unit (5) is producing power.

Description

I
Method of maintenance of a vacuum building connected to a plurality of nuclear reactor units of the CAN DU type, corresponding suppression and filtered containment discharge system The present invention concerns the maintenance of a vacuum building of a nuclear power station comprised of several nuclear reactor units of the CANDUO type.
As explained in the document "Introduction to CANDU", reference # 150279 R002, August 2016, published by Bruce Power, the vacuum building is a defense in depth safety feature unique to the CANDUO nuclear power station operating in Ontario, Canada.
The vacuum building is a part of the Negative Pressure Containment (NPC) System of the nuclear power station.
Each nuclear reactor unit has a reactor containment. The reactor containment is a substantially leak tight envelope that contains all nuclear systems that could release radioactivity. It is the fourth of the five barriers preventing the release of radioactivity to the public.
The NPC System comprises an overall containment envelope including the containments of the nuclear reactor units and the vacuum building, which is connected to the containments of the nuclear reactor units by several pressure relief ducts.
The NPC System has two functions.
During normal operation, the inner volume of the reactor containments is kept slightly negative with respect to atmospheric pressure so that any leakage is into the containments rather than out. There is normally some tritium and other potential airborne contamination present inside the containments. To minimize releases of radionuclides, the air is dried and filtered prior to being released.
The second function is to contain the radioactive steam and other contaminants that would be present following a potential LOCA (Loss Of Coolant Accident) in one of the nuclear reactor units. On receipt of a LOCA signal, all penetrations into the reactor containments close. Steam created by the heavy water leaking from the primary cooling circuit (Heat Transfer System) of the nuclear reactor, is ducted from the reactor vault, via the fueling machine tunnel and the pressure relief duct to the vacuum building.
The radioactive steam and other contaminants passes through large Pressure Relief Valves (PRV) located in the PRV manifold around the vacuum building.
The pressure in the vacuum building is kept much lower than in the containments of the nuclear reactors. The steam is thus drawn into the main chamber of the vacuum building. As the internal pressure of the vacuum building rises, it forces light water that is stored in the emergency water storage tank at the top of the building into a spray header.
From the spray header, the water quenches the steam and thus reduces the main Date Recue/Date Received 2020-06-30

2 chamber and the reactor containments pressure. Within one or two minutes, the reactor containments pressure returns to sub atmospheric. The process may repeat if large amounts of steam are being produced from the LOCA.
The resultant radioactive atmosphere is then sealed inside NPC system and maintained for as long as possible at a sub-atmospheric pressure. The reason for this retention is to allow the short lived radioactive isotopes, particularly 1-131, to decay.
The vacuum building inspection and maintenance program (Vacuum Building Outage - "VBO") occurs roughly every 10 -12 years. It requires all the nuclear power units protected by a single vacuum building to be shut down, for a period of approximately 20-30 days.
The corresponding loss of revenue is extremely high.
It is possible to optimize the VBO in order to decrease the duration of the VBO or extend the intervals between two VB0s. However, the impact of said optimization on the loss of revenue is low.
Accordingly, there is a need for a method of maintenance of a vacuum building which significantly decreases the revenue lost during the maintenance operations.
To this end, according to a first aspect, the invention is directed to a method of maintenance of a vacuum building connected to a plurality of nuclear reactor units of the CANDUO type, the nuclear reactor units having respective reactor containments, said reactor containments having respective inner volumes in fluid connection with one another through fueling machine tunnels and in fluid connection with the vacuum building through at least one pressure relief duct the maintenance method comprising the following steps:
- obtaining a suppression and filtered containment discharge system;
- safely connecting the suppression and filtered containment discharge system to the inner volumes of the reactor containments, the suppression and filtered containment discharge system being designed for ensuring the following functions:
* condensation of steam generated inside the inner volumes of the reactor containments in case of accidents;
* keeping the inner volumes of the reactor containments at a slightly negative pressure;
* discharging the atmosphere from the inner volumes of the reactor containments to the environment after separation of most of airborne radioactive elements;
- safely isolating the vacuum building from the inner volumes of the reactor containments;
- implementing a maintenance operation in the isolated vacuum building isolated from the inner volumes of the reactor containments while the at least one reactor unit is producing power.
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3 The suppression and filtered containment discharge system connected to the inner volumes of the containments allows the continuous operation of the nuclear power units. It ensures the functions previously ensured by the vacuum building.
The cost of the system is moderate compared to the loss of revenue due to the shutdown of the nuclear power units for the complete duration of the maintenance operation.
With the present maintenance method, the nuclear power units can continue to operate, or are briefly stopped, while a bulkhead is installed in the pressure relief duct and isolate the vacuum building from the inner volumes of the reactor containments and also to tie-in the suppression and filtered containment discharge system.
The maintenance method may present one or more of the following features, considered independently or according to any technically feasible combination:
- the vacuum building is safely isolated from the inner volumes of the reactor containments by building at least one bulkhead;
- the at least one bulkhead is built in the at least one pressure relief duct;
- the method comprises a step of arranging at least one passive cooling device in at least one of the reactor containments, designed for passively cooling the atmosphere in the inner volume of said at least one reactor containment;
- the at least one reactor unit is producing power during the step of safely isolating the vacuum building;
- after the end of the step of maintenance, the suppression and filtered containment discharge system remains as a redundant system with the vacuum building.
According to a second aspect, the invention is directed to a suppression and filtered containment discharge system designed to be used in the maintenance method having the features above, - a steam condenser designed to condense steam generated inside the inner volumes of the reactor containments in case of accidents;
- a blower designed to keep the inner volumes of the reactor containments at a slightly negative pressure;
- a filtered containment ventilation system designed to discharge the atmosphere from the inner volumes of the reactor containments to the environment after separation of most of airborne radioactive elements.
The suppression and filtered containment discharge system may present one or more of the following features, considered independently or according to any technically feasible combination:
- the steam condenser comprises a rotary vacuum condenser;
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4 - the rotary vacuum condenser comprises a bottom vessel, a cylindrical wall inwardly defining a rotation chamber, an upper recycling tank located above the rotation chamber and recycling pipes wherein an upper part of the rotation chamber opens up in the upper recycling tank and the recycling pipes bring the upper recycling tank in communication with the bottom vessel, and wherein tangential venturi devices bring the rotation chamber in communication with the bottom vessel;
- the steam condenser comprises a natural circulation condenser with a heat exchanger having a first side arranged to be in thermal contact with the steam and a second side in fluid communication with a body of coolant, the heat exchanger being arranged such that, when the first side is heated by the steam, a natural circulation of coolant is established in the second side with the body of coolant;
- the filtered containment ventilation system comprises a wet filtration device, a dry metal-fibers filter and a molecular sieve;
- the wet filtration device comprises a Venturi scrubber;
- the Venturi-scrubber comprises Venturi nozzles having respective length adapted to said slightly negative pressure kept inside the reactor containments.
Other features and advantages will become apparent from the following description, given purely as an example, in reference to the following figures:
- Figure 1 is a diagram showing in a simplified manner a nuclear power station comprising four nuclear reactor unit of the CANDUO type and the corresponding vacuum building, the nuclear power station being equipped with the suppression and filtered containment discharge system of the invention ;
- Figure 2 is a diagram showing in a simplified manner the suppression and filtered containment discharge system of the figure 1;
- Figure 3 is a diagram showing in a simplified manner the structure of the Rotary Vacuum Condenser of the suppression and filtered containment discharge system of the figure 3;
- Figure 4 is a diagram showing in a simplified manner a natural circulation condenser of the suppression and filtered containment discharge system of the figure 3; and - Figure 5 is a diagram showing in a simplified manner the Venturi scrubber of the suppression and filtered containment discharge system of the figure 3.
The present method is specially adapted to the maintenance of a vacuum building 1 of a nuclear power station 3 comprising a plurality of nuclear reactor units

5 of the CANDUO type.
Date Recue/Date Received 2020-06-30 CANDUO nuclear reactors are well known and will not be described in details here.
CANDUO nuclear reactors are pressurized nuclear reactors, using natural uranium as a fuel and heavy water as a moderator. The primary coolant is heavy water.
CANDUO
nuclear reactors are operating in Canada and in other countries such as Pakistan, India, Argentina, South Korea, Romania and China.
The vacuum building is a safety feature unique to the CANDUO power stations of Ontario, Canada. Several such power stations are currently operating:
Darlington, Bruce A, Bruce B and Pickering.
In such nuclear power stations 3, the nuclear reactor units 5 have respective reactor containments 7.
As indicated above, the reactor containment 7 is a substantially leak tight envelope that contains all nuclear systems of the reactor unit that could release radioactive materials. Said systems include at least the reactor core, with the calandria and the fuel bundle, the primary loop, and the moderator cooling loop.
As shown on the figure 1, the reactor containments 7 have respective inner volumes in fluid connection with one another through fueling machine tunnels 9 and in fluid connection with the vacuum building 1 through at least one pressure relief duct 11.
In a CANDUO nuclear reactor, the fuel bundles are contained in metal pressure tubes of about 10 cm diameter. The pressure tubes are contained in the calandria, which contains the heavy water moderator. Each pressure tube is enclosed in a calandria tube. Carbon dioxide gas in the gap between the two tubes acts as an insulator.
The pressure tubes are refuelled on-line, using refuelling machines. Two refuelling machines are positionned on opposite faces of the calandria and open the end caps of a pressure tube. One machine pushes in the new fuel, whereby the depleted fuel is pushed out and collected at the other machine.
A spent fuel storage is attached to the central service area (or auxillary building) 13. The containment 14 extends through a portion of the central service area that connects all the reactor units 5, called auxilliary containment in the present description, the inner volume of which is connected to the inner volumes of the reactor containments 7 by additional fueling machine tunnels 9.
In the example depicted on the figure 1, the nuclear power station 3 comprises four nuclear reactor units 5.
The nuclear reactor units 5 and the auxilliary building 13 are arranged along a line, the auxilliary building 13 being center with two nuclear reactor units 5 on each side.
The two nuclear reactor units 5 on each side are connected to one another by a fueling machine tunnel 9.
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6 The auxilliary building 13 is connected to the nuclear reactor units 5 which are on both sides of the auxilliary building 13 by two additional fueling machine tunnels 9.
The fueling machine tunnels 9 are arranged such that the fuelling machines can travel from the auxilliary building 13 to each of the nuclear reactor unit 5 along the fueling machine tunnels 9. The fueling machines can thus bring fresh fuel to each nuclear reactor unit 5 and remove spent fuel from each nuclear reactor unit 5.
The vacuum building 1 is located on one side of the line, and is arranged for protecting all the nuclear power units 5 of the station 3.
The nuclear power station 3 comprises in the present example two pressure relief ducts 11, connected to the two additional fueling machine tunnels 9 on both sides of the auxilliary building 13.
The reactor containments 7 of the nuclear reactor units 5, the vacuum building 1, the pressure relief valve manifold 19 and the pressure relief duct(s) 11 are parts of a Negative Pressure Containment (NPC).
The NPC comprises as well the auxilliary containment 14 of the auxilliary building 13.
The vacuum building 1 comprises a main chamber 15, in the upper part of which are arranged an emergency water storage tank and spray headers (not shown on the figures).
Pressure relief valves 17 (PRV) are arranged in a PRV manifold 19 extending around the vacuum building 1.
The PRV manifold 19 is in fluid connection with the the pressure relief duct(s) 11.
In normal operation, the PRV isolate the PRV manifold 19 from the main chamber 15 of the vacuum building 1.
The part of the NPC comprising the reactor containments 7, the auxilliary containment 14, the fueling machine tunnels 9, the pressure relief duct(s) 11 and the PRV
manifold 19 is maintained at a slightly negative pressure with respect to the atmosphere.
Said pressure is typically about -3 kPa.
The part of the NPC comprising the main chamber 15 of the vacuum building is kept at significantly lower pressure. Said pressure is typically about -90 kPa with respect to the atmosphere.
As indicated above, a first function of the NPC is making sure that any leakage is into the reactor containments 7 rather than out.
A second function is to contain the radioactive steam and other contaminants that would be present following a potential LOCA (Loss Of Coolant Accident) in a nuclear Date Recue/Date Received 2020-06-30

7 reactor unit or any other accident involving pressure buildup inside the containments of the reactor pressure units.
On receipt of a LOCA signal, all penetrations into the NPC close and the PRV
open. Steam created by the heavy water leaking from the primary cooling circuit is ducted from the reactor containments, via the fueling machine tunnels and the pressure relief duct(s) to the main chamber 15 of the vacuum building 1.
As the internal pressure of the main chamber 15 rises, it forces light water that is stored in the emergency water storage tank into the spray header. The water is projected by the spray header into the main chamber 15, quenching the steam and thus reducing the main chamber and containment pressure.
According to the invention, the maintenance method comprises the following steps:
- obtaining a suppression and filtered containment discharge system 21 ;
- safely isolating the vacuum building 1 from the inner volumes of the reactor containments 7;
- safely connecting the suppression and filtered containment discharge system 21 to the inner volumes of the reactor containments 7, - implementing a maintenance operation in the isolated vacuum building 1 isolated from the inner volumes of the reactor containments 7 while the at least one nuclear reactor unit 5 is producing power.
Preferably, the at least one nuclear reactor unit 5 is producing power during the step of safely isolating the vacuum building 1.
It continues producing power as well during the step of safely connecting the suppression and filtered containment discharge system 21.
More generally, that at least one nuclear reactor unit 5 is never stopped and continues producing power at every step of the process.
According to another embodiment, the at least one nuclear reactor unit 5 is stopped and does not produce power during the step of safely isolating the vacuum building 1. It is restarted after said step is completed.
In this case, the at least one nuclear reactor unit 5 is stopped and does not produce power also during the step of safely connecting the suppression and filtered containment discharge system 21.
The suppression and filtered containment discharge system 21 is designed for ensuring the following functions:
* condensation of steam generated inside the inner volumes of the reactor containments 7 in case of accidents, especially in case of LOCA;
* keeping the inner volumes of the reactor containments 7 at a slightly negative pressure;
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8 * discharging the atmosphere from the inner volumes of the reactor containments 7 to the environment after separation of airborne radioactive elements to meet regulatory guidelines.
In other words, the suppression and filtered containment discharge system 21 is designed for duplicating the functions ensured by the vacuum building in normal operations of the nuclear power station.
The suppression and filtered containment discharge system 21 is described in detail further down.
The suppression and filtered containment discharge system 21 is arranged in the vicinity of the nuclear reactor units 5.
The suppression and filtered containment discharge system 21 is arranged advantageously on a platform which is qualified for seismic resistance to meet regulatory requirements.
The vacuum building 1 is safely isolated from the inner volumes of the reactor containments 7 by building at least one bulkhead 23.
Typically, the bulkhead(s) 23 is built in the pressure relief duct(s) 11. One bulkhead 23 is built in each pressure relief duct 11 and can be anywhere along the pressure relief duct that provides easy installation and removal.
The bulkhead 23 is designed to fluidly isolate an upstream part 25 of the pressure relief duct 11 communicating with the reactor containments 7 from a downstream part 27 communicating with the vacuum building 1.
The bulkhead 23 is designed to withstand the maximum design pressure occurring inside the NPC in case of LOCA or other postulated accident.
It is for example made of metal panels and seals, or constructed in any other adapted manner.
The suppression and filtered containment discharge system 21 is connected to anywhere upstream of part 25 of the pressure relief duct(s) 11, but typically is connected to the auxiliary building containment 14 According to another embodiment, the suppression and filtered containment discharge system 21 comprises several sub-systems, each able to ensure the functions identified above. Said sub-systems are connected anywhere upstream of the bulkhead 23.
Typically, the suppression and filtered containment discharge system 21 is tied-in to an existing spare containment penetration. Such existing spare penetrations are equipped with isolation valves, making it possible to connect the system while at least one nuclear reactor unit 5 is producing power.
Date Recue/Date Received 2020-06-30

9 According to another embodiment, a part or the complete suppression and filtered containment discharge system 21 is installed inside existing structures of the nuclear power station 3.
Preferably, the method comprises a step of arranging at least one passive cooling device 29 in at least one of the reactor containments 7. The at least one passive cooling device 29 is designed for passively cooling the atmosphere in the inner volume of said reactor containment 7.
Said at least one passive cooling device 29 is arranged in the fueling machine tunnels 9.
For example, one such passive cooling device 29 is arranged in each fueling machine tunnels 9.
The passive cooling device 29 is of any adapted type such as closed loop heat pipes, containment cooling condensers or emergency condensers that utilize basic laws of physics, such as gravity and natural convection, enabling them to function without any power or actuation by instrumentation and control (I&C) equipment.
As shown on the figure 2, the suppression and filtered containment discharge system 21 comprises:
- a steam condenser 31, designed to condense steam generated inside the inner volumes of the reactor containments 7 in case of accidents;
- a blower 33, designed to keep the inner volumes of the reactor containments 7 at a slightly negative pressure;
- a filtered containment ventilation system (FCVS) 35, designed to discharge the atmosphere from the inner volumes of the reactor containments 7 to the environment after separation of most of airborne radioactive elements.
For example, the steam condenser 31, the blower 33 and the FCVS 35 are arranged in series, as shown on the figure 5.
The steam condenser 31 has an inlet directly fluidly connected to the NPC.
The blower 33 has a suction inlet fluidly directly connected to the outlet of the steam condenser 31.
The FCVS 35 has an inlet directly fluidly connected to the discharge outlet of the blower 33.
The FCVS 35 has an outlet fluidly connected to a stack 37 or any other mean for releasing a purified gas stream to the environment.
According to another embodiment, the blower 33 is a part of the FCVS 35.
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10 According to another embodiment, the blower 33 is connected upstream the steam condenser 31, or downstream the FCVS 35, or is interposed between different components of the FCVS 35.
According to another embodiment, the steam condenser 31 is independent from the blower 33 and the FCVS 35. It is for example arranged inside the NPC, with its inlet and outlet in direct fluid communication with the inner volume of the NPC.
The steam condenser 31 advantageously comprises a rotary vacuum condenser 39.
An example of a type of rotary vacuum condenser is a jet-vortex condenser that has been used as pressure suppression devices in the confinement systems in nuclear power plants (NPP) equipped with VVER-440 reactors such as Kola NPP units 1 and 2, Novovoronezh NPP units 3 and 4, and Kozloduy NPP units 3 and 4.
A rotary vacuum condenser (RVC) 39 is depicted on the figure 3.
The RVC is a fully passive device, with no moving parts.
It mainly comprises a bottom vessel 41, a cylindrical wall 43 inwardly defining a rotation chamber 45, an upper recycling tank 47 located above the rotation chamber 45, and recycling pipes 49.
The bottom vessel 41 is partially filled with a body water. The inlet of the RVC
opens up in the atmosphere above the body of water. The bottom of the cylindrical wall 43 is submerged in the bottom vessel 41.
Tangential venturi devices 50 are located at bottom of the cylindrical wall 43 and bring the rotation chamber 45 in communication with the bottom vessel 41.
The upper part of the rotation chamber 45 opens up in the upper recycling tank 47.
The recycling pipes 49 have upper ends opening in the upper recycling tank 47 and lower ends opening in the bottom vessel 41. The lower ends are submerged in the body of water.
In case of an accident such as a LOCA, the pressure in the NPC rapidly increases.
As result of this, the body of water in the bottom vessel 41 is pushed through the venturi devices 50 into the rotation chamber 45. The venturi devices 50 create an acceleration of the water, and the water starts to circulate inside the rotation chamber 45. A
vacuum channel is created inside the rotation chamber. The water level in the bottom vessel 41 decreases whereas simultaneously the level in the rotation chamber 45 increases. When the water level at the periphery of the rotation chamber 45 reaches its upper edge, the water starts to flood the upper recycling tank 47 and flows through recycling pipes 49 back into bottom vessel 41. The water spinning in the rotation chamber 45 ensures an efficient Date Recue/Date Received 2020-06-30

11 condensation of the steam from the steam-gas mixture passing through the RVC.
Non-condensable gases are directed to the outlet.
A secondary design feature of the venturi devices 50 is that with motive flow from the bottom vessel 41, suction flow from within the rotation chamber 45 and resulting entrainment in the exit of the venturi device, the large shear surfaces created in the flow are able to capture radioactive elements.
The suppression and filtered containment discharge system 21 advantageously comprises a bypass line 72 arranged for bypassing the steam condenser 31. The bypass line is equipped with an isolation valve 73.
Indeed, the steam condenser 31 is used with Filtered Containment Ventilation System 35 during the initial high-pressure spike from the accident and then the FCVS 35 continues to run on its own for the longer-term.
During the pressure spike, the non-condensable gases are pulled through the steam condenser 31 and through the Filtered Containment Ventilation System 35 prior to discharge. The isolation valve 73 is closed at this stage.
Once the pressure spike is done at the beginning of the accident, the isolation valve 73 is opened to allow the non-condensable gases to flow through the bypass around the steam condenser 31. Actually, without the pressure spike, a steam condenser 31 such as the Rotary Vacuum Condenser doesn't allow flow as it is sealed with water.
The bypass line 72 and the Filtered Containment Ventilation System 35 draw a negative pressure for containment with the blower 33 and exhaust the filtered air outside.
In another embodiment the steam condenser 31 can comprises a natural circulation condenser 51, potentially together with a RVC.
The natural circulation condenser 51 comes in addition to or instead of the RVC
39.
The natural circulation condenser 51 is another fully passive device, with no moving parts.
As shown on the figure 4, the natural circulation condenser 51 comprises a heat exchanger 53 having a first side arranged to be in thermal contact with the steam 54 and a second side in fluid communication with a body of coolant 55.
The heat exchanger 53 is arranged such that, when the first side is heated by the steam, a natural circulation of coolant is established in the second side with the body of coolant 55.
The filtered containment ventilation system 35 comprises a wet filtration device 57, a dry metal-fibers filter 59 and a molecular sieve 61 (figure 2).
Date Recue/Date Received 2020-06-30

12 The wet filtration device 57, the dry metal-fibers filter 59 and the molecular sieve 61 are typically serially arranged, in that order, as shown on the figure 2.
These devices may be separate (figure 2) or combined into one single device (figure 5).
The wet filtration device 57 comprises advantageously a Venturi scrubber 63.
As shown on the figure 5, the Venturi scrubber 63 comprises a tank 65 partially filled with a scrubbing solution. An inlet 67 of the Venturi scrubber 63 is connected to a header 69 distributing the gas into Venturi nozzles 71 at least partially submerged in the scrubbing solution. Each Venturi nozzle 71 has submerged holes to let the scrubbing solution penetrate into the nozzle.
As the gas passes through the throat of the Venturi nozzles 71, the incoming gas flow develops a suction that causes scrubbing solution to be entrained with it. Due to the large difference between the velocity of the scrubbing solution and that of the incoming flow, more than 99.5% of the aerosols are removed. At the same time, the entrained scrubbing solution provides a large mass transfer surface inside the throat of the nozzle, permitting effective sorption of iodine.
The Venturi nozzles 71 have respective lengths adapted to said slightly negative pressure kept inside the containments.
The dry filtration device 59 comprises several metal fibres filter elements, each including several fine layers of metal fibres with increasing density. These filter elements provide a high retention efficiency, especially for the small fraction of micro aerosols penetrating the Venturi scrubber 63.
The filtered gas is then passed through the molecular sieve 61 and from there to the ventilation stack 37.
The molecular sieve 61 is mainly used for retention of iodine, which will increase with prolonged duration of the accident.
The FCVS 35 is designed to provide a removal efficiency of greater than 99.9%
for aerosols and is designed to meet release limits per regulatory guidelines.
The blower 33 is typically a turbo-blower.
The blower is designed to maintain a slightly negative pressure inside the reactor containments 7 during the maintenance operation, for example between -1 kPa and -5 kPa with respect to the atmospheric pressure.
Due to the suppression and filtered containment discharge system 21 at least one, if not all of the nuclear reactor unit 5 can continue to operate independent of the isolation of the vacuum building 1.
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13 The maintenance operation in the isolated vacuum building 1 is implemented with the vacuum building 1 isolated from the inner volumes of the reactor containments 7 and while the at least one nuclear reactor unit 5 is producing power.
It typically lasts between 20 and 30 days.
Said maintenance operation typically comprises the following tasks:
= inspections of spray header & hangers at the top of the vacuum building = inspect dousing tank with divers = inspect concrete = inspect J tube piping = inspect PRVs = maintenance on equipment & airlocks = maintenance on Emergency Filtered Air Discharge System = maintenance on Vacuum Pumps.
After the maintenance operation is completed, the suppression and filtered containment discharge system 21 can be separated from the NPC and put in a storage mode.
The communication between the vacuum building 1 and the inner volumes of the reactor containments 7 is re-established.
For that, the bulkhead(s) 23 are dismantled and evacuated.
.In another embodiment, after the end of the step of maintenance, the suppression and filtered containment discharge system 21 remains as a redundant system with the vacuum building 1, so that the probabilistic risk assessment for any large release frequency during normal operation would be orders of magnitude less for off-site releases.
Date Recue/Date Received 2020-06-30

Claims (12)

14

1.- Method of maintenance of a vacuum building (1) connected to a plurality of nuclear reactor units (5) of the CANDU type, the nuclear reactor units (5) having respective reactor containments (7), said reactor containments (7) having respective inner volumes in fluid connection with one another through fueling machine tunnels (9) and in fluid connection with the vacuum building (1) through at least one pressure relief duct (11), the maintenance method comprising the following steps:
- obtaining a suppression and filtered containment discharge system (21) ;
- safely connecting the suppression and filtered containment discharge system (21) to the inner volumes of the reactor containments (7), the suppression and filtered containment discharge system (21) being designed for ensuring the following functions:
* condensation of steam generated inside the inner volumes of the reactor containments (7) in case of accidents;
* keeping the inner volumes of the reactor containments (7) at a slightly negative pressure;
* discharging the atmosphere from the inner volumes of the reactor containments (7) to the environment after separation of most of airborne radioactive elements;
- safely isolating the vacuum building (1) from the inner volumes of the reactor containments (7) ;
- implementing a maintenance operation in the isolated vacuum building (1) isolated from the inner volumes of the reactor containments (7) while the at least one reactor unit (5) is producing power.

2.- Method according to the claim 1, wherein the vacuum building (1) is safely isolated from the inner volumes of the reactor containments (7) by building at least one bulkhead (23).

3.- Method according to the claim 2, wherein the at least one bulkhead (23) is built in the at least one pressure relief duct (11).

4.- Method according to anyone of the claims 1 to 3, wherein the method comprises a step of arranging at least one passive cooling device (29) in at least one of the reactor containments (7), designed for passively cooling the atmosphere in the inner volume of said at least one reactor containment (7).
Date Recue/Date Received 2020-06-30

5.- Method according to anyone of the claims 1 to 4, wherein the at least one reactor unit (5) is producing power during the step of safely isolating the vacuum building (1).

6.- Method according to anyone of the claims 1 to 5, wherein, after the end of the step of maintenance, the suppression and filtered containment discharge system (21) remains as a redundant system with the vacuum building (1).

7.- Suppression and filtered containment discharge system (21) designed to be used in the method of any one of the claims 1 to 6, comprising:
- a steam condenser (31) designed to condense steam generated inside the inner volumes of the reactor containments (7) in case of accidents;
- a blower (33) designed to keep the inner volumes of the reactor containments (7) at a slightly negative pressure;
- a filtered containment ventilation system (35) designed to discharge the atmosphere from the inner volumes of the reactor containments (7) to the environment after separation of most of airborne radioactive elements.

8.- Suppression and filtered containment discharge system according to the claim 7, wherein the steam condenser (31) comprises a rotary vacuum condenser (39).

9 ¨ Suppression and filtered containment discharge system according to the claim 8, wherein the rotary vacuum condenser (39) comprises a bottom vessel (41), a cylindrical wall (43) inwardly defining a rotation chamber (45), an upper recycling tank (47) located above the rotation chamber (45), and recycling pipes (49), wherein an upper part of the rotation chamber (45) opens up in the upper recycling tank (47) and the recycling pipes (49) bring the upper recycling tank (47) in communication with the bottom vessel (41), and wherein tangential venturi devices (50) bring the rotation chamber (45) in communication with the bottom vessel (41).

10.- Suppression and filtered containment discharge system according to any of the claims 7 to 9, wherein the steam condenser (31) comprises a natural circulation condenser (51), with a heat exchanger (53) having a first side arranged to be in thermal contact with the steam and a second side in fluid communication with a body of coolant (55), the heat exchanger (53) being arranged such that, when the first side is heated by the steam, a natural circulation of coolant is established in the second side with the body of coolant (55).
11.- Suppression and filtered containment discharge system according to anyone of the claims 7 to 10, wherein the filtered containment ventilation system (35) comprises a wet filtration device (57), a dry metal-fibers filter (59) and a molecular sieve (61).
Date Recue/Date Received 2020-06-30 12- Suppression and filtered containment discharge system according to the claim

11, wherein the wet filtration device (57) comprises a Venturi scrubber (63).
13.- Suppression and filtered containment discharge system according to the claim

12, wherein the Venturi-scrubber (63) comprises Venturi nozzles having respective length adapted to said slightly negative pressure kept inside the reactor containments (7).
Date Recue/Date Received 2020-06-30