The reactors of the Ignalina NPP are protected by a pressure suppression type containment which, because of its specialized nature, is referred to as the Accident Confinement System (ACS). It differs from the standard pressure suppression containments in several respects. The major of which are :
First three features are similar to the confinement system used in the VVR - 440/Mod213 Soviet built PWR's, the last feature seems to be unique. The original design documentation for this system was sparse, and a limited number of studies were available which analyzed the response of the ACS to accidental conditions. For this reason considerable effort was devoted to the determination of relevant ACS parameters and to the evaluation of ACS response to a broad range of DB and SB-LOCA events. Design parameters were determined from construction drawings and where uncertainties persisted, they were verified by direct measurement in the plant. The accident types evaluated include the entire spectrum of break sizes and break locations. Results of these studies are available in [86].
6.3.1 Purpose and Applicability
A characteristic feature of nuclear power plants built in the West is the containment, or protective shell. This is a large, especially strong, steel and reinforced concrete building, usually semi cylindrical in shape, which encloses the reactor and its cooling circuits. The Ignalina NPP does not have such a containment structure. Almost all components of the power plant are located in large, interconnected, traditional buildings. This external image of the power plant has contributed to the widespread opinion that the Ignalina NPP has no containment system. That is not true. In fact, both the reactor and a large portion of the cooling circuit are enclosed in reinforced protective shells. In addition, a complicated large scale system is provided whose purpose is to condense the steam erupting from the cooling circuit during a possible accident. This equipment takes up a significant fraction of the volume of the central power plant building.
The Ignalina nuclear power plants are cooled by high pressure water. While the reactor is on line, most of the thermal energy is found, not in the core, but in the cooling circuit. Therefore, if an accident occurs which damages the cooling circuit (pipe break, valve stuck open, etc.), the main function of the containment system is to prevent the released high pressure steam-water mixture from reaching the atmosphere. Generally, there are two containment types which achieve this result.
The most common type is called "dry containment". This is also the simplest type: a sufficiently large and strong building, which can withstand the pressure reached when the entire contents of the cooling circuit is suddenly lost from the circuit (within half a minute). In this case, it is possible for the pressure to reach 4 atmospheres. Containment buildings are correspondingly large and expensive.
The second, or "pressure suppression" containment type uses additional equipment which condenses a part of the released steam to reduce the peak pressures which can be reached during an accident. Internally, the containment structure is divided into at least two major volumes, which are interconnected by a flow path submerged under a permanent pool of water. When the steam enters the first section, the pressure rises, and in order to reach the second section, the steam-air mixture must "bubble" through the water. This condenses most of the steam, reducing the peak pressure attained.
The system used at the Ignalina NPP [50] belongs to the second category of containment. It differs from containment systems common in Western nuclear plants in several important aspects:
In many other respects the Ignalina NPP ACS has features similar to traditional pressure suppression systems used in the West. Spray nozzles are located in several regions where steam can accumulate, It has provisions for condensing steam released during operational transients (e.g. opening of main safety valves) and the spray water is cooled by the condenser tray cooling system.
The ACS performs its function of limiting pressure buildup and containment of potentially radioactive gases released from the primary system in the following way:
The venting of uncontaminated air and the use of sprays makes it possible to lower the ACS pressure with respect to the atmospheric pressure. This reduces the probability of release of radioactive material to the environment. In addition the ACS serves as a water reservoir. The stored water can be used for emergency core cooling, as well as to condense steam released by the pressure relief valves.
6.3.2 Design Characteristics of the ACS
An overall schematic of the ACS representing a cross-section through the main power plant building is presented in Fig. 6.2. As shown in the diagram, a significant fraction of the total volume of this building is assigned to the accident confinement system. Its principal components are two interconnected ACS towers, each of which contain five vertically-positioned condensing trays. The water in the pools of these trays, is the medium which serves as the first barrier to the steam emitted during a LOCA event. The water of the pools forms a barrier which divides the entire system into two distinct parts:
1 - The compartments in front of the pools. They encompass the primary circuit piping where a potential break and the release of high pressure water is most likely.
2 - The compartments beyond the condensing pools. The non-condensibles present in the first part of the ACS are pushed into t hese compartments. Most of the released steam is condensed as the s team-air mixture bubbles through the condensing pools.
Fig. 6.2 Principal ACS schematic
1 - fuel channel, 2 - main circulation pump, 3 - suction header, 4 - pressure header, 5 - group distribution header, 6 - ECCS header, 7 - condensing pools, 8 - ACS condenser tray cooling system, 9 - air discharge pipe section, 10 - pipe for removal of contaminated steam from protection valve, 11 - pipe for removal of contaminated steam from broken fuel channel
The compartments which surround the reactor and the components of the primary circuit, are reinforced and capable of withstanding elevated pressures. Their design pressure is shown in Table 6.5. They can be divided into the following two subgroups:
The principal design features of the Accident Confinement System are presented in Table 6.6. The capacity of system is designed to isolate the steam escaping from the primary circuit in case of the Design Basis Loss of Coolant Accident (DB-LOCA) which in the case of an RBMK plant is taken to be the break of the pressure header. The design capacity of the ACS in this accident situation is shown in Table 6.5.
6.3.3 ACS Structural Characteristics
The structures housing the accident confinement system are divided into two parts, which are located in separate towers (Fig. 6.2). This feature was dictated by the plant layout requirements. However, both halves of the system function simultaneously, and analysis show that they receive an approximately equal portion of the released steam, except for one accident type, which will be discussed below.
The bottom section of the towers contain the steam reception chamber, the steam corridors, and the hot-condensation chamber. In case of an accident, the steam flows through these compartments and subsequently up to the steam distribution channels, which are positioned vertically on both sides of the tower. The lower section of the hot-condensation chamber stores the water reserves for the emergency core-cooling system and serves as a collection pool for the condensed steam during a LOCA event. The hot- condensation chamber is connected to the steam reception corridors by ten overflow
Table 6.5 Principal ACS design characteristics [50,51]
|
Parameter |
Quantity |
||
|
Maximum accident steam flow into the system, kg/s |
14700 |
||
|
Condensed steam before starting the spray system, kg |
30300 |
||
|
Discharged clean air, kg |
25000 |
||
|
Maximum water temperature, oC: |
|||
|
- pools 1 - 4 |
88 |
||
|
- pool 5 |
98 |
||
|
Maximum permissible excess pressure (MPa) in the: |
|||
|
- reinforced leaktight compartments |
0.3 |
||
|
- steam reception chamber |
0.1 |
||
|
- bottom water pipes compartment |
0.08 |
||
|
- gas-holding chamber |
0.08 |
||
pipes. The purpose of these pipes is to direct part of the condensate to the steam-reception corridors. On the corridor side the pipes have hydrolocks to prevent steam from flowing into the hot condensate chamber.
The hot-condensate chamber is joined to an air removal channel to the gas-holding chamber, which is located in the upper part of the ACS tower. This channel conducts the air forced from the hot-condensate chamber and the condensation pools to the holding chamber. The spill-over water from the condensing trays falls through this channel to the bottom of the hot-condensate chamber sump.
Sprays are used in the top part of the reception chamber, and also in the gas-holding chamber, to condense the steam which remains in the ACS compartments. The over-pressure valve is built into the wall of the reception chamber, their purpose is to release air from the compartment of heat exchangers of condensate tray cooling system to the reception chamber, when the pressure there decreases below atmospheric levels.
Five concrete trays holding condensation pools are located above the hot-condensate chamber. The pools contain the water reserves needed for condensing the steam released during a LOCA event. A schematic diagram of the first four condensation pools (numbering from the bottom) is shown in Fig. 6.3. The space above the pools is connected by air passageways to he gas-holding chamber. The height of the overflow barrier (2.1 m) was chosen, by considering the expansion of the water caused by the bubbling of the steam. To maintain the initial water level of 1.1 m, there are 100 mm holes. Rectangular holes, distributed at a height of 1.2 m, allow the condensate and hot-water to overflow and spill into the hot- condensate chamber.
Table 6.6 ACS components [50,51,62]
|
Parameter |
Quantity |
|
|
Volume (m3) of water in condensing pools: |
||
|
- pool 1 - 4 |
2860 |
|
|
- pool 5 |
745 |
|
|
Volume of cooling system water, m3 |
1000 |
|
|
Initial water temperature in the pools, oC |
30 |
|
|
Volumes (m3) of ACS facility: |
||
|
- compartments containing main circulation pumps, suction and pressure headers, ECCS headers (reinforced leaktight compartments) |
2 x 6000 |
|
|
- compartments up to the ACS towers (reinforced leaktight compartments) |
4000 |
|
|
- compartment enclosing the pipes leading to the fuel channels and spaces below the reactor |
4200 |
|
|
- summed volume of the steam receiving chambers in the ACS tower, and the steam distribution headers in the bottom four condensing pools (volume to the top of the barbotage) |
2 x 2300 |
|
|
- total spaces above the water surface in the condensing pools, the air venting channel, and the channel to the first gas holding chamber |
2 x 4935 |
|
|
- gas-holding chambers, not including the channel to the first chamber |
2 x 10170 |
|
Fig. 6.3 Condensing compartment and pool
1 - condensate overflow to hot-condensate chamber, 2 - cooled water supply, 3 - air and non-condensing gas removal to gas holding chamber, 4 - water supply for filling first tray, 5 - water to purification system, 6 - contaminated steam from steam reception chamber
The bottom four condensation pools are intended for the condensation of steam from the compartments in which the main circulation pumps and the group distribution headers are located. They are al similar, except for the bottom pool which is somewhat smaller (see Fig. 6.3). Part of the cross-section of the bottom pool is taken up by the steam reception chamber. In each pool, there are 10 steam distribution devices, each about 20 m long. The bottom pool has 7 devices 20 m long, and 3 devices 10 m long. These are 800 mm diameter pipes connected to rectangular, sheet metal downcomers which have a total height of 1.2 m and are submerged to a depth of 0.9 - 1.05 m in the condensation pool water. A scaled representation of the vent and the rectangular channels including a summary of the geometrical characteristics of vents and downcomers is presented in Fig. 6.4. There are 10 headers per tray, this results in a total flow area of 11.93 m2 for condensation trays 2 to 4. At the exiting end the rectangular downcomer channels are provided with a saw-tooth edge in order to better distribute the steam and to reduce condensation type oscillations. The steam distribution devices connect to the steam distribution channels, and through them to the steam reception chamber.
|
Condensation pool |
Length of devices, m |
Number of steam distribution devices in both ACT |
Downcomers length L, m |
Downcomers number |
|
1 |
19.9 9.95 |
14 6 |
1.6 1.6 |
336 72 |
|
2 - 4 |
19.9 |
60 |
1.6 |
1440 |
|
5 |
8.5 7.5 |
38 2 |
2.29 2.10 |
456 20 |
Fig. 6.4 Sketch of steam distribution devices
The fifth (top) pool (Fig. 6.5) is intended for the condensation of steam from the pressure relief valves. 600 mm diameter pipes from the SDV-A and MSVs conduct steam into the rectangular chambers in the center of the pool. The structure of the steam distribution devices is analogous to those described earlier. The rectangular downcomers are connected to each other through holes in the columns.
The fifth pool of the left ACS tower is equipped with an additional 23 m long steam distribution device, which has downcomers submerged 1.5 m deep under water. This device is intended for the condensation of steam released during a potential rupture of a fuel channel. Steam is transported to this device from above, by a 600 mm diameter pipe. All of the condensing pools are equipped with cold water distribution headers. Water is provided to these headers from the condensate tray cooling system.
The compartment for confining potential "contaminated" gases is located above the condensation pools, Fig. 6.6. This compartment is divided by vertical partitions into seven channels, positioned in such a manner that air, or other non-condensing contaminated gases coming from the pools during an accident, would pass through them sequentially. A grating is installed in each channel in order to even out the flow along the cross-section.
Ten 800 mm diameter pipes, through which clean air is released into the atmosphere are located in the last compartment. The entrance end of these pipes reaches into a separate section of the compartment. Hollow metal spheres are placed at the bottom of each pipe. In their nominal position they lie on the bottom so that there is a 300 mm space between the ball and the pipe. The water level in the section is kept at about 300 mm. This is adequate for covering the ends of siphons coming from a 100 m3 storage tank located in the top part of the gas-holding chamber. The volume of the storage tank is filled with water and it is connected by a 1500 mm diameter two pipes to the steam reception corridor. In the event of
Fig. 6.5 Fifth steam-condensing pool (left ACS tower)
1 - cooling water, 2 - section for condensing steam from protection valve, 3 - section for condensing steam from fuel channels, 4 - steam pipe from protection valve, 5 - steam pipe from ruptured fuel channel
a LOCA which produces a pressure rise in the inner ACS compartments, a pressure pulse is transmitted through this pipe to initiate the flow of water in the siphon.
The top part of the gas-holding chamber is provided with a set of sprays. In the event of an accident which produces moderate steam releases over a long time period, their purpose is to reduce the partial pressure of the steam in the gas holding chamber and to cool the compartment. Water is provided to these sprays from the condensate tray cooling system.
6.3.4 Condenser Tray Cooling System
The Condenser Tray Cooling System (CTCS) is designed to remove the energy deposited in the condenser tray pools by condensation of LOCA generated steam, and to maintain the prescribed water reserves in the condensing pools during normal and off-normal plant operation. During long-term transients the CTCS becomes the major sink for the removal of the decay energy released as steam into the ACS.
The CTCS is located in enclosures beyond the re-enforced compartments of the ACS. A representation of the CTCS serving one of the ACS towers is shown in Fig. 6.7. The tower itself is shown schematically in the upper portion of the figure, a diagram of the main components of the CTCS and their inter-connections is shown below. The main CTCS components are:
Fig. 6.6 Gas-delay chamber
1 - cooling water, 2 - section of gas-holding chamber, 3 - 100 m3 water tank, 4 - air discharge to environment, 5 - pipe transmitting the pressure pulse, 6 - siphon, 7 - floating ball
Fig. 6.7 Condenser tray cooling system
1 - heat exchanger, 2 - valve, 3 - water pump, 4 - check valve, 5 - mass flow rate gauge, 6 - ACS tower
As shown in Fig. 6.7, the main water supply is the ACS Hot Condensate Chamber (HCC), water is drawn from the HCC pool through five 400 mm diameter pipes. The valves provided on the suction side of the system are normally maintained open. To ensure reliable operation in the case any two pumps are not available, a 600 mm interconnection is provided at both the pump suction and pressure sides. An alternate water supply is the sump of the Bottom Steam Reception Chamber (BSRC) which receives the overflow condensate from the HCC and the runoff from the BSRC sprays. At both the inlet and the outlet of the heat exchangers valves are normally maintained open.
The CTCS pumps deliver water to the following systems of the ACS Tower:
In Fig. 6.7 the lines supplying the sprays are indicated in the upper left hand and lower right hand corners of the schematic representing the ACS tower.
The secondary side of the ACS heat exchangers is supplied with service water via a 1200 mm header at a flow rate of 8400 m3/h. The minimum flow through any heat exchanger is rated at 1500 m3/h. Operator control of the pump units is maintained from the MCR. In case auxiliary power is lost, the pump units and valves are fed off the emergency power sources.
The operation of the ACS condenser tray pool cooling system is initiated when the temperature of water at the four bottom pools reaches 35oC or at the fifth pool - 50oC [88]. This system can also be activated by the ECCS start up signal two minutes after initiation of a LOCA.
For normal power operation, in addition to the outlined CTCS, a separate pump system is available. It is designed to maintain the prescribed water inventory in the level #5 condensing pools and has the capability to exchange water between the two ACS towers during scheduled maintenance. This system consists of two X90/85 K-1-2G type pumps with a capacity of 90 m3/hour generating a hydraulic head of 8.5 kgf/cm2. If the situation warrants it, the auxiliary deaerator makeup system pumps can also be used to supply water to the HCC pool. This system consists of 4 pumps and a 1500 m3 water tank. In addition, the deminiralized water distribution system which includes 3 pumps and 2 tanks of a 5000 m3 capacity each can be switched to supply water both to the HCC pool and to the auxiliary deaerator makeup system.
The availability of the CTCS is one of the requirements of normal plant operation. Thus operating procedures require the shut down of the reactor unless the following equipment failures or deviation from the design norms can be corrected:
6.3.5 Reactor Cavity Overpressure Protection System
Protection of the reactor cavity against overpressurization is an important part of the safety system of RBMK-1500. This system is intended to ensure reactor vessel leaktigthness and integrity of reactor metal structures under accidents caused by a rupture of a fuel or special channel, with resulting pressure growth in reactor cavity. The protection system provides:
A simplified schematic of reactor pressure relief system is shown in Fig. 6.8. In the reactor pressure relief system, a steam-gas mixture is discharged from the top of the reactor cavity through four 300 mm diameter and one 600 mm diameter pipes located along the periphery of upper metal structure. The pairs of 300 mm diameter pipes and 600 mm diameter pipe are connected to four 400 mm diameter pipelines that penetrate the boundary of ACS. Within the ACS, each pair of the 400 diameter mm pipes is connected to one 600 mm diameter pipeline which terminates at the header of the steam distribution devices within the fifth suppression pool, and they also connected to one group of the three membrane safety devices (350 mm diameter) within the ACS. The bottom pipes terminate in the second group of the three membrane safety devices within the ACS. When the membrane safety devices perforate (i.e., if setpoint of 59 kPa i s exceeded), the steam- gas mixture is discharged into the ACS suppression pools 1 to 4.
The requirement for overpressure protection system is to have the capacity for coping with design basis accident involving one pressure tube rupture. Cavity pressure exceeding 0.314 MPa has been described as having the possibility to lift the upper-head shield assembly breaking the reactor seal, the pressure tubes, and affecting the operation of other safety functions.
The efficiency analysis of the reactor pressure relief system installed at Ignalina plant in 1996 showed that the capacity of the existing system makes it possibly to withstand multiple rupture of up to 9 pressure tubes under a conservative assumption of simultaneous guillotine ruptures.
Fig. 6.8 Simplified schematic of the reactor pressure relief system
1 - reactor, 2- the fifth ACS suppression pool, 3 - suppression pools 1-4, 4 - steam distribution devices, 5 - membrane safety devices (350 mm diameter)
6.3.6 Description of ACS Response
The reactor cooling circuit of an RBMK-type reactor is rather complicated. The core is cooled by 1661 separate channels. These provide steam to four steam separators. The water and condensate mixture from each separator is returned to the core by three (of four) pumps. Thus the system consists of pipes of various diameters. The impact on the accident confinement system, i.e. the quantity of steam and hot water released, will depend on the size of the broken pipe. To be conservative, it is assumed that it is the biggest pipe in the in a given piping section. The DB-LOCA event, that is the largest break for the entire system is a complete guillotine rupture of the 900 mm diameter pressure header.
6.3.6.1 Accident in the Reinforced Leaktight Compartment
The ACS response to a loss-of-coolant-accident depends to a marked degree upon the location and the nature of the break which leads to primary system coolant loss. The DB-LOCA analysis assumes the rupture of a Pressure Header (PH) (4) (see Fig. 6.2) The initial blowdown rate of the steam produced by such an event is about 14.7 t/s [38]. From the break compartment the flashed vapor and compartment air first flows through the steam-release tunnels to the BSRC of both ACS towers. From there, it passes along the corridors leading to the steam distribution headers of the bottom four condensation pools and is forced to bubble through the pool water. During barbotage the steam condenses, while the air and other non-condensable gases pass through and continue to the air- removal channel and the gas-holding chamber. From there part of the clean air is vented to the atmosphere.
The venting systems functions as follows. The pressure increase in the steam-removal tunnels, is transmitted to the 100 m3 volume tank (3) (Fig. 6.5) through the impulse-transmitting pipe. The additional pressure initiates flow in the siphons (6) and water starts to flow into the air venting compartment sump. When the water level rises, the hollow metal spheres (7) float and close the inlets of the pipes (4). The pipes are closed within ~5 minutes.
After ECCS signal initiation (1-2 seconds after the break) the CTCS pumps starts. During next 5 minutes the nominal flow rate is reached. Cooled water is provided to the four condensation pools and to the sprays in the steam-reception and the gas holding chambers (Fig. 6.2). The condensed steam in the pools leads to an increase in the water level from 1.1 to 1.2 m, there it overflows weirs and spills out into the hot-condensate chamber. Since the height of the overflow barrier is 100 mm above the initial level of water in the pool, overflow from the pool begins ~20 seconds after the start of steam condensation. This prevents entrainment of water in the air current into the air removal channel. The gas velocity at this time is lower than 3 m/s, and the amount of spray carried away is small.
The hot-condensate chamber can hold additional 152 t of condensate, the excess, (~360 t) flows through the overflow openings into the steam-removal corridor and reception chamber.
Due to the condensation of the steam in the pools and the reception chamber (by the spray system), the pressure at the accident location begins to decrease. When the pressure at the accident location is lower by about 5 kPa compared to the pressure in the compartment of heat exchangers at condenser tray cooling system, the over-pressure valves open and air from the compartment of heat exchangers flows to the steam-reception chamber.
6.3.6.2 Rupture of the Group Distribution Header
The Group Distribution Headers (GDH) are important components of the primary system which accept coolant water from the PH and distribute it to the core channels. A schematic of the piping between the PH and the GDH is shown in Fig. 6.9. As shown in the diagram, in order to reach the GDH (6), coolant water must pass through a flow limiter (2), a valve (3), a check valve (4) and enter the GDH through a ‘water mixer’(5) which in the event of a LOCA combines the primary system flow with ECCS water. The schematic also shows a bypass line (8) and three entry ports for ECCS water (10).
The rupture of a GDH has disrupts the coolant supply to ~40 fuel channels and generates a pressure transient in the ACS compartments. A complete rupture of the GDH leads to a reversal of the coolant flow direction in fuel channels (7) and an adequate cooling of the fuel. Of more concern is a partial rupture which could lead to flow stagnation and heat up of the fuel. This problem has been addressed in [87].
Fig. 6.9 GDH rupture after the check valve
1 - pressure header, 2 - flow limiter, 3 - valve, 4 - check valve, 5 - mixer, 6 - GDH, 7 - fuel channels, 8 - bypass line, 9 - flow meter, 10 - ECCS headers
The presence of a check valve and their influence on the direction of break and ECCS flow divides potential GDH breaks into two general types:
a) break before the check valve,
b) break after the check valve and the ECCS mixer, but before the piping to the fuel channels (Fig. 6.9).
Case b generates a more intense break flow and is of more interest as a possible challenge to the ACS. Reference [86] analyzes a broader range of scenarios, here only the case which has the maximum impact on the ACS will be considered.
Through the rupture the steam-water mixture discharges into the bottom water pipes compartments. After the pressure of steam at these compartments exceed 59 kPa, the special valves are opening and steam-water mixture flows through the steam release tunnel to both ACS towers. Further, the system works analogously to an accident in the Reinforced Leaktight Compartment. A GDH break represents a ‘mid-size’ LOCA. Thus the short-term loads imposed by the accident are substantially smaller then in the case for the DB-LOCA case considered in Subsection 6.3.6.1. They approach a peak value of ~130 kPa and thus do not threaten ACS integrity. The real challenge to the ACS is to ensure that the long term confinement capability is not impaired. This has a special importance for this event, because there is a small probability that a GDH break event could be accompanied by partial fuel damage and thus could have radiological consequences.
Long term analysis have been carried out which indicate that the ‘long term’ peak pressure (which occurs approaches, but does not exceed the peak pressures achieved short after the break [86].
6.3.6.3 Rupture of a Fuel Channel
Rupture of a fuel channel releases steam and liquid water into the inner spaces of the reactor graphite block. The graphite has a large heat capacity and elevated temperature, thus most of the liquid is vaporized. The steam-gas mixture from the reactor load goes through eight 300 mm diameter and one 600 mm diameter pipes, then through four 400 mm diameter headers to the steam distribution devices (4) and to the membrane safety devices (5) (see Subsection 6.3.5, Fig. 6.8).
After the temperature of water at the fifth pool reaches 50oC, the CTCS is switched on, cooled water is provided to the fifth condensation pool and to the sprays in the gas-holding chamber. Meanwhile, the water pipes from the cooler to CTCS pools 1 - 4 are disconnected. The hot water pours through the overflow holes into the hot condensate chamber, from where it again goes to the CTCS coolers.
The volume of air released to the atmosphere will be equal to the amount of gas coming with the steam until the closing of the release pipes (maximum 1000 m3). Water is provided to the section in order to close the release pipes (see Subsection 6.3.3, Fig. 6.6) by the MCR operator's signal, or automatically, on a signal from radioactivity meters in the outlet of the air release pipes.
6.3.6.4 Inadvertent Opening of the MSV’s
The RBMK-1500 plant is provided with a total of 12 MSV’s. The steam released from the MSV’s during over-pressure events is directed to the top-most condenser trays of the ACS towers. When a protection valve has actuated, one or both ACS towers operate, depending on which high pressure steam loop experienced the abnormal pressure increase and the actuation of the SDV-A or MSVs. In the rest of this description, the operation of a single ACS tower is described. The operation of the other two is analogous. The steam goes through 600 mm diameter pipes to the rectangular chambers in the fifth condensation pool (see Subsection 6.3.3, Fig. 6.5). Afterwards, it goes to the steam distribution devices and then, bubbling through the water, is condensed. Air, displaced from the supply pipes and chambers by the steam, gets to the holding chamber, while an equal volume of clean air is released through the release pipes to the atmosphere. The two valves (vacuum breakers) are build into the wall of these rectangular chambers. Their purpose is to release air from the gas-holding chamber to rectangular chamber, when the pressure inside the chamber is lower by about 1.5 kPa compared to the pressure in the gas-holding chamber.
After the temperature of water at the fifth pool reaches 50oC (no later that five minutes after the opening of the valve) the CTCS switches on, and cooled water is provided to the condensation pool, where it is distributed by means of the water header. Water heated to the maximum temperature, 98 oC, is poured over the barrier to the hot-condensate chamber. There, it mixes with the cold ECCS water. From the hot-condensate chamber, the water again goes to the cooler pumps.
At the same time, cold water from the CTCS coolers is provided to the sprays in the gas-holding chambers. The water pipes to the other pools are closed. The closing of the valves in the lines to pools 1 - 4 and opening to pool 5 and to the spray system occurs automatically. The provision of the water necessary for the closing of the air-release valve is switched on by the operator or by the radioactivity detectors at the exit of the gas release pipes. This reduces the probability that contaminated air would be released to the atmosphere. Due to the long gas path through the channels, the contaminated air cannot reach the release pipes by diffusion during the valve closure period (5-10 minutes). Eliminating the consequences of the opening of the protection valves, water from the hot-condensate chamber and the top condensation pool is cleaned in the power plant's water purification system.
6.3.6.5 Small Pipe Break Accident
The accident events analyzed up to this point assumed that the biggest pipe in the room is the one that breaks and that the pipe ruptures completely. This maximizes the amount of steam and hot water that is released. However, in reality, smaller breaks have a higher probability of occurring. A characteristic of accidents involving small pipe breaks in the MCP room or the bottom water pipes compartments, is that air from the pipes and the accident rooms is pushed out at a slower rate. Therefore, there it is probable that a relatively large amount of air remains in the inner ACS enclosures after the venting pipes in the gas-release room are shut by the action of the siphon. The larger air content increases the gas-holding chamber pressure. A mitigating factor is that the air enters sufficiently slowly that it can be cooled. The spray system is turned on in the gas holding chamber at the time when the gas release pipes are closed, consequently the pressure does not exceed 160 kPa, even in the case when no air is vented at all. The closure time of the gas release pipes is designed to be ~5 minutes so that they would not close before the initiation of the spray system, ensuring isothermal compression of the air. In other respects, the operation of the system does not differ from its operation in the event of a large break accident.
6.3.6.6 Hydrogen Release
During LOCA type accidents hydrogen is released together with steam-water mixture in the ACS compartments. The hydrogen arises in the reactor core from process of radiolysis of water and steam-zirconium interaction of high temperature.
In all of the accident cases considered, if pressure header of MCP breaks, approximately 85 m3 (7.7 kg) of hydrogen is released in the ACS compartments [85]. It is difficult to calculate the hydrogen concentration in the gas- holding chamber accurately, because it is not known what proportion of it is released into the atmosphere with the air. But, assuming the worst case that all of the hydrogen is still in the gas-holding chamber, then its concentration would be 0.29 % of the entire volume. That is much less than the allowed limit, 0.4 % [50], and it is not necessary to take any special precautions to prevent formation of an explosive mixture.
In other pipes and compartments where there is a danger of hydrogen accumulation (steam reception chamber, steam distribution channels, top condensation pool isolation compartments, the top part of the gas-holding chamber), there must be hydrogen concentration meters and forced circulation. If necessary, the hydrogen concentration can be reduced to allowed limits by means of forced circulation.
There are an alarm instrumentation for monitoring hydrogen content in the ACS compartments (32 alarm sensors for each unit). This monitoring capability reduces the probability that explosive hydrogen concentrations could accumulate in the ACS compartments.
6.3.7 ACS Leakage Testing
Leak testing of the ACS is conducted at least once a year during preventive maintenance by visual examination of valves, seals of hatches, leaktight doors, tunnels of pipelines, devices and cables. Results of the testing are recorded.
Pressure testing is performed using plant compressed air or by employing a venting device of the re-circulating system. The BWPC are tested by air pressure up to about 1 kPa and the Reinforced Leaktight Compartments (RLC) and the ACS tower by an excess pressure of about 2 kPa. Test results are compared with the results measured at the start of the unit's operation. During leakage tests visual and audio inspections are conducted in the pressurized compartment. After a stable pressure is reached the pressure source is turned off and the rate of pressure decrease determined using manometers. The experimental results are employed to evaluate the leakage rate and the equivalent leakage area.
Criteria for leakage of the ACS compartment of the Ignalina NPP unit 1 state that total equivalent leakage area should not exceed 0.486 m2 . During testing of the ACS of the Ignalina NPP unit 2 at the start of operation, the equivalent area for the reinforced leaktight compartments was found to be 0.0069 m2. For the ACS towers these corresponding values were 0.0049 (left) + 0.0059 (right) = 0.0108 m2. For the bottom water communication compartments it was 0.0217 m2. The total equivalent leakage area of the ACS for unit 2 was 0.0394 m2. Typical results for the ACS of unit 1 and unit 2 leakage test are shown in Tables 6.7 and 6.8, respectively.
Table 6.7 ACS leakage test results of the Ignalina NPP unit 1 [52]
|
Compartment |
Excess testing pressure, kPa |
Range of pressure changes, kPa |
Time of pressure drop, s |
Equivalent area of leakage, m2 |
|
RLC |
1.45 |
1.45 to 0.5 |
13 |
0.294 |
|
BWPC+RLC |
1.20 |
1.20 to 0.5 |
11 |
0.0455 |
|
ACS towers |
0.80 |
0.80 to 0.5 |
14 |
0.219 |
Table 6.8 ACS leakage test results of the Ignalina NPP unit 2 [53]
|
Compartment |
Excess testing pressure, kPa |
Range of pressure changes, kPa |
Time of pressure drop, s |
Equivalent area of leakage, m2 |
|
RLC+BWPC+ACS towers* |
3.96 |
2.0 to 1.0 |
130 |
0.0406 |
|
RLC+BWPC+ACS towers** |
4.40 |
3.0 to 2.0 |
206 |
0.0850 |
|
RLC** |
4.60 |
2.0 to 1.0 |
125 |
0.0308 |
|
BWPC** |
0.50 |
0.5 to 0.2 |
13 |
0.0356 |
|
Left ACS tower** |
0.80 |
0.80 to 0.5 |
117 |
0.0142 |
|
Right ACS tower** |
0.80 |
0.80 to 0.5 |
98 |
0.0170 |
*
By special preparation of ACS for testing**
Without any additions to the structure