ABSTRACT
DESING AND IMPLEMENTATION OF FORCED COOLING TOWERS FOR LOVIISA
NPP SAFETY- AND RESIDUAL HEAT REMOVAL (RHR) COOLING CIRCUITS
S.Tarkiainen, T.Hyrsky, I.Paavola, A.Teräsvirta
Fortum Nuclear and Thermal Power - NECON, Espoo , FINLAND
Nuclear power plants must be able to remove the residual heat from the nuclear fuel
immediately after the plant trip when the turbine island no longer consumes the generated
steam, and on the long term when the plant is in the shutdown states. Also the spent fuel stored
in the plant area must be cooled at all times. Certain types of external events may be able to
compromise the conventional plant cooling system operations. These events are usually very
rare, but the impact on the plant system operations can be significant.
The twin unit Loviisa VVER-440, located in the proximity of city of Loviisa by the Baltic
Sea, utilizes sea water as a medium for the safety and RHR system cooling. The dependence
from available seawater for plant cooling purposes during extreme accident conditions was
recognized by studies of residual heat transfer to the ultimate heat sink. This paper describes
the design and implementation of the cooling towers to fulfill the task of reactor core and fuel
pools heat removal by independent air cooling in accident conditions.
The chosen solution was air-cooled cooling tower connected to the plant cooling systems for
RHR and safety systems. The tower capacity and dimensions were iterated to fit the wide range
of cooling and operational requirements. Towers are forced draught - type to keep the design
compact and reliable. The heat exchangers are of water-to-air - type, consisting of finned tube
bundles and connecting piping. The bundles are arranged in two-pass cross-counterflow formation to allow compact connections with pipes and air fans. The towers are also weather
protected to cope with the Finnish winter conditions.
The requirements set by the postulated accidents in different operating stages and the
arrangement of the RHR and fuel pool cooling led to a design solution of two separate towers
for each unit. The capacity, location and the connections for the towers were designed to allow
manual operation of the towers together with existing cooling circuits to fulfill the cooling task.
By implementing the cooling towers, the total plant core melt risk can be reduced by 12 % for
all operating stages in both units.
DESING AND IMPLEMENTATION OF FORCED COOLING TOWERS FOR LOVIISA
NPP SAFETY- AND RESIDUAL HEAT REMOVAL (RHR) COOLING CIRCUITS
S.Tarkiainen, T.Hyrsky, I.Paavola, A.Teräsvirta
Fortum Nuclear and Thermal Power - NECON, Espoo , FINLAND
ABBREVATIONS :
NPP - Nuclear Power Plant
STUK - Radiation Protection Center, Finland
PRA - Probabilistic Risk Analysis
CDF - Core Damage Frequency
TG - Reloading pool cooling system
RR - Residual heat removal system
TF - Intermediate component cooling system
VF - Auxliary Seawater Circuit
TJ,TH,TQ -HPSI, LPSI, Spray safety systems
RHR - Residual Heat Removal
CCS - Component Cooling Syste,
LOCA - Loss-of-Coolant Accident
VT - Cooling tower for Component Cooling System
VS - Cooling tower for Residual Heat Removal System
SAFETY IMPROVEMENTS AFTER FUKUSHIMA IN LOVIISA
Loviisa 1&2 NPP's are Russian origin VVER-440 with western safety modifications. The
plants were built in 1970's, with commercial operation for Lo1 1977 and Lo2 1980. Both units
have excellent production and safety records. During the operations, the plants have undergone
power uprates, from 440 MWe up to gross (net) capacity of 520 (496) MWe in the last
modernization project.
In the operating licence renewal 2007, the plant was obliged by the Finnish regulator (STUK)
to perform a periodic safety review in 2015. In connection to this review, to complement a
power uprate project, a set of safety improvements were also collected and evaluated. The
basis for this evaluation was the plant risk study and economical considerations. The
Fukushima accident in 2011 enhanced the safety aspects in the considerations.
PRA-studies for external events
Loviisa NPP PRA studies has been conducted from 1989, first concentrating on the internal
accident sequences and later extended to the fires, floods and other external events. Over the
years of operations, the plant has designed and implemented system improvements and
installed additional systems for plant risk mitigation. 2012 Loviisa plant risk spectra was very
balanced showing only few risk drivers. The seawater channels have been subject of studies for
different reasons in the past. Phenomenas like high water level, frazil ice, algae, oil spills and
special types of molluscs have been studied as a threat for the plant heat sink.
The list of most effective plant upgrades varied from small internal changes to bigger
upgrades. The single most effective was upgrading the plant ultimate heat sink. The most
influential events were sea vegetation and oil hazards (Fig1).
Figure 1 : Loviisa Plant Risk Analysis for Weather Events
EFFECT OF SEAWATER LOSS FOR LOVIISA PLANT SYSTEMS
Decay heat sources in different plant operation states
During the plant power operations, the reactor core releases its power thorough steam
generators to the turbine and condensators. Other decay heat sources are reloading fuel pools
inside the containment and spent fuel pools in separate building in the plant area. After reactor
trip when the reactor vessel is closed and intact, the steam generators are the primary way to
decay heat removal from fuel to the heat sink. During the plant outage after reactor shutdown,
the reactor vessel is opened and fuel is transported to the pool for reloading. The amount of
transported fuel varies from 1/3 of the core in the normal reloading outage to the full core when
the pressure vessel is inspected. Latter case has the biggest heat load for the reloading pool
cooling system (TG). During power operations, the recently removed hot fuel is cooled in the
refueling pool with TG and TF-systems.
The heat load from spent fuel is mainly constant. The spent fuel cooling is arranged via
intermediate component cooling system (TG, TF), which in turn is also cooled by seawater
system (VF).
Heat sink for Loviisa
The designed heat sink for Loviisa is seawater. A cooling system (VF) circulates seawater for
in-plant consumers for ex. intermediate cooling systems (TF), residual heat removal (RR) and
safety systems. In the plant accident conditions, only the cooling systems dedicated to the
residual heat removal from the core (RR,TF, safety systems) and fuel pools (TG) are in
operation.
Safety systems
Safety systems are designed to mitigate the accident progression and remove the residual heat
from the damaged reactor pressure vessel (Fig 2).
Both units have similar safety systems for accident mitigation and heat removal. High
pressure safety injection (HPSI, TJ) systems are connected to the letdown system piping. Low
pressure safety injection (LPSI, TH) lines are connected to the circulation loops and they are
both passive and active operated systems. For heat removal from the containment, a safety
spray (TQ) system is built inside the containment to remove heat and mitigate the pressure rise.
The plant safety systems are 4-redundant systems for the active components and 2-redundant
for the passive components, as lines or tanks.
The water sources in the beginning of the loss-of-coolant accident (LOCA) are safety water
tanks (TH), from where all systems have suction lines except the separate passive boron water
hi-pressure tanks (TH) and ice condensers. The two tanks contain 1000 m3 of borated water,
(12 %, 60 C), and all active systems take suction from this tank. On full operation, the tank will
be empty in 30 minutes. When tank is empty, the suctions switches to the separate recirculation
line sumps inside the containment. The recirculation mode forces cooling water from the
containment floor through the heat exchanger to the reactor and back to the containment floor
via the primary circuit openings. This recirculation mode is a closed circuit and requires
electric power and sea water cooling for continuing operation.
RHR System (RR)
Residual Heat Removal systems are designed to remove the residual heat from the reactor
core thorough secondary side systems. The secondary side systems remove heat from reactor
by circulating desalinated feed water thorough the steam generators, first by boiling the water
and releasing steam from safety valves to atmosphere, then condensing the steam back to feed
water circulation with sea water cooling and ultimately by filling the steam generator with
water to act as a heat exchanger without steam phase. The reactor operates in natural circulation
mode from reactor core to the steam generators for heat removal when reactor is undamaged.
The RR system has an additional separate system to operate with water-to-water heat removal
phase, with separate pump house outside the reactor hall. Also this operation requires the sea
water cooling and electric power feed for pumps.
Fuel pool cooling system (TG)
Both Loviisa units have reloading pools inside the containment. For the spent fuel storage
there is a special facility inside the plant perimeter. During refuelling outage, the reloading
pools serve as storage space for fuel removal and reloading from and to the reactor. Some 1/3
of the core fuel is removed each year from the reactor. After the refueling outage, the recently
removed hot fuel is cooled inside the containment for one year and later transferred into the
spent fuel storage inside the plant perimeter for longer term storage.
Fuel pools have a dedicated cooling system (TG), which circulates the pool water and cools it
with intermediate component cooling water (TF). The cooling system has the capacity to cool
all transferred fuel inside the pool, also on those rare cases when the whole core is moved from
the reactor during reactor vessel inspections. The spent fuel facility is cooled by the Loviisa
unit 2 TG-system.
Figure 2 : Diagram of safety systems in Loviisa NPP
Loss of heat sink
Assuming that the seawater cannot be used for cooling for longer periods, plant has only few
options to cool the reactor and fuel pools. After reactor trip from full power, the reactor decay
heat has maximum values, but during only a few days it is diminished to only a fraction of the
maximum values. The available method in this phase for heat removal is dumping steam to the
atmosphere thorough the steam generators and providing additional feedwater with dieseldriven pump. This cooling circuit is open and requires regular feedwater additions to the
feedwater tanks.
The heat load in the fuel pools during the power operations is low and allows few days of
inoperation for the cooling system. Ultimately the tanks will reach boiling point, and start to
release heat to the atmosphere by boiling the pool water and losing water inventory. Simple
manual operations can be used to replenish the tanks, but this will require almost constant
upkeeping from the plant personnel. In the long term the sea water cooling can be restored, but
these methods of restoration must be conducted ad-hoc and are not pre-planned.
Active safety systems require electric power feed for operations in the Loviisa plant
configuration. Plant safety electric system is arranged with 4-redundant power trains, which
have diesel generator back up for outside grid failure. Diesel generators are also seawater
cooled with separate seawater channels directly from the seawater inlet surge chamber.
CALCULATIONS FOR DECAY HEAT LOAD
The heat load from the reactor and fuel pools was different for each power option. Also plant
configurations resulted to some design options, mainly the need to have full heat removal
capacity in both RHR and CCS towers. The accident conditions apply for the heat
transportation systems. The significant cases are normal power operation, and reloading outage.
Heat load for the long term cooling is divided into two sources. Decay heat from the reactor
vessel via secondary circuit systems and decay heat from fuel pools and safety systems via the
component cooling system. Also some time considerations was used to choose the correct
decay heat.
Reactor core
The heat load during the power operations is released from the reactor vessel. The heat load
from the reactor 72 h after the reactor trip is the determining case for heat removal to the RHRsystem, and is also the envelope case for the RR-towers. The time delay (72 h) is covered by
heat removal to the atmosphere with steam generator blowdown. If heat sink is lost when plant
is in refueling outage, we estimate 5 days delay from the reactor shutdown when the reactor is
still intact, and evaluate the decay heat for open reactor vessel after this delay.
Fuel pools
Loviisa plant has spent fuel cooling pools in the containment buildings. The reloading fuel
pools are open to the containment atmosphere and containing the recently changed fuel
elements and during the reloading the fuel removed from the core for reloading on both units.
In some special cases, the whole contents of the core is transferred to the reloading pool to
allow inspection on the reactor vessel. This case is the determining case for heat transfer load
for the TG-system, directly after the fuel transportation. The delay from reactor trip to complete
fuel removal is 6 days, and the system can allow additional 36 hrs delay for starting of the
cooling.
The long term cooling for spent fuel is arranged in separate building housing the mid-term
storage for spent fuel. The heat load is 1,7 MW, and it remains mainly constant since the spent
fuel is transferred to the storage after one year from removal from the reactor core. The water
inventory is large and offers a buffer when mixed together with reloading pool cooling. The
cooling task from spent fuel source is for the Loviisa 2 TG- and TF-system.
Figure 3: Decay heat from core, ANSI + 10 % [kW]
Considered loads
Following loads were developed to take into account the possible plant configurations, time
delays and decay heat loads from different sources. Additional loads are main circulating
pumps during RHR-operations and additional 0,25 MW for additional loads for the TF-circuit.
These were further developed into design cases for all towers. The parameters for the heat
removal cases are in the Table 1.
Table 1 : Heat load sources for design cases R1,R2,T1 and T2 (Fig 3)
Heat from core, 72 hrs
Heat from core, 5 days
Heat from core in reloading pool, 6 days
Heat from core, in reloading pool 7,5 days
Heat from spent fuel storage 1
Heat from spent fuel storage 2
Heat from fuel pools
PCP operating
P
5,9 MW
4,8 MW
4,5 MW
4,1 MW
0,6 MW
1,1 MW
0,25 MW
1,0 MW
Cases R1 and R2
R1
R2
Cases T1 and T2
T1
T2
T1 and T2
T1 and T2
T1 and T2
R1 and R2
CONSIDERED OPTIONS FOR SEAWATER-INDEPENDENT COOLING
Cooling can be arranged from external sources, for example sweet water lakes in the vicinity
of the Loviisa city, or using geothermal storage for heat sink. Both these options were studied.
Water from the lakes can be used to produce new feedwater for the steam generators. The
location of the lake is inland, and to use large amounts of lake water for plant cooling purposes
requires investment for the pumping capacity and water purification. To constantly pump water
for cooling purposes in the long term, also during winter, will cause changes in the lake water
sources. Together with required investment to pumping capacity, this option was not very
lucrative.
Also geothermal storage was considered as a ultimate heat sink. The problematic issue was
the heat transfer to the bedrock, since the heat transfer from the reactor and fuel pools was
possible only with similar water pumping facilities as in the other options, and bedrock's ability
to receive large amount of heat in short time did not prove to be adequate.
Both these heat sink options require investment to the new heat transfer system, and in some
cases, the source is outside the plant area, and therefore additional maintenance problem for the
plant. Both these options did not solve all the problems with decay heat removal.
Air cooling as a ultimate heat sink was promising option in this situation. The air cooling
technology is proven in the conventional power production and also in some nuclear solutions.
In a case when seawater was not available, the air cooling can provide a long term solution for
cooling both the reactor core and fuel pools. The challenge was to design a suitable solution for
Loviisa plant, taken into account the existing intermediate systems between seawater and plant
cooling systems and necessary operating conditions for all-round year demand. The
environmental parameters for air cooling must be according to the most demanding situation
for Finland both in summer and winter weather.
DESCRIPTION OF THE SOLUTION
Air cooling for Loviisa decay heat removal requires necessary in-plant connections for fuel
coolant. These can be provided by the existing intermediate cooling systems (VF, TF) so, that
the normal plant operations are not changed, and require no major changes for the licenced
equipment. Both intermediate systems allow serial connections to the main cooling heat
exchangers. A simple serial configuration for the cooling tower heat sink can be designed
without challenging the original operation of these intermediate systems. The dimensioning of
the air cooling must be such, that the heat removal tasks can be performed at any case without
complicating the air cooling system or oversizing the system. Since the reasons for this kind of
accident are extreme, also the design basis for environmental parameters are extreme for
finnish climate (Table 2).
Table 2 : Environmental parameters for tower design
Air ambient conditions for all cooling towers
Temperature, dry bulb
28,6 oC
Humidity
60 % RHR
Temperature dry bulb
36,0 oC
Humidity
< 40 % RHR
Temperature, dry bulb
-40 oC
Air pressure
1012 hPa
Daily average 24 hr
Daily extreme 6 hr
Lowest operating /Standby
temperature
Installation level +10-15
m above sea level
AIR COOLING SYSTEM FOR DECAY HEAT REMOVAL
The air cooling system for the Loviisa plant consists of two cooling towers per unit, which
will be used for removing decay heat from the reactor and the spent fuel pools and cooling of
other equipment critical from the nuclear safety point of view. All cooling tower equipment
will be located in three square buildings, each measuring about 10 x 15 meters and about 10
meters in height. The towers used for decay heat removal from the reactors will be located in
the common building for units 1 and 2. Cooling towers are placed on roofs of the Loviisa NPP
buildings. TF towers are placed on the roof of tank area of the unit in question, while RR
towers will be placed above the reserve residual heat removal system (RR) building close to
turbine building for both units.
Figure 4 : Cooling tower locations in Loviisa plant
The cooling towers will be connected to existing systems of Loviisa NPP. One cooling tower
is connected in reserve residual heat removal system RR and the other cooling tower is
connected to intermediate component cooling system TF in each unit. Figure 4 shows the
planned locations for Loviisa Unit 2
Operations before the tower cooling
When the reactor is shut down from full power, steam from the steam generators can be
vented into the atmosphere before the cooling tower is taken into operation. During the steam
venting additional water is fed into the steam generators from diesel driven pumping system
with dedicated water storage tanks. After 72 hrs, the cooling towers can be taken into operation.
Preparing the towers includes powering up the local control systems and warming the heat
exchanger tubing in case of low winter temperatures before the tower can be filled with water.
HEAT REMOVAL FROM THE REACTOR WITH COOLING TOWER
Reserve residual heat removal system RR removes decay heat generated in the reactor core
through secondary circuit after shutdown of the reactor. Heat is transferred from the reactor
core by natural convection to primary circuit and into the steam generators. The RR system
cools the secondary side of the steam generators. A sea water circuit (VF) will transfer the heat
to the sea water. If sea water is not available, the RHR air cooling tower (coded VS) will
provide cooling to RR circuit by replacing the VF-connection. Existing pumps and
measurements of RR systems are utilized when operating the system with cooling towers
(Figure 5 and Table 3).
Figure 5 : RHR tower (VS) connection principle
Cooling towers are able to provide sufficient cooling capacity in order to bring the plant first
into hot shutdown state and later into cold shutdown state with closed circuit cooling.
Table 3 : Desing parameters for reactor cooling tower VS
Thermal design parameters for 10VS and 20VS
Heat load
< 7,4 MW
Design point 1 for VS
o
Water inlet temperature
140 C
Daily extreme 6 hr conditions (36 oC)
o
Water outlet temperature
< 122 C
Thermal design parameters for 10VS and 20VS
Heat load
< 6,4 MW
Design point 2 forVS
Water flow
100 kg/s
Water inlet temperature
95 oC
Daily average conditions (28,6 oC)
o
Water outlet temperature
< 79,8 C
Other parameters for 10VS and 20VS
Design Pressure
12 bar
Design temperature
140 oC
Cooling liquid
Feed water
Water flow
100 kg/s
HEAT REMOVAL FROM THE REACTOR FUEL POOLS WITH COOLING TOWER
The intermediate component cooling system TF cools the fuel pool cooling systems TG and
other safety relevant components and systems such as emergency core cooling system,
containment heat removal system, boron injection system and some air-conditioning systems,
transferring heat to the sea water. If sea water cooling is not available the VT air cooling tower
will provide cooling by replacing the sea water cooling circuit (VF). The refueling fuel pool is
located in the reactor building of each unit for short term storage and refuelling purposes, and
long term storage facility is located at unit's 2 auxiliary building and cooled by unit 2 TFsystem. The cooling tower can be filled with TF-system water and taken into operation by
manually operating the connection valves (Figure 6 and Table 4).
Figure 6 : Fuel pool cooling tower VT connection principle
Table 4: Desing parameters for fuel pool and component cooling tower VT
Thermal design parameters for 10VT (Design point)
Heat load
< 5,4 MW
Design point for 1VT
Water flow
135 kg/s
Water inlet temperature
59,5 oC
Daily extreme 6 hr conditions (36 oC)
Water outlet temperature
<50 oC
Thermal design parameters for 20VT (Design point 1)
Heat load
< 5,4 MW
Design point for 2VT
Water flow
158 kg/s
Water inlet temperature
58,2 oC
Daily extreme 6 hr conditions (36 oC)
o
Water outlet temperature
<50 C
Thermal design parameters for 20VT (Design point 2)
Heat load
< 6,3 MW
Design point for 2VT
Water flow
158 kg/s
Water inlet temperature
59,5 oC
Daily extreme 6 hr conditions (36 oC)
o
Water outlet temperature
<50 C
General parameters for VT towers
Desing Temperature
100 oC
Design Pressure
12 bar
Cooling liquid
Deionized
water
ELECTRIC SUPPLY
Electric power for the towers is provided from the plant network. The tower consumers are
fans, heaters, door and roof motors and automatics. Every plant supplies electric power to the
dedicated tower from two feeds. For the reactor cooling towers, the power supply is divided to
allow cross-feeding towers from both plants due to operational requirements. Under normal
conditions, the power is provided from the plant production or external grid if the plant is in
shutdown. If external grid is not available and plant emergency generation is not possible, onsite diesel generator power plant can be used to provide electricity for air cooling system. New
10 MW air cooled diesel power plant EY07 was constructed at the Loviisa NPP site in 2011.
The design provides connection to the tower feeding cubicles. This new diesel power plant as
well as external grid are able to provide power for the both Loviisa NPP units in a situation
where the cooling towers are needed.
THE STRUCTURE OF THE AIR COOLING TOWER
The cooling tower consist of finned tube heat exchanger bundles, fans with drive units,
connection piping and steel support structures and covers. Finned tube bundles are arranged as
deltas on top of the structure and the fans force air from bottom and sides thorough the heat
exchange surfaces. The water-air heat exchangers are of two-pass cross-counter-flow type
(Figure 7), with forced draught induced by air fans. The cooling towers will be encased in
separate buildings to provide protection from weather during standby. For the component
cooling towers, a separate control container is located near the tower to allow the operation of
the tower. Local automation system controls the cooled water temperature so that the actual
cooling system parameters are close to the normal operations.
Figure 7 : Principle of the two-pass water-air heat exchanter
The cooling towers are stored drained to protect from freezing when not in service. During
cooling operations, towers are filled with water from cooled system and the electric power is
switched on. Automation system opens the roofs and doors to allow air flow, starts the fans and
controls the outlet water temperature. Figure 8 represents the cooling tower operating principle.
Tower buildings will be equipped with electric heating for startups in winter conditions.
Automation pre-heats the tower when it is taken into operation by circulating heated air inside
the tower and thorough the heat exchangers. The towers have a dedicated automation system
for each tower. The automation system controls the outlet water temperature by adjusting the
rotation speed of the cooling fans. Tower automation system must be operated locally from the
tower control cubicle and it is not connected to the plant automation system.
Figure 8 : Principle of cooling tower and example of configuration for Loviisa NPP without weather protecting building.
The system is designed to remove decay heat generated in reactor core and fuel pools both in
short and long term. Power operation and shutdown conditions, as well as extreme weather
conditions possible at Loviisa NPP site are accounted for. The design point for VT cooling
tower is dimensioned to remove decay heat from the fuel pool when all fuel from the reactor
has been transferred into the pool due to maintenance reasons. The physical size of VT tower is
twice the size of VS-tower due to the different heat transform design parameters.
Figure 9 : Cooling tower for LO2 component cooling system near finished
IMPLEMENTATION
The project to implement the cooling towers to power plant was approved in 2012 as a part of
the Loviisa modernization program. The project was divided into a major contract for the
towers manufacture, design works for the connections and electrical supply, civil constructions
for tower foundations, erection works for the towers and installation works for electric supply
and automation. The erection works were finished in February 2015, and tower is taken into
operation during year 2015 (Fig 9).
CONCLUSIONS
The Loviisa NPP has a long history of improving the plant safety by recognizing the safety
deficiencies and designing and implementing procedures and facilities to mitigate or remove
risks. The probabilistic risk assessment is a powerful tool to recognize and analyze the risks to
the plant operations, also taking into account the plant external events such as oil spills,
powerful wind loads etc. Probabilistic analysis allows to compare risks and make decisions on
measures to implement. The design of these measures requires knowledge of the plant systems
and new innovations to complement the existing system design. With the cooling tower, the
plant ultimate heat sink can be assured in most extreme conditions.