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 diesel-driven 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 RHR-system, 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)

P Cases R1 and R2 Cases T1 and T2 Heat from core, 72 hrs 5,9 MW R1 Heat from core, 5 days 4,8 MW R2 Heat from core in reloading pool, 6 days 4,5 MW T1 Heat from core, in reloading pool 7,5 days 4,1 MW T2 Heat from spent fuel storage 1 0,6 MW T1 and T2 Heat from spent fuel storage 2 1,1 MW T1 and T2 Heat from fuel pools 0,25 MW T1 and T2 PCP operating 1,0 MW 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 Humidity

28,6 oC 60 % RHR

Daily average 24 hr

Temperature dry bulb Humidity

36,0 oC < 40 % RHR

Daily extreme 6 hr

Temperature, dry bulb -40 oC Lowest operating /Standby temperature

Air pressure 1012 hPa 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 Water inlet temperature 140 oC Daily extreme 6 hr conditions (36 oC) Water outlet temperature < 122 oC 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) Water outlet temperature < 79,8 oC 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 TF-system. 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) Water outlet temperature <50 oC 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) Water outlet temperature <50 oC 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, on-site 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.