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Design of Buildings, Systems and Components Sites with More Than One Reactor

In case of multiple-unit sites, the following measures have to be considered:

• Strict separation of safety related systems and components, and

• Provision of a plant arrangement which prevents common cause failures for safety related systems and components,

There is no specific technology required; the design is related to well-known technologies that have to fulfill the specific design requirements.

The PSA should be the tool that enables identification of the areas that must be considered to strengthen the safety of multi-unit sites. Off-Site and On-Site Electricity Supply

In case of an external event like an earthquake, the off-site electricity supply is very diffi or even impossible to maintain. This results not only from the direct effect on the grid structure like masts and cables, but also from the fact that other plants which feed into the grid may also be affected and consequently have to be shut down. Nevertheless, it should be evaluated whether it is possible to enforce the grid design, which may result in a higher chance for survivability of parts of the off-site grid connection.

Since the large uncertainty exists for the maintainability of the off-site grid in case of an earthquake, the way to substitute off-site electricity supply is mainly to provide mobile power supply systems or addition of diesel generators or other power sources such as gas turbines. These components must be protected against external events by bunkering or locating at positions which cannot be affected by e.g. tsunami waves.

NRA requirements for existing Japanese plants

In order to prevent common cause failures due to events other than natural phenomena, the measure against power failures is strengthened. For off-site power, independence of two circuits was not required before, but is required. For on-site AC power source, two permanently installed units, two more mobile units and storage of fuel for seven days are required. For on-site DC power source, one permanently installed system with a capacity for 30 min was required before, but increase of the capacity to 24 h duration and addition of one mobile system and one permanently installed system both with 24 h duration are required. Additionally, it is required that switchboards and other equipment should not lose their operational capabilities.

Loss of power supply and 3rd grid connection

To ensure that operational and safety-related components maintain their AC supply, in Germany nuclear power plants are forced to use a tiered back-up system: the main grid connection, the stand-by grid connection, the emergency power supply (ordinary back-up AC power source), and the emergency feed power supply (diverse AC power source). The different stages of the AC power supply allow it to cover different failures of the AC grid. An additional third grid connection is also available [6].

Robustness of emergency power supply

The measures to enforce the on-site power supply are in general the protection of the existing components against external events, to extend the capacity and timely availability, and provide diverse components.

In case of the Olkiluoto 3 Nuclear Power Plant (NPP) [7] the reactor plant electrical power system is divided into four parallel and physically separated subdivisions designed against external events. The power supply to equipment critical for safety of each division is backed up with a 7.8 MVA diesel generator. The Olkiluoto gas turbine plant can also supply the bus bars of the diesel generators. In case of the loss of all external power supplies, the malfunction of all four diesel generators at once, i.e. the complete loss of all AC power, the plant unit has two smaller diesel generators with an output of approximately 3 MVA each. These units are bunkered and can ensure power supply to safety-critical systems even in such a highly exceptional situation.

Another example is the “SUSAN” system of the Muehleberg NPP in Switzerland [8]. “SUSAN” is an acronym for “Spezielles Unabhängiges System zur Abführung der Nachzerfallswärme,” which means a special independent residual heat removal system. The main tasks are (1) to remove residual heat from the reactor pressure vessel (RPV) in the long term, (2) fast shutdown and isolation of the reactor and (3) limit and reduce the primary circuit pressure. The system is designed to resist design earthquake, protection against sabotage, flooding and airplane crash. The main system parts and equipment of SUSAN are located in a dedicated building, which is protected against impact from outside. Two 100 % emergency diesel generators are used to supply necessary pumps and systems with power in case of station blackout. Bunkering of Buildings with Safety Related Systems

Emergency Feed Building Recent German PWRs are equipped with a second four-fold emergency power supply (emergency diesel sets) [9]. These second emergency cooling systems can cool the reactor core (via steam generators) as well as the spent fuel pool (via auxiliary emergency cooling chain or emergency systems). Emergency diesel sets are equipped with diesel and water reserves conservatively lasting for at least 10 h and more. Emergency buildings (similar to regular emergency diesel housings) are also designed according to design basis regulations including fl A building arrangement of a typical emergency feed building is shown in Fig. 12.3. Air ventilation shafts and air suction holes are located in the upper part of the building, indicated by the circles in Fig. 12.3. The emergency feed building is designed for airplane crash, explosion pressure wave, flooding, explosive gases, and earthquake, and is located separately from

other buildings of the plant. It encloses the following:

1. Four additional EDGs (so called D2 Diesels): They serve for power supply in case of loss of offsite power (LOOP) and unavailability of the four main EDGs (D1 Diesels).

Fig. 12.3 Bunkered emergency feed building for recent German PWRs. The circles indicate air ventilation shafts and air suction holes located in the upper part of the building

2. Four trains of emergency feed water pumps: Directly driven by the D2 Diesels (but can also be power supplied by the D1 Diesels, if available): An emergency control room (RSS), including wash room, toilet, plant documentation.

3. Safety related instrumentation and control (I&C).

4. Safety related switchgears.

5. Dedicated heating, ventilation, air conditioning (HVAC) system (also powered by D2 Diesel).

6. Mobile equipment for secondary side bleed and feed.

Robustness of Cooling Chain in BWRs and PWRs

An example for the implementation of an additional cooling system and therefore for the robustness of the cooling chain of a BWR is described in the Stress test Report for German nuclear power plants [6].

An additional independent residual heat removal (RHR) system was installed in separated new building for Philipsburg 1 NPP, Brunsbuettel NPP both in Germany, Oskarshamm 1&2 NPP in Sweden, and Muehleberg NPP in Switzerland.

It serves as an independent heat sink for residual heat removal and power supply by diesels including cooling of the independent diesels in a separated new building. It is also possible to diversify, for example, by air-cooled cell cooling towers, wells etc. As another example, the ZUNA system of Gundremmingen 1&2 NPP may serve. This is a retrofitted, independent, additional residual heat removal and feed water system with a diverse heat sink by means of wet well cooling towers and diverse emergency power diesels (station blackout diesels). The ZUNA system is protected against external and internal events.

An example for the robustness of a fuel pool cooling system is the wet storage of spent fuel pool of Goesgen NPP in Switzerland [10]. The cooling during normal operation is provided by natural circulation. The temperature of the pool is 45 °C maximum with support of fans in case of high outside temperature and fully loaded fuel pool. The cooling in case of accidents is provided by natural circulation without need of electrical supply. The temperature of the spent fuel pool depends on the type of accident, but up to max 80 °C. Passive Components and Systems Using Natural Forces

Passive components do not need external power since they rely on laws of physics such as gravity, heat transfer by temperature difference or pressure increase though heating of enclosed fluids.

Isolation Condenser

Isolation condenser (IC) is a passive system of BWRs for emergency cooling located above containment in a pool of water open to atmosphere. The scheme is

Fig. 12.4 Isolation condenser [11, 12]

shown in Fig. 12.4 [11, 12]. Under normal condition IC system is not activated, but the top of the IC is connected to the reactor's steam lines through an open valve. Steam enters the IC until it is filled with water. When the IC system is activated, a valve at the bottom of the IC is opened which connects to a lower area on the reactor. The water flows to the reactor via gravity, allowing the condenser to fill with steam, which then condenses. This cycle runs continuously until bottom valve is closed. In case of electricity failure, the valve closes automatically and operators have to open them manually. Fail-open valves and lines need to be installed for severe accidents.

Gravity Driven Cooling System

The gravity-driven cooling system (GDCS) injects water to the RPV by gravity. The GDCS pool locates at higher elevation than the RPV. Squib valves from the DC safety related power from batteries activate the system. The schematic diagram of ESBWR GDCS is provided in [11, 12].

Passive Containment Cooling System

Passive containment cooling system (PCCS) of ESBWR consists of a set of heat exchangers located in the upper portion of the reactor building. The steam from the reactor fl ws through the containment to the PCCS heat exchangers where the steam is condensed. The condensate drains backs from the PCCS heat exchangers where the steam is condensed to the GDCS pools. For more detail, refer to [11, 12].

The passive safety systems of ESBWR are discussed in [11, 12]. In the events where the reactor pressure boundary remains intact, the isolation condenser system(IC) is used to remove decay heat from the reactor to transfer it outside containment. In the events where the reactor pressure boundary does not remain intact and water inventory in the core is lost, the PCCS and GDCS work in concert to maintain the water level in the core and remove decay heat from the reactor and transferring it outside containment. When the water level of the RPV drops to a predetermined level, the reactor is depressurized and the GDCS is initiated. Both IC and PCCS heat exchangers are submerged in a pool of water large enough to provide 72 h of reactor decay heat removal capability. The pool is vented to the atmosphere. It is located outside of the containment. It will be refilled easily with low-pressure water sources via pre-installed piping.

Emergency Condenser

Emergency condensers (ECs) are used for residual heat removal from the RPV. The residual heat is released into the core fl pool inside the containment, not outside of it as the isolation condenser. The schematic drawing of the ECs is shown in Fig. 12.5 [13]. Each of the four ECs consists of a steam line (to connection) leading from an RPV nozzle, and a condensate return line (lower connection) back to the RPV. Each return line is equipped with an anti-circulation loop. The ECs are connected to the RPV without any isolating element and

Fig. 12.5 Emergency condenser [13]

are actuated by a drop of the RPV water level. In the event of water level drop in the RPV, steam from the RPV enters the heat exchanger tubes of the ECs, located in the core fl pools and condense inside the pipe. The condensate returns back into the RPV. This system assures core cooling even at high RPV pressure.

The ECs are used for the KERENATM (formerly SWR-1000) reactor, an advanced BWR in Germany. The cross section of the KERENATM reactor contain-

ment is shown in Fig. 12.6 [13]. Shielding/Storage pool is on top of the containment. It is used as a heat sink to remove the heat from the containment. The water inventory is sufficient to ensure passive heat removal for at least 3 days.

Fig. 12.6 Section through the KERENA reactor containment [13]

Containment Cooling Condenser

In case the ECs are in operation or when the safety relief valves are opened in case of LOCA, the water of the core flooding pool starts to evaporate and the pressure in the containment will increase. Containment cooling condensers (CCC) are installed above the core flooding pools as seen in Fig. 12.7.

The heat exchanger tubes are slightly inclined. Both inlet and discharge lines are connected to the shielding/storage pool and are open during normal operation. When the temperature increases inside the containment, the water in the CCC starts to heat up so that a natural circulation flow establishes in the system.

Passive Pressure Pulse Transmitter

The passive pressure pulse transmitters (PPPT) function without electric power supply, external media, or actuation via I&C signals. The PPPTs serve to initiate scram, containment isolation of main steam lines, and automatic depressurization of the RPV. The PPPT consists of a small heat exchanger connected to the RPV via a non-isolatable pipe, as shown in Fig. 12.8.

The secondary side of the heat exchangers is connected to a diaphragm pilot valve via a pipe. During normal operation the PPPTs are fi with water. In case of water level drop inside the RPV, the water level in the tube of the PPPTs drops as well. When the primary side of the heat exchanger is fi with steam it will condense and drains back into the RPV while in the secondary side of the heat exchanger the temperature rises until the water starts to evaporate. The design of the heat exchanger is such that the activation of the systems is done in the required time. By means of the increased pressure, a function is triggered via the diaphragm pilot valve.

Fig. 12.7 Containment

cooling condenser [13]

Fig. 12.8 Passive pressure pulse transmitter [13]

Passive Residual Heat Removal System

The passive residual heat removal system (PRHR) of advanced PWR, AP1000TM provides reactor cooling by natural circulation through the core as shown in Fig. 12.9 [14].

The heat exchanger of PRHR is located in the in-containment refueling water storage tank (IRWST). The decay heat is transferred to the cooler water in the IRWST. The reactor coolant water in PRHR becomes cooler and denser and cools the core. The cycle continues until the water of the IRWST is depleted. Large amount of water is, however, stored in the IRWST. The decay heat is transferred to the water of IRWST in the containment vessel (CV) with PRHR and steam is generated. The IRWST is vented to the containment vessel and increase its pressure.

Passive Containment Cooling System

The passive containment cooling system (PCS) of AP 1000TM is shown in Fig. 12.10 [14].

Passive containment cooling water storage tank (PCCWST) is located in the roof structure of the containment building. The water will be dispersed via gravity to the top of the CV from PCCWST. The water film covers the steel surface of the CV. The airflow through the annulus removes the heat from the CV by evaporation of the water.

The outside air flows into the outer annulus from the inlet louvers. It flows down and flows up in the inner annulus between the CV wall and the air baffle. Evaporating water is applied to the top of the CV from PCCWST. The steam is exhausted through the chimney area to the atmosphere. PRHR heat exchanger

Fig. 12.9 Passive residual heat removal system (PRHR) [14]

transfers decay heat to the in containment refueling water storage tank to the containment atmosphere. The steam is condensed by PCS operation and returned via gravity-drain gutters to the IRWST again.

Advanced Accumulator

An advanced accumulator (ACC) is a passive device leading to a discharge characteristic of high and low flow rate using a vortex flow damper to cope with large break loss of coolant accident (LOCA) of a PWR [15–17]. High flow rate is required for the refill of RPV after large break LOCA, but low flow injection is necessary for reflooding of the core. The function was provided by an accumulator firstly and low head injection pump secondly in the current system. The switching off the systems is necessary. The new system of ACC operates at high flow rate firstly and low flow rate secondly by means of the vortex flow damper. It can eliminate the low head injection pumps and storage tank for safety injection of the present system.

A vortex chamber is provided at the bottom of accumulator tank as shown in Fig. 12.11. A standpipe is connected to the vortex chamber that is connected to the injection pipe. At high water level, water comes into both large and small flow pipes. Since the mass flow through the standpipe is large and is radially directed

Fig. 12.10 Passive containment cooling system [14]. 1 Core, 2 PRHR, 3 IRWST, 4 Gutters, 5

CV, 6 Louvers, 7 PCCWST, 8 Atmosphere

Fig. 12.11 Principle of advanced accumulator [17]. a Large flow rate (RV refilling). b Water levels in accumulator tank. c Small flow rate (core reflooding)

to the vortex chamber, it dominates the injection mass flow at the outflow without forming a vortex in the vortex chamber. Consequently the coolant is injected at high flow rate. At low water level, water stops flowing into the standpipe. The flow from the small flow pipe connected circumferentially to the vortex chamber forms a strong vortex in the vortex chamber. The coolant is injected with small flow rate due to the vortex. Actual Japanese NRA Requirements Related to Buildings, Systems and Components

Installation of permanent backup facilities designed as “specialized safety facility” is required as the measures against intentional air craft crashes, etc.

Measures are strengthened for fire protection and internal flooding which trigger simultaneous loss of all safety function due to common cause.

Measures are required to prevent core damage even in the event of loss of safety functions due to the common cause. For example, a safety-relief valve(SRV) is opened by using mobile power sources to reduce the RPV pressure and water is injected using mobile water injection system.

Measures are required to prevent CV failures in the event of core damage. For example a filtered venting system is installed to reduce the pressure and temperature of CV and to remove radioactive materials. A system such as mobile pumps, hoses etc. are to be prepared to inject water into the lower part of the CV to prevent its failure. It is shown in Fig. 12.12.

In order to suppress radioactive materials dispersion in the event of CV failure, deployment of outdoor water spray equipment is required to douse the reactor building and prevent a plume of radioactive materials contaminating the atmosphere.

Fig. 12.12 Measures to prevent containment vessel failure [3]

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