solid base rock

Figure 2,2.1 Site plot plan conceptual model (cross section)

Figure 2.2.2 Overall plant layout of a bwr npp

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about 119m

reactor building

reactor building

Figure 2.2,3 Typical equipment layout of a 1,100 MWe class plant

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floor of the reactor building near the turbine building

50. Reactor building

The reactor building houses a reactor pressure vessel (RPV) which generates steam, a primary containment vessel (PCV) which acts as the primary barrier against release of radioactivity in the event of an accident, pumps and heat exchangers for the safe shutdown system, an emergency diesel generator system which provides emergency power to the safety system components, a storage pool for spent fuel assemblies, a water cleanup system for the reactor water and the moisture-separator pit water, a standby gas treatment system (SGTS) which prevents release of radioactive gases in the event of an accident by keeping a negative pressure inside the reactor building. The RPV is installed in the center of the building and it is enclosed by the PCV while other equipment is located outside the containment vessel. The PCV is a steel self-standing structure fixed to the reactor building foundation. Its outer surface is covered with a thick reinforced concrete wall which acts as a biological shield. The reactor building is a reinforced concrete structure from its foundations to the operating floor and the building acts as the secondary containment to confine radioactive materials. In order to ensure that function, the building is kept at a negative pressure by the heating, ventilating and air conditioning (HVAC) system.

Three different types of PCVs have been adopted at present including that of the ABWR in Japan. Their features are summarized in Figure 2.2.4.

Among these, MARK-1 and MARK- II containment vessels were introduced from the United States and constructed as they were. However, based on the constructional and operational experiences with BWR power plants in Japan, improvements have been added to their original design and they have been adopted as the Japanese Improvement and Standardization Program design. Figure 2.2.5 compares the original design and the Japanese Improvement and Standardization Program design.

Dimensions of the reactor building are determined by the size of the PCV, reactor components layout and the operating floor lay-down space. The size and layout of the bottom of the reactor building

are determined by the PCV or the suppression pool outside of the biological shield for the Mark- I containment vessel, pumps of the reactor safe shutdown system which utilize the corners of the building effectively, and the emergency diesel generator system.

Dimensions of the upper operating floor are determined by the spent fuel pool, temporary equipment pits, temporary spaces for disassembled parts of the reactor system during the outage, personnel passages, stairs and a large equipment hatch. The reactor building horizontal cross section is nearly square. The height of the building is determined by the total height of the PCV, the required depth of water for shielding during transfer of spent fuel assemblies from the reactor core, spent fuel cask lifting height and the installation height of the overhead traveling crane. The height of each floor and the number of stories in the building is determined by the arrangement of auxiliary system components optimizing the system flow from one floor to the next

Pumps for the emergency core cooling system (ECCS) that function to cool the core in an accident as a safety system are located on the lowest floor of the building just above the foundation in order to effectively pump the suppression pool water. This system, including piping and electrical cabling, consists of multiple trains separated physically by concrete separation walls.

Access routes to each separate room are also separated from each other for fire and flood protection. Assuming a very severe accident condition where the external power is lost simultaneously with a loss of coolant accident and a single failure occurs, the ECC capability must be fulfilled. A vertical type core cooling pump is used with an equipment hatch and lifting devices on the floor above and there is ample space for disassembly and maintenance of the pump.

The primary loop recirculation (PLR) pumps which recirculate reactor coolant are located within the PCV with enough space for moving the pump through a hatch from the reactor building. Also a provision must be made for transporting out disassembled pumps for maintenance. Control rod drive mechanisms are set in the housing welded to the bottom head of the RPV and control rod drive

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NSRA, Japan

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(keep enough space for maintenance)

inside space of RPV pedestal

(keep enough space for CRD, LPRM replacing)

(increase of internal space) cooler

(optimized anamgemml) stairs

(new installation) pipe whip structure (optimized arrangement) recirculation pump

(keep enough space ¹ for maintenance)

pressure

suppression

chamber

(increase of inside diameter and height)

monorail for SRV removal

(new installation)

SRV cany in / out hatch

(new installation)

MS I V

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