FIG. 76. Modern control room.
5.3.2.Advanced methods
With the above types of advances in electronics and design, control rooms have reduced in size by half or more; and with the advances in the lift capacity of cranes, the control panels and associated equipment have become a target area for complete room modularization.
5.4. FUEL BUILDING — CONSTRUCTION ACTIVITIES AND TECHNOLOGIES
5.4.1.Conventional methods
The fuel building is used for the receipt, inspection, storage and transfer of new fuel and spent fuel. The building encloses the spent fuel pool. This is a water-filled pool which contains the storage racks for the spent fuel assemblies. The spent fuel racks are typically stainless steel in combination with other absorber (poison) materials. Poison materials control the chain reaction inside the fuel racks by absorbing the neutrons emitted from the still active but spent nuclear fuel. Some of the main poison materials are borated stainless steel, boraflex and boral.
The fuel building houses the fuel pool cooling heat exchangers and fuel pool cooling pumps. These items of equipment and their associated systems clean and remove heat from water that is recirculated to the spent fuel pool. Cranes overhead in the building, and above and bridging the pool, are used to move individual fuel assemblies and casks containing multiple assemblies for transport and storage.
Figures 77–80 show the conventional stick-build approach to constructing the fuel building. Note the pool liner assembly in Fig. 78, conventionally erected in place, which is now a candidate for modularization.
5.4.2.Advanced methods
For those NSSS designs that utilize a building layout where the fuel building is separate yet adjoined to the containment structure, in contrast to those that have the fuel pool as a part of the reactor building structure, the potential targets for application of large preassembly modularization with placement by VHL crane are the fuel pool and storage racks, as well as other aforementioned equipment. The potential benefit of modularization in this case is that it allows delayed sequencing of the fuel building erection. With the fuel building pushed to the right of the schedule, the VHL cranes can be placed in the location of the future fuel building for part of the construction duration. This enables the VHL cranes to place more complete and larger modules in the adjacent building structures, including the containment, auxiliary/reactor, control and radiation waste (radwaste) building.
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FIG. 77. Fuel building foundation work.
FIG. 78. Fuel pool liner assembly.
FIG. 79. Fuel pool in operation.
As per IAEA safety guide (IAEA Safety Series No. 116), a leakage detection and leakage collection system in the spent fuel storage bay is recommended. Advanced construction methodology has now adopted ‘tank in tank’ technology for the spent fuel storage building, which provides the following:
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FIG. 80. Fuel handling equipment above fuel pool.
FIG. 81. Spent fuel storage tank (credit: NPCIL).
—A well designed leakage detection and leakage collection system;
—A mechanism that prevents any contamination of the groundwater, even in case of any remote leakage due to failure of metallic liner on the inside faces of the spent fuel storage pool;
—Adequate space for in-service inspection of the inner pool during the operating life of the plant. Figure 81 shows typical details of the spent fuel storage building.
The spent fuel storage building also has a facility for inspection of spent fuel bundles. In the event of any leak observed in the bundles and transferring mechanism, they are moved to a pre-decided location inside the pool. Any inadvertent dropping of a spent fuel cask during handling is provided for by adequate reinforcement in the raft, as well as thick metallic embedded plates in the cask handling area. Use of the cask handling crane is limited to the cask handling area only.
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6. CONSTRUCTION ACTIVITIES AND
TECHNOLOGIES SPECIFIC TO
GROUP B (TURBINE ISLAND) BUILDINGS AND STRUCTURES
The significant construction activities for turbogenerator buildings are:
—Excavation;
—Setting up of the dewatering system;
—Construction of the base slab;
—Construction of the condenser cooling water piping;
—Installation of the condenser;
—Casting of the turbogenerator pedestal;
—Erection of structural steel.
These activities are described in the following paragraphs.
6.1.EXCAVATION
Excavation is one of the critical construction activities for the turbine building. This is especially true for coastal sites where tidal elevations are significant. This results in deep turbine buildings with excavation depths greater than 20 m being quite common.
It is recommended that advanced excavation methods such as precision line blasting or chemical foam expansion be used for deep turbine buildings in order to achieve a short excavation schedule.
6.2. SETTING UP OF DEWATERING SYSTEM
A comprehensive dewatering system must be set-up after the excavation is completed. This is especially required for deep turbine buildings in rock sites, with multiple sumps strategically located over the whole footprint of the turbine building.
6.3. CONSTRUCTION OF BASE SLAB
The construction of the base slab is also a crucial construction activity, since the base slab is the largest concrete element in many NPPs. This is because many designs recommend mass stability for this building. Some designs use rock anchors to provide stability to the base slab. This activity can take significant time, since the number of rock anchors will be large, and sometimes instrumentation is set-up to monitor the performance and condition of the rock anchors and the differential settlement of the turbine building.
It is recommended that advanced construction methods such as large pours with pumped concrete be used to construct the base slab. Refer to Paragraphs 5.1.1.1. and 5.1.1.2 for the conventional and advanced methods, respectively.
6.4. CONSTRUCTION OF CONDENSER COOLING WATER PIPING
The condenser cooling water piping is (sometimes) located below the turbine building base slab, and must therefore be constructed before the base slab. Though the construction is relatively traditional, construction
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