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Generation III/III+ BWR Designs

ABWR.

The ABWR was originally designed by General Electric in 1990s and the standard design was certified by the NRC in 1997. ABWRs are the evolutionary design of the generation III. Several NPPs had been constructed by Hitachi and Toshiba and currently are in commercial operation in Japan and two ABWRs are in construction is Taiwan. The South Texas Project Electric Generating Station (STP) plans to build COL a Toshiba ABWR design with rated power of 1350 MWe; the STP COL is currently being reviewed by the NRC. The ABWR design incorporates features of the BWR designs in Europe, Japan, and the United States, and uses improved electronics, computer, turbine, and fuel technology. The design is expected to show improvement in plant availability, operating capacity, safety, and reliability. Improvements include the use of internal recirculation pumps, control rod drives that can be controlled by a screw mechanism rather than a step process, microprocessor-based digital control and logic systems, and digital safety systems. The design also includes safety enhancements such as containment over-pressure protection, passive core debris flooding capability, an independent water makeup system, three emergency diesels, and a combustion turbine as an alternate power source.

The ABWR containment utilized the same pressure suppression concept as GE’s previous generations of BWR designs, and was evolved from the Mark II type of containment (Fig. 9.2).

The ABWR containment is significantly smaller that the Mark III containment because the elimination of the recirculation loops translates into a significantly more compact containment and reactor building [25]. The containment structure is made of reinforced concrete with a steel liner from which it derives its name — RCCV, or reinforced concrete containment vessel (Fig. 9.3). The RCCV consists of an Upper Drywell, a Lower Drywell, a Suppression Pool Chamber (also called wetwell), and the vents connecting the drywells and wetwell. The height of the containment is 31.8 m measured from the top of the basemat to the top of the upper drywell with an inside diameter of 29 m. The ABWR containment is about 10 m shorter than the Mark II design.

As shown in Fig. 9.3, the upper drywell (UD) surrounds the reactor pressure vessel (RPV) and houses the steam and feedwater lines and other connection of the reactor primary coolant system, safety/relief valves (SRVs) and drywell HVAC coolers. The UD is a cylindrical, reinforced concrete structure with a removable

FIG. 9.2

COMPARISON BETWEEN MARK II AND ABWR CONTAINMENTS [25]

FIG. 9.3 RCCV, REACTOR INTERNALS AND MAJOR PENETRATIONS FOR ABWR CONTAINMENTS [25]

steel head and a reinforced concrete diaphragm floor. The lower drywell (LD) accommodates the reactor internal pumps, under vessel components and servicing equipment. The pedestal consisting of cylindrical prefabricated concrete-filled steel structure, which supports the RPV, is connected rigidly to the diaphragm floor and separates the LD from the wetwell. Ten drywell connecting vents (DCVs) are built into the pedestal and connect the UD and LD. The DCVs are extended downward via steel pipes, each having three horizontal vent outlets into suppression pool (Table 9.2). In addition, the LD floor is topped with a think basaltic concrete pad under the RPV. In an event of ex-vessel relocation of molten core melt through the lower head of

TABLE 9.2

KEY DESIGN CHARACTERISTICS OF CONTAINMENT FOR ABWR [20]

Containment

Primary

Type

Pressure suppression

Construction

Reinforced concrete with steel liner

Drywell

Concrete cylinder

Wetwell

Concrete cylinder

Design pressure, MPa

0.31

Containment ultimate pressure capacity, Mpa

0.97

Design leak rate, % free volume/day

0.5 — Excluding MSIS leakage

Drywell free volume, m3

7350

Wetwell free volume, m3

5960

Suppresion pool water volume, m3

3580

Number of vertical vents

10

Vertical vent diameter, m

1.2

Number of horizontal vents/vertical vent

3

Horizontal vent diameter, m

0.7

Secondary

Type

Controlled leakage

Construction

Reinforced concrete with steels

the RPV, the heated core melt will be both caught and held on the basaltic pad, which is subsequently flooded and cooled by wetwell’s water supplied through fusible links within the wall separating the wetwell from the lower drywell. The pressure retaining concrete walls of the RCCV are lined with leak-tight steel plates and for the pressure boundary of the primary containment system.

In addition, the RCCV is structurally integrated with the reactor building and forms the a major structural part of this building as shown in Fig. 9.2b. The reactor building is separated into four compartments, three house the three independent divisions of the emergency core cooling system (ECCS), and one is reserved for non safety systems. Because the RCCV and reactor building are monolithically connected, the overall structural stiffness is increased, therefore is capable of resisting higher dynamic and shear loads. To illustrate this, the reactor building is designed for a safe shutdow earthquake (SSE) of 0.3 g peak ground acceleration (PGA); however, a seismic margin assessment indicated that the ABWR plant level seimsic capacity in terms of high confidence and low probability of failure (HCLPF) is estiamted at two times the SSE at the sequence level. This ensure very little possibility of a core damage event as a result of earthquake.

In addition, the ABWR containment incorporates the containment overpressure protection system (COPS). The COPS protects the containment if a less likely severe accident occurs which increases the containment pressure to a point where containment structural integrity is threatened. The COPS will release this containment pressure through a line connecting the wetwell atmosphare to the plant stack, thus providing a filtered release path from the wetwell airspace to preclude an uncontrolled containment failure due to over pressurization.

The COPS consists of a relief line connecting the wetwell airspace to the plant, containing two rupture disks in series. The COPS is normally closed even at the design pressure. When the containment is overpressurized above the design pressure but below a setpoint pressure of approximately 1.0 MPa, corresponding to the ASME Service Level C capability of the containment structures, the rupture disks would open, allowing the pressure to be relieved in a manner that forces escaping fission materials to pass through the suppression pool water, thereby preventing the release of radioactive materials to the environment. After the containment pressure has been reduced and normal containment heat removal capability has been regained, the operator can close two normall open air-operated valves in the relief path to reestablish containment integrity. Note that the COPS is completely passive, and no power is required for initiation or operation of the COPS.

 
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