Desktop version

Home arrow Engineering arrow Containment structures of U.S. nuclear power plants

ESBWR.

The ESBWR, which is designed by GE Hitachi, is a generation III+ reactor that utilizes complete passive features and natural circulation principles for accident preventions and mitigations. The ESBWR is rated at 1560 MWe. The reliance on natural circulation and passive safety systems enhances the plant performance and simplifies the design for the ESBWR. The passive safety features eliminate the need for safety-grade pumps and AC powers. Design simplification also results on a reduction in building volume compared to the ABWR.

The ESBWR containment also used the pressure suppression concept as GE’s BWR designs but allowed different configurations and arrangements from the ABWR containment to accommodate natural circulations and passive safety systems inside the containment (Fig. 9.4).

Most noticeably are the elevated pools to allow for natural circulations and the ECCS system within the containment which consists of gravity driven cooling system (GDCS) and automatic depressurization system (ADS). Similar to the ABWR, the ESBWR containment structure consists of a reinforced concrete cylindrical structure with a steel liner from which it derives its name — RCCV, or reinforced concrete containment vessel (Fig. 9.4). The RCCV consists of an Upper Drywell, a Lower Drywell, three GDCS pools, a Suppression Pool Chamber (also called wetwell), and the vents connecting the drywells and wetwell. The RCCV is capped off with a steel head and is submerged in the PCCS/IC pools which are part of the reactor building. As shown in Fig. 9.5, the RCCV and the reactor building are structurally integrated through reinforced concrete walls and slabs. Therefore, the reactor building can serve as a secondary barrier to accidental release of radioactive materials to the environment.

The upper drywell of the RCCV consists of reinforced concrete cylindrical structure with an inside diameter of 36 m and is 19.95 m high. The wall and roof slab have a thickness of 2 m and 2.4 m, respectively. It

FIG. 9.4 SCHEMATIC VIEW OF CONTAINMENT SYSTEM FOR ESBWR [26]

surrounds the RPV and houses three GDCS pools and a large suppression pool. It also has a large airspace to allow the accumulation of large amount of steams released as a result of a postulated reactor accident. The lower drywell is a cylindrical airspace directly below the RPV and is surrounded by the reactor pedestal on the side and basemat at the bottom. The lower drywell has an inside diameter of 11.2 m and is 15.05 m high. The foundation mat is 5.1 m thick. The reactor pedestal is a 2.5-thick reinforced concrete cylindrical structure which is part the containment boundary and also provides support to the RPV. In addition, at the bottom of the lower drywell cavity and above the basemat is a BiMAC (basemat internal and melt arrest and coolability) device (Fig. 9.6) which consists of a sacrificial refractory concrete slab with embedded inclined pipes which are connected to GDCS pools. In a severe core melt event, the molten core melt would breach the bottom of the RPV and relocate in the lower drywell cavity. BiMAC is passive system actuated by thermocouples which detect the relocated core melt, which triggers the opening of the squib valves that allow the GDCS pool water flow through the BiMAC pipes to cover and cool the molten core melt. It should be noted that the design of BiMAC is not available for other designs and therefore is unique for the ESBWR.

The ESBWR design consists of four main passive systems [26, 27]:

  • 1. Automatic depressurization system (ADS) — The ADS consists of 10 safety relief valves (SRVs) mounted on top of the main steamlines which discharge steam to the suppression pool, and 8 depressurization valves (DPVs) that discharge steam to the drywell airspace;
  • 2. Gravity driven cooling system (GDCS) — The GDCS provides the makeup water through gravity flow into the vessel after the ADS depressurizes the RPV. The GDCS and ADS together become the plant’s ECCS;
  • 3. Isolation condenser system (ICS) — The ICS removes delay heat from the reactor following transient events involving reactor scram including station blackout. The ICS consists of four independent high pressure loops, each containing a heat exchanger which condenses steam on the inside of the tube wall.

FIG. 9.5

SCHEMATIC VIEW OF REACTOR BUILDING AND CONTAINMENT FOR ESBWR [26]

The tubes are located in a large pool above the RCCV. The system uses natural circulation to remove decay heat; and

4. Passive containment cooling system (PCCS) — The PCCS removes heat from the RCCV following a LOCA. The system consists of four safety-related low pressure loops. Each loop has a heat exchanger open to the containment, a condensate drain line and a vent discharge line submerged in the suppression pool. Similar to the isolation condensers, the PCCS heat exchangers are located in cool pools external to the containment.

The drywell structure is designed for the pressure and temperature transients associated with the rupture of any of the primary system piping inside the drywell. It is also designed for the negative differential pressures associated with containment depressurization events, when the steam in the drywell is condensed by the PCCS, the GDCS, the fuel and auxiliary pool cooling system (FAPCS), and cold water cascading from the break following post-LOCA flooding of the RPV.

FIG. 9.6

CONCEPTUAL VIEW OF BIMAC DEVICE FOR ESBWR [26]

In a LOCA event, the increased pressure inside the drywell forces a mixture of non-condensable gases, steam and water vapor though the PCCS vertical and horizontal vent pipes into the suppression pool where the steam is rapidly condensed. The non-condensable gases are contained in the airspace of the wetwell. The containment design pressure is 0.31 MPa (45 psig) (Table 9.3).

As described above, the RCCV is an integral part of reactor building structure which also houses structures, systems, and components above the RCCV, the buffer pool which is used to store the dryer,

TABLE 9.3

KEY DESIGN CHARACTERISTICS OF CONTAINMENT FOR ESBWR [26]

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

1.0

Design leak rate, % free volume/day

0.5 — excluding MSIS leakage

Drywell free volume, m3

7206

Wetwell free volume, m3

5432

Suppresion pool water volume, m3

4424

Number of vertical vents

12

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 equipment storage pool for storing the chimney partitions and the separator, and steam tunnels, refuel area and other supporting systems. The reactor building is a rigid box type shear wall structure, constructed of reinforce concrete. Similar to the ABWR, the ESBWR RCCV is also monolithically connected to the reactor building. The reactor buidling not only increases the structural performance of the RCCV but also provide a secondary barrier to accidental release of radioactive fission products into the environment.

The reactor building is designed for a safe shutdown earthquake (SSE) of 0.3 g peak ground acceleration (PGA); however, a seismic margin assessment indicated that the ESBWR plant level seismic capacity in terms of high confidence and low probability of failure (HCLPF) is estimated at 1.67 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 ESBWR 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 via a remote manual valve. The COPS is normally closed even at the design pressure. The containment pressure builtup would take some time to reach 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, which would allow time for the operator to manually open the valve 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. This function of the containment inerting system is called the manual containment overpressure protection system (MCOPS).

 
Source
< Prev   CONTENTS   Source   Next >

Related topics