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US EPR.

The US evolutionary pressurized water reactor (USEPR) is developed based on the European pressurized water reactor (EPR) design derived from the German Konvoi types with features from the French N4 reactors. The USEPR design certification application was submitted to the U.S. NRC in 2007 and is currently under NRC review. This section provides a brief description of the USEPR reactor features as they relate to containment designs, based on the information from publicly available information documents [33-37].

FIG. 9.10

AP1000 IN-VESSEL RETENTION OF MOLTEN CORE DEBRIS [28]

The USEPR is a four-loop evolutionary generation III+ PWR with rated power at about 1600 Mwe. The USEPR design is built and improved upon the established technologies from the well-proven N4 and Konvoi reactors in France and Germany, and incorporates a combination of active and passive safety features to ensure safety and reduces the probability of a core accident at power by a factor of 10 compared to previous generations of reactors. The main design features for the USEPR include:

  • • Proven four-loop RCS design with increased NSSS component volumes allowing extended time to response to transients and accidents
  • • Four completely independent trains of safety systems housed in four divisions of safeguard buildings
  • • Double shell containment building to protect against commercial aircraft impact and other external hazards
  • • Increased redundancy and physical separation of safety trains
  • • Top-mounted in-core instrumentation system eliminates penetrations on RPV lower head
  • • Four emergency diesel generators located in two physically separated buildings to provide AC power to four trains of safety systems
  • • Prestressed post-tensioned concrete containment with grouted tendons to withstand internal design basis pressure of 0.43 MPa (62 psi) with ultimate pressure capability of 0.82 MPa (119 psi) at Service Level D limits
  • • In-containment refueling water storage tank (IRWST) as water source for both safety injections and severe accident mitigations
  • • Dedicated primary depressurization system to prevent high-pressure core melt in event of severe accidents
  • • Passive ex-vessel melt stabilization, conditioning, and cooling using core catcher and IRWST
  • • Passive containment spray system
  • • Control of hydrogen concentration using passive autocatalytic recombiners
  • • Predicted core damage frequency less that 1 E-06/yr and large and early release frequency less than 1E - 07/yr
  • • Core is designed for 12- to 24-month fuel cycle
  • • Refueling outages can be conducted in 17 days or less
  • • Advanced cockpit control room design
  • • Plant design life of 60 years without replacement of the reactor vessel

The USEPR containment and other primary Category I structures are built on a common basemat called the nuclear island (NI), including: the reactor containment building (RCB), the containment shield building (RSB), the fuel building, and four safeguard buildings (Fig. 9.11). The NI basemat is a cruciform shape with dimensions of 108 m (360 ft) by 108 m (360 ft) by 3.048 m (10 ft) thick, and embedded below grade by approximately 12.2 m (40 ft). Also indicated in Fig. 9.16 are two Category I emergency power generating buildings (EPGB) housing four emergency diesel generators. The EPGBs are separated from the NI basemat structures.

Not shown in Fig. 9.11 are four Category I emergency service water buildings (ESWB) housing four trains of the essential service water system which cools the component cooling water system (CCWS) heat exchangers with water from the ESWS tanks as the ultimate heat sink during normal operating conditions and accident conditions.

The RCB is a large, post-tensioned reinforced concrete shell structure consisting of an upright cylinder capped with a spherical dome and a free volume of 79,287.2 m3 (2.8 million cubic feet), and is located within the RSB (Fig. 9.12). The design leak-rate is less than 0.25% per day at design pressure of 0.43 MPa (62 psi). The containment is 66.45 m (218 ft) high and has an inner diameter of 47 m (153.5 ft) with the wall thickness of 1.3 m (4.3 ft) and a steel liner plate of a thickness of 6.35 mm (^ inches) covers the entire inner surface of the containment to provide a leak-tight membrane to airborne radioactivity following postulated design

FIG. 9.11

US EPR PLANT BUILDING LAYOUT [33]

basis accidents (DBA). The containment houses the NSSS components separate from the rest of containment using a “two room” concept. The major NSSS components inside the equipment space are located in separate compartments; concrete wall are erected between the individual coolant loops and between the hot leg and cold leg piping of each loop. This design feature reduces possibility of hydrogen concentrations. In addition, the passive autocatalytic hydrogen recombiners (PARs) are installed in different compartments to keep the average concentration below 10% at all times to avoid the risk of deflagration or detonation.

The wall and dome of the RCB are post-tensioned with hoop, vertical, and gamma tendons to withstand the design basis internal pressure. Reinforced steel bars are also placed in RCB concrete wall and dome for crack control and to provide strength to accommodate seismic and other loads. The USEPR is the only standard design in the United States to date which utilizes post-tensioned prestressing concrete containment with fully grouted tendons. The RCB uses the Freyssinet C-range 55C15 tendon system, each tendon consisting of 55 seven-wire strands. Three groups of tendons are provided for the RCB: horizontal hoop, vertical, and gamma tendons. The horizontal hoop tendons are provided around the cylindrical shell of the RCB. The tendons terminate at the three vertical buttresses around the outside of the containment wall at azimuths of 0-, 112-, and 230-degree apart. Each hoop tendon extends the full circumference of the wall to allow the termination of tendons to alternate among the three vertical buttresses. The vertical tendons extend up along the cylindrical shell and terminate at the top of the ring girder that is provided at the transition of the wall to the spherical dome. Two groups of gamma tendons placed at 90-degree apart also extend vertically up through the wall where they then wrap around over the dome and terminate at the ring girder on the opposite side of the wall. The bottom of both the vertical and the gamma tendons terminate at the tendon gallery located directly under the circumference of the cylindrical containment wall below the NI foundation basemat. This gallery provides access for installing and maintaining the lower terminations of the tendons.

FIG. 9.12

US EPR REACTOR CONTAINMENT BUILDING AND CONTAINMENT SHIELD BUILDING [33]

After tensioning of the strands, the tendon sheath is injected with cement which acts as corrosion inhibitor. Both the quality of the cement material and the tendon installation process follow the procedure provided in Regulatory Guide 1.107 [38]. In addition, the USEPR employs Alternative B of Regulatory Guide 1.90 [39] for monitoring deformation under pressure during the periodic in-service inspections as required by General Design Criterion (GDC) 53, “Provisions for Containment Testing and Inspection.” Membrane compression is maintained and the maximum stress in the tensile reinforcing bars will be limited to one-half the yield strength of the reinforcing steel (0.5 fy) under the peak expected pressure for in-service inspection tests.

Another salient feature of the containment is the design of the reactor pit area consisting of the transfer channel, the corium spreading area and the IRWST pool, located in the RCB lower part below the reactor pressure vessel (Fig. 9.13). The USEPR severe accident mitigation management relies upon the ex-vessel retention strategy to minimize the possibility of an energetic corium water interaction. In a severe accident scenario where core melt is assumed to have occurred, a passive melt plug at the bottom of the RPV opens to flush the full core of corium to the reactor pit and through the transfer channel into the corium spreading area (Fig. 9.14). The corium in the spreading area is passively cooled by gravity injection of water from the IRWST. The cooling water flows directly under the spreading area to protect the containment basemat.

Furthermore, the RCB is surrounded by the containment shield building (RSB) for protection against external hazards such as winds, tornadoes and hurricanes, as well as missile impact of a commercial aircraft crash. The

FIG. 9.13

CONCRETE STRUCTURAL COMPARTMENTS FOR IRWST AND CORIUM SPREADING AREA [33]

RSB is a heavily reinforced concrete structure consisting of a cylindrical shell and a dome. The RSB, which is approximately 56.7 m (186 ft) in diameter by 70 m (230 ft) high, completely encloses the RCB. The RSB also serves as an additional preventive barrier to the release of radiation from the RCB. In addition, the plant layouts as shown in Fig. 9.11, especially the spatial separations of the four safeguard buildings, ensure the safe shutdown of the plant in the event of any postulated external hazards including a commercial aircraft impact.

FIG. 9.14

US EPR PASSIVE CORIUM RETENTION EX-VESSEL [33]

 
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