Of the BWR plants that have been licensed for com­mercial operation in the United States, ^30% utilize either reinforced or prestressed concrete primary containments. Leak tightness of each of these con­tainments is provided by a steel liner attached to the containment inside surface by studs (e. g., Nelson studs) or by structural steel members. Exposed sur­faces of the carbon steel liner are typically painted to protect against corrosion and to facilitate decontami­nation should it be required. A portion of the liner toward the bottom of the containment and over the basemat is typically embedded in concrete to protect it from damage, abrasion, etc. due to corrosive fluids and impact. A seal to prevent the ingress of fluids is provided at the interface around the circumference of the containment where the vertical portion of the liner becomes embedded in the concrete. BWR con­tainments, because of provisions for pressure sup­pression, typically have ‘normally dry’ sections (drywell) and ‘flooded’ sections (wetwell) that are interconnected via piping or vents. Requirements for BWR containments include the following:

1. Provide an ‘essentially’ leak-tight barrier against the uncontrolled release of radioactivity to the environment for all postulated design basis acci­dent conditions.

2. Accommodate the calculated pressure and tem­perature conditions resulting from a loss-of — coolant accident.

3. Withstand periodic integrated leak-rate testing at the peak-calculated accident pressure that may be at levels up to and including the containment design pressure.

4. Permit appropriate periodic inspection of all impor­tant components and surfaces, and the periodic test­ing ofthe leak tightness ofcontainment penetrations.

The containment vessel can also provide structural support for the NSSS and other internal equipment. The containment foundation, typically a basemat, provides the primary support and transfer of load to the earth below. Figure 2 presents a cross-section of a BWR Mark I reinforced concrete containment.

Each of the three BWR primary plant types (Mark I, Mark II, and Mark III) incorporates a number of reinforced concrete containment internal structures. These structures may perform singular or several functions, including the following:

1. Radiation shielding;

2. human accessibility provisions;

3. NSSS and other equipment anchorage/support/ protection;

4. resistance to jet, pipe whip, and other loadings produced by emergency conditions;

5. boundary of wetwells and pool structures, and allow communication between drywell and wetwell (Mark II and III);

6. lateral stability for containment;

7. transfer of containment loads to underlying foun­dation; and

8. transfer of fuel to reactor (Mark III).

As many of these functions are interrelated with the required containment functions, these structures are considered to be safety-related.

Of the BWR plants that utilize steel primary containments, all but the pre-Mark plant type have reinforced concrete structures that serve as second­ary containments or reactor buildings and provide support and shielding functions for the primary con­tainment. Although the design parameters for the secondary containments of the Mark I and Mark II plants vary somewhat, the secondary containments are typically composed of beam, floor, and wall struc­tural elements. These structures typically are safety — related because they provide additional radiation shielding; provide resistance to environmental/opera — tional loadings; and house safety-related mechanical equipment, spent fuel, and the primary metal con­tainment. Although these structures may be massive in cross-section to meet shielding or load-bearing requirements, they generally have smaller elemental

thicknesses than primary containments because of reduced exposure under postulated accident loadings. These structures may be maintained at a slight nega­tive pressure for collection and treatment of any air­borne radioactive material that might escape during operating conditions.

Other structures include such things as founda­tions, walls, slabs, and fuel/equipment storage pools. The spent — and new-fuel storage pools, and the pools for reactor internals storage, typically have a four- wall-with-bottom-slab configuration. The walls and slab are composed of reinforced concrete members lined on the interior surface with stainless steel. Cross-sections of these members are generally large because they must support a large pool of water
and heavy fuel/component loads produced by high-density fuel storage considerations. The fuel storage pool in Mark III plants is located within the primary containment.