All commercial NPPs in the United States contain structures whose performance and function are necessary for the protection of the safety of

Figure 1 Pantheon, built 119-128 AD. From Http://en. wikipedia. org/wiki/file:Pantheon_rome_2005may. jpg

plant-operating personnel and the general public, as well as the environment. The basic laws that regulate the design (and construction) of NPPs are contained in Title 10ofthe Code of Federal Regulations (CFR),10 which is clarified by Regulatory Guides (e. g., R. G. 1.29),11 NUREG reports, Standard Review Plans (e. g., Con­crete and Steel Internal Structures of Steel or Concrete Containments),12 etc. In addition, R. G. 1.29 and Part 100 to Title 10 of the CFR state that NPP structures important to safety must be designed to withstand the effects of earthquakes without the loss of function or threat to public safety. These ‘safety-related’ structures are designed as Seismic Category I. Seismic Category I structures typically include those classified by the American Society of Mechanical Engineers (ASME) and the American Nuclear Society (ANS) as Classes 1, 2, and 3 (i. e., safety related).

Initially, existing building codes such as the ACI Standard 318, Building Code Requirements for Reinforced Concrete, were used in the nuclear industry as the basis for the design and construction of concrete structural members. However, because the existing building codes did not cover the entire spectrum of design requirements and because they were not always con­sidered adequate, the United States Nuclear Regu­latory Commission (USNRC) developed its own criteria for the design of Category I structures (e. g., definitions of load combinations for both operating and accident conditions). Current requirements for nuclear safety-related concrete structures, other than concrete reactor vessels and concrete containments, are also based on ACI 318, but have incorporated modifications to accommodate the unique perfor­mance requirements of NPPs. These requirements were developed by ACI Committee 349 and first published in October 1976.13 This Code has been endorsed by the USNRC as providing an adequate basis for complying with the general design criteria for structures other than reactor vessels and contain — ments.14 USNRC15 provides additional information on the design of seismic Category I structures that are required to remain functional if the Safe Shutdown Earthquake (SSE) occurs. Current requirements for concrete reactor vessels and concrete containments were developed by ACI Committee 359 and first published in 1977.16 Supplemental load combination criteria are presented in Section 3.8.1 of the USNRC Regulatory Standard Review Plan} However, since all but one of the construction permits for existing NPPs have been issued prior to 1978, it is unlikely that endorsed versions of either ACI 349 or ACI 359 were used in the design of many of the concrete structures at these plants. Older plants that used early ACI codes, however, have been reviewed by the USNRC through the Systematic Evaluation Program to determine if there were any safety concerns.18

Each boiling water reactor (BWR) or pressurized water reactor (PWR) unit in the United States is located within a much larger metal or concrete con­tainment that also houses or supports the primary coolant system components. Although the shapes and configurations of the containment can vary signifi­cantly from plant to plant, leak tightness is ensured by a continuous pressure boundary consisting of non­metallic seals and gaskets and metallic components that are either welded or bolted together. There are several CFR General Design Criteria (GDC) and ASME Code sections that establish minimum requirements for the design, fabrication, construc­tion, testing, and performance of the light-water reac­tor (LWR) containment structures. The GDC serve as fundamental underpinnings for many of the most important safety commitments in licensee design and licensing bases. General Design Criterion 2, Design Bases for Protection Against Natural Phenomena, requires the containment to remain functional under the effects of postulated natural phenomena such as earthquakes, tornadoes, hurricanes, floods, tsunami, and seiches. General Design Criterion 16, Containment Design, requires the provision of reactor containment and associated systems to establish an essentially leak — tight barrier against the uncontrolled release of radio­activity into the environment and to ensure that the containment design conditions important to safety are not exceeded for as long as required for postulated accident conditions. Criterion 53, Provisions for Con­tainment Testing and Inspection, requires that the reactor containment be designed to permit (1) appropriate periodic inspection of all important areas, such as penetrations; (2) an appropriate surveillance program; and (3) periodic testing at containment design pressure of leak tightness of penetrations that have resilient seals and expansion bellows. Current LWR contain­ments are considered as a significant element of the USNRC’s safety policy, which employs a defense- in-depth approach (i. e., successive compensatory measures are exercised to prevent accidents or miti­gate damage if a malfunction, accident, or naturally caused event occurs). The defense-in-depth philoso­phy ensures that safety will not be wholly dependent on any single element of the design, construction, maintenance, or operation at a nuclear facility (e. g., the facility in question tends to be more tolerant of failures and external challenges).

From a safety standpoint, the containment is one of the most important components of an NPP because, independent of the fuel barrier and reactor coolant pressure boundary barrier, it serves as the final barrier to the release of fission products to the outside environment under postulated accident conditions. During normal operating conditions, the containment is subject to various operational and environmental stressors (e. g., ambient pressure fluc­tuations, temperature variations, earthquakes, and wind storms). In some containment designs, the prin­cipal leak-tight barrier is surrounded by another structure (e. g., reactor or shield building) that pro­tects the containment from external events. Ensuring
that the structural capacity and leak-tight integrity of the containment has not deteriorated unacceptably because of aging or environmental stressor effects is essential to reliable continued service evaluations and informed aging management decisions. More detailed information on containments is available.1

In addition to the containment, a myriad of concrete-based structures are contained as a part of an LWR plant to provide foundation, support, shield­ing, and containment functions. Table 1 presents a listing of typical safety-related concrete structures that may be included as part of an LWR plant.20 Relative to general civil engineering reinforced con­crete structures, NPP concrete structures tend to be

Table 1 Typical safety-related concrete structures in LWR plants and their accessibility for visual examination

Concrete structure

Primary containment Containment dome/roof Containment foundation/basemat Slabs and walls Containment internal structures Slabs and walls

Reactor vessel support structure (or pedestal) Crane support structures Reactor shield wall (biological)

Ice condenser dividing wall (ice condenser plants) NSSS equipment supports/vault structures Weir and vent walls (Mark III)

Pool structures (Mark III)

Diaphragm floor (Mark II)

Drywell/wetwell slabs and walls (Mark III) Secondary containment/reactor buildings Slabs, columns, and walls Foundation

Sacrificial shield wall (metallic containments) Fuel/equipment storage pools Walls, slabs, and canals Auxiliary building Fuel storage building Control room (or building)

Diesel generator building

Piping or electrical cable ducts or tunnels

Radioactive waste storage building

Stacks

Intake structures (including concrete water intake piping and canal embankments)

Pumping stations

Cooling towers

Plant discharge structures

Emergency cooling water structures

Dams

Water wells Turbine building

Internal liner/complete external

Internal liner (not embedded) or top surface

Internal liner/external above grade

Generally accessible Typically lined or hard to access Generally accessible Typically lined Lined or hard to access Generally accessible Lined with limited access Lined

Lined with limited access Internal liner/partial external access

Accessible on multiple surfaces Top surface

Internal lined/external accessible

Internal lined/partial external

Generally accessible

Generally accessible

Generally accessible

Generally accessible

Limited accessibility

Generally accessible

Partial internal/external above grade

Internal accessible/external above grade and waterline

Partially accessible Accessible above grade

Internal accessible/external above grade and waterline

Limited accessibility

External surfaces above waterline

Limited accessibility

Generally accessible

Source: Hookham, C. J. In-Service Inspection Guidelines for Concrete Structures in Nuclear Power Plants, ORNL/NRC/LTR-95/14; Lockheed Martin Energy Systems, Oak Ridge National Laboratory: Oak Ridge, TN, 1995.

more massive and have increased steel reinforcement densities with more complex detailing. Information pertaining to a particular structure at a plant of interest can be obtained from sources such as the plant’s safety analysis report or docket file. Concrete structures that are considered to be ‘plant specific’ or unique have not been addressed in the discussion later, but some information provided for similar structures may be applicable. Additionally, the names of certain structures may vary from plant to plant depending on the nuclear steam supply system (NSSS) vendor, architect engineering firm, and owner preference. Typical safety-related concrete structures contained in LWR plants may be grouped into four categories: primary containments, contain­ment internal structures, secondary containment/ reactor buildings, and other structures.