4.13.3.2.1 Materials of construction

Nuclear safety-related concrete structures are com­posed of several constituents that, in concert, perform multiple functions (e. g., load-carrying capacity, radi­ation shielding, and leak tightness). Primarily, these constituents can include the following material sys­tems: concrete, conventional steel reinforcement, pre­stressing steel, and steel liner plate. The quality of these materials is established through regulations, qualification tests, and certification, followed by check­ing throughout construction. More detailed informa­tion on materials of construction than provided later is available elsewhere.35,58-60

Concrete is a composite material consisting of a binder (cement paste) and a filler of fine or fine and coarse aggregate particles that combine to form a synthetic conglomerate. Cement is a mixture of compounds made by grinding crushed limestone, clay, sand, and iron ore together to form a homoge­neous powder that is then heated at very high tem­peratures ranging from 1400 to 1600 °C to form a clinker.59 After the clinker cools, it is ground and mixed with a small amount of gypsum to regulate setting and facilitate placement. This produces the general-purpose Portland cement, which is mixed with water to produce cement paste that binds the aggregate particles together. (Current generation cements have higher tricalcium silicate (C3S) contents and are ground finer than previous cements. Current cements attain most of their compressive strength within a 28-day period, whereas the previous cements continued to gain strength after 28 days.6,61)

Portland cements are composed primarily of four chemical compounds: (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). The type of Portland cement produced (e. g., general purpose, moderate sulfate resistance and heat of hydration, high early strength, low heat of hydration, and sulfate resistant) depends on the relative amounts of the four basic chemical compounds and fineness (high early strength). The calcium silicate hydrates (C-S-H) constitute about 75% of the mass. The C-S-H gel structure is made up of three types of groups that contribute to bonds across surfaces or in the interlayer of partly crystal­lized tobermorite material: calcium ions, siloxanes, and water molecules. Bonding of the water within the layers (gel water) with other groups via hydrogen bonds determines the strength, stiffness, and creep properties of the cement paste.

There are also a number of alternative or sup­plementary cementing agents that have been used in conjunction with Portland cement, and these are pulverized fly ash, ground granulated blast furnace slag (GGBFS), and silica fume. Fly ash is collected from the exhaust flow of furnaces burning finely ground coal and reacts with calcium hydroxide in the presence of water to form cement compounds consisting of calcium silicate hydrate. GGBFS is a by-product of the iron-making process and is formed by taking the hot slag, rapidly chilling or quenching it, and grinding into a powder. When mixed with water in the presence of an alkaline environment provided by the Portland cement, GGBFS hydrates to form cementing compounds consisting of calcium silicate hydrate. Silica fume is the condensed vapor by-product of the ferrosilicon smelting process. Silica fume reacts with calcium hydroxide in the presence of water to form cementing compounds consisting of calcium silicate hydrate. High alumina cement, con­sisting mainly of calcium aluminates, has been utilized as a cementitious material because of its rapid set and rapid strength gain characteristics and resistance to acidic environments, sea water, and sulfates. However, owing to certain conditions of temperature and humidity, the cement converts over time to a different hydrate having reduced volume (i. e., increased poros­ity and reduced strength), it is recommended that calcium aluminate cements not be used for structural applications (particularly in wet or humid conditions above 27 °C).62

Selection of the proper water content of concrete is critical, since too much water reduces the concrete strength and insufficient water makes the concrete un­workable. Hardening of concrete occurs as a result of hydration, which is a chemical reaction in which the major compounds in the cement form chemical bonds with water molecules and become hydrates. The hard­ened cement paste consists mainly of calcium silicate hydrates, calcium hydroxide, and lower proportions of calcium sulfoaluminate hydrate either as ettringite or monosulfate. About 20% of the hardened cement paste volume is calcium hydroxide. The pore solution is normally a saturated solution of calcium hydroxide within which high concentrations of potassium and sodium hydroxides are present. Proper curing of the concrete during this stage is essential, as it affects the concrete’s durability, strength, water-tightness, abrasion resistance, volume stability, and resistance to freezing and thawing.

Since cement is the most expensive ingredient in concrete, it is desirable to utilize the minimum amount necessary to produce the desired properties and characteristics. Aggregate typically occupies 60-75% of the volume of concrete and therefore its characteristics strongly influence the chemical, physical, and thermal properties of concrete, its mix proportions, and economy. (The balance of the concrete mix generally consists of 10-15% cement, 15-20% water, and air (5-8% if entrained).) Aggre­gates thus are important with respect to the concrete durability. The aggregates come in various shapes, sizes, and material types ranging from fine sand par­ticles to large coarse rocks. Selection of the aggregate material is determined in part by the desired char­acteristics of the concrete. Aggregate materials are available ranging from ultra-lightweight (e. g., ver — miculite and perite) to lightweight (e. g., expanded clay shale or slate-crushed brick), normal weight (e. g., crushed limestone or river gravel), and heavy­weight (e. g., steel or iron shot). Sometimes chemical or mineral admixtures are added during the mixing process to enhance durability (air entrainment), improve workability (enhanced placement and com­paction), modify hardening and setting characteris­tics, aid in curing, reduce heat evolution, or provide other property improvements.63

The concrete typically used in nuclear safety — related structures consists of Type II Portland cement,59 fine aggregates (e. g., sand), water, various minerals, or chemical admixtures for improving properties or performance of the concrete and either normal-weight or heavy-weight coarse aggregate.

American Society of Testing and Materials (ASTM) C 150,64 Type II Portland cement, typically has been used because of its improved sulfate resistance and reduced heat of hydration relative to the general — purpose Type I Portland cement. Both the water and fine and coarse aggregates are normally acquired from local sources and subjected to material charac­terization testing prior to use. Coarse aggregate can consist of gravel, crushed gravel, or crushed stone. Chemical (e. g., air-entraining or water-reducing) or mineral (e. g., fly ash or ground granulated blast fur­nace slag) admixtures have been utilized in many of the mixes to impart improved characteristics or per­formance. For those concrete structures in NPPs that provide primary (biological) radiation shielding, heavy-weight or dense aggregate materials, such as barites, limonites, magnetites, and ilmenites, may have been used to reduce the section thickness and meet attenuation requirements.

The constituents are proportioned and mixed to develop Portland cement concrete that has specific properties. Depending on the characteristics of the specific structure, the concrete mix may be adjusted to provide increased strength, higher durability, or better workability for placement. The hardened concrete typically provides the compressive load­carrying capacity for the structure. Specified con­crete unconfined compressive strengths typically have ranged from 13 to 55 MPa, with 35 MPa being a typical value achieved at 28 days age.

Concrete tensile strength is about one-tenth to one-fifth of its compressive strength, so concrete cannot be relied upon to withstand very high tensile stresses. This limitation is overcome by embedding steel reinforcement in the concrete so that the concrete and steel work in concert. In addition to resisting tensile loads, the bonded steel reinforce­ment is used to control the extent and width of cracks, especially where it is desirable to reduce member cross-sections. Steel reinforcement is also used in compression members to safeguard against the effects of unanticipated bending moments that could crack or even fail the member. The effectiveness of rein­forced concrete as a structural material depends on the interfacial bond between the steel and concrete so that it acts as a composite material, the passivating effect of the highly alkaline concrete environment to inhibit steel corrosion (see next section), and the similar coefficients of thermal expansion of the con­crete and steel. Most of the mild, or conventional, reinforcing steels used in NPPs to provide primary tensile and shear load resistance/transfer consist of plain carbon steel bar stock with deformations (lugs or protrusions) on the surface. These bars typically conform to ASTM A61565 or A 70666 specifications. The minimum yield strength for the steel reinforce­ment ranges from 280 to 520 MPa, with the 420 MPa strength material being most common.

Post-tensioning is a method of reinforcing (or strengthening) concrete with high-strength tendons to resist tensile loadings and to apply compressive forces to the concrete to provide increased resistance to concrete cracking. A number of NPP concrete containment structures utilize post-tensioned steel tendons that are designed to have (1) consistently high strength and strain at failure, (2) serviceability throughout their lifetime, (3) reliable and safe pre­stressing procedures, and (4) ability to be retensioned and replaced (nongrouted systems). The tendons are installed within preplaced ducts in the containment structure and post-tensioned from one or both ends after the concrete has achieved sufficient strength. After tensioning, the tendons are anchored by button-heads, wedges, or nuts. Corrosion protection is provided by filling the ducts with wax or corrosion- inhibiting grease (unbonded) or portland cement grout (bonded). (Although bonded post-tensioning tendons are less vulnerable to local damage than ungrouted tendons, ungrouted tendons have been primarily used in the United States because the grouted tendon systems cannot be visually inspected, mechanically tested, or retensioned in the event of a larger than anticipated loss of prestressing force.) Supplemental conventional reinforcing is also used to minimize shrinkage or temperature effects and to provide local load-carrying capacity or load transfer. Three major categories of post-tensioning system exist depending on the type of material utilized to fabricate the tendons: wire, strand, or bar that con­form to ASTM specifications A 421,67 A 416,68 and A 7 2 2,69 respectively. Minimum tensile strengths range from 1620 to 1725 MPa for the A 421 material and 1725 to 1860 MPa for the A 416 material. The A 722 material has a minimum tensile strength of 1035 MPa. Typical NPP tendon systems group sufficient numbers of wires, strands, or bars to have minimum ultimate strengths ranging from 2000 to 10 000 kN. The trend has been to increase the strength of the tendons to reduce the total number (e. g., in the early 1970s, the typical tendon had a capacity of 3000 kN and since then has progressed to capacities of 10 300 and 15 300 kN).19 With the exception of Robinsion 2 (bar tendons) and Three Mile Island 2 (strand tendons), plants that have post-tensioned containments utilize unbonded tendons so that the tendons can be inspected and replaced (if necessary). Bellefonte and Ginna each has grouted tendons (rock anchors) to which tendons are attached.

Leak tightness of reinforced and post-tensioned concrete containment vessels is provided by a steel liner plate. A typical liner is composed of steel plate stock <13 mm thick, joined by welding, and anchored to the concrete by studs (Nelson studs or similar conforming to ASTM A 10870), structural steel shapes, or other steel products. PWR containments and the drywell portions of BWR containments are typically lined with carbon steel (ASTM A 3671 or A 51672). The liners of LWR fuel pool structures typically consist of stainless steel (ASTM A 27673 or A 30474). The liners of wetwells also have used car­bon steel materials such as ASTM A 285,75 A 516, and A 53 7.76 Certain LWR facilities also have used carbon steel clad with stainless steel weld metal for liner members. Although the liner’s primary function is to provide a leak-tight barrier, it acts as part of the formwork during concrete placement and may be used in the support of internal piping/equipment. The liner is not considered to contribute to the strength of the structure.