Concrete containment structures for nuclear power plants
This study provides a summary of analysis and design and pre-operational inspection and testing requirements for concrete containments. Both conventionally reinforced as well as prestressed concrete containments are included.
Author(s): Shen Wang
Nuclear containments serve the critical function of providing an external protection and a leak proof boundary for containing radiation in nuclear power plants. Failure can result in catastrophic safety consequences as a result of radiation. The size of the containment and available free volume depends upon equipment layout and operational requirements. The available free volume within the containment affects the magnitude of pressure that can be generated during an accident. Conventionally reinforced concrete type has been applied for small containments with low to moderate internal design pressure. However, large containments with relatively high internal design pressure require either a prestressed concrete or a steel containment. In general, selection of containment type (conventional concrete, prestressed concrete or steel) depends upon considerations for the equipment size and layout, design pressure and other loadings, construction cost and schedule.
Design of concrete containments in the United States is now carried out under ASME Section III, Div 2 /ACI 359 Code  for Concrete Containments – Joint ACI/ASME Technical Committee on Concrete Pressure Components for Nuclear Service under the sponsorship of the American Concrete Institute and the American Society of Mechanical Engineers. The older containments were designed in accordance with NRC SRP section 3.8.1 and the American Concrete Institute (ACI) Building Code Requirements for Structural Concrete (ACI 318). Some licensees have incorporated some of the requirements of the ASME III-Div. 2 Code, as part of reconstitution efforts during license renewal.
Conventionally reinforced containments are generally cylindrical structures with a hemispherical dome supported on a flat slab . Conventional bonded reinforcement is provided in the structure to carry all applicable load actions including tension, shear and flexure. The design and detailing provisions are given in ASME Section III, Div 2 Code and generally follow the basic principles of reinforced concrete design given in older editions ACI 318. The ASME Code is currently being updated to the latest editions of ACI 318 . Special detailing is provided around boundaries, discontinuities and openings. The concrete cylindrical shell is usually between 42 to 52 inch thick and the dome is 30 to 36 inch thick. The base slab is usually 8–10 foot thick. The concrete sections are, generally, heavily reinforced with up to eight layers of No. 11 to No. 18 rebars with a reinforcement ratio of 2–4%. The inside surface of the concrete is lined with carbon steel liner plate. The liner plate in the base slab is covered with an additional 2 foot thick concrete slab.
Prestressed concrete containments are cylindrical structures with a hemispherical or sphere torus dome and a flat basemat slab with a tendon gallery running under the cylindrical wall . As indicated above, prestressing is required when containment pressure requirements become large and the consequent design loads cannot be handled by conventional reinforced concrete. Early prestressed containments in the United States, such as, Palisades and Turkey Point were designed for 1.5 times the accident design pressure (1.5 Pa). They consisted of a cylinder, shallow dome with a ring girder and six vertical buttresses. The ring girder is provided to accommodate vertical and dome prestressing tendons end anchorages. The buttresses are used to accommodate hoop prestressing tendons end anchorages. In latter designs, the prestress level was subsequently reduced to 1.2 Pa with three buttresses and about 1000 ton capacity tendons. The shallow dome was replaced with a hemisphere. The vertical inverted U-tendons were used which start in the tendon gallery below the base slab ran over the dome and anchored at the tendon gallery on the opposite side. This eliminates the need for the ring girder and number of tendons is significantly reduced. However, loss of prestress in the longer inverted U-tendons due to friction and elastic shortening is higher than those in the earlier containments. Use of inverted U-tendons also eliminates potential corrosion of prestressing anchors due to limited exposure to the environment. The tendons comprised of wire with buttonheads or strands. However, due to high loss of prestress due to relaxation of steel, especially at higher temperatures, more recent plants use low relaxation prestressing wires or strands. Except for a few earlier plants (i.e. Robinson and TMI-2), all US nuclear power plant containments use unbonded post-tensioning system. Unbonded post-tensioning allows periodic surveillance and inspection of the tendons over the life of the plant for corrosion and degradation. The authors in [2,4–8] give a detailed discussion on various aspects of prestressed containment design and testing.
Because of the level of prestressing, the concrete strength used in prestressed containments generally range from 5000 to 6000 psi and should exhibit low creep properties. Creep and shrinkage of concrete and relaxation of prestressing steel are important aspects of prestressed concrete design as they affect the magnitude of prestress loss in the long-term.
Modelling and analysis considerations
SRP 3.7.2  provides general outline for the required analysis approach. In general, elastic structural finite element analysis is used for containment analysis. The analysis should include cracking that is likely to soften the structure and change its dynamic characteristics and damping. In case of post-tensioned containments, it is possible that cracking may not be significant due to level of prestress present during, for example, a safe shutdown earthquake. Since the analysis also needs to include the soil structure interaction (SSI) effects, a two-step analysis approach is commonly used: one for static loading (non-seismic loads) using commercial software programs such as ANSYS or GTDTRUDL and other for a separate analysis involving SSI using, for example, using SASSI program. The static model may include soil springs as approximate and simplified assumptions for soil boundary effects. The SSI model, on the other hand, includes detailed soil impedance and embedment effects. In order to minimise differences between the two models, a SASSI model is usually developed from the static ANSYS or GTSTRUDL model. The SASSI analysis is carried out for a suite of site specific ground motions representing various soil conditions that may be present. The results of the SASSI seismic analyses are then combined with analysis of the structure for other non-seismic loading using appropriate load combinations given in ASME Code (Table CC 3230-1).
For prestressed containments, the post-tensioning effect is modelled either directly by discrete modelling of the tendons or indirectly through temperature or pressure loading. In case of discrete modelling, the tendon slip/movement may also be captured for an unbonded tendon.
Concrete containment design criteria
The loads and load combinations are given in CC-3230 of ASME Section III, Div 2 for concrete containments. The design philosophy for concrete containments is based on ensuring that concrete under any set of load conditions does not reach its nominal or crushing capacity and there is no significant yielding of steel to result in any general yield condition. To accomplish this, appropriate safety margins against these conditions are built into the Code provisions in terms of allowable stresses for various service and factored load conditions. The material allowable stresses are given in the Code for both service and factored loads and provide a healthy margin of safety in design of concrete containments.
For service loads, the load factors are unity since the design allowable are well below concrete compressive strength and steel yield strength. Therefore, the structural response under service load conditions is kept well within the material capacities with adequate safety margins. For factored loads, the load factors may be greater than one depending on the probability of load occurrence, confidence in load magnitude and the safety margins required for a guaranteed performance.
The containment is designed for all of the applicable load combinations with SSE applied simultaneously in three orthogonal directions generally combined using the square root sum of the squares method or 100:40:40 rule as recommended in NRC Regulatory Guide RG 1.92. In most cases, the most critical load combination for concrete containments results due to the combined effect of the design accident pressure and SSE at the base of the containment shell in the vertical direction or 1.5 times the accident pressure (Pa) load combination in the hoop direction at mid height of the containment shell.
The allowable stresses discussed above are given in ASME Section III, Div 2 for various load effects such as primary and secondary or a combination thereof. This code categorised membrane forces, moments and shears as either primary or secondary depending upon their location in the containment and the source that the forces come from or origin. Primary mode behaviour is the ability of internal forces to equilibrate applied loads. Secondary resistance is exhibited when internal forces are not required to balance external forces, or the external loads are self-limited. For example, membrane forces, moments and shears are all considered primary for externally applied loads at critical locations of the containment. On the other membrane forces and moments from volume change effects are considered secondary.
Secondary force may be due to (i) a local, internal, force, or moment that results from applied loads, but is not required to equilibrate such loads; or (ii) a local, internal force, or moment that results from non-load, volume change effects, such as shrinkage strain and thermal strain.
Analyses is carried out in stages for primary membrane, primary membrane plus bending, primary plus secondary membrane and primary plus secondary membrane and bending of critical containment sections. For each of these analysis steps there is an allowable concrete and reinforcement stress/strain that needs to be satisfied as given in Tables CC 3421-1 and CC 3431-1 of ASME Section III, Div 2 code.
The code requires an allowable stress design approach for both service as well as factored loads. The intent is to ensure that concrete and steel both remain within their respective capacity limits for all load situations in order to safeguard the containment integrity. Although allowable stress for reinforcement for load resisting purpose should not exceed 0.9fy, some limited and local yielding (or partial yield) is allowed in some situations, for example, where multiple layers of reinforcement are provided for bending, in diagonal reinforcement and in reinforcement adjacent to large openings. These exceptions do not include design for shear in which case reinforcement should always remain below yield.
There is no guidance in the code  on how to treat thermal effects. ACI 349-06  may be used for guidance; however, the requirements of ASME Section III, Div. 2 must be satisfied. Gurfinkel  and Wang  give detailed discussion on thermal effects in reinforced concrete containments.
Note that because of lack of data, the development lengths in areas of bi-axial tension are increased by 25%. As a result of this requirement, couplers or Cadwelding is generally used to avoid use of excessively long splices.
Design consideration for prestressed concrete containment
The level of prestress is generally designed to provide sufficient balancing force to resist all of the accident design pressure load and in some cases also some or all of the tangential shear loads due to SSE. Thus depending upon the level of prestress, the following three design situations may arise in design of post-tensioned containments:
- A containment with enough prestressing to cover both design pressure and tangential shear so no conventional reinforcement is needed for these actions.
- A containment which relies on conventional reinforcement to carry some of the tangential shear while some of the shear is carried by concrete in compression as a result of prestressing.
- A containment where all tangential shear is carried by conventional reinforcement and concrete does not carry any shear.
In cases 1 and 2, concrete is expected to carry shear which may be affected by cracking as a result of accident thermal forces. In order to reduce effects of thermal cracking, higher level of prestress may be required which in turn will result in thicker walls and thus induce higher earthquake forces. Because of this thermal cracking issue, this type of design is both complicated and uneconomical. In case 3, the design is simple as it requires designing prestress for accident pressure and provide conventional reinforcement for seismic loads. Under thermal loads, concrete will crack and thus relieve some of the thermally induced forces. Since concrete is considered cracked, it cannot carry any shear or tension for which conventional reinforcement is provided.
The tangential shear reinforcement is generally provided to resist lateral loads due to seismic or wind effects in two orthogonal directions. If this orthogonal reinforcement is not sufficient, additional layers inclined at 45° may be required for reinforced concrete containments. For detailed discussion on design for tangential shear effects see Oesterle .
Prestress losses need to be conservatively computed in order to ensure that sufficient prestress force remains available to counter accident pressure loads during the life of the containment . A detailed discussion of prestressing losses is provided in RG 1. 35.1 . Appropriate design and reinforcement detailing is required in the tendon anchorage region to make sure that there is no excessive bursting cracking that could jeopardise the design prestress level in the tendon. Adequate corrosion protection is also required to ensure long-term durability of the prestressing system. For more discussion on tendon anchorage design and testing see [15,16] (Fig 1).
Fig 1: Prestressed anchorage block testing at a buttress 
Prestressed containments also require careful consideration of radial tension reinforcement because of several instances of delaminations . It is recommended that prestressing tendons should not be placed near a free edge. In addition, radial tension reinforcement be provided if detailed analysis is not carried out to confirm that delamination is not likely to occur [18,19].
Liner and liner anchor design
A liner plate, generally 6 mm (1/4 in) to 12 mm (1/2 in) thick, is attached to inside face of concrete containment for leak protection. The liner is erected first during construction process and serves as a form for concrete placement. The liner is braced with structural steel sections (channels and angles at regular spacing) or welded studs which serve as embeds in the concrete. Although the code does not allow the liner to be considered as a stiffness or a strength element contributing to capacity, it should be ductile enough to accommodate the stresses/strains due to various imposed load conditions including accident thermal without jeopardising the leak tightness. Under accident thermal and pressure loads it is important to include the liner – concrete interaction to adequately determine stresses/strains in concrete and liner. The liner can, however, be used as a structural element to resist construction loads during concrete placement and small attachment loads from cable tray, conduit support and ladders. For large attachments, equipment and personal hatches, and piping penetrations, the liner plate is thickened locally to transfer loads to the concrete. The authors in [20–23] provide a comprehensive background on liner behaviour, design, detailing and testing (Fig 2).
Fig 2: Liner and Anchor behaviour and test simulation [20,21]
The limiting condition for liner is the strain and its impact on anchorage in concrete. There are no strength or buckling stability issues with liner as the liner is well restrained by the concrete and anchors. However, buckling needs to be considered for temporary loads during construction and installation prior to concrete placement. Hence, as long as the liner is able to perform its leak-tight boundary function, there is no limit on its strength or stability. The only issue relates to the effect of imposed deformations of the liner on anchors. The integrity of anchorage system must be maintained during the design. The liner has to be a ductile element so that it can exhibit good deformation capacity beyond yield. Consequently, full penetration butt welds are necessary to ensure leak tightness and adequate ductility of the liner plate.
The liner anchors are generally tested for the imposed ultimate deformation from the liner plate. The liner may assume an initial inward curvature during thermal accident load which is acceptable as there is no structural consequence of this deformation or resulting buckling. The force induced in the liner depends upon the liner strength and strain hardening. Therefore, liner strength is kept to a minimum with well-known yield and post-yield behaviour, if possible. Since it is practically difficult to get a well-defined yield for the plate material, higher anchor loads are induced due to higher actual yield for which high strength ductile anchors need to be used. Note that local crushing around the anchors is considered acceptable during a rare accident condition. Anchors also need to be designed for mechanical loads that may come from small attachments to the liner plate during construction or operation.
Pre-service inspection and testing
The pre-service (CC-5000) and in-service inspection and testing requirements for concrete containments (both reinforced and unbonded post-tensioned) are given in Subsection IWL-1000 of ASME Section XI of the Code. This section does not include requirements for bonded (grouted) tendons. This section should be used together with Subsection IWA for General Requirements to ensure structural and leak-tightness of the containment. Subsection IWE applies to containment liner (see discussion on steel containments in Section 3-2.7). USNRC RG 1.90 provides guidance for in-service inspection of prestressed concrete containments with grouted tendons.
The pre-service inspection and testing is used to establish a successful start and to serve as a baseline for inspection and testing during operational service of the containment. The pre-service examination includes 100% of the concrete surfaces of the containment and post-tensioning system based on plant construction record. Pressure integrated leak rate testing or local leak rate testing is carried out per Appendix J of 10CFR Part 50 which provides containment leakage testing in terms of Type A, B, and C tests. The Type A test is a measurement of the overall integrated leakage rate of the primary containment; whereas Type B and C tests are local leak rate tests designed to detect and measure local leakage across each pressure-containing or leakage-limiting boundary for primary containment.
A professionally registered responsible engineer is assigned the responsibility of evaluating the concrete and post-tensioning system to ensure the structural and leak-tightness of the concrete containment.
The visual examination should include inspection of concrete surfaces for any apparent distress/cracks/voids, detailed condition of concrete around anchorages and detailed inspection of strands and anchorage hardware. ACI 349.3R provides detailed guidance and quantitative inspection criteria for concrete surface examination.
The structural integrity test (SIT) requirements given in Section CC-6000 of ASME Section III, Div. 2 Code  are a pre-requisite for Code acceptance and stamping before start of operation. The test is performed at 1.15 times the containment design pressure to evaluate design compliance and quality of construction. A detailed test procedure is required that uses calibrated instrumentation to measure strains/displacements, crack widths and ambient conditions. Cracks exceeding 0.01 inches in width are recorded before the test, at peak pressure and after depressurisation. These and other representative measurements are compared against the acceptance criteria given in CC-6400.
Severe accident analysis
Containment performance at beyond design basis accident internal pressure and temperature is required as an input for determining the offsite consequences and accident progression of the containment during a severe accident. Extensive research and scale model testing of reinforced and prestressed concrete containments to determine behaviour at beyond design basis accident pressure has been performed in the last 25 years at Sandia National Laboratories  and Central Electricity Generating Board, England . Concrete containments start to leak before a complete rupture or failure. It is extremely difficult to accurately predict the location and leakage rate of the concrete containment due to beyond design basis internal pressure and temperature. Hessheimer and Dameron , and Dameron et al.  provide guidance for predicting leak area and leak rate in containments. Hessheimer and Dameron  recommend a non-linear finite element analysis of the concrete containment to predict containment performance and leakage.
Hessheimer and Dameron  have concluded that global, free field strain of 1.5 to 2.0% for reinforced and 0.5 to 1.0% for prestressed concrete can be achieved before failure or rupture. In addition, leakage in concrete containment increases appreciably after the rebars and liner plate yield. Furthermore, under gradual increase in internal pressure, containment leakage continues to grow without failure and rupture. Sheikh  provides the simplified approach for predicting containment performance during a severe accident. This approach has been used in the NRC’s state-of-the-art consequence analyses project .
The results using this simple approach are quite consistent with detailed finite element analyses using state-of-the-art computer codes and test data .
Nuclear concrete containments are critical components for safety of nuclear power plants. In this article, concrete containments in nuclear power plants are discussed in terms of structural configuration, modelling and analysis practice, and design consideration. Furthermore, other aspects unique to nuclear concrete containments are also addressed, including liner and liner anchor, pre-service inspection and severe accident analysis. The purpose is to develop an up-to-date, scholarly work on nuclear concrete containment structures. It would benefit the nuclear industry as its transitions to a new generation of designers, constructors and regulators.
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