Safety-related Nuclear Facilities Research Articles

Effects of Accident Thermal Loading on In-Plane Shear Behavior of Steel-Plate Composite Walls

Structural walls in safety-related nuclear facilities are required to be designed for seismic and accident thermal (due to postulated high-energy pipe break events) loading combination. Current U.S. and international codes provide limited guidance for analysis and design of walls for this loading combination. This paper describes the experimental results and observations from tests conducted on a laboratory-scale (1:4 to 1:5) test unit representing steel-plate composite (SC) walls subjected to combined in-plane (seismic) and accident thermal loading. The test unit was subjected to surface temperatures of up to 450°F in combination with cyclic in-plane loading. Results of similar experiments recently conducted in Japan are also summarized (with surface temperatures up to 570°F). Surface heating combined with the low thermal conductivity and high specific heat of concrete resulted in nonlinear thermal gradients through the thickness of the specimens. These nonlinear thermal gradients and the associated self- or internal restraint led to extensive concrete cracking. This concrete cracking reduced the initial and secant stiffness of the specimens. The initial stiffness of the heated specimens reduced to 30 to 40% of the initial stiffness of the control (unheated) specimen. The secant stiffness of the heated specimens reduced up to 50% of the secant stiffness of the control (unheated) specimen. However, the in-plane shear strength of the heated SC specimens was still approximately 10 to 30% greater than the nominal in-plane shear strength calculated, for the limit state of steel plate von Mises yielding, using AISC N690 equations and measured material properties. Evaluation of the experimental results and observations suggests that the in-plane shear strength of SC walls subjected to typical accident surface temperatures (up to 570°F) can be estimated conservatively using the current provisions of AISC N690. The stiffness for accident thermal loading combinations can be considered to reduce from cracked composite stiffness at ambient temperature to fully cracked—that is, steel-only stiffness as the surface temperature increases up to typical accident value.

Steel-Plate Composite Wall to Reinforced Concrete Wall Mechanical Connection – Part 2: In-Plane and Out-of-Plane Shear

In safety-related nuclear facilities, steel-plate composite (SC) walls are often used in combination with reinforced concrete (RC) walls or foundations. The design demands need to be transferred between the two different structural systems through appropriate connection design. A design procedure was developed by the authors, and it was evaluated by conducting two full scale tests for SC wall-to-RC wall mechanical connections subjected to out-of-plane flexure. This paper presents a brief description of the design procedure as well as the experimental and numerical investigations conducted to further evaluate the design procedure. The focus was on the performance, strength, and governing failure mode of SC wall-to-RC wall mechanical connection under in-plane and out-of-plane shear. The investigation results include global force-displacement and applied force-strain responses. The paper also presents overall damage progression in terms of concrete cracking patterns. The experimentally observed and numerically predicted results indicate that the proposed connection design procedure is suitable and conservative for SC wall-to-RC wall mechanical connections.

Evaluation of design equations for out-of-plane shear strength of deep concrete sections in nuclear power plant buildings

Reinforced concrete walls in nuclear power plant buildings, which are generally more than one-meter thick, are typically designed to resist out-of-plane shearing force using concrete resistance only, namely, through-thickness shear reinforcement is not provided. Standard ASCE 43, which addresses design of safety-related nuclear facilities in the United States, references ACI 349 for design of reinforced concrete nuclear structures and components, with exceptions as deemed necessary. The code ACI 349–13, assumes, identically to ACI 318–08 and ACI 318–14, that the nominal shear stress capacity of longitudinally reinforced concrete sections is independent of cross-section depth: an assumption not supported by data. In the last revision cycle of ASCE 43, the ACI 349–13 design equation for concrete shear stress capacity was evaluated for conformance with the performance target of the standard, namely, a 98% or higher probability of exceedance, and found to be inadequate. A correction to the ACI 349–13 equation was developed and implemented in ASCE 43–19 to account for the effects of effective depth and longitudinal reinforcement ratio on concrete shear stress capacity. In parallel with the revision of shear provisions in ASCE 43, a size-effect modification factor, λs, was introduced in ACI 318–19 to account for the reduction in concrete shear stress capacity with increasing effective depth. In this paper, the technical basis for the correction factor included in ASCE 43–19 is developed and the ACI 318–19 equation is assessed to judge whether it achieves the ASCE 43 performance target. The assessment utilized the test data of longitudinally reinforced beams assembled by Reineck and his co-workers for slender sections (ratio of shear span-to-effective depth greater than or equal to 2.4) and non-slender sections (ratio of shear span-to-effective depth less than 2.4). For slender sections, the probability of exceedance of the ASCE 43–19 (ACI 318–19) design strength is greater than 98% (99%). For non-slender sections, the exceedance probabilities are greater than 99%. Because the ACI 318–19 equation meets the ASCE 43 performance target for both slender and non-slender sections, the correction invoked in ASCE 43–19 could be retired once ACI 349 adopts the shear provisions of ACI 318–19 for longitudinally reinforced concrete.

MASTODON: An Open-Source Software for Seismic Analysis and Risk Assessment of Critical Infrastructure

Seismic analysis and risk assessment of safety-critical infrastructure like hospitals, nuclear power plants, dams, and facilities handling radioactive materials involve computationally intensive numerical models and coupled multiphysics scenarios. They are also performed in a strict regulatory environment that requires high software quality assurance standards, and in the case of safety-related nuclear facilities, a conformance to the American Society of Mechanical Engineers Nuclear Quality Assurance (NQA-1) standard. This paper introduces the open-source finite-element software, MASTODON (Multi-hazard Analysis of Stochastic Time-Domain Phenomena), which implements state-of-the-art seismic analysis and risk assessment tools in a quality-controlled environment. MASTODON is built on MOOSE (Multi-physics Object-Oriented Simulation Environment), which is a highly parallelizable, NQA-1 conforming, coupled multiphysics, finite-element framework developed at Idaho National Laboratory. MASTODON is capable of fault rupture and source-to-site wave propagation using the domain reduction method, nonlinear site response, and soil-structure interaction analysis, implicit and explicit time integration, automated stochastic simulations, and seismic probabilistic risk assessment. When coupled with other MOOSE applications, MASTODON can also solve strongly and weakly coupled multiphysics problems. This paper presents a summary of the capabilities of MASTODON and some demonstrative examples.

Design of concrete walls and slabs for wind-borne missile loadings

Exterior walls and roofs in safety-related nuclear facilities in the United States, including nuclear power plants, must be designed to resist the loadings imposed by missiles (steel pipes, steel spheres, automobiles) borne by hurricanes and tornadoes. For confinement or containment, these walls and roofs are constructed from reinforced and/or prestressed concrete. The minimum required thicknesses of a reinforced panel to prevent scabbing and perforation for an impacting steel pipe are typically established using empirical formulae calibrated for rigid missiles and high velocities. To enable improved design of concrete panels to prevent scabbing and perforation by wind-borne steel pipes, numerical simulations were performed to: (1) investigate the effects of panel thickness, pipe mass and diameter, pipe velocity, and concrete uniaxial compressive and tensile strength on impact resistance, and (2) establish minimum panel thicknesses to prevent scabbing and perforation for the pipes and impact velocities specified in standards of practice. The numerical simulations used axisymmetric SPH models validated using data from tests at Sandia National Laboratory in the 1970s of four reinforced concrete panels impacted by Schedule 40 pipes. The values of panel thickness and concrete compressive strength considered in the parametric study are typical of those in existing nuclear power plant structures in the United States. The Schedule 40 pipe is the missile used for simulations because it is referenced in the United States Nuclear Regulatory Commission (USNRC) Standard Review Plan and the Department of Energy (DOE) Natural Phenomena Hazards Analysis and Design Criteria for DOE Facilities that point to Regulatory Guide (RG) 1.76, RG 1.221, and ANS-2.3. The impact velocities bound the maximum velocities recommended in the USNRC and ANS documents for design against the impact of Schedule 40 pipes. A considerable number of design parameters have a meaningful effect on the impact resistance of reinforced concrete panels but the most important, aside from panel thickness and impact velocity, is tensile strength of concrete. Results of the parametric study enable the writing of guidance on the minimum thickness of reinforced concrete panels to resist 152 mm (6 in), 203 mm (8 in) and 254 mm (10 in) diameter Schedule 40 pipes at impact velocities identified in RG 1.76, RG 1.221 and ANS-2.3. Panel thickness ranging from 305 mm (12 in) to 914 mm (36 in) are required to prevent perforation and scabbing caused by the impact of these Schedule 40 pipes. The required panel thickness decreases with increasing tensile strength.

Steel-Plate Composite Wall-to-Wall T-Joints: Joint Shear Strength

Steel-plate composite (SC) walls are used in the containment internal structures (CIS) of safety-related nuclear facilities. CIS are typically composed of multiple SC walls intersecting each other and forming SC wall-to-wall joints. These joints should preferably be designed to be stronger than the connected SC walls so that the walls develop flexural yielding prior to joint failure. To ensure that flexural yielding of the connected SC walls occurs first, it is important to estimate the shear strength of the SC wall-to-wall joints. This paper presents the results of an experimental investigation on the fundamental behavior and the strength of SC wall-to-wall T-joints. The test parameters included in the investigation were the detailing of the joint region, which they included the presence and number of tie bars and distribution of stud anchors. The results from the investigation suggest that the structural behavior of SC wall T-joints is remarkably similar to the behavior of reinforced concrete beam-column joints. In addition, it appeared that the effects of detailing the joint region on the behavior and strength are not significant. The measured joint shear strength compared favorably with the design equations for reinforced concrete beam-column joints.

Multi-hazard investigation and testing of steel-plate composite (SC) wall piers: Seismic and thermal loads

Design of structural walls in safety-related nuclear facilities needs to consider the combination of seismic and accident thermal loads. However, current codes provide limited procedural guidance for the analysis and design of walls for this load combination. Experimental studies were conducted to evaluate the cyclic in-plane response of steel-plate composite (SC) wall pier test units at ambient and typical accident temperatures (up to 232 °C). The test unit was subjected to two durations (1-hour and 3-hour) and magnitudes (149 °C and 232 °C) of surface temperatures in combination with cyclic in-plane loading. Accident temperatures result in the evolution of non-linear thermal gradients through the thickness of the test unit. The thermal gradients result in extensive concrete cracking, thus reducing the stiffness of the specimens. For surface temperature of 149 °C, the secant stiffness of the test unit reduced by 20% in comparison to the ambient secant stiffness. The reduction in secant stiffness was 40% for surface temperature of 232 °C. The lateral strength of flexure-controlled SC wall pier test unit subjected to typical accident temperatures was 25% higher than the strength corresponding to the initiation of compression yielding in the faceplates (at ambient temperatures). Evaluation of the experimental results suggests that the ambient strength equations for flexure-controlled SC wall piers can also be used to calculate the strength at typical accident temperatures (up to 232 °C). The stiffness for accident thermal loading combinations can be considered to linearly reduce from cracked stiffness at ambient temperature to fully cracked, i.e., steel only stiffness as the surface temperature increases up to typical accident values.

Design of steel-concrete composite walls subjected to low-velocity impact

Steel-concrete composite (SC) walls are composed of two steel plates and a concrete core. Experiments have shown that SC walls have excellent resistance to impact loadings, which is a great advantage for the applications in safety-related nuclear facilities. This paper presents an energy method for evaluating the maximum deformation of SC walls under low-velocity impact. The design requirements and flow for SC walls are proposed in three aspects: (i) local failure, (ii) maximum deformation, and (iii) damage degree. The parameters affecting the local failure and damage degree are discussed. The design method is applied to a sample SC wall in the AP1000 nuclear facility to validate its feasibility and provides a simplistic tool for researchers and engineers.

Seismic isolation of nuclear power plants: Past, present and future

Abstract Seismic isolation of nuclear power plants is in its infancy, with only a small number of applications worldwide. This outcome is due in part to the construction of only a small number of new build nuclear power plants since base-isolation technology became mainstream in the 1990s, perceived concerns regarding the long-term mechanical properties of isolation bearings, and a lack of guidance, codes and standards related to isolation of safety-related nuclear facilities. This paper charts the history of seismic isolation, identifies the research that led to the first implementation of isolation for buildings and bridges in the modern era, summarizes the first applications of the technology to nuclear facilities, and describes important research and developments, including the writing of nuclear standards, in the past 20 years. Future research and development needs are identified.

Minimum Requirements and Section Detailing Provisions for Steel-Plate Composite (SC) Walls in Safety-Related Nuclear Facilities

Steel-plate composite (SC) walls are comprised of concrete walls sandwiched between steel faceplates located on the exterior surfaces. These faceplates are anchored to the concrete infill using steel anchors and connected to each other using ties. SC walls have been used extensively in the third generation of nuclear power plants and are also being considered for small modular reactors (SMRs) of the future. The American Institute of Steel Construction (AISC) has published Supplement No. 1 to AISC N690, which includes Appendix N9 for the design of SC walls in safety-related nuclear facilities. This paper presents the minimum requirements for SC walls and the section detailing provisions from Appendix N9 along with their basis. The minimum requirements include requirements for minimum and maximum SC wall thickness, faceplate thickness, and steel and concrete material strengths. The provisions of Appendix N9 are applicable to SC walls that satisfy these minimum requirements. The design provisions account for the effects of interaction between out-of-plane shear demands (in both x- and y-directions) and the corresponding interfacial shear demands while accounting for the differences in behavior between yielding and nonyielding steel anchors and ties as classified by AISC N690 Supplement No. 1.

Behavior and design of steel-plate composite wall-to-wall corner or L-joints

Steel-plate composite (SC) walls used in the containment internal structures of safety-related nuclear facilities intersect each other at corners of compartments to form wall-to-wall corner or L-joints. This paper focuses on joint shear behavior and strength of wall-to-wall corner L-joints, the understanding of which is imperative for the design of wall-to-wall joints irrespective of the failure mode. This paper presents the results of a large-scale test conducted on an SC wall-to-wall corner or L-joint. The test specimen was designed to undergo joint shear failure prior to flexural yielding of the connected SC walls. The test specimen did not include any tie bars in the joint shear region, which would represent the lower bound case for these joints. The specimen was subjected to cyclic loading until failure, and the test results included the joint shear force-displacement and joint shear force-shear strain responses of the specimen along with observations of concrete cracking and crushing in the joint region. The results and observations were evaluated to establish the joint shear strength and governing failure mode. The experimentally measured joint shear strength was compared with the ACI 349-06 code equation for calculating the joint shear strength of comparable reinforced concrete beam-to-column joints. A detailed nonlinear finite element model was developed to gain additional insight into the observed behavior. The model was benchmarked using experimental results. The experimental and analytical results were compared with ACI 349-06 code provisions for calculating the joint shear strength of reinforced concrete beam-to-column joints. The results from experimental and analytical investigations indicate that the ACI 349-06 code equation could be used for estimating the joint shear strength of steel-plate composite wall-to-wall L-joints with γ of 8.

Seismic probabilistic risk assessment for seismically isolated safety-related nuclear facilities

In the United States, seismic probabilistic risk assessment is performed on nuclear power plant (NPP) designs to calculate mean annual frequencies of unacceptable performance, including core damage and large early release (of radiation). Seismic (base) isolation is a viable strategy to protect NPPs from extreme earthquake shaking but it has not yet been employed in the United States, in part due to a lack of clear regulatory guidance. Guidance and standards for seismic isolation of NPPs are now becoming available, but they do not explicitly address risk calculations.This paper presents seismic risk calculations for safety-related nuclear structures, including NPPs, isolated using nonlinear bearings, with a focus on assessing risk associated with the isolation system and the safety-related umbilical lines. Fragility curves are developed for the isolation systems and umbilical lines of NPPs located at eight sites of nuclear facilities across the United States assuming that the performance goals outlined in a forthcoming NUREG/CR focusing on seismic isolation and Chapter 12 of ASCE Standard 4-16 are satisfied. Risk is computed for isolation systems in NPPs with and without a stop. The mean annual frequency of unacceptable performance at each site is less than 1×10-6(1×10-5) following the guidance (requirements) set forth in the NUREG/CR (ASCE 4) if a stop or displacement restraint is provided.Three strategies for reducing the calculated mean annual frequency of unacceptable performance are investigated and quantified, namely, 1) testing more prototype isolators to achieve greater confidence, 2) testing isolators for a larger displacement and corresponding axial force at a given confidence level, and 3) providing a stop. The annual frequency of unacceptable performance of the isolation system (and umbilical lines) is greater than 1×10-6 (the assumed target annual frequency of unacceptable performance), if the isolation system is designed per the forthcoming NUREG/CR and a stop is not provided, even though there is considerable reduction in risk if the isolators are tested for a greater displacement (and corresponding axial force) and/or with a greater confidence level than that required by the forthcoming NUREG/CR. The risk is well below 1×10-6 if a stop is provided. A stop is needed to achieve the corresponding target annual frequency of unacceptable performance of 1×10-5 if the isolators (and umbilical lines) are designed and tested per ASCE 4-16. The risk calculations were performed setting the design factor (the factor by which the ordinates of the design basis response spectrum per the ASCE 4-16 and ASCE 43-05 are increased) equal to 1.0. Because the achieved annual frequency of unacceptable performance of the isolation system (and umbilical lines) is less than the corresponding target set in the forthcoming NUREG/CR (a United States Nuclear Regulatory Commission report) and ASCE Standard 4-16 (that has developed from United States Department of Energy guidance), if the earthquake risk is dominated by horizontal ground shaking and a stop is provided, the design factor can be set equal to 1.0 for a seismically isolated NPP.

Direct shear strength of rebar-coupler anchor systems for steel-plate composite (SC) walls

This paper focuses on the direct shear behavior of rebar-coupler anchor systems, and their use for anchorage of steel-plate composite (SC) walls to the concrete basemat of safety-related nuclear facilities. Large-scale rebar-coupler anchor specimens were tested under direct shear loading until failure. The results included the applied load-slip displacement responses of the specimens, the direct shear strength, and the observed failure mode. The American Concrete Institute (ACI) 349 code equation for calculating the direct shear strength of embedded anchors was compared with the direct shear strengths from the tests. The code equation underestimated the direct shear strength of the anchor system, because shear failure was assumed to occur in the rebars, whereas experimental observations indicated that shear fracture failure occurred in the couplers rather than the rebars. The design equation was updated to utilize the net shear area of the couplers instead of the rebars, after which the direct shear strengths from the tests could be calculated with reasonable accuracy. The experimental results were also used to propose an empirical model for the shear force vs. slip displacement response of rebar-coupler anchor systems.

Correlation of horizontal and vertical components of strong ground motion for response-history analysis of safety-related nuclear facilities

Design standards for safety-related nuclear facilities such as ASCE Standard 4-98 and ASCE Standard 43-05 require the correlation coefficient for two orthogonal components of ground motions for response-history analysis to be less than 0.3. The technical basis of this requirement was developed by Hadjian three decades ago using 50 pairs of recorded ground motions that were available at that time. In this study, correlation coefficients for (1) two horizontal components, and (2) the vertical component and one horizontal component, of a set of ground motions are computed using records from a ground-motion database compiled recently for large-magnitude shallow crustal earthquakes. The impact of the orientation of the orthogonal horizontal components on the correlation coefficient of ground motions is discussed. The rules in the forthcoming edition of ASCE Standard 4 for the correlation of components in a set of ground motions are shown to be reasonable.

Hysteretic model for steel–concrete composite shear walls subjected to in-plane cyclic loading

Steel–concrete composite (SC) shear walls are being widely used as an alternative to reinforced concrete walls. Investigations on seismic behavior of SC walls have been conducted to develop design specifications for safety-related nuclear facilities. However, there is a lack of hysteretic models that can be used to predict structural performance as the structure approaches collapse. This paper presents (a) the analysis of experimental results of 32 SC wall specimens, and (b) the derivation and calibration of a quadri-linear backbone with negative post-peak stiffness and associated hysteretic rules. Different cross section shapes and loading configurations were used to test the SC wall specimens. Based on the experimental results, equations for stiffnesses and loads are derived from a mechanics based model, and basic hysteretic rules are employed to describe the response of SC walls subjected to in-plane cyclic loading. Calibrations are conducted to suggest the reduction factors for the Young’s moduli of concrete and steel that reflect the plasticity extension and damage accumulation.

Static resistance function for steel-plate composite (SC) walls subject to impactive loading

Steel-plate composite (SC) walls consist of a plain concrete core reinforced with two steel faceplates on the surfaces. Modules (consisting of steel faceplates, shear connectors and tie-bars) can be shop-fabricated and shipped to the site for erection and concrete casting, which expedites construction schedule and thus economy. SC structures have recently been used in nuclear power plant designs and are being considered for the next generation of small modular reactors. Design for impactive and impulsive loading is an important consideration for SC walls in safety-related nuclear facilities. The authors have previously developed design methods to prevent local failure (perforation) of SC walls due to missile impact. This paper presents the development of static resistance functions for use in single-degree-of-freedom (SDOF) analyses to predict the maximum displacement response of SC walls subjected to missile impact and designed to resist local failure (perforation). The static resistance function for SC walls is developed using results of numerical analyses and parametric studies conducted using benchmarked 3D finite element (FE) models. The influence of various design parameters are discussed and used to develop idealized bilinear resistance functions for SC walls with fixed edges and simply supported edges. Results from dynamic non-linear FE analysis of SC panels subjected to rigid missile impact are compared with the maximum displacements predicted by SDOF analyses using the bilinear resistance function.

SEISMIC ISOLATION OF NUCLEAR POWER PLANTS

Seismic isolation is a viable strategy for protecting safety-related nuclear structures from the effects of moderate to severe earthquake shaking. Although seismic isolation has been deployed in nuclear structures in France and South Africa, it has not seen widespread use because of limited new build nuclear construction in the past 30 years and a lack of guidelines, codes and standards for the analysis, design and construction of isolation systems specific to nuclear structures. The funding by the United States Nuclear Regulatory Commission of a research project to the Lawrence Berkeley National Laboratory and MCEER/University at Buffalo facilitated the writing of a soon-to-be-published NUREG on seismic isolation. Funding of MCEER by the National Science Foundation led to research products that provide the technical basis for a new section in ASCE Standard 4 on the seismic isolation of safety-related nuclear facilities. The performance expectations identified in the NUREG and ASCE 4 for seismic isolation systems, and superstructures and substructures are described in the paper. Robust numerical models capable of capturing isolator behaviors under extreme loadings, which have been verified and validated following ASME protocols, and implemented in the open source code OpenSees, are introduced.

Steel-plate composite walls: Experimental database and design for out-of-plane shear

The seismic behavior and design of steel-plate composite (SC) walls in safety-related nuclear facilities is typically governed by their in-plane shear strength and ductility. However, the out-of plane shear (and flexure) behavior can govern the design near foundations and connections where SC walls interact with other structures. The out-of-plane shear behavior of SC walls is similar to that of reinforced concrete (RC) walls, and has been evaluated experimentally by conducting beam tests in one-way bending. The beam tests were conducted on specimens that were representative of strips taken in the longitudinal and transverse directions of full-scale SC walls. This paper includes a summary of the experimental database of SC beam shear tests conducted in Japan, S. Korea, China and the US. Different loading configurations were used to test the SC beam specimens. The parameters considered were the shear span-to-depth ratio, steel faceplate thickness (reinforcement ratio), stud anchor spacing, and the presence (or absence) of shear reinforcement. The out-of-plane shear strengths from the tests in the experimental database are summarized and compared with shear strength calculations based on applicable design code equations to estimate the lower bound design shear strength of SC walls. Reliability analysis is conducted to suggest associated strength reduction (ϕ) factors.

Steel-plate composite (SC) walls for safety related nuclear facilities: Design for in-plane forces and out-of-plane moments

Steel-concrete (SC) composite walls being considered and used as an alternative to conventional reinforced concrete (RC) walls in safety-related nuclear facilities due to their construction economy and structural efficiency. However, there is a lack of standardized codes for SC structures, and design guidelines and approaches are still being developed. This paper presents the development and verification of: (a) mechanics based model, and (b) detailed nonlinear finite element model for predicting the behavior and failure of SC wall panels subjected to combinations of in-plane forces. The models are verified using existing test results, and the verified models are used to explore the behavior of SC walls subjected to combinations of in-plane forces and moments. The results from these investigations are used to develop an interaction surface in principle force (Sp1–Sp2) space that can be used to design or check the adequacy of SC wall panels. The interaction surface is easy to develop since it consists of straight line segments connecting anchor points defined by the SC wall section strengths in axial tension, in-plane shear, and compression. Both models and the interaction surface (for design) developed in this paper are recommended for future work. However, in order to use these approaches, the SC wall section should be detailed with adequate shear connector and tie bar strength and spacing to prevent non-ductile failure modes.