Ronald F. Kulak

NEPTUNE: a system of finite element programs for three-dimensional nonlinear analysis

This paper describes the NEPTUNE system of finite element codes that is being developed to assist in the design, safety evaluation, and licensing of metal and concrete reactor structures subjected to static or transient mechanical loads. The implementations of (1) a quadrilateral plate element for concrete, (2) a family of interface elements to treat contact and/or impact between structures, and (3) a silent boundary element are highlighted. Terse descriptions of the remaining elements in the code are given. The solution of the static equilibrium equations are obtained with the dynamic relaxation algorithms, and the equations of motion are solved with the central difference algorithm. The wide range of applicability of the code system is demonstrated with four examples. The propagation of a wave through a silent boundary is first illustrated. Then the impact of a deformable cylindrical container against a rigid floor is simulated. The final two problems deal with structural safety evaluations. The transient response of a liquid metal reactors head assembly is first described, and then, a numerical simulation of the response of a 16-scale reinforced concrete light water reactor, containment model to static overpressurization is presented.

Multi-objective optimisation and sensitivity analysis of a paratransit bus structure for rollover and side impact tests

Paratransit buses are heavily used in the United States. A paratransit bus consists of custom passenger compartments mounted onto separate cutaway chassis. The lack of dedicated national crashworthiness standards, along with different construction methods used by paratransit fleet manufacturers, can result in a wide variance of passenger compartment structural strength. In August 2007, the Florida Department of Transportation (FDOT), to ensure adequate crashworthiness performance, introduced a standard stipulating that newly acquired buses must be tested for rollover and side impact conditions. The rollover test is performed using a tilt table test according to UN-ECE Regulation 66. The side impact test involves the impact of a bus by a common sport utility vehicle or pickup truck. In the current study, an original finite element model of a paratransit bus was used in LS-DYNA® simulations of both the rollover and the side impact testing procedures per FDOT standard. Using LS-OPT®, a metamodel-based approach was used to perform multi-objective optimisation of the bus structure for the rollover and the side impact tests. The linear ANOVA and the Sobols indices approach were used for sensitivity analysis. The structural components of the bus having the greatest influence on the bus performance in the simulated test scenarios were identified. The simulation results show that the original bus design would pass the FDOT testing procedure. However, appropriate redistribution of the mass can noticeably increase its strength for the side impact case.

Numerical modeling of concrete under thermal loads

Abstract The formulation needed for the conductance of heat by means of explicit integration is presented. The implementation of these expressions into a transient structural code, which is also based on explicit temporal integration, is described. Comparisons of theoretical results with code predictions are given both for one-dimensional and two-dimensional problems. The coupled thermal and structural solution of a concrete crucible subjected to a sudden temperature increase predicts the history of cracking. The extent of cracking is compared with experimental data.

EVALUATION OF PIPE WHIP IMPACTS ON NEIGHBORING PIPING AND WALLS OF THE IGNALINA NUCLEAR POWER PLANT

Presented in this paper is the transient analysis of a Group Distribution Header (GDH) following a guillotine break at the end of the header. The GDH is the most important component of reactor safety in case of accidents. Emergency Core Cooling System (ECCS) piping is connected to the GDH piping such that, during an accident, coolant passes from the GDH into the ECCS. The GDH that is propelled into motion after a guillotine break can impact neighboring GDH pipes or the nearest wall of the compartment. Therefore, two cases are investigated: • GDH impact on an adjacent GDH and its attached piping; • GDH impact on an adjacent reinforced concrete wall. A whipping RBMK-1500 GDH along with neighboring concrete walls and pipelines is modeled using finite elements. The finite element code NEPTUNE used in this study enables a dynamic pipe whip structural analysis that accommodates large displacements and nonlinear material characteristics. The results of the study indicate that a whipping GDH pipe would not significantly damage adjacent walls or piping and would not result in a propagation of pipe failures.Copyright

Modeling of containment structures on high performance computers

With recent advances in parallel supercomputers and network-connected workstations, the solution of large scale structural engineering problems, such as containment structures, has now become tractable. High-performance computer architectures, which are usually available at large universities and national laboratories, now can solve large nonlinear problems. At the other end of the spectrum, network connected workstations can be configured to become a distributed-parallel computer. A description of the development of a parallelized finite element computer program for the solution of static nonlinear structural mechanics problems is presented here. Also, a finite element methodology is presented for use in finding the structural capacity of reinforced concrete structures. The method is applicable to both cylindrical and rectilinear geometries. Containment structures for nuclear reactors are the final barrier between released radionuclides and the public. Containment structures are constructed from steel, reinforced concrete, or prestressed concrete. US nuclear reactor containment geometries tend to be cylindrical with elliptical or hemispherical heads. The older Soviet designed reactors do not use a containment building to mitigate the effects of accidents. Instead, they employed a sealed set of rectilinear, interconnected compartments, collectively called the accident localization system (ALS), to reduce the release of radionuclides to the atmosphere during accidents. As an illustrative example, the methodology developed herein is applied to a generic VVER-440/V213 design subjected to internal overpressure.

Finite element formulation for thermal stress analysis of thin reactor structures

Abstract This paper describes the formulation of a finite-element procedure for the thermal stress analysis of thin wall reactor components. A general temperature-dependent constituent relationship is derived from a Gibbs potential function and a temperature-dependent yield surface. This form of constitutive relationship is applicable to problems of small strain. A similar form of a hypoelastic-plastic type is also developed for large strains. The variation of the yield surface with temperature is based upon a temperature-dependent, work-hardening model. The model uses a temperature-equivalent stress-plastic strain diagram which is generated from isothermal unaxial stress-strain data. The above constitutive relationships are incorporated into two computer codes and a previously developed numerical algorithm is used with minor modifications. A set of problems is presented validating the thermal analysis capability of the computer codes to a variety of problem types.

Dynamic structural response of LMFBR head closures to hypothetical core disruptive accidents

Abstract Presented here is an investigation of the dynamic structural response of the primary vessels head closure to a hypothetical core disruptive accident (HCDA). Two head-closure designs were considered: the first represents a loop-type design and the second represents a pool-type design. Using representative configurations of liquid metal fast breeder reactors (LMFBR), independent models were used (1) to derive loading pressure histories and (2) to study the structural response of the head closures. Results for loading pressures, displacement histories, deformed profiles, stress magnitudes and plastically deformed regions are presented.

Pretest analysis of a 1:6-scale reinforced concrete containment model subject to pressurization

Abstract Pretest predictions were made by the Reactor Analysis and Safety Division of Argonne National Laboratory for the response of the 1 :6-scale reinforced concrete model to be tested by Sandia National Laboratories. For this purpose a series of axisymmetric models were studied with the two-dimensional computer program TEMP-STRESS and a three-dimensional circumferential segment model with the program NEPTUNE. The two-dimensional models predicted failure at 175–190 psig (1.207–1.310 MPa). However, two different failure mechanisms were indicated: (1) hoop failure of the vessel at midheight following failure of a splice in this area, (2) failure of a weld in the liner near the basemat due to excessive strains. The three-dimensional model predicted failure at an internal pressure of 180–185 psig (1.241–1.276 MPa) by failure of the splices of the hoop rebars just above cylinder midheight in a region away from the equipment hatch opening.

Integrated probabilistic analysis of nuclear power plant building damage due to an aircraft crash

In order to ensure that nuclear power plant buildings are reliable and safe in case of external loading, it is very important to evaluate uncertainties associated with loads, material properties, geometrical parameters, boundaries and other parameters. Therefore, a probability-based analysis was developed as the integration of deterministic and probabilistic methods using existing state-of-the-art software. The subject of this paper is the integrated analysis of building failure due to impact by a commercial aircraft. The Monte Carlo Simulation, First-Order Reliability and the combined Monte Carlo Simulation and Response Surface methods were used for the probabilistic analyses. During an aircraft crash, the dynamic impact loading is uncertain. Therefore a relation expressed by the probability of failure of impacted wall and loading function was determined. With failure defined as concrete cracking and rebar rupture, the failure probabilities of the impacted wall were calculated as a function of the peak impact load. The integrated deterministic and probabilistic analysis approach was applied to the Ignalina Nuclear Power Plant in Lithuania. The conclusions from this analysis was that a through-the-wall crack in the concrete element of a plant wall may occur with a probability of 0.0266, but the failure probability of the reinforcement bars is very small, that is, near zero. Thus, no perforation of the impacted wall by structures of the aircraft should occur. The importance of performing a probabilistic analysis of crash events is shown by comparing results to those obtained by a mean value deterministic approach.

Design and analysis of seismically isolated structures

Abstract This paper describes developing methodology for performing three-dimensional (3D) analysis of individual seismic bearings and for simulating the 3D system response of isolated reactor structures (superstructure, isolators, basemats, etc.) including the surrounding soil under earthquake excitation. In addition, programs for elastomer research and quality control are discussed. Several sample problems are presented to illustrate the 3D capability.