Design strength evaluation of RC beams under radiation environments for nuclear power plants

Abstract

Neutron irradiation changes the behavior of construction materials such as strength and ductility, and thus structural design equations or their safety margins should accordingly be updated for the design of nuclear power plants (NPP) under irradiation. However, current design codes do not account for such changes in material strength. In this study, a framework is proposed to evaluate the change of the safety margins in design equations of reinforced concrete (RC) flexural members under radiation environments. Material strength changes are approximated on the basis of a collected test database, and the design strengths of RC beams are evaluated considering these material strength changes. The evaluation results demonstrate that the design strength of an under-reinforced flexural member can increase while the design strength of an over-reinforced member generally decreases. These results are associated with the material strength changes such that the yield strength of steel increases and the compressive strength of concrete decreases with the fluence of neutron radiation. Current NPP design codes need to further consider this un-conservative design possibility due to the design strength reduction of flexural members under irradiation.

Introduction

Neutron irradiation is one of the major concerns associated with the construction and operation of a nuclear power plant (NPP). Irradiation not only causes damage to living organisms but also changes the behaviors of construction materials, which can influence the structural performance of NPP. In relation to high radiation levels, this study focuses on typical light water reactor (LWR) configurations and considers reinforced concrete (RC) structures close to a reactor pressure vessel such as biological shield walls and reactor vessel supports. Field et al. (2015) conducted the radiation transport simulations and showed that the estimated neutron fluence exceeds the 1.0 × 10¹⁹ n/cm² at 40 years of a nominal design life at the surface of the biological shield, when fast neutron with energies above 0.1 MeV are considered as a conservative estimate.

There have been investigations on the behavior of construction materials under irradiation, especially concrete and steel. They have included experiments on the change of material behaviors under neutron and/or gamma irradiation such as the compressive and tensile strength of concrete (Field et al., 2015, Hilsdorf et al., 1978, Fujiwara et al., 2009, Kontani et al., 2010, Vodák et al., 2005), the porosity of cement paste (Vodák et al., 2010), the density of aggregates (Ichikawa and Koizumi, 2002), and the yield stress of mild steel (Murty, 1984a, Murty, 1984b). Recently, Mirhosseini et al. (2014) investigated the effects of neutron irradiation on RC 2D panels while only considering the change of the concrete compressive strength. Therefore, most existing studies mainly focused on experimental and theoretical investigation of radiation effects on material properties, and their impact on the resistance and design of structural members has not been investigated experimentally or statistically. Consequently, these impacts have not been seriously considered in the design of structural members of NPP. For example, the observed material behavior changes due to irradiation have not yet been applied to current NPP design codes including ACI 349-06 (ACI 349-06, 2007), KTA-GS-78 (KTA-Sachstandsbericht., 2005), and DIN 25449 (DIN 25449, 2008).

Basically, the current NPP design codes (ACI 349-06, 2007, KTA-Sachstandsbericht., 2005, DIN 25449, 2008) require more conservative safety margins compared to the design standards for ordinary concrete structures (American Concrete Institute, 2011, Korea Concrete Institute, 2007, Standards Australia International Ltd, 2009) to avoid the catastrophic consequences of NPP failures. In these standards, more conservative safety margins are achieved by providing larger load factors only; the safety margins included in resistance models are kept the same as those for ordinary structures. The safety level achieved by the greater load factors are quantitatively well-supported by the literature including the work by Hwang et al. (1987), Han (1998), Han et al. (1991), and Han and Ang (1998). However, the current safety margins for resistance involve no consideration of the effects of long-term radiation exposure on the resistance of structural members. As mentioned earlier, the strength change of construction materials such as steel and concrete can affect the performance of structures, which can result in unsafe predictions of structural resistance and improper choices of safety margins. Therefore, design standards need to take into account the quantified effect of neutron radiation on the behavior of structural members and provide proper safety margins to ensure the integrity of the design standards of NPP.

In the present paper, the safety of flexural members is evaluated when steel and concrete are exposed to neutron radiation affecting the change of the material and structural strength. Then, the safety margins in current design standards are recalibrated by proposing a Eurocode-based statistical safety factor calibration framework that considers the effects of radiation exposure of structural materials. In this framework, the effects of neutron radiation on the material strengths of steel and concrete are represented by statistically approximating the material strength changes. The modeling error of design equations is estimated through using a collected experimental database of ordinary RC beams. We herein limit our application to the ultimate flexural failure of RC beams as a representative structural member type. The reliability analysis in this study is based on the ultimate state only, and the serviceability limit state is not considered following the practices in current structural design codes. The serviceability related long-term effects (Mohanty et al., 2003) need to be considered in the future work to reflect all failure modes realistically.

The remainder of this paper is organized as follows. In Section 2, literature on the material behavior of concrete and steel under neutron radiation is briefly reviewed, and prediction functions for the material strength changes of concrete and steel given the effects of radiation are developed. In Section 3, the nominal bending capacity of RC beam sections are evaluated with the developed prediction functions. In Section 4, a safety margin calibration method is proposed in order to ensure the applicability of current code provisions to extreme radiation environments. Section 5 provides a collected database of the ultimate flexural strength of RC beams which is used to estimate the modeling error of design equations. In Section 6, the safety margins in current design code equations are recalibrated in conjunction with the changes in material strength due to radiation exposure. Finally, conclusions and discussions are drawn in Section 7.