Ultra-high gamma irradiation of calcium silicate hydrates: Impact on mechanical properties, nanostructure, and atomic environments

Abstract

The concrete biological shield in a nuclear power plant receives ~100–200 MGy gamma dosage during an 80-year design life. However, precise changes in the mechanical properties and atomic environments of C-S-H at ultrahigh irradiation dosages have not been systematically documented. Here, we report that irradiation decreases C-S-H basal spacing (~ 0.6 ± 0.1 Å for 189 MGy) and increases its Young's modulus, which is attributed to the lower basal spacing as the nano porosity potentially increased and microporosity remained unchanged. Irradiation also decreased the molecular water content and increased hydroxyl groups in C-S-H, showing that interlayer water removal reduces the basal spacing. Finally, ¹H and ²⁹Si NMR results indicate some disorder in the local proton CaO-H species and slight depolymerization of the silicate structure. Together, these results indicate that the C-S-H gel stiffens upon ultrahigh gamma irradiation dosage, a finding which concerns long-term nuclear power plants operations worldwide.

Introduction

Nuclear powerplants all over the world are aging, about 67% of all existing nuclear reactors are more than 30 years old [1]. Currently, the US Nuclear Regulatory Commission is evaluating the license extension of some light water reactors (LWRs) from 60 to 80 years. Extending the safe operation of these reactors over 80 years will require ensuring the durability of different nuclear reactor components—including the nuclear fuel rod cladding [2], [3], cables [4], reactor pressure vessel (RPV) [5], [6], and concrete [7]—under increasing neutron and gamma irradiation dosages. One important component of an LWR is the concrete wall that surrounds the RPV, known as the biological or primary shield, that absorbs radiation created in the fuel rods and provides structural support in some specific designs. This concrete wall is irreplaceable over the lifetime of a nuclear plant, making it necessary to ensure its structural integrity over an extended period before granting a renewal for the LWR. Over 80 years, the concrete in the biological shield wall can be exposed to about 100–200 MGy gamma irradiation [8], and neutron fluxes up to 6 × 10¹⁹ n/cm² (E > 0.1 MeV) [9]. High-energy neutrons can cause radiation-induced volumetric expansion (RIVE) through the amorphization of the different minerals present in the aggregates [9], [10], whereas gamma irradiation can cause drying, accelerated carbonation, and radiolysis in the cement paste [11], [12].

Radiolysis of water generates H₂ gas and H₂O₂, which can accelerate the carbonation of cement paste [11], [13]. Gamma irradiation has been reported to induce carbonation, which favored vaterite and aragonite formation rather than calcite formation in the natural carbonation of pristine cement paste [11]. Specifically, vaterite formed inside the small pores around Calcium Silicate Hydrate (C-S-H), the major binding phase of cement paste, reducing the porosity and increasing the strength of the irradiated samples. An increase in calcite content due to accelerated carbonation under gamma irradiation has also been reported [13], indicating some unresolved questions regarding gamma irradiation effects on different CaCO₃ polymorph formations. Gamma irradiation up to 200 MGy at different levels of drying of cement paste had a hydrogen generation rate proportional to the gamma irradiation dosage rate, even at different irradiation temperatures [8], [14]. Moreover, the G-value (i.e., number of hydrogen molecules produced per 100 eV of energy absorbed) decreased with the irradiation period and increased with the free-water content of paste. Previously, Tajuelo Rodriguez et al. studied pure synthetic C-S-H by using gamma irradiation up to 1.39 MGy and reported no significant changes in morphology, mean silicate chain length, basal spacing, and viscous and elastic response in C-S-H [15], [16]. Likewise, C-A-S-H (Calcium AluminoSilicate Hydrate) present in slag-blended cement was not affected by 4.77 MGy of gamma radiation [17]. The majority of studies available in the literature employ a relatively low gamma irradiation dose (Fig. 1) compared with the expected dosage level in the 80-year proposed design life of an LWR. Therefore, an irradiation study over a range of irradiation dosage is required. Additionally, although past literature mostly focuses on the irradiation effects on the nano- and microstructure of cementitious materials, the nature of irradiation-induced damage on the local atomic environments of C-S-H is largely unexplored.

In this study, we examine the effects of gamma irradiation on the mechanical properties, microstructure, nanostructure, and ¹H and ²⁹Si atomic environments of pure C-S-H samples at a range of ultralow to ultrahigh dosages (0.1–189 MGy) for three different Ca/Si ratios: 0.75, 1.0, and 1.33. Solid-state Nuclear Magnetic Resonance (NMR) is a state-of-the-art characterization tool that can resolve crucial atomic structure details in cementitious materials [29]. Thus, to understand the effect of radiolysis and find the source of hydrogen generation due to gamma irradiation, ¹H NMR was used to evaluate the change in chemical environments of hydrogen species with increasing radiation dosage. Potential changes in local silica environments were investigated by using ²⁹Si NMR. X-Ray Diffraction (XRD) and Thermal Gravimetric Analysis (TGA) were used to record the reduction in basal spacing and the removal of interlayer water due to the drying effect of gamma irradiation. X-ray Computed Tomography (XCT) and Ultra Small and Small Angle X-ray Scattering [(U)SAXS] were used to evaluate the change in porosity. Finally, nanoindentation and Ultrasonic Pulse Velocity (UPV) were used to evaluate the change in static Young's modulus, and dynamic Young's modulus, shear modulus, and Poisson's ratio after irradiation. Briefly, ²⁹Si NMR showed the depolymerization of the minor Q³ silicate structure, and ¹H NMR indicated disorder in select C-S-H proton environments. XRD showed a decrease in Basal spacing. The mechanical properties of the C-S-H samples increased after irradiation and the porosity of C-S-H both in micro and nanoscale was mostly unchanged after irradiation.