The proposed project objectives are to study the changes in viscoelastic behavior, chemical structure and morphology of calcium silicate hydrates after a gamma dose of 200 MGy. Calcium silicate hydrates are considered the glue of cement and hence are important for the stability of the concrete bio-shield in nuclear power plants. The impact of high gamma doses on the viscoelastic properties of calcium silicate hydrates is not fully understood. Results from previous experiments with samples irradiated to 2.24 MGy indicate that there may be a creep reduction with gamma irradiation. The proposed experiment will be a first of its kind to study the impact of a high gamma dose (200 MGy) to inform models that predict the degradation of cement and concrete with irradiation. Since creep has been proven to delay the onset of irradiation damage, a reduction in creep will certainly impact the level of predicted damage. The outcome of the experiments is mainly the production of stress relaxation and creep curves after irradiation, accompanied by the study of chemical-structural properties to understand the factors that may drive the changes in viscous response, such as the loss of water from the interlayer and/or the precipitation of pseudomorphs. The chemical-structural changes will be studied with X-ray diffraction. Morphology and compositional changes will be examined with TEM. Viscoelastic properties will be obtained through stress relaxation and creep nanoindentation experiments. The main goal is to inform irradiation damage predictive models to support the license renewal of the light water reactor US fleet to 80 years of operation. The samples will be available at the end of January, and the experiments will be finished in 3 sessions of 5 days each during April, May and June.

The proposed research advances DOE’s Office of Nuclear Energy mission to meet the nation’s energy, environmental, and national security needs, since it will produce data to understand irradiation degradation mechanisms in the concrete biological shield of LWRs. This research also supports the aim of the DOE NE Light Water Reactor Sustainability Program’s (LWRS) objective to support the U.S. Nuclear Commercial Fleet Second License Renewal (SLR) up to 80 years of safe and economically viable operation. Therefore, it is tied to the NE objective of enhancing the long-term viability and competitiveness of the existing U.S. reactor fleet. The jointly-sponsored NRC-DOE Expanded Materials Degradation Analysis demonstrated the need to address urgently the effects of radiation on concrete. Among the various aspects requiring attention, the effects of gamma irradiation on the constituents of the hardened cement paste are poorly understood. The effects of the expected radiation dose at the planned extended life cycle (approx. Gamma dose of 100-200 MGy) on mechanical properties of cement and concrete in the biological shield need to be predicted to ensure the materials will remain operative and safe. This project will complement previous data sets obtained through the NSUF RTE framework to understand the effect of a medium gamma dose on the order of 2.24 MGy on calcium silicate hydrates (C-S-H), by producing data sets for a much higher dose (200 MGy). Calcium silicate hydrates constitute the major part of the volume of cement paste and are the phases that provide strength to cement. Hence, it is important to understand how radiation affects the chemical stability, morphology and mechanical properties of these phases. Viscoelasticity is a crucial property of C-S-H to relax stresses that occur due to a combination of radiation shrinkage of cement paste and radiation induced volumetric expansion of the aggregates, and therefore limits damage propagation in concrete. The results of this research will then serve to inform models to predict irradiation damage in concrete, in particular, to take into account possible radiation induced changes in viscoelastic properties of the cement paste.