|Fig. 1: Seabrook Power Station Containment Building.  (Source: Wikimedia Commons)|
The Atomic Energy Act (IAEA) and regulations of the United States Nuclear Regulatory Commission limit commercial power plants to 40 years with a possible renewal of 20 years.  The 40-year permit is based on economic factors, among others, but is not limited by the nuclear technology. Given the initial permitting timeframe, it could be possible that plants were engineered to have an infrastructure service life of 40 years. Before a license is extended, a thorough inspection must take place. Natural degradation of concrete is taken into account, but the influences of radiation are harder to quantify. Specifically, concrete could degrade faster than expected given the constant bombardment of radiation year after year. The current state of research related to irradiated concrete will be discussed.
Nuclear power in the US accounts for approximately 20% of the total electric energy, making it the world's largest supplier of commercial nuclear power (almost double that of it's second contender, France).  The average age of the US commercial nuclear power plant fleet is 33 years, with plants already into an operating renewal license. If shut down after this initial period, commercial providers would be taxed with decommissioning costs and these lost electrons will need to be replaced. With many of these plants up for renewal, the major concern is the evaluation of safety-related systems, which are either cost prohibitive or near impossible to replace. The biggest concern isn't large mechanical and electrical systems, that mostly can be replaced, but the large concrete structures used for containment. Data is limited for the long term, 40- to 80-year, performance of concrete used in nuclear plants. Of the studies that have gathered data, it comes in the form of concrete cores from the plants as well as concrete of similar age to the plant.  While this data relates to the degradation of concrete under environmental conditions, valuable data, there is a need for long-term effects of radiation on load-bearing concrete. Given that the age of the US nuclear power fleet is only 40 years, it is near impossible to obtain such information.
Nuclear facilities around the world rely on concrete that is typically made up of locally sourced products. These raw materials make up about 75% of the concrete's volume, which adds variance to data gathered, making the process of characterizing irradiated concrete all the more difficult.  The attenuation of radiation in concrete can have three different major damaging effects: embrittlement, localized heat generation, and dehydration. These damaging effects can be one in the same, i.e. heat generation may result in higher than expected stresses as well as dehydration. In order to quantify the effects of radiation, it is important to differentiate between the induced higher temperature and radiation; however, it is hard to subtract the effect of this induced heat because the occurrence is simultaneous.
In 1978 there was a study completed by Hilsdorf et al. to quantify the amount of radiation concrete could withstand without negatively impacting the strength and elasticity.  The upper safe limit (no long-term effects on concrete) concluded from this study was a radiation flux of 1 × 1019 neutrons per square centimeter (n/cm2). For reference, the same study found that a biological containment wall might exceed 5 × 1019; five times the concluded safe limit. There is of course debate whether the proposed values should be taken as fact, as there are some assumptions within the data. However, given there are only a few publications between 1980 and 2005, the importance of this work found its way into the standards of the American Concrete Institute, the Nuclear Regulatory Commission and the IAEA.  These organizations all have set threshold levels for radiation fluxes through concrete based on Hilsdorf's work from 1978.
The increasing number of nuclear power plants coming to the end of their original licensing with the possibility of an extension has lead to an increase in research in irradiated concrete. A report out this month by Field, et. al. has concluded, among other points, very similar results to the 1978 study.  That is that the point of diminishing compressive strength, elastic modulus and tensile strength all happen around the 1 × 1019 n/cm2 mark. The report goes on to quantify the decrease in the mechanical properties of the concrete, but does mention that the relationships used may be different for irradiated concrete. Part II of this report explains how a micromechanical model for irradiated concrete can be developed and provides insights on the importance of distinguishing between various modes of degradation on cement paste. In other reports it is believed that attention on this matter is only needed if the lifetime of nuclear plants exceeds 60 years, which is to say the plants are structurally stable to withstand an initial license extension 
While the answer to the question 'Does radiation speed up the degradation of concrete?' isn't completely solved, there are sound studies proving that something does happen to the concrete, but more studies need to be carried out. In order to obtain valid data collection, various literature sources mention steps that should be taken: documentation of initial concrete mixture ratios as well as their source; various radiated concrete core samples from across different parts of the plant, core samples from non-radiated concrete, and computer simulation models.  The cores allow for concrete, irradiated and not, of the same origin, and under similar operational conditions to be tested. This comparison will allow for an in-depth study of the effects of induced heat and radiation on one end of the core while observation of the effects when neither is present.
© Bryce Anzelmo. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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