|
|
| Fig. 1: Nuclear Power Plant Design. (Source: Wikimedia Commons) |
The majority of global operating nuclear power plants have been in service since a few decades ago. In the United States, most of the currently operating plants began work in the 1970s and 1980s. Given the original licensing period of 40 years for existing nuclear power plants, most NPPs are approaching the end of their lifetime, where closure or extension under inspection are the two possible options. [1] The factors determining whether a plants lifetime can be extended are economic, safety and environmental. One of the technical aspects of aging of NPPs is concrete degradation, so it is important to understand how concretes behavior changes when exposed to radiation, in order to formulate well-supported technical decisions.
Concrete is present in multiple aspects of the power plant. For decades, it has been used to make the containment building of the plant, depicted in Fig. 1, to resist accidents and natural disasters. [2] Its thickness can range up to 2 meters. [3] Concrete is also widely used as a biological shield, which means an absorbent material around the nuclear reactor to reduce the radiation to levels safe for humans. [4] The concrete mix used differs depending on the concretes location in the NPP. Therefore, the concrete used in the biological shielding needs to have certain properties to function properly, considering its proximity to the reactor pressure vessel. The issue with nuclear radiation is that it influences materials properties, including concretes mechanical characteristics. It is therefore crucial to know the critical radiation dose below which the compressive and tensile strength, elasticity, and thermal and shielding properties remain unaffected.
There are different types of radiation: there are gamma rays which do not easily displace atoms, making them relatively non-threatening to concretes functional properties. Most research seems to disregard gamma radiation as it bears little effect on concretes strength and general properties. [5] Hence, the research is mainly geared towards the second type of radiation: neutron radiation.
Neutron radiation refers to the emission of electrically neutral particles as a result of radioactive decay of heavy elements with more neutrons than protons. [4] It can be categorized into two main radiation types: slow and fast neutrons. Fast neutrons have energies above 0.1 MeV, even reaching over 1 MeV, whereas slow neutrons only have energies up to a few eV. [5] Based on current experimental data, as well as the lack of, there is no sufficient evidence showing that fast neutron radiation is more detrimental to slow neutron radiation, although it is important to know that fast neutrons are always accompanied by slow neutrons. [5]
An early study carried out by Hilsdorf et al. in 1978, which investigated the topic of concrete degradation due to radiation as a whole, led to several interesting findings. The first and most important one is that for the average concrete, the critical value of neutron radiation fluence above which compressive and tensile strength decrease is around 1 × 1019 n/cm2. [4] That same study shows that the radiation dose in the reactor vessel is likely high enough to cause deterioration of concrete. Additional data presents separate effects due to fast or slow neutrons; for instance, irradiated concretes modulus was found to be 10% to 20% lower than its nonirradiated counterpart when the fast neutron fluence reached values between 7 × 1018 and 3 × 1019 n/cm2, and a similar percentage change was found when slow neutron fluence was around 3 × 1019 n/cm2. [6] It is estimated that the biological shield will experience neutron fluences at levels that will damage the concretes mechanical characteristics after 40 years of operation, which is when knowledge of the concretes composition will be used in the risk assessment for extending NPP lifetime. [5] Since it was found that some concrete mixes are more radiation resistant than others, advances have been made to utilize concrete mixes that are more resistant to radiation. [3,4]
In conclusion, while studies show that concretes properties deteriorate as a result of their operations in a power plant, a large amount of these studies are from the 1960s and 1970s, when nuclear power plants were rapidly developing. That being said, it is important to conduct more research on irradiated concrete, for a twofold reason; firstly, to ensure that we have enough and more recent data, especially given the advancements in scientific techniques which could generate even more accurate results; and secondly, to further investigate the long-term effects of radiation on concrete, since many power plants are reaching the end of their lifetime (the concern of which creates another compelling factor for additional research). Another reason is to have a more accurate understanding of how different concrete compositions react to radiation, perhaps to obtain optimal concrete mixes for enhanced performance and NPP lifetime.
© Jad Fidawi. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
[1] B. Anzelmo, "Irradiated Concrete," Physics 241, Stanford University, Winter 2015.
[2] S. Mirhosseini, M. . Polak and M. Pandey, "Nuclear Radiation Effect on the Behavior of Reinforced Concrete Elements," Nucl. Eng. Des. 269, 57 (2014).
[3] L. Katz, "Nuclear Containment Building Stability," Physics 241, Stanford University, Winter 2017.
[4] H. K. Hilsdorf, J. Kropp, and H. J. Koch, Der Einfluss radioaktiver Strahlung auf die mechanischen Eigenschaften von Beton (Ernst u. Sohn, 1976) ["The Effects of Nuclear Radiation on the Mechanical Properties of Concrete," American Concrete Institute, Special Publication 55, 223 (1978)].
[5] K. G. Field, I. Remec, and Y. Le Pape, "Radiation Effects in Concrete for Nuclear Power Plants - Part I: Quantification of Radiation Exposure and Radiation Effects," Nucl. Eng. Des. 282, 126 (2015).
[6] B. Pomaro, "A Review on Radiation Damage in Concrete for Nuclear Facilities: From Experiments to Modeling," Model. Simul. Eng. 2016, 1, (2016).
