Concrete Degradation: Why You Should Care
By Sarah Doenmez for C-10 Research and Education Foundation, 16 April 2015
- WHERE IS CONCRETE USED IN NUCLEAR POWER PLANTS? WHY SHOULD I CARE?
- WHY DOES CONCRETE DEGRADE?
- WHAT IS ASR?
- EVALUATING THE EXTENT OF CONCRETE DEGRADATION: WE NEED TO KNOW WHAT’S ON THE INSIDE
- C-10 RESEARCH & EDUCATION FOUNDATION HAS TAKEN ACTION
- WALLS FOR 100 YEARS, WASTE FOR MILLIONS OF YEARS
- WHAT YOU CAN DO:
I. Where is Concrete used in nuclear power plants? Why should I care?
Nuclear power plants use a lot of concrete to shield people and the environment from the radioactivity that is created in the production of the electricity. Every reactor has a containment dome made of concrete, as well as shielding walls, storage facilities, floors, and other structures. Most if not all structures for the storage of nuclear waste are made of concrete too. Concrete walls are considered the primary way of keeping the radiation created by nuclear power plants out of our world.
“Not only is concrete used in nuclear power stations, but it is also an issue for spent fuel pools, dry cask storage, burial of nuclear waste – whether a “facility” like WIPP, or burying it in concrete lined trenches or burying concrete casks in unlined trenches. Also it may be used for above ground temporary storage of nuclear waste.” (Nuclear Power Stations Concrete Damage) Concrete is used extensively in nuclear waste storage facilities.
However, our nuclear power plants are aging and the concrete structures that form their structures are showing signs of age. Depending on where the plant is, the signs can be different.
Seabrook in particular is known to have Tier 2 level cracks. These are cracks which are actively expanding. In 2010 it was found that there are cracks in the walls at Seabrook Nuclear Power Plant in NH. The cracks are thin but, as a recent report says, “by the time they are visible, the degradation is usually at an advanced stage…ASR capable of being detected visually, however, is probably in a fairly advanced stage of development.“ (William, Xi, Naus 2013 83, 88-89) 131 cracks have been visually detected in the walls at Seabrook, as noted in an NRC document.
The cracks have been found in all major structures.
Seabrook has not yet fully inspected the containment building and not inspected the building where the fuel rods are kept, but is scheduled to do so in 2015. It is expected that there will be signs of cracking there too.
Seabrook is the first nuclear power plant known to have ASR: Alkali-Silica Reaction, a form of concrete degradation.
The concrete that protects us from the radioactive by products of nuclear energy is crumbling.
III. Why Does Concrete Degrade?
There are different factors that contribute to the degradation of concrete: contact with aggressive water (water which is high in certain chemicals), contact with salt, changes in temperature, contact with radioactivity. Some factors affect the structure of the atoms in the concrete. Others affect the paste that holds concrete together, others create space between the concrete and the paste. All concrete degrades over time, and degrades in different ways. Different types of degradation can affect the strength, elasticity, ductility, and load-bearing capacity of concrete.
One significant conclusion reached in a study done for the NRC shows that temperature changes, either up or down, can cause degradation in concrete, and that such temperature changes take place in nuclear power reactors. “Elevated temperatures have two different effects on concrete. One involves phase transformations of the concrete constituents under high temperatures; the other refers to mechanical damage in the form of spalling. Both effects may result in significant deterioration of the stiffness, strength, and ductility of concrete and concomitant loss of load-bearing capacity . Phase transformations depend primarily on the temperature level, but not on the rate of temperature changes. Spalling damage occurs under high heating or cooling rates. A moderate elevation in temperature can occur in concrete exposed to gamma radiation.“ (William, Xi, Naus 2013 1) The radiation in a nuclear power plant causes temperatures to rise. We can be sure that this type of degradation is occurring in nuclear power plants.
Another significant finding in the study is that the radioactivity in a power plant can cause changes in the atoms of the elements which form the concrete, as well as in the water in the paste that holds the concrete together.
“Interactions between nuclear radiation and the internal structure of concrete produce geometric changes of the structure resulting from displacements of atoms from their lattice sites and phase transformations of the concrete constituents resulting in a reduction in porosity and/or formation of microcracks….Gamma rays … can result in the destruction of anisotropic chemical bonds such as covalent bonds. Water in the concrete can be decomposed by gamma rays by a process called radiolysis and can be converted to hydrogen, oxygen, and hydrogen peroxide. Water can also be removed from the concrete by evaporation due to heat generated by gamma radiation. Because most of the water in concrete is contained in the cement paste, gamma radiation has a greater effect on the cement paste than it has on the aggregate materials. In the instance of neutron irradiation, the neutrons do not interact with the electrons but with the nuclei of atoms… Because neutrons interact with the nuclei of atoms, the lattice spacing within the material may change after the collisions. Therefore, the neutrons have a more significant effect on dense and well-crystallized materials (e.g., aggregates) than on randomly structured materials with high porosity (e.g., cement paste).” (William, Xi, Naus 2013 93)
Contact with radioactivity in its different forms creates concrete degradation. Since we know that the reactor cores of nuclear power plants contain radioactivity, we have to assume that the concrete of the containment domes and the storage vessels is degrading and will degrade further.
IV. What is ASR?
ASR is alkali-silicon reaction. It is a form of concrete degradation. It happens when water is absorbed by concrete. Silicon can become reactive in the presence of water. ASR leads to the formation of a gel in the concrete which can create cracking. In addition, raised temperatures can make concrete susceptible to developing ASR over time. “As plants age, the potential of ASR to occur in structures forming the biological shield or support for the reactor pressure vessel may increase as these structures are located in areas in which they are subjected to moderate elevated temperature in combination with radiation.“ (William, Xi, Naus 2013 88)
It is known that ASR is taking place at Seabrook, but the extent of the cracking has not been explored beyond the surface of the walls.
There is no known remedy or repair for ASR.
V. Evaluating the extent of concrete degradation: why we need to know what’s on the inside
There are different ways to examine concrete to evaluate its material properties, such as strength and elasticity. Visual inspection of the surface can show that degradation is occurring but does not show anything about the specific forms of degradation or its extent. Sampling of cores to look at the interior, lab analysis, and petrographic ultrasound are ways of determining the extent of degradation in concrete. The American Concrete Institute wrote a report entitled “Evaluation of Existing Nuclear Safety-Related Concrete Structures” in 2002 (Janowia et. al), and the NRC has recommended but not required Seabrook to follow the procedures detailed in the report.
So far the NRC is requiring Seabrook only to do visual inspections, which do not show the full state of the concrete walls. Scientists know of several methods which can be used to assess the health of concrete: “Quantifying and determining the nature of distress in concrete are generally accomplished through removal of cores or other samples using an established procedure. When core samples are removed from areas exhibiting distress, a great deal can be learned about the cause and extent of deterioration through property determinations and petrographic studies. Additional uses of concrete core samples include calibration of NDT devices, conduct of chemical analyses, visual examinations, determination of steel reinforcement corrosion, and detection of the presence of voids or cracks. Information on methods for examining hardened concrete to characterize materials and to identify and quantify degradation is available (Walker et al. 2006). Qualitative or quantitative characterization of the microstructure can be done using techniques such as nanoindentation, mercury intrusion porosimetry, XRD, thermal gravimetric analysis, nuclear magnetic resonance, SEM, and energy-dispersive spectroscopy.” (William, Xi, Naus 2013) Scientists confirm that concrete degrades and cracks, know several methods for examining and determining the health of concrete, and feel that further study of this field is important to better understand which factors are most significant in concrete degradation and the pace at which degradation occurs. We need to know what is happening on the inside to understand the extent of concrete degradation, the rate its occurring, and make any judgements about the viability of the structures impacted.
The NRC has acknowledged that the ACI report contains industry-standard and valuable methods for determining the health of concrete, and the 2013 study provides additional detail for methods that could and should be used. Seabrook must be required to analyze the interior of the concrete to determine the extent of the degradation, the pace at which it is occurring.
VI. C-10’s Action
C-10 has filed a petition with the NRC asking that the concrete in the nation’s 104 nuclear power plants be thoroughly examined to determine the safety of the plants. The NRC accepted this petition for review. C-10 is demanding that the NRC hold Seabrook to the standards it has recommended and require Seabrook to do further analysis of the degradation of the concrete at Seabrook nuclear power plant.
“The NRC has made clear multiple times that no decision will be made by the agency on a license renewal application for Seabrook until the extent of the concrete degradation is fully understood.” (Grinnell) However the NRC is allowing Seabrook to delay the additional inspections needed.
C-10 continues to study the issue of concrete in nuclear power plants and to monitor the safety of the Seabrook plant. Debbie Grinnell of C-10 points out that “We are a long way from having standards. The recent (summer 2013) the NESCC, NIST and ANSI collaborated to issue a report on codes and standards for the repair of nuclear plant structures. They reported ‘research has not reached a level that would allow the repair and prediction of service life. A combination of measurement techniques and models is necessary to be able to monitor the progression of any SR and to predict the remaining service life before and after repair. Protocols for the Selection of repair materials and monitoring also need to be developed.’ (Nuclear Energy Standards Coordination Collaborative –Codes and Standards for the repair of Nuclear Plants). “ The watchdog work of C-10 is integral to keeping the local communities informed of the situation and safe.
VII. Walls for 100 years, waste for millions more
The rule established by the NRC says that concrete has to be strong for 100 years. But we know that nuclear waste will remain radioactive and dangerous to life for millions of years (the half–life of Uranium 238 is 4.5 million years.) If the concrete structures we use to shield us from radioactivity last 100 years, what then? If they crack sooner and release radioactivity, what then?
We know this happening. The situation needs to be focused on and studied, and solutions need to be devised. Seabrook needs investigation of the interior of the concrete to know the extent and rate of degradation. The American fleet of nuclear reactors all use concrete, and we now know that concrete is degrading.
Grinnell, Debbie C-10 Research and Education Foundation. Personal Communication. 16 April 2015
Janowiak, Ronald J., Chair ACI Committee 349 Evaluation of Existing Nuclear Safety-Related Concrete Structures American Concrete Institute 2002
Nuclear Power Stations Concrete Damage, Part II Mining Awareness Plus. N.p., 29 Mar. 2015. Web. 03 Apr. 2015. <https://miningawareness.wordpress.com/2015/03/29/part-ii-nuclear-power-stations-need-testing-for-concrete-damage-comment-deadline-monday-march-30th-11-59-et/>.
William, Kaspar University of Houston Department of Civil and Environmental Engineering Houston, TX 77204-4003; Xi, Yunping University of Colorado Department of Civil, Environmental and Architectural Engineering Boulder, CO 80309-0428; Naus, Dan Oak Ridge National Laboratory Materials Science and Technology Division Oak Ridge, TN 37831-6069; Graves, Herman L., III, Technical Monitor NRC A Review of the Effects of Radiation on Microstructure and Properties of Concretes Used in Nuclear Power Plants November 2013 NRC Job Code N6978 Prepared for Division of Engineering Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 NUREG/CR-7171 ORNL/TM-2013/263