Degradation of Concrete Structures in Nuclear Power Plants

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1		Pathath Abdul Rasheed	--	3977	2022-11-07 06:28:09	\|
2	Reference format revised.	Lindsay Dong	Meta information modification	3977	2022-11-07 07:27:21	\| \|
3	Adjust second-level title	Lindsay Dong	Meta information modification	3977	2022-11-14 01:54:13	\|

This entry is adapted from the peer-reviewed paper 10.3390/en15218011

Concrete, an integral part of a nuclear power plant (NPP), experiences degradation during their operational lifetime of the plant. The damage mechanism could be chemical or physical. The major causes of chemical degradation include alkali–aggregate reactions, leaching, sulfate attack, bases and acids attack, and carbonation. Physical degradation is a consequence of both environmental and mechanical factors combined. These factors are mainly elevated temperature, radiation, abrasion and erosion, salt crystallization, freeze–thaw distortions, fatigue and vibration. Additionally, steel reinforcements, prestressing steels, liner plates, and structural steel also experience degradation.

nuclear power plants concrete reinforcement structures degradation

1. Introduction

In nuclear power plants (NPPs), concrete structures are the main components used to support, contain, and protect the mechanical and electrical systems. Concrete structures are found in the primary containment building, the biological shield wall, secondary and internal containment as well as in cooling towers. The biological shield wall is the only structure expected to receive the radiation from the plant which is designed very close to the reactor vessel to absorb radiation and also acting as a load-bearing structure which supports the reactor vessel ^[1] The concrete helps in preventing radiation releases, radiation attenuation (gamma radiation, neutron, and other irradiation), provide structural support for the nuclear steam system and different equipment in NPPs ^[2]. The degradation issues resulting from the nuclear power generation mainly occur in the containment building while other structures that surround the reactor building are not significantly affected by the nuclear power generation process. Hence, different types of concretes have been used in NPPs according to the location and function of each structure in NPPs considering the safety, significance and environmental exposure.

From the NPP operational experience, it is obvious that concrete can be highly reliable without maintenance if it is properly designed for the exposed environment along with proper construction methods with high quality control ^[3]. In spite of all the understanding and consequent precautions, certain environments can damage the concrete either due to chemical or physical mechanism mainly by affecting the aggregates in the concrete or the performance of the cement-paste matrix ^[4].

Significant extension of the service life of NPPs is possible mainly by preventing the material degradation of the concrete elements, hence it is necessary to evaluate the major causes of degradation and its mechanism ^[5]. In general it is observed that the performance of concrete structures in the NPPs is fairly satisfactory during initial 40 years of service life, subsequently, the mechanical properties start diminishing, even at lower ranges of neutron fluence values (mostly E > 0.1 MeV) ^[6]. The performance sustained during the earlier service life could largely be attributed to the integrity of calcium–silicate–hydrate gel, mostly constituting the tobermorite mineral formed during the hydration of cement, resulting in the required strength characteristics. However, degeneration of the mineralogical structure of aggregtaes due to long-term irradiation and radiation-induced swelling leads to considerable degeneration of concrete structures. These factors which are specific to NPPs in combination with the general possibility of degeneration due to common physical and chemical attacks of concrete, lead to the general agreement that the service life of concrete structures in NPPs cannot extend beyond 40 years. However, a proper understanding of the various mechanisms including radiation-induced degradation mechanisms can help mitigate these and thereby result in the design of NPPs which can last much longer than stipulated today.

2. Concrete Degradation by Chemical Attack

Chemical degradation on any concrete structure may originate from the chemical reactions between the cement paste/coarse aggregate and the environment. Although the chemical reactions generally occur at concrete surfaces and the areas between cracks, the entire cross-section of the concrete structure can be damaged after continuous exposure for a long time. The factors affecting the degree of chemical damage on the concrete materials are mainly the pH of the attacking fluid and the concrete’s properties, such as alkalinity, permeability, and reactivity. The major mechanisms of chemical attack on concrete are alkali–aggregate reactions, leaching, effect of bases and acids, salt crystallization and sulfate attack.

2.1. Alkali–Aggregate Reactions

Portland cement contains large amounts of alkali ions and hence alkali–aggregate reactions occur in the presence of reactive silica, silicate, or carbonate in aggregate materials. Alkali–silica reactions (ASRs) cause the development of alkali–silica gel (e.g., calcium sodium silicate hydrate and calcium potassium silicate hydrate) which swells when in contact with water ^[7]. A combination of reactive aggregate, high moisture levels and high alkalis can enhance this process. The reactivity of aggregates is closely related to the mineralogical evolution of the parent rock, whereas the presence of moisture and alkali is a consequence of the local environmental condition and the pore-solution characteristics of the concrete ^[8]. The amount of ASR gel mainly depends on the reactive nature of the silica, the amount of hydroxyl ions in the pore-solution, and the internal structure of the aggregate along with other factors such as gel composition, gradation of aggregates, reaction temperature, type and proportions of the reacting materials ^[9].

The swelling of alkali–silica gel can cause an increase in hydraulic pressure in the concrete which results in significant cracking of the concrete and loss of the mechanical properties (i.e., stiffness and tensile strength). This would in turn cause the reduction in the lifespan of NPPs. The primary structures susceptible to alkali–aggregate reactions are the concrete structures exposed to water (rain, cooling or ground water, or humidity inside containment areas. It is also found that neutron irradiation can enhance the reactivity of silica-rich aggregates leading to increased ASR gel formation ^[10]^[11]. The alkali-aggregate reactions cause damage to the concrete structures typically in the first 10 years of the NPP’s lifespan. However, the presence of less reactive silica delays the concrete deterioration up to 20 years ^[12].

The addition of mineral admixtures in concrete provides partial protection against ASRs (Table 1).

Table 1. Important admixtures that can be used to prevent ASR damages in the concrete structures.

Mineral Admixture	Replacement Level	Results	Ref
High-reactivity metakaolin (HRM)	20%	Reduction in alkali concentration, Reduction in long term concentrations of OH⁻, Na⁺, and K⁺	^[13]
Fly ash (FA)	25–40%	Reduction in expansion and cracking	^[14]
Silica fume	8% to 12%	Able to control ASR	^[15]
Steel fibers	2%	Controls ASR expansion, mechanical properties can be restored	^[16]

2.2. Leaching

Leaching is a diffusion–reaction phenomenon, which alters the cement paste matrix as a result of dissolving calcium-containing elements through hydrolysis. The leaching process results in an increase in pores and increases the permeability of the concrete thereby causing harmful effects on the long-term durability of the concrete. The concrete areas exposed to water are the most vulnerable to the leaching process. However, leaching can be seen even on dry surfaces of concrete structures of NPPs which might lead to a reduction in alkalinity of the concrete followed by corrosion of the reinforcements and surroundings ^[17]. The leaching process lowers the compressive strength, increases the vulnerability to environmental attacks, and ends up in corrosion of the steel reinforcements due to chloride penetration.

Generally, leaching can be controlled by using a low water-to-cement (w/c) ratio thereby reducing the water leaching ratio, increasing the curing time, and by using cement with low alkali content ^[18]^[19]^[20]. The commonly used solution against leaching is the use of mineral admixtures in the concrete since the silica-rich mineral reacts with the calcium hydroxide which increases the formation of impermeable C–S–H gel ^[4].

2.3. Sulfate Attack

Exposure to sulfates present in the surrounding environment of NPPs leads to sulfate attack on concrete which is a major deterioration mechanism. Sulfate exposure is one of the major causes of the loss in compressive strength and expansion in volume of concrete ^[21]. Sulfates react with different concrete components in concrete to form ettringite and gypsum, which results in the expansion and disintegration leading to loss of strength ^[22]. The two main factors which affect the rate of sulfate attack are the concentration of sulfate and reactivity of the cement paste ^[23]. Anything above 1200 ppm (parts per million) concentration of sulfate in the environment can aggressively attack concrete in NPPs.

During the normal concrete curing process, ettringite is formed by the combining calcium aluminate with sulfate ^[24]. However, at higher curing temperatures, the ettringite formation can be inhibited. The reformation of ettringite in hardened concrete leads to the generation of extensive pressures, and cracking of the concrete in the presence of moisture, which is denoted by delayed ettringite formation (DEF). DEF is the result of high initial temperatures, more than 70–80 °C, in which the usual formation of ettringite is prevented or the decomposition of already formed ettringite. DEF can be enhanced by the use of cements with high aluminate and sulfate content ^[24]. The degree of DEF also depends on the availability of water in concrete, presence of microcracks and the temperature.

Similar to the mitigation techniques for other deterioration mechanisms, the use of mineral admixtures has been effective in minimizing sulfate attack by reducing the mobility of water and dissolved ions as well as by varying the early hydration process. Another possible solution is the use of Type V cement with a lower C₃A content blended with natural pozzolan to protect against sulfate attack ^[25]. The potential of mineral admixtures such as FA and slag in sulfate attack mitigation even in concrete with lower quality materials, such as recycled aggregates, has also been reported. However, the higher content of recycled aggregates may reduce the resistance of concrete against sulfates due to the high porosity and defects in recycled aggregates ^[26]. In addition, the replacement of a portion of cement with FA, slag or silica fume can significantly prevent DEF damage.

2.4. Bases and Acids Attack

Concrete structures are chemically attacked by acids and bases, in which acidic solutions are typically more reactive to the basic concrete/cement structures. The cement/concrete mixture may not react with other basic solutions since the mixture is highly alkaline with a pH of 12.5 or higher. However, when concrete is exposed to bases for a prolonged peior, deterioration can occur by chemical processes other than reacting with hydroxide ions. The possible acid solutions present in the NPP’s surroundings are sulfuric acid, carbonic acid, phosphoric acid, acetic acid, etc. All these have a distressing effect on conventional as well as NPP concrete structures ^[4]. The rate of acid attack on concrete structures strongly depends on the pH of the surrounding fluid and the period of exposure. Acids reacts with the calcium components available in hydrated cement paste, such as calcium–silicate–hydrate, calcium hydroxide, and calcium aluminate hydrate, producing calcium salts which enhance the porosity and permeability of concrete ^[27]^[28]^[29].

2.5. Carbonation

Carbonation is the rarest degradation issue in NPPs, which is a chemical degradation process as a result of the reaction between Ca²⁺ and CO₃²⁻ ions ^[30]. In addition, the diffusion of CO₂ from the surrounding environment is the driving force behind carbonation. Carbonation results in the reduction of pH which may accelerate the corrosion of reinforcing bars in the concrete through the dissolution of the protective thin oxide passive layer. On the contrary, it is known that carbonation refines pore structures, decreases transport properties, and increases the strength of cement-based materials. The carbonation process increases with increasing temperature since penetration of CO₂ occurs at high temperature.

To protect concrete structures from carbonation, different methods can be adopted such as increasing the CO₂ binding capacity, increasing capillary condensation and applying surface coatings ^[31]^[32]. Portland cement has the highest resistance to carbonation due to its high CO₂ binding capacity ^[31]. The CO₂ binding capacity can be increased by increasing the available CaO in Portland cement with a low level of binder replacement and using a higher binder-to-aggregate ratio. Other than CO₂ binding capacity, capillary condensation and porosity are additional parameters determining the carbonation rate ^[31].

3. Concrete Degradation by Physical Attack

Concrete can be damaged by physical attack mainly due to mechanical and environmental effects. Most of the prominent degradation mechanisms in NPPs are due to physical attack. The different processes causing physical damage to concrete materials are elevated temperature, irradiation, abrasion and erosion, freeze–thaw distortion, and salt crystallization.

3.1. Elevated Temperature

Generally, the exposure of the majority of concrete structures is limited to a maximum temperature of 65 °C based on their NPP specifications. However, the temperature can be raised up to the temperature of the steam system coolant in certain areas. The heat transfer medium, liquid sodium, may undergo accidental leakage in NPPs leading to a high temperature when it comes into contact with water ^[33]. The concrete performance at elevated temperatures depends on the temperature and duration of exposure, moisture content of the concrete, type of cement aggregates, and size of the structural elements. ^[34]. Usually, at higher temperatures the cement paste shrinks, while the aggregate mostly expands and this causes thermal stresses at the interface which results in cracking ^[35].

The concrete may lose its compressive strength and modulus of elasticity at 90 °C, and the loss will be higher when the temperature reaches 200 °C. The breakdown of cement gel starts at 180 °C since the free water is expelled by the dehydration process at this temperature ^[36]. At temperatures of around 400 °C, the decomposition of calcium hydroxide to quick lime and water happens, which induces secondary internal stresses ^[37]. At temperatures above 430 °C, the loss of strength is greater in concrete comprising siliceous aggregates compared to concrete with light-weight aggregates ^[38].

Since the concrete temperature can be increased by both heat and radiation, concrete with low radiation penetrability can be used for better performance at high temperatures in NPPs. Sakr et al. found that the concrete with ilmenite aggregates exhibited an improved mechanical performance compared with concretes containing barite or gravel ^[39]. The concrete attenuation coefficient of ilmenite concrete was higher than gravel and baryte concrete by 39.8% and 8%of ⁶⁰Co at laboratory temperatures, respectively. Horsczaruk et al. showed that the compressive strength of the concrete with magnetite aggregate initially increased up to 300 °C and then reduced when the temperature reached 450 °C, followed by a gradual decrease in compressive strength at 800 °C ^[40].

Concrete containing OWA (olive waste ash) showed good performance at elevated temperatures compared to control concrete ^[41]. In addition, the performance of OWA concrete was diminished when the OWA content was increased from 7% to 22% and the performance of OWA concrete containing tuff aggregate was better compared to OWA concrete containing basalt aggregate. They also found that the OWA concrete with a w/c ratio of 0.5 was more resistant than a w/c ratio of 0.7, and an air entrained OWA concrete was better than non-air entrained OWA concrete. The concrete containing palm oil fuel ash (POFA) showed no change in compressive strength up to 400 °C and a substantial strength loss beyond 600 °C ^[42].

3.2. Abrasion and Erosion

Abrasion is observed in the areas of cooling water intake and discharge, and floor elements in NPP structures. The abrasion–erosion damage is usually observed in spillway aprons, stilling basins, and tunnel linings ^[43]. Abrasion–erosion damage results from the friction between particles and the components at the concrete’s surface during hydraulic structure operation and leads to the loss of material at the concrete’s surface ^[44]. By enhancing the quality and compressive strength of the concrete, the abrasion–erosion resistance can be improved ^[45]. The abrasion–erosion resistance of NPP concrete can be enhanced by adding mineral admixtures, such as FA, using low w/c ratios and appropriate aggregates ^[45].

3.3. Radiation-Induced Degradation

The degradation mechanisms induced by the irradiation of concrete is specific to NPPs, especially in containment vessels and waste storage facilities. Concrete structures of NPPs, especially the biological shields and radioactive waste storage facilities, are exposed to gamma and neutron radiation ^[46]. It was observed that the interaction of concrete with nuclear radiation caused aggregate expansion by geometric changes of solid phases and phase transformations which led to changes in the properties of concrete ^[47]^[48]. The compressive strength and stiffness of concrete diminished after exposure to the neutron irradiation due to the expansion of aggregates called radiation-induced volumetric expansion (RIVE) and internal damage in the concrete material which affected the transport properties of the degraded concrete ^[49]^[50]. Gamma rays can decompose water content in the concrete to hydrogen, oxygen, and hydrogen peroxide by radiolysis, and the by-product of radiolysis may react with cement paste ^[51]. This also leads to shrinkage of cement paste and subsequent damage to the concrete depends on the exposure temperature.

3.4. Freeze–Thaw Distortions

Freeze–thaw distortion is a fundamental issue of NPPs in cold climates in which moisture accumulates on the surfaces leading to degradation of concrete structures ^[52]. The water in the concrete expands as it freezes which results in an increase in hydraulic pressure in the concrete and ends up in cracking of the concrete especially when repeated freeze–thaw cycles occur. Hence, proper evaluation of the effect of freeze–thaw cycles must be taken into account before designing the lifetime of NPPs in locations where this could be critical. The freeze–thaw damages are visible on the outer surfaces of NPP components; hence it can be identified before the loss of the concrete’s structural properties. Freeze–thaw distortions are the most commonly reported degradation issue in NPPs.

Freeze–thaw can reduce the concrete’s resistance to the entry of harmful chemicals. The prevention of such chemical diffusion can be done by using mineral additives or kaolinite clay in the cement. Chung et al. found that both FA- and silica fume-enriched concrete at lower water-to-cementitious ratios with proper curing showed resistance to freeze–thaw damage ^[53].

3.5. Salt Crystallization

Salt crystallization is another issue in NPPs especially when they are located in rural/coastal areas and use sea water as a coolant. The water containing dissolved salts can permeate the concrete and salt crystallization occurs within the concrete pores when it evaporates. This salt deposit increases the stress enough to create micro-cracks in the concrete and hence causes a substantial threat for NPP concrete.

The use of low-permeability concrete can reduce the salt crystallization damage when exposed to water containing dissolved salt. Maes et al. found that the chloride penetration increased with more sulfate content at short immersion periods, except for high-sulfate-resistant concrete ^[54]. Different coating methods can be used to prevent chloride penetration, such as silane, polyurethane and acrylic coatings ^[55]^[56]. Other than coatings, different admixtures have also been used against salt crystallization damage.

3.6. Fatigue and Vibration

Fatigue and vibration are mechanical causes of damage to concrete structures due to the fluctuations in loading, moisture content and temperature. The damage by fatigue in concrete structures starts from small cracks in the cement paste near to reinforcing steel, in large aggregate particles, or in defective areas. This will lead to large scale concrete failure when excessive cracking/deflections, or brittle fractures occur. So far, there are no significant fatigue-related concrete failures that have been reported in NPPs.

4. Degradation of Mild Steel Reinforcement

The degradation of the mild steel reinforcements within NPP concrete structures is caused mainly by corrosion along with other causes, such as elevated temperature, fatigue and irradiation. The main reasons for the corrosion in mild steel reinforcements are the thermal and mechanical stresses, corrosive external and internal environments, moisture content, presence of microorganisms and stray electrical currents. Electrochemical corrosion can occur either by the formation of a galvanic cell with two different metals in the concrete or by the formation of concentration cells due to the presence of different concentrations of dissolved ions. Generally, the high alkalinity of concrete (pH > 12) provides protection to steel from anodic activity. However, the ingress of chloride ions and sulfate ions can reduce the pH to less than 11, resulting in rust formation on the reinforcing steel.

Concrete corrosion mechanism becomes complex when chloride and sulfate ions interact with hydrated cement phases. It becomes more complex when the cations associated with chloride and sulfates are present ^[57]. Zuquan et al. found that the presence of sulfate in the sulfate–chloride composite can prevent the entry of chloride into the concrete at an initial exposure period, though, it increases the entry of chloride at a later stage of the exposure period ^[58]. It is found that reinforcement corrosion doesn’t initiate when exposed to only sulfate ions while considerable corrosion can occur in a mixed chloride–sulfate solution ^[59]. The presence of sulfate ions in a chloride environment is independent of the initiation time of reinforcement corrosion: the concentration of magnesium sulfate and sodium sulfate can enhance the corrosion current density ^[60]. Thus, it may be established that for parts of NPPs located in areas prone to chloride exposure, the unavoidable presence of sulfate ions creates favorable conditions for accelerated corrosion and hence, appropriate mitigation strategies are imperative in such situations.

A typical mechanism of corrosion in NPPs is the microbially influenced concrete corrosion (MICC). It is known that concrete is typically highly alkaline and normal microbes cannot survive at this pH. However, the adhesion of several microbes are possible when the pH of the concrete is reduced to 9.5 by any of the attacks discussed in the above sections ^[61]. Sulfur compounds can be oxidized to sulfuric acid as a result of microbial reaction, which leads to the corrosion and degradation of concrete structures. Sulfuric acid may react with free lime and form calcium sulfate which results in the formation of a corroding layer on the concrete’s surface and it may penetrate into the concrete ^[62].

The effective means of mitigating the concrete corrosion is to prevent the ingress of moisture and chlorides ^[63]. By increasing the concrete covering of steel bars, and by enhancing the quality of concrete, the corrosion-related degradation of concrete can be minimized ^[64]. Similarly, the use of corrosion inhibitors is recommended for mitigating the corrosion provide that it should provide corrosion protection during all stages of plant operation ^[65]. The use of alternative reinforcement with high quality material, using corrosion-resistant mineral admixtures, and coatings providing a physical barrier against corrosion can be considered as other strategies of corrosion mitigation.

5. Degradation of Prestressing Steel

Prestressing steel in NPP concrete structures can be damaged by corrosion, irradiation, fatigue, elevated temperature, and losses of prestressing forces ^[4]. The majority of corrosion-related damage to prestressed parts occurs in localized areas; however, this can also happen uniformly throughout the steel. Localized corrosion attack can be caused by pitting, stress–corrosion cracking (SCC) and hydrogen embrittlement. Pitting is an electrochemical process and leads to the loss of materials at the tendon surface of the prestressed steel which reduces its capacity to support loads. SCC can normally cause fractures of a ductile material under stress, while hydrogen embrittlement is the entry of hydrogen atoms into the metal lattice which reduces its ductility. To protect the corrosion of prestressed steel, organic corrosion inhibitors, such as petrolatum or Portland cement grout can be used to fill the ducts containing the post-tensioned tendons ^[4].

Elevated temperatures can possibly affect the steel wires. However, it was found that short-term heating of 3–5 min at around 400 °C was not harmful to the prestressed wire ^[66]. Generally, the damage of prestressed steel occurs only at particular localized areas after long-term exposure at around 200 °C. Radiation affects the prestressed steel in a similar manner as reinforcing steel. Fatigue in prestress steel results in concrete failure, mainly due to flexural compression, shear forces, and flexural and tensile stress variations. Most of the fatigue failures normally occur at the tendons by stress concentrations at crack locations.

6. Degradation of Liner Plate and Structural Steel

Corrosion is the major degradation process of the liner plate and structural steel of NPPs similar to the corrosion process of reinforcing steel. Corrosion of the liner plate and structural steel include galvanic corrosion, pitting and crevicing. Structural steel is protected from the corrosive environment since it is embedded in the concrete; however, the presence of pores and high concrete permeability may allow fluids to reach the steel and increase the corrosion rate.

Fatigue is the other cause of degradation of liner plate and structural steel due to load cycles and vibration. Fatigue problems occur only when abnormal circumstances occur, such as material flaws and stress concentration factors. The fatigue sites in the liner plates are commonly at base metal delaminations, weld defects and arc strike areas, structural attachments and concrete-to-floor boundaries.

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Pathath Abdul Rasheed

Sunitha K. Nayar

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Update Date: 14 Nov 2022

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