A major potential problem is crud. Crud can cause an increase in background radiation levels. When two dissimilar metals are in close vicinity, the opportunity for ion transfer is high.The result is a perfect environment for galvanic corrosion. Because of the numerous types of materials used in nuclear facilities, galvanic corrosion is a major concern.

EO 1.13 DEFINE the following terms:

a. Crud b. Scale c. Galvanic corrosion

EO 1.14 IDENTIFY the five problems associated with the presence or release of crud into reactor coolant.

EO 1.15 STATE the four causes of crud bursts.

EO 1.16 STATE the two conditions that can cause galvanic corrosion.

EO 1.17 EXPLAIN the mechanism for galvanic corrosion.

EO 1.18 IDENTIFY the two locations that are susceptible to galvanic corrosion.

EO 1.19 STATE the five control measures used to minimize galvanic corrosion.

Crud

In addition to the corrosion film, corrosion products in the form of finely divided, insoluble oxide particles called crud become suspended in the reactor coolant or loosely adhere to metal surfaces. Crud has several undesirable characteristics. It can be transported throughout the reactor coolant system. As a result, it can accumulate and foul heat-transfer surfaces or clog flow passages. The most undesirable characteristic of crud, however, is that it becomes activated when exposed to radiation. Because crud can be transported throughout the reactor coolant system, it can collect outside the reactor core, causing radiation hot spots that increase ambient radiation levels. Hot spots caused by collections of crud may occur at the entrance to the purification heat exchanger and other areas of low flow velocity. Crud that is loosely adhered to metal surfaces can suddenly become suspended in the reactor coolant.

The crud release can result from an increased oxygen concentration, a reduced (or significantly changed) pH, a large temperature change (heatup or cooldown), or a physical shock to the system. Physical shocks include starting, stopping, or changing pump speeds, or other evolutions like a reactor scram or a relief valve lift. The result is a sudden increase in reactor coolant activity. The release of crud in this fashion is termed a crud burst. Crud bursts often lead to the removal of protective corrosion films and make the freshly exposed metal more susceptible to additional corrosion. In addition to the corrosion film and crud, some of the corrosion products are soluble and are easily transported throughout the system.

High crud concentrations in the system can also complicate disposal of primary coolant. Many of the corrosion products have relatively long half-lives and represent significant biological hazards. If, therefore, primary coolant is drained or leaks from the plant shortly after a crud burst, additional procedures may need to be utilized to minimize the effects of this condition.

Therefore, if the conditions mentioned previously (O₂, pH) are changed, the solubility of these corrosion products will change, and they can then be transported to and deposited anywhere in the reactor coolant system.

Another corrosion byproduct is scale, which is made up of deposits on surfaces from the formation of insoluble compounds from normally soluble salts. Most common are calcium or magnesium carbonates (CaC0₃ or MgC0₃).

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Galvanic Corrosion

Galvanic corrosion is the corrosion that results when two dissimilar metals with different potentials are placed in electrical contact in an electrolyte.

Of all the different types of corrosion, galvanic corrosion corresponds most closely to the electrochemical cells described previously in this module because galvanic corrosion occurs when two electrochemically dissimilar metals are joined together (in electrical contact) in a conducting medium (electrolyte). It may also take place with one metal with heterogeneities (dissimilarities) (for example, impurity inclusions, grains of different sizes, difference in composition of grains, differences in mechanical stress); abnormal levels of pH; and high temperatures. A difference in electrical potential exists between the different metals and serves as the driving force for electrical current flow through the corrodant or electrolyte. This current results in corrosion of one of the metals. The larger the potential difference, the greater the probability of galvanic corrosion. Galvanic corrosion only causes deterioration of one of the metals. The less resistant, active metal becomes the anodic corrosion site. The stronger, more noble metal is cathodic and protected. If there were no electrical contact, the two metals would be uniformly attacked by the corrosive medium as if the other metal were absent. Two locations susceptible to galvanic corrosion is a piping transition from one metal to another and a sacrificial anode (such as zinc).

Figure 10 illustrates that galvanic corrosion occurs when two different metals are in contact and exposed to an electrolyte.