Concrete attack

Concrete Attacked by hydrochloric acid, but to a lesser extent by sulphuric or organic acids... 

As discussed in Chapter 17 under portland cement coating, when water is added to portland cement a chemical reaction takes place during the hardening. This reaction produces calcium hydroxide and tricalcium silicate hydrate. The alkalinity of concrete is provided by the presence of calcium oxide from the cement. Consequently, concrete attack can be due to chemicals that react with the Portland cement binder and form conditions that physically deteriorate the material. Any material that will cause the calcium oxide or hydroxide to be removed, lowering the pH of the cement mix, will cause instability and solution of the cement hydrates. 

The application of coatings and sealers provides resistance to concrete attack by minimizing the ingress of corrosive species into concrete and increasing the durability of the concrete. They belong to several types depending on applications. The sealers may be applied as lining for the pores which make concrete water repellent. Silicone resins have been successfiilly used. They deposit... 

In addition to debris bed dryout, there is a second possibility for non-coolable core debris. If a molten pool is contacted by an overlying water pool, a crust may form, preventing the further contact of water with the melt. In this case, core-concrete attack may continue unabated, as discussed in Section 4.4. 

One potential benefit of an ex-vessel steam explosion is that the core debris may be dispersed in the containment, reducing the concerns of core-concrete attack, and possibly making the debris more coolable. On the other hand, the benefit of such an event depends on exactly where the debris ends up and the continuing availability of long-term containment heat removal. 

The most obvious concern about CCIs is the compromising of the containment structure. In addition to basemat meltthrough, CCIs can lead to failure of vessel supports and other local structures that can indirectly lead to containment failure. The ensuing discussions of concrete attack are intended to include all of these possibilities. 

As noted in Section 4.3, the presence of an overlying water pool does not guarantee that the debris will be coolable. A crust may form over the melt or the boiling rate may simply not be sufficient to remove the decay heat. However, it is possible that water will have some beneficial effect and at least slow down the concrete attack. 

As concrete attack progresses, concrete begins to fail (lose its structural integrity) even before gross melting of its constituents occurs. The loss of structural integrity accompanies the release of water and carbon dioxide from the concrete in three phases ... 

Figures 4.4-2 and 4.4-3 show examples of calculations of concrete attack." The contours in Figure 4.4-3 represent the movement of the ablation front downward and radially outward with time (one hour per contour). An important aspect of basemat meltthrough is that, even if it occurs, one would expect that many hours would be available to initiate emergency response plans, including evacuation and sheltering, so that offsite health effects can be minimized.

Figure 4.4-2 Calculation of concrete attack in a BWR Mark II containment during a station blackout sequence...

Depending on the timing and location of containment failure, the suppression pool may also be effective in scrubbing the release occurring during core-concrete attack or reevolved from the reactor coolant system after vessel failure. In the NUREG-1150 analyses for Peach Bottom (Mark I containment), containment failure was found to be likely to occur in the drywell early in the accident. Thus, in many scenarios the suppression pool was not effective in mitigating the delayed release of radioactive material. 

Severe fuel damage starts soon after blowdown, and the vessel penetration and corium concrete interaction starts in about 2h 10 after the initial event. Containment fails late, in about five hours, by overpressurization produced by the noncondensible combustible gases released from the corium concrete attack. 

The probability of forming a coolable debris bed and avoiding core-concrete attack is also higher with water on the drywell floor. The NUREG-1150 values for cases with a replenishable water supply were used to represent the case with a flooded drywell floor. 

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