Containment mitigation measures

This section on protective measures discusses three elements (1) containment, (2) instrumentation and detection of a runaway, and (3) mitigation measures. For each element, examples are given to illustrate the principles discussed. This section is basically a summary of protective measures, not an exhaustive treatise. Protective measures are necessary considerations, and in fact, safety requirements, when handling reactive substances and exothermic reactions. 

Mitigation measures can also be passive safeguards, meaning that they require no human intervention and no engineered sensing and actuation system to work. Examples of passive mitigation measures are secondary containment systems, blast-resistant and fire-resistant structures, insulated or low-heat-capacity spill surfaces to reduce the rate of evaporation, and an increased distance between the hazardous materials and energies and the sensitive receptors. 

Flare stacks that safely bum organic material released into vent headers are an example of a prerelease mitigation measure. The flare destroys the hazardous organic material before it reaches the environment. A dike around a storage tank is an example of a postrelease mitigation measure. The dike contains the release in a small area, reducing the total evaporation rate from the spill and so reducing the impact of the release. 

Containment uses a physical barrier to prevent an uncontrolled release of materials to the environment. The walls of a vessel or pipe serve as the primary containment barrier that encloses harmful materials. Redundant (secondary) containment serves as a safeguard if the primary barrier fails, and is considered a postrelease mitigation measure. Containment can take many forms, depending on factors such as the system or process to be contained, the risks involved with a release, and the cost benefit of the additional secondary containment. 

As discussed in Section 3.1.2.1, a liquid that is uncontained is one over which there is no control and which will result in potentially severe consequences. If a dike is placed around the tank containing the refrigerated liquid ammonia and the ammonia spill is confined within it, a much reduced hazard zone can be obtained because we have limited the surface area available for vaporization and additional postrelease mitigation measures can be applied. As pointed out in Chapter 3, combinations of postrelease mitigation measures will provide the best overall response to an accidental release. 

The estimated impact is then compared to hazard acceptance criteria to determine whether the consequences are tolerable without additional loss prevention and mitigation measures. If the identified consequences are not tolerable, the next step is to estimate the ffequency/probability of occurrence of the identified failure modes leading to loss of containment. For simple cases, frequency estimates are combined with consequences to yield a qualitative estimate of risk. For complex cases, fault tree analysis is used to estimate the frequency of the event leading to the hazard. These estimates are then combined with the consequences to yield a measure of risk. The calculated risk level is compared to a risk acceptance criterion to determine if mitigation is required for further risk reduction. 

In the first phase, soon after TMI, mitigation measures against the certain consequences of a core melt (the slow over-pressurization of the containment up to its burst and the attack of the containment bottom by the molten core deposited there after reactor vessel perforation) were implemented. 

Containment and Severe Accident -Double Containment -Cavity Flooding System(CFS) -Hydrogen Mitigation System - Single Containment In-Vessel Retention - Replacement of Fusible Plug with MOV (Motor Operated Valve) Passive Auto-catalytic Recombiner -i- Igniter Accident mitigation Measure such as IVR adopted... 

During the initial screening of the asset, a variety of data can be collected. By way of example, known information on existing characteristics and features of a particular asset can be obtained. The asset can then be broken down into subsystems, and each subsystem can be analyzed to identify mechanisms that can potentially lead to the loss of asset performance in terms of capacity, injectivity, and containment (process). The mechanisms can then be evaluated and ranked, and appropriate mitigation measures can be determined for some or all of the mechanisms (risk mitigation measures). Uncertainties can also be identified and characterization needs and solutions can be prioritized. 

Level 2 PSA, which identifies ways in which radioactive releases from the plant can occur and estimates their magnitude and frequency. This analysis provides additional insights into the relative importance of accident prevention and mitigation measures such as the use of a reactor containment. 

The purpose of this question was to find out whether any additional iodine mitigation measures have been planned or already implemented other than using additives in the spray and containment sump water and using controlled containment venting with a filter. Other than these measures, if already implemented, there are no other mitigation measures reported by the participating organizations. 

Plant Name Passive iodine removal Iodine mitigation measures implemented/ planned Measurement of airborne iodine (gaseous and particulate form) activity in containment Measurement of controlled iodine (gaseous and particulate form) release into environment Other AM measures to mitigate airborne iodine (in gaseous/particulate form) activity in containment Comments... 

All of the above discussed radioactive contaminated media and waste could also contain non-radioactive environmental pollutants, e.g., boron and other water treatment chemicals in reactor coolant effluents. The mitigation measures taken to prevent release of radioactive pollutants into the environment will also mitigate the release of some non-radioactive pollutants. With regard to the identification of non-radioactive pollutants that could result in a major environmental accident, this section only considers the storage/use of substances that does not involve exposure to (and possible contamination with) radionuclides. 

---