Mitigation of At-power accidents

Radionuclide release from the reactor containment to the environment during accidents initiated while the reactor is at power and the containment is initially intact follow three paths  

Plants with a double containment are offered some additional ways to cope with containment releases. With an intact containment, the space between both containments may be held subatmospheric and the leakages collected and filtered before release. The ventilation system used for that purpose usually has a large design margin and can therefore process a primary containment leakage rate far in excess of the normal value (if AC power is available). 

Some plants, equipped with a steel primary containment, are also contemplating or have installed a spray system in the space between the two containments (e.g. at Loviisa [C-8]). The main purpose of such a system is to cool the primary contaimnent in case all other means are lost, but of course it will have a scrubbing effect on the accidental releases. 

Typical mechanisms for aerosol removal from gas streams by filters are diffusion to surfaces, interception and impaction. Very large particles can be removed by gravitational settling. These mechanisms are quite dependent on the particle size and it is usually found that conventional filters have a minimum in filter efficiency for particles in a narrow size range less than 1 im. When the gas is hot relative to the filter, thermophoresis can enhance particle removal. When the aerosol laden gas stream contains elevated concentrations of steam that condenses within the filter, difflisiophoresis will enhance particle removal. These phoretic enhancements of filtration are attractive because filtration efficiencies by these mechanisms are not especially dependent on the aerosol particle size. Washed Venturi scmbbers involve the injection of water droplets into the aerosol laden gas and these water droplets act much like spray water droplets to remove aerosol particles. Electrostatic precipitation is, in principle, a very attractive decontamination process, but it is difficult to assure that the necessary power will be available to operate the precipitators under accident conditions. 

If the leak is trough a containment penetration, the penetration should be isolated (e.g. by closing a valve in the leakage path). If this is not sufficient or not possible, the containment spray system should be started (if not yet done) in order to (1) reduce the airborne fission products inventory in the containment, and (2) reduce the driving force for the leak. 

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