Source term mitigation for severe nuclear accidents
Following a nuclear accident, it may be necessary to proceed to controlled release of radioactive air from the reactor to prevent damage of the containment building and so large and uncontrolled radioactive release. Many countries had already installed filtered containment venting systems (FCVS) systems prior to the 2011 Fukushima accident and most of the remainder followed suit after the accident. The EU-funded PASSAM (Passive and active systems on severe accident source term mitigation) project investigated suitable air-cleaning technologies. The work focused on FCVSs. The team investigated existing systems, plus novel technologies including acoustic agglomerators, high-pressure sprays, electrostatic precipitators, advanced zeolites and combined wet-dry filtration systems. Researchers tested the various systems under conditions realistically simulating severe nuclear accidents. The work was intended to improve mitigation, and to demonstrate the new systems’ abilities to achieve greater attenuation. PASSAM devoted the greatest study to pool scrubbing. Researchers showed that in some circumstances the reality of gas hydrodynamics is substantially different from what is described in accident analysis codes. In addition, they demonstrated the necessity of maintaining alkaline conditions in the scrubber, to prevent delayed release of iodine. The team also checked the filtration efficiency of sand bed filters and metallic pre-filters, regarding gaseous molecular and organic iodine. Other experiments demonstrated that caesium iodide aerosols trapped in the sand filter are unstable, and may constitute a delayed source term. Such was not the case for the same particles trapped in a metallic pre-filter. Studies into acoustic agglomeration and high-pressure spray systems showed an increase in particle size upstream of the FCVS, thereby enhancing filtration efficiency. The team also noted a decrease in aerosol mass concentration. Although an increase in particle size could not be measured for the high-pressure spray, the system showed acceptable efficiency that permitted reduction of airborne particle concentration. Tests of trapping gaseous molecular and organic iodine using wet electrostatic precipitators (WESPs) confirmed the importance of optimising WESP design and operating procedures. Researchers also confirmed the trapping efficiency of zeolites, and demonstrated good retention of gaseous organic iodides given a combination of wet scrubber and zeolite filtration. PASSAM yielded a database that will prove valuable for implementing or enhancing mitigation systems for nuclear reactors.