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The information notice below was in 1999, but the problem remains with the topic showing up in the US NRC “DRAFT 10 CFR Part 21 Reporting of Defects and Noncompliance, Meeting with the Industry and Stakeholders March 16, 2016“, where they note that a HEPA filter failure may mean that radioactive discharges are less filtered than usual, leading to higher levels of exposure to radiation by the public and environment (20 times or more greater than allowed by the US EPA).
NRC INFORMATION NOTICE 99-01: DETERIORATION OF HIGH-EFFICIENCY PARTICULATE AIR FILTERS IN A PRESSURIZED WATER REACTOR CONTAINMENT FAN COOLER UNIT, p. 1
NRC INFORMATION NOTICE 99-01: DETERIORATION OF HIGH-EFFICIENCY PARTICULATE AIR FILTERS IN A PRESSURIZED WATER REACTOR CONTAINMENT FAN COOLER UNIT, p. 2
p. 3 NRC INFORMATION NOTICE 99-01: DETERIORATION OF HIGH-EFFICIENCY PARTICULATE AIR FILTERS IN A PRESSURIZED WATER REACTOR CONTAINMENT FAN COOLER UNIT, p. 3
http://www.nrc.gov/docs/ML0310/ML031040519.pdf
In March of 2016 the USNRC states: “High Efficiency Particulate Air (HEPA) filters and/or dust collection systems on airborne effluent stacks. The equipment failure of a HEPA or a dust collection system on a high-flow airborne effluent stack could result in a release of licensed material“. [i.e. more Radioactive Discharges than legally allowed] “If the equipment failure is unidentified due to an unrelated failure in monitoring, a release of radiological material contributing to a total effective dose equivalent exceeding 0.5 rem is possible. A defect that could create a potential failure might be improperly constructed air filters as supplied by the vendor, an air filter being too small for the encasing, equipment errors with the associated radiation detection equipment, etc.” “DRAFT 10 CFR Part 21 Reporting of Defects and Noncompliance,Meeting with the Industry and Stakeholders March 16, 2016http://www.nrc.gov/docs/ML1607/ML16070A266.pdf 0.5 rem is 500 mrem or 5 mSv, compared to the 0.25 mSv (25 mrem) per year allowed by the US EPA and 1 mSv per year maximum total from nuclear facilities “recommended” by international agencies (e.g. ICRP). Medical exposure adds to the exposure and risk.

There were at least two reports of HEPA filters for nuclear, which failed pressure tests, within the last few years: http://www.nrc.gov/docs/ML1307/ML13079A320.pdf http://www.nrc.gov/docs/ML1403/ML14034A190.pdf