Studies on in-place test of high efficiency particulate air filters in nuclear fuel facilities sampling and mixing of test aerosol

In-place test of high efficiency particulate air (HEPA) filter systems in a nuclear facility is important for assuring that the filter system retains the expected overall collection efficiency. Axial mixing distances in different ducts were examined for sampling the representative aerosol at sufficiently mixed position in the upstream and downstream of the HEPA filter. Various duct diameters and air velocities were modeled corresponding to practically used conditions, and the effective mixing distances along the duct axis were determined for the modeled conditions. Main factors affecting on the effective mixing distance were regarded as air velocity, duct diameter, injection procedure of aerosol, and presence of orifice or bend. Effectiveness of multiple sampling with a multi-hole nozzle was also studied. Results show the followings: In the case of duct diameter 250 mm and air velocities 4 -- 16 m/sec (Reynolds number 6.6 x 104 -- 2.5 x 10), the effective mixing distance to reduce the deviation of aerosol concentration from the average to less than 10 % was 35 times the duct diameter (about 9 m). In other ducts of different diameters the distances were shorter than the above. In the case of injection of aerosol particles at a point adjacent to the duct wall the effective mixing distance was about three times that in the case of injection at the duct center, to obtain the same deviation of aersol concentration. In the case of the duct with a orifice or a bend the effective mixing distance was about one third that in the case of the duct without them. Sampling techniques with multi-hole nozzle were very useful for estimation of average concentration of aerosol. (author)