Because of the high quantity of dust generated by various cutting/dismantling processes, frequent replacement of high-efficiency sub-micron particulate air filters is necessary. If such filters could be cleaned during service, costs for the replacement of the filters, radiation exposures and the amount of secondary waste could be reduced.

The effectiveness in long-term operation (approx. one year) of high-efficiency submicron particle air filters will be investigated in the framework of the dismantling of the Niederaichbach nuclear power station (KKN) in Germany.

A high-efficiency submicron particle air filter system will be exposed to heavy dust generation during the remote-controlled dismantling of KKN primary circuit pressure tubes, and therefore must be dedusted periodically. The dust is radioactively charged (essentially cobalt-60 and iron-55). The radioactivity could amount to approximately 1E5 Bq/g (pressure tubes and moderator tank) and the dose rate to 0.1 Sv/h. There is at present no experience on the effectiveness and the long-term behaviour of high-efficiency submicron particle air filters that are dedustable during operation.
Because of the high quantity of dust generated by various cutting/dismantling processes, frequent replacement of high efficiency submicron particulate air filters is necessary. If such filters could be cleaned during service, costs for the replacement of the filters, radiation exposures and the amount of secondary waste could be reduced. The effectiveness in long term operation of high efficiency submicron particle air filters is investigated in the framework of the dismantling of the Niederaichbach nuclear power station (KKN) in Germany.

A 2 stage filter system (HFA and ARFA), each equipped with high efficiency aerosol filters, was installed. ARFA is equipped with an automatic dust cleaning device. At the beginning, ARFA showed an average pressure difference of 150 Pa, which rose steadily to nearly 1000 Pa. The pressure differences registered averaged 1000 Pa. The exhaust air temperature remained at approximately 24 C throughout the entire work period, whereas the relative humidity of the exhaust air in the above period averaged at 33 per cent fluctuations up to 12 per cent may occur within 24 hours. Dismantling work has mainly been carried out with grinder, plasma are and milling. The first trial discharge of the filter boxes showed that about 60 kg of duxt had accumulated.
WORK PROGRAMME

1. Installation of the filters

2. Determination of the main parameters of the clean filter station

3. Continuous measurements (pressure pickups, air humidity and temperatures) during cutting of KKN primary cooling circuit (activated cooling channel tubes inside the reactor vessel)

4. Final evaluation including radiation exposure of workers, secondary waste arisings, specific costs, effectiveness and long-term behaviour of the filter system.