Chemical Technology October 2016

WASTE MANAGEMENT

Table 4: A selection of treatment methods for gaseous and airborne waste. Information and data on the components and elements of air cleaning and gas processing systems are the subject • Building supply and extract systems. The principle species to be treated in the THORP off-gas system are 129I, -C, NOx, fuel dust particles and aerosols containing plutonium and/or mixed fission products. THORP is designed on the principle of cascading depressions between areas to provide barriers against the spread of contamination. The main ventilation streams are kept separate until they enter the 125 m stack from which they are discharged into the atmosphere. The prime task of the dissolver off-gas (DOG) system is to remove nitrogen oxides (NOx) generated by the dissolution of the UO 2 fuel, together with the major volatile radioac- tive species released as the fuel is dissolved. The off-gas streams from different parts of the plant or from different types of equipment are combined into a series of “headers”, which feed into the COG system at an appropriate point ac- cording to the type of decontamination required. Overview of technology options Table 4 gives a selection of treatment methods for gaseous and airborne waste [29].

Treatment Method

Secondary Waste

Features

Limitations

Glass Fibre filter media, high efficiency 99,97%, widespread use, retention of sub-micron particles 0.3µ Chemically impregnated charcoal or zeolites to remove inorganic and organic iodine in reactors. Kr in offgases adsorbed on solid sorbent like charcoal. Operates at elevated pressure and reduced temperature. Kr can be recovered and sorbent used multiple times. Used for decay of short lived noble gases Scrubbing solution targets compounds and particulate matter. Used for process offgas treatment. Can be as simple as water or reagents targeting specific compounds

HEPA High efficiency particulate filtration

Humidity control and prefilters required to protect HEPA filters

HEPA and prefilters

Humidity control and charcoal has limited operating temperature. High cost.

Sorption

Spent media

• Cyclones; • Electrostatic precipitators; • Recombiners (H 2 -O 2

Further processing for storage is required. Commercial experience limited.

Spent sorption media

Cryogenic Trapping

Acknowledgement This article is based on extracts taken from IAEA TECDOC No. 1744, IAEA, Vienna (2014): International Atomic Energy Agency, “Treatment of Radioactive Gaseous Waste” and is published here with the kind permission of the IAEA. For the complete publication, please see http://www-pub.iaea.org/books/IAEABooks/10741/ Treatment-of-Radioactive-Gaseous-Waste. © IAEA ); • Other considerations (fans, stacks, etc); • System testing; • New technologies. References References for this article are available from the editor, Glynnis Koch, at chemtech@crown.co.za ) and (NO x -NH 3

Large beds for retention time required

Delay / Decay

None

Not practical for high volume gaseous stream treatment

Liquid waste streams

Wet Scrubbing

of a separate report. In addition to that the report covers: • Fibrous filters, medium and high efficiency; • Granular bed and sand filters; • Iodine adsorbents;

• Modular iodine adsorbers; • Monolithic iodine adsorbers; • Mist eliminators, coalescers, etc.; • Scrubbers and condensers;

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Chemical Technology • October 2016