"Module 5"Please note this article was first published in 2002, circumstances and events may have changed opinions. Jeff Charlton
Recovery from a Terrorist CBR Attack London Underground
5.01 IntroductionThis module looks at the specific threat from biological contamination only, emanating from a terrorist event where platform and public access areas have been intentionally contaminated. The module excludes trackside decontamination which is found in module 4. Public or passenger decontamination is also excluded.
Overview The decontamination will be undertaken following total evacuation and isolation of the affected areas and local containment or removal of source materials where possible. See modules 2 and 12.
5.02 Facility problemsThe natural ventilation and difficult access of the stations, especially section 12 deep level areas, pose various problems regarding decontamination. This includes the large areas involved together with the need for efficient and safe works. Typical problems include;
- Limited access from the street
- Limited electric outlets to public areas in terms of power supply
- Natural ventilation causing spread thru ought the network
- Extensive Ceiling voids
- Substantial pre existing dirt and debris reservoirs in non visible areas
- Initial Facility solutions
- Utilise the natural but controlled ventilation to prevent leakage
- Utilise the substantial line power supply network
5.03 Hypothesis for clearanceIt must be assumed in the absence of proof to the contrary, that contamination would have spread via environmental and natural ventilation systems. Clearance will depend on substantial clearance protocol to be established.
There is therefore a reasonable requirement to either decontaminate or isolate all potentially affected areas. All decontamination should follow a protocol which results in compliance of clearance protocol. In any event, disturbed air sampling is essential. The decontamination protocol must therefore be designed to fulfil HSE clearance hypothesis which at present is unknown.
5.04 Specific Risk and Hazard assessmentThe hazard already established or considered will be seen as airborne. Therefore the air as a transport medium requires scrubbing to remove free floating contaminates.
5.05 Initial ResponseThe affected station must where possible be contained to prevent contamination from the initial site travelling to secondary locations. This can be achieved by the erection of barriers to the entry and exit tunnel points. Inflatable balloons can quickly be installed with the aid of compressed air from either tanks or compressor. To reduce all potential exposure to the decon team a knockdown procedure will be followed to reduce or destroy all surface and accessible contaminate.
This takes the form of a thermal fog introduced by pulse jet equipment. The fog containing a compound of both wetting agents and hydrogen peroxide will be utilised. The hot fog will continuously expand to fill all voids and cool to condense where dwell and surface contact will be improved.
5.06 Chemical EfficacyWhile many toxic biocides are available most will create problems regarding residue or have intrinsic application problems such as temperature, humidity, or deactivation by organic material, (dirt). Equally extended dwell periods may be required to penetrate and denature contaminates. The procedures and materials provided in modules 12 and 13 take these issues into account.
5.07 Primary ControlsDue to the constant and continual air flow found in underground systems there is a requirement to prevent secondary contamination from occurring. This may be from the push, pull effects of trains within tunnel complex or from the venturi action of ventilation shafts. Substantial Negative air pressure with Heppa filtration coupled to sealing off flow routes is recommended. See temporary shrink wrap.
5.08 Secondary ControlsThere is a need to move and collect all loose and air transportable contaminated particulates. Laminar air movement is seen as the most effective solution to this, coupled to substantial collection and filtration devices.
5.09 Vacuum extractionThree methods of collection are envisaged.
- The use of large Heppa air scrubbers located at the lowest point of the affected area. Power will be provided from the line network. Equipment delivered by train. (module 12)
- A specially constructed train carriage module as described in module12. Note: Train specific equipment must be engineered as a proactive measure and made available on any line or subsequent equipment manufactured for use on other lines where line transfer is impossible. This equipment is modified from existing rolling stock.
- Laminar air movement
Air will be forced from the uppermost accessible area to flow in laminar streams to be collected by the vacuum extraction. Turbulent air streams will be introduced to void or dead spots. (See module 12.) The air movement will be created either by water powered fans utilising water hydrant power, or electric fans installed at the station entrance and powered by mobile generators.
5.10 DecontaminationSuccessful decontamination of biological agents depends on contact of a suitable agent or source removal. (cleaning). Three types of surfaces are considered for decontamination;
In any event cleaning and source removal is always the most effective and sometimes the only method of decontamination.
- 1. Hard non porous
- 2. Porous
- 3. Entrapped and soft finishes
- Hard non porous include painted surfaces, laminates, glazed tiles, metallic finishes.
- Stone, brickwork, concrete.
- Fabrics, dirty surfaces, carpets etc.
Clearly cleaning to such a large complex coupled to likely time constraints may make this impracticable. The application of either wet biocidal spray’s, thermal or wet fogging techniques, coupled to foaming agents with prolonged dwell and contact time can however be considered. The relatively clean condition of most visible surfaces may facilitate this operation.
Due to extensive pores and possible reservoirs these materials are best treated with foaming agents to prevent run off and allow adequate dwell periods.
Soft and permeable materials should be considered for controlled waste disposal due to difficulty in decontamination and low values.
5.11 Void areasWhere access is difficult or impossible, a temporary isolation, encapsulation or containment should be considered. This should take the form of an elastomeric film or encapsulate as described in (section 13) or physical barriers. Negative air pressure coupled to Heppa filtration are also considerations. This will enable the prompt clearance of contaminated areas but will require further and future decontamination under controlled conditions within an acceptable time frame.
5.12 ConclusionThe requirement for urgent decontamination and public confidence in the safe re-commencement of service may depend on temporary measures to facilitate the reopening of stations. This may become a major concern where multiple sites are targeted or spread is confirmed. The limitations and availability of suitably trained and equipped decontamination teams may dictate the need for temporary solutions too.