5.1 Scene Size-Up & Hazard Risk Assessment

5.1 Scene Size-Up & Hazard Risk Assessment

Scene size-up is the continuous, dynamic process of gathering, analyzing, and evaluating information to make sound, risk-based tactical decisions at a hazardous materials emergency. Unlike structural firefighting where size-up focuses primarily on building construction, fire location, and fuel load, hazardous materials size-up requires evaluating complex interactions between chemical hazards, container integrity, environmental conditions, and potential human exposures.

To conduct a rigorous hazard risk assessment, Operations-level responders utilize structured decision-making models, most notably Ludwig Benner's General Emergency Behavior Model (GEBM) and the DECIDE model (Detect, Estimate, Choose, Identify, Do, Evaluate). Understanding how containers fail, how hazardous substances disperse, and how to quantify risk is vital for preventing responder casualties and mitigating emergency situations.

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Ludwig Benner's General Emergency Behavior Model (GEBM)

Benner's General Emergency Behavior Model (GEBM) provides a logical, predictive framework explaining how hazardous materials incidents progress. GEBM establishes that every hazardous materials release follows a predictable, six-step sequential chain of events:

$\text{Stress} \longrightarrow \text{Breach} \longrightarrow \text{Release} \longrightarrow \text{Dispersion/Engulfment} \longrightarrow \text{Exposure/Contact} \longrightarrow \text{Harm}$

If emergency responders can successfully intervene and break any single link in this sequential chain, the incident can be stabilized, preventing or significantly minimizing harm to responders, the public, and the environment.

Step 1: Container Stress

External or internal energy applied to a container alters its physical state and structural integrity. Container stress occurs in three primary forms:

1. Thermal Stress: Radiant, conductive, or convective heat applied to a container (e.g., direct flame impingement on a liquefied petroleum gas [LPG] tank car, intense sunlight expanding liquid contents, or extreme cryogenic cold embrittling carbon steel). Thermal stress increases internal vapor pressure while simultaneously weakening the structural tensile strength of container walls.
2. Chemical Stress: Internal or external chemical reactions that degrade container walls, gaskets, or relief valves. Examples include corrosive acid eating through a 55-gallon steel drum, runaway polymerization generating exothermic heat and internal pressure, or incompatible chemical mixing producing violent internal degradation.
3. Mechanical Stress: Physical force applied to a container from impact, gouging, scoring, dropped loads, or internal overpressurization. Mechanical stress creates structural deformities, micro-fractures, or structural tears that reduce the container's pressure rating.

Step 2: Container Breach

When total applied stress exceeds the container's structural design limits, a breach occurs. GEBM classifies container breaches into five distinct physical types:

• Disintegration: Total catastrophic failure of the container wall into multiple flying fragments (shrapnel), typical of Boiling Liquid Expanding Vapor Explosions (BLEVEs) or explosive detonations.
• Runaway Cracking: A crack that propagates rapidly through the vessel wall under tension, instantly splitting open the entire length of the tank without fragmenting into multiple small pieces.
• Closures Opening / Failing: Failure or accidental displacement of pressure relief devices, valves, manways, gaskets, or drain plugs under excess internal hydraulic or vapor pressure.
• Punctures: Physical penetration of the container wall by an external object (e.g., forklift tines penetrating a drum wall or derailment debris piercing a tank car shell).
• Split or Tear: Longitudinal failure along welded seams or stress points caused by mechanical impact, roll-over stress, or internal hydraulic pressure.

Step 3: Release Mechanics

Once breached, container contents escape into the surrounding environment. The velocity, volume, and physical state of release depend on internal energy levels and breach size:

• Detonation: Instantaneous release of chemical energy (occurring in less than 1 microsecond) accompanied by supersonic blast waves.
• Violent Rupture: High-speed release of pressurized liquid or gas (e.g., BLEVE), producing high-velocity shrapnel and rapid vapor cloud expansion.
• Rapid Relief: Controlled or uncontrolled release through pressure relief valves or medium-sized openings over seconds or minutes.
• Spill or Leak: Low-velocity release of liquid or gas through small cracks, open valves, gaskets, or punctures over an extended timeframe.

Step 4: Dispersion & Engulfment Patterns

Released hazardous materials disperse into surrounding geometry, creating distinct physical shapes based on vapor density, physical state, wind speed, atmospheric stability, and terrain topography:

• Hemispheric: Spherical cloud expanding outward and upward in all directions from an unconfined high-pressure gas release.
• Cloud: Airborne mass of gas or vapor moving with prevailing wind patterns while staying suspended above ground level.
• Plume: Elongated, continuous stream of vapor or gas moving downwind from a continuous release source.
• Cone: Wide-spreading triangular dispersion pattern typical of pressurized liquid or gas releases from a point source.
• Stream: Liquid flow following terrain contours, drainage ditches, storm drains, or rivers.
• Pool: Accumulation of liquid in low-lying depressions or flat uncontained surfaces.
• Irregular: Unpredictable dispersion pattern caused by turbulent indoor HVAC ventilation or complex structural geometry.

Step 5: Exposure & Contact

Target populations, environmental receptors, or physical structures coming into direct contact with the dispersed hazardous material. Exposures are categorized into:

• People: Emergency responders, facility workers, and the general public situated downwind or down-gradient.
• Environment: Waterways, soil, air basins, wildlife habitats, and agricultural lands.
• Property: Surrounding buildings, critical infrastructure, transportation networks, and industrial equipment.

Step 6: Harm (TRACEM-N Hazards)

The resulting physical, biological, or chemical damage inflicted upon exposures. Types of harm are categorized using the standard TRACEM-N acronym:

• Thermal: Burns from intense heat, fireballs, or cryogenic frostbite.
• Radiological: Alpha, beta, gamma, or neutron radiation exposure.
• Asphyxiation: Displacement of atmospheric oxygen (simple asphyxiants like nitrogen) or disruption of cellular oxygen transport (chemical asphyxiants like cyanides and carbon monoxide).
• Chemical: Systemic toxicity, organ damage, or skin burns from toxic, hazardous substances.
• Etiologic / Biological: Disease or infection from biological pathogens, toxins, or medical waste.
• Mechanical: Physical trauma from blast pressure waves, flying shrapnel, or structural collapse.
• Corrosive: Tissue destruction and chemical destruction from strong acids or bases.

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Hazard Risk Assessment & Tactical Prioritization

During scene size-up, the Incident Commander and Operations-level responders perform a Risk-versus-Benefit Analysis guided by standard fire service risk principles:

• Risk A Lot to Save Savable Lives: Responders will accept calculated, high-risk tactical interventions (e.g., hot-zone rescue) ONLY when there is a realistic probability of saving savable human lives.
• Risk A Little to Save Property: Responders will accept limited, low-risk defensive tactics to protect environment or high-value property.
• Risk Nothing for Lives or Property Already Lost: Responders will NOT risk responder safety or enter contaminated environments if lives are already lost, containers are beyond control, or property is unrecoverable.

Strategic Mode Selection

1. Offensive Mode: Direct entry into the Hot Zone to control the leak, plug punctures, close valves, or perform rescue. Requires specialized Hazmat Technicians and full Chemical Protective Clothing (CPC).
2. Defensive Mode: Operations conducted from the outside perimeters (Warm/Cold zones) to isolate the area, dike spill pathways, apply vapor-suppressing foam, or evacuate downwind populations. Standard mode for Operations-level responders.
3. Non-Intervention Mode: Allowing the incident to run its natural course without active responder entry when risk assessment indicates entry would result in catastrophic responder casualties (e.g., massive pressure tank fire with imminent BLEVE threat).

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Summary of Container Stress, Breach, and Release Relationships