The correct answer is B (50%) because in a closed or confined vessel , a much larger fraction of the released thermodynamic energy is converted into pressure (overpressure) compared to unconfined conditions.

CCPS explains that the degree of confinement and congestion strongly influences explosion severity. In an unconfined environment , only a small fraction of energy (typically 2–5%) contributes to overpressure because gases can expand freely. However, in a closed vessel , expansion is restricted, causing the energy released during combustion to be efficiently converted into pressure rise within the vessel .

This leads to significantly higher pressures, often approaching the adiabatic constant-volume explosion pressure , where a large portion of the energy contributes directly to overpressure. Values around 50% or higher are commonly used as engineering approximations for confined systems.

Options A (2%) applies to unconfined vapor cloud explosions, while C (17.5%) may apply to partially congested systems. Option D (0%) is incorrect since overpressure is the defining feature of an explosion.

CCPS highlights that confinement dramatically increases explosion severity , making proper design, relief systems, and hazard analysis critical for vessels handling flammable materials.