ABSTRACT
Small Modular Reactors (SMRs) present two different safety philosophies for emergency core
cooling: passive designs relying on natural circulation and gravity-driven actuation without
external power, and active designs employing motor-driven pumps, control logic, and electrical
power systems. While both approaches have undergone regulatory review in North America and
the United Kingdom, no published study has performed a systematic, quantitative comparison of
their reliability for equivalent initiating events through the lens of deployment feasibility in
emerging markets. This paper addresses that gap through event tree and fault tree analysis of
passive and active SMR emergency core cooling systems — illustrated by the NuScale
VOYGR™ reactor vent valve assembly and the BWRX-300 isolation condenser system,
respectively — responding to a common initiating event of loss of normal heat removal
(LONHR, frequency 10⁻² to 10⁻¹ per reactor-year). Using beta-factor common cause failure
modeling (β = 0.10 passive; β = 0.15 active) and NUREG/CR-2300 methodology, fault tree
quantification yields system failure probabilities of 2.3×10⁻⁴ (passive) and 2.55×10⁻⁴
(active) — a 9% difference that falls within PRA uncertainty bounds and is hence not the
primary finding. The critical result is qualitative: passive systems exhibit common cause failure
dominance (60% of total probability), concentrating risk in manufacturing quality assurance and
design uniformity across redundant trains, while active systems are dominated by a shared
physical heat sink vulnerability (48%) that bypasses train-level redundancy entirely. The
different ways these systems fail creates some pretty lopsided risks for newcomer regulatory
authorities, project financiers, and operational workforce planning — factors that govern SMR
deployment feasibility in resource-constrained markets is what really moves the needle, way
more than just marginal reliability differences.