1. Introduction

On October 17, 2025, a fire broke out at SK Energy’s hydrogen manufacturing plant in Ulsan, South Korea, injuring five workers—four of them seriously—after a pipe explosion during scheduled maintenance. The blaze erupted in the hydrogen production unit of the Fuel Cracking Complex in Yongyeon-dong, where residual hydrogen ignited during line-opening operations.

Although the fire was extinguished within 20 minutes and other hydrogen lines remained operational, the incident once again highlighted a persistent challenge within the hydrogen economy: ensuring safety in high-pressure hydrogen systems, particularly during shutdown and maintenance phases.

As global hydrogen production expands to support decarbonization goals, the lessons from Ulsan underscore the urgency of embedding advanced monitoring technologies directly into process infrastructure.

2. The Hydrogen Safety Paradox

Hydrogen is one of the cleanest and most promising energy carriers, but it is also unforgiving when mishandled. With an ignition energy threshold of only 0.02 mJ and a flammability range spanning 4 to 74 percent in air (and up to 94 percent in oxygen), hydrogen is among the most volatile gases in industrial use.

Its physical properties—extremely low molecular weight, high diffusivity, and an invisible flame—make it exceptionally difficult to detect and contain. In Ulsan, as in several previous accidents, even trace residual hydrogen inside maintenance piping proved enough to trigger an explosion.

The paradox is clear: hydrogen’s potential as a clean energy source is matched by its demand for the highest level of engineering discipline. The fuel of the future requires safety systems that are as advanced as its technology.

3. Historical Lessons: A Pattern of Recurring Causes

The Ulsan explosion joins a growing list of hydrogen-related incidents that share similar root causes—insufficient purging, oxygen ingress, or static discharge.

• Gangneung, South Korea (2019): An electrolyzer membrane malfunctioned during low-load operation, allowing oxygen crossover into a hydrogen buffer tank, causing an explosion.
• Santa Clara, USA (2019): A hydrogen tube-trailer explosion occurred during refilling due to a leaking valve and miscommunication between operators.
• Muskingum River Power Plant, USA (2007): A hydrogen vent-stack failure during routine transfer caused gas accumulation and ignition inside a confined structure.

Each of these incidents reinforces the same lesson: hydrogen safety depends on continuous monitoring, effective purging, and strict adherence to functional-safety standards.

4. Safety Standards: Engineering Out the Risk

Global frameworks such as ISO 22734, NFPA 2, IEC 60079, and IEC 61511/61508 provide robust guidelines for safe hydrogen system design and operation. Their central principles include:

• Avoidance of Flammable Mixtures: Systems must prevent the formation of hydrogen–oxygen mixtures through design provisions for crossover, membrane failure, or gas mixing.
• Continuous Gas Monitoring: Real-time detection of oxygen and hydrogen concentrations is mandatory for identifying leaks or hazardous buildup.
• Automatic Venting and Purging: Equipment must purge residual gases before maintenance or shutdown operations.
• Bonding and Earthing: All components must be grounded to prevent static discharge.
• Critical Failure Management: Safety instrumented functions (SIFs) should comply with SIL-2 or higher, ensuring fail-safe operation under defined performance criteria.

While compliance with these standards forms the foundation of safe operation, recent incidents demonstrate that regulation alone cannot eliminate risk. Effective hydrogen safety requires real-time, in-situ monitoring to detect and respond to hazards instantly.

5. The Analytical Challenge: Measuring Oxygen in Hydrogen

Accurate detection of oxygen within high-pressure hydrogen systems has long posed a technical challenge. Conventional methods—paramagnetic, zirconia, electrochemical, or tunable diode laser analyzers—each have limitations under real-world process conditions.

• Paramagnetic analyzers provide precision but require complex extractive sampling systems that introduce leak points.
• Zirconia sensors demand high temperatures and are unsuitable for combustible gas mixtures.
• Electrochemical cells are low-cost but degrade rapidly and need frequent calibration or replacement.
• Tunable diode laser (TDL) analyzers offer advanced capability but struggle with pressure effects and require depressurization of samples.

These shortcomings mean that traditional analyzers often introduce the very hazards they are meant to prevent: leaks, sampling delays, and maintenance risks—none of which are acceptable in modern hydrogen plants.

6. Optical Innovation: The MOD-1040 and the In-Situ Revolution

To address these limitations, Modcon Systems Ltd. developed the MOD-1040 Optical Oxygen Analyzer, a luminescence-based, non-extractive solution engineered specifically for hydrogen and refinery-gas environments.

Unlike conventional analyzers, the MOD-1040 operates directly in the process line, eliminating the need for sampling, conditioning, or pressure reduction. It uses fluorescence quenching of a proprietary dye immobilized on a support foil, illuminated by red light, and measured in the near-infrared range. The luminescence lifetime correlates precisely with oxygen partial pressure, enabling fast and accurate measurement—even at pressures exceeding 200 bar.

Key advantages:

• In-situ installation: No sample lines or potential leak paths; instantaneous real-time readings.
• Explosion-proof design: Certified to ATEX/IECEx Zone 1 for hazardous environments.
• Functional safety: SIL-2 certified in accordance with IEC 61508-2:2010.
• High stability and precision: Immune to temperature drift and gas-cross sensitivities.
• Minimal maintenance: No consumables or calibration gases required.

By integrating the MOD-1040 directly into the hydrogen process line, operators achieve uninterrupted oxygen surveillance—even during maintenance or transient states—precisely when most accidents occur.

7. System-Wide Benefits: Beyond Safety Compliance

The adoption of in-situ oxygen analyzers extends benefits far beyond compliance:

• Reduced hazardous-zone classification: Removing extractive sampling points may permit reclassification of some areas as general-purpose zones, reducing the need for explosion-proof instrumentation and specialized cabling.
• Lower project and operating costs: Simplified system design decreases capital expenditure and maintenance burden.
• Enhanced process optimization: Continuous data on oxygen concentration supports tighter control of purity and yields, enabling predictive maintenance and longer uptime.
• Increased reliability: With no moving parts or consumables, optical sensors provide exceptional long-term stability.

These improvements make in-situ analysis not just a safety enhancement but a business advantage—improving both asset reliability and profitability.

8. Lessons from Ulsan: Safety as a Foundation for Growth

The Ulsan fire is a sobering reminder that hydrogen’s potential cannot be separated from its risks. As production capacity scales globally, even small oversights during maintenance or startup can have catastrophic consequences for both safety and public confidence.

The path forward lies in engineering safety into every layer of hydrogen production—from plant design and automation to real-time gas monitoring. Technologies like the MOD-1040 exemplify this approach, merging innovation with compliance to create a system that continuously verifies safety conditions rather than assuming them.

When safety is built into the process, not bolted on, hydrogen becomes a truly sustainable fuel for the future.

9. Conclusion

Hydrogen’s promise as the cornerstone of the clean-energy transition will only be realized if safety evolves at the same pace as technology. The incident in Ulsan demonstrates that accidents often arise not from negligence, but from the inherent complexity of high-pressure hydrogen systems and the limitations of conventional monitoring tools.

Advanced in-situ analyzers such as the MOD-1040 Optical Oxygen Analyzer transform this landscape. They replace delayed sampling with continuous awareness, providing the real-time insight necessary to prevent flammable mixtures and protect both personnel and infrastructure.

In the green hydrogen production era, safety is not merely a regulatory requirement—it is the enabling technology that allows innovation, scalability, and public trust to coexist.