Supporting Nuclear Safety: How Zircoa's Refractory System Enabled a Nuclear Power Plant Life Extension

The Challenge: Meeting IAEA Standards for Severe Accident Protection

The nuclear power plant operator was pursuing a 20-year operating license renewal for one of their generating facilities as part of their commitment to long-term plant safety and modernization.

Meeting the International Atomic Energy Agency (IAEA) Safety Aspects of Long-Term Operation standards required the operator to enhance protective structures throughout the facility. One critical requirement involved creating a sacrificial barrier system in the reactor sump — a containment area designed to capture and contain debris in the event of a severe accident.

In the unlikely event of a core meltdown, the reactor would produce corium: an extremely hot mixture of molten nuclear fuel, fission products, control rods, and structural materials from the reactor core. This molten material can reach temperatures between 2300°C and 2500°C and remains dangerously hot for extended periods. The operator needed a barrier system capable of protecting the reactor sump from this corium for up to 36 hours, allowing time for other critical safety protocols to be executed.

To be effective, the barrier system had to address a set of extraordinary anticipated design criteria, including the need to:

• Provide sustained thermal resistance against corium — routinely estimated at 2300-2500°C — to help delay melt progression for an extended period (targeted at up to 36 hours)
• Demonstrate high resilience and structural stability when subjected to extreme water quenching and high-pressure steam
• Achieve exceptionally low porosity to resist melt penetration and corium ingress
• Provide predictable, reliable performance as a single-use sacrificial structure

Without a solution that met these demands, the operator faced serious consequences: failure to meet IAEA safety standards, inability to obtain operating license renewal, potential catastrophic damage to critical mechanical equipment, and significantly reduced safety margins for the facility.

The Solution: Engineered Zirconia Refractory System with Validated Performance

Zircoa began working with the operator’s engineering team in early technical discussions to understand the exposure scenarios and performance expectations for the barrier system. The collaborative process involved evaluating suitable zirconia compositions, considering various brick geometries including layered and interlocking designs, and reviewing critical properties like porosity, density, and thermal-mechanical behavior.

Based on these discussions, Zircoa recommended a refractory system based on magnesia partially stabilized zirconia (MgO-PSZ). The solution consisted of custom-geometry refractory bricks in Composition 3004 paired with a compatible zirconia mortar in Composition 1640.

This refractory system offered several critical advantages:

• Exceptional resistance to molten metallic and oxidic materials – MgO-PSZ composition provides superior chemical inertness and erosion resistance against both metallic and oxidic phases of corium
• High mechanical integrity at extreme temperatures – Maintains structural stability even under exposure to 2300-2500°C conditions
• Predictable performance after water or steam exposure – Reliable behavior across the range of environmental conditions in a reactor containment scenario
• Reduced risk of system failure under severe conditions – Custom geometries and engineered mortar joints prevent melt penetration or corium ingress

Before committing to full-scale supply, the operator required thorough validation. In 2023, Zircoa supplied sample bricks and mortar for comprehensive testing. The operator’s evaluation process examined:

• Thermal stability under extreme temperature conditions
• Ablation and melt resistance when exposed to corium-simulating materials
• Mechanical strength under load, or Refractoriness Under Load (RUL)
• Behavior and thermal shock resistance during water quenching
• Mortar compatibility and joint performance

The 2023 testing program validated the material performance across all critical criteria. The samples demonstrated the thermal stability, melt resistance, and mechanical properties required for this mission-critical application. Based on these successful test results, the operator approved Zircoa’s refractory system for full-scale supply.