Safety central in nuclear re-start
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Experts have shown that restarting a nuclear reactor is not just a matter of flicking a switch.
The Australian Nuclear Science and Technology Organisation (ANSTO) has resumed operation of its OPAL Reactor following a six-month shutdown dedicated to essential maintenance and facility upgrades.
The authority says the methodical restart of the multi-purpose reactor was a showcase of its commitment to nuclear safety and precision in reactor operations.
Similar to the extensive checklist required before an aircraft takes off, the OPAL Reactor’s restart followed a detailed sequence, with operators closely monitoring conditions at each stage.
“You cannot just flick a switch to start a reactor,” said ANSTO engineer Simon Thomas.
“Detailed inspections and safety checks come first. Every part of the reactor is examined to make sure it’s in top condition.”
The process began with refuelling the reactor and engaging all support systems, which were thoroughly checked for reliability. Once these systems were in place, safety mechanisms were tested rigorously before the reactor was activated at a low-power setting.
Bringing the reactor to full operation was deliberately gradual.
Operators carefully increased power from 5 MW to the reactor’s full 20 MW capacity, ensuring stability at each incremental stage.
“Once we’re confident that everything is stable, operators slowly increase reactor power in stages,” Thomas explained.
Achieving criticality - when the reactor reaches a self-sustaining nuclear chain reaction - was a key milestone.
According to the experts, criticality indicates that the reactor is operating in a stable, controlled state rather than suggesting an imminent hazard.
The OPAL Reactor's control rods, made primarily from hafnium, play an essential role in maintaining this state by absorbing neutrons to manage reactivity, which is the degree of deviation from criticality.
“Think about control rods like the throttle and brake on a car,” Thomas said.
“They allow us to control the reactor’s power. In less than a second, the control rods can automatically drop to shut down the reactor completely if needed.”
In the main control room, the atmosphere reportedly remained calm during the reactor’s return to criticality, with only the occasional sound of operators moving control rods or verbally confirming system checks.
The team relied on real-time data monitoring, with the reactor control system tracking over 5,000 signals to oversee key parameters such as temperature, pressure, and flow rates.
Once the OPAL Reactor achieved stable criticality, operators observed a blue Cherenkov glow in the 13-metre-deep pool where the core is located.
This glow, visible when particles exceed the speed of light in water, is a standard phenomenon in nuclear operations and a visual sign of the reactor's activity.
The OPAL Reactor, which ANSTO uses for medical, scientific, and industrial purposes, is now operating with enhanced facilities, ready to serve a range of high-tech sectors.
