Author: PPD Team Date: 23/05/2025

 

In nuclear power generation, not all shutdowns are emergencies. Many are planned well in advance for regular refuelling, equipment maintenance, or regulatory inspections. These scheduled outages are an essential part of operating a safe and efficient nuclear power plant. But how long do they take? And how do the timelines compare across global reactors and Indian facilities, especially those using Pressurised Heavy Water Reactors (PHWRs) or Pressurised Water Reactors (PWRs)?

Understanding this process requires a detailed look at reactor operations, international benchmarks, and India’s specific approach to planned nuclear outages.

Planned outages: purpose and process

Planned outages are routine, structured, and executed with precision. They are usually scheduled every 12 to 24 months, depending on the reactor design, fuel type, and regulatory requirements. Unlike emergency scrams, which shut down reactors within seconds to prevent hazards, scheduled outages involve a gradual power reduction to ensure thermal and mechanical stability.

The process begins by transitioning the plant from full-power operation to a subcritical state, where the nuclear chain reaction has stopped, and the residual decay heat can be managed safely. Operators must carefully control temperature changes, depressurisation, and system isolation to ensure both worker safety and equipment longevity. This approach also allows time for diagnostics, component testing, and preparation for the maintenance phase.

How long does it take globally?

Globally, operators in countries such as the United States, France, and Canada follow broadly consistent steps during scheduled reactor shutdowns.

Reactor power is typically reduced gradually—often by 1% to 10% of thermal capacity per hour—to ensure stability across heat exchangers, piping systems, and turbine components. In certain scenarios, ramp-downs may occur faster, approximately 1% per minute, depending on grid requirements and plant design. This step generally brings the reactor down to about 15% of its nominal power output in approximately 90 minutes.

At this lower power level, the reactor may be held steady for one to two hours. This holding period allows for gradual cooling and system adjustments. Operators often perform turbine valve testing, switch feedwater systems, and begin preparing for disconnection from the grid.

The reactor is then made subcritical by inserting control rods or injecting neutron absorbers like boron or gadolinium. This phase typically takes 30 minutes or less. Once subcritical, shutdown cooling systems are activated to reduce the reactor’s temperature and pressure further. Over the next 6 to 8 hours, temperatures drop below 93°C, allowing systems to safely reach what is known as “cold shutdown.”

The final step before maintenance begins involves flushing coolant loops, draining non-essential systems, and tagging equipment for safe access. Radiation monitoring and shielding preparations are also done during this phase. Depending on the plant, this stage takes between 24 and 72 hours.

Overall, transitioning from full power to cold shutdown typically takes 12 to 24 hours. The duration of maintenance and refuelling outages varies globally, with recent data indicating an average of 24 days in 2021, down from 51 days in 2020.

Shutdown practices in India: PHWRs

India’s operating nuclear fleet is predominantly made up of Pressurised Heavy Water Reactors. These reactors are unique because they support on-power refuelling, which reduces the need for frequent shutdowns due to fuel depletion. However, PHWRs still require regular planned outages for pressure tube inspections, system maintenance, turbine overhauls, and regulatory verifications.

The Nuclear Power Corporation of India Limited (NPCIL) oversees these processes, following guidelines set by the Atomic Energy Regulatory Board (AERB). While shutdown times vary across reactor units and locations, AERB reports and NPCIL documents offer a general picture.

Power coast-down in Indian PHWRs is typically conducted over 8 to 12 hours, involving a gradual reduction in reactor power, often in steps of 5% to 10% per hour. Operators carefully monitor pressure and temperature while phasing out auxiliary systems like feedwater heaters and turbine-driven pumps. This controlled approach is important to avoid thermal stress on reactor components.

Once the reactor reaches about 10% to 20% of its thermal capacity, it is tripped using control rod insertion or neutron absorber injection. Subcriticality is achieved within 30 to 60 minutes, after which cooling systems are activated to lower the reactor temperature to ambient levels. This cooldown process generally takes another 6 to 8 hours.

The next 24 to 72 hours are used to isolate systems for maintenance. Equipment is depressurised and tagged to prevent accidental reactivation. Radiation levels are assessed, and ventilation systems are adjusted to limit exposure. Decontamination steps are also initiated to maintain safety during inspection.

The core maintenance window can last anywhere from 10 to 30 days, depending on the complexity of the work. For PHWRs, pressure tube inspections are critical due to material ageing and stress corrosion risks. Other key tasks include generator rewinds, pump replacements, and control system upgrades. Once all work is complete, the plant undergoes hydrostatic pressure tests and system checks. Restart typically takes one to two days, as power is increased gradually to ensure thermal expansion occurs safely.

Shutdown timelines for Indian PWRs: Kudankulam case

Kudankulam Nuclear Power Plant (KKNPP) uses Russian-designed VVER reactors, a type of Pressurised Water Reactor. These follow shutdown procedures more aligned with Western PWR standards.

At KKNPP, it generally takes 6 to 8 hours to reach the shutdown state. Full cooldown and maintenance preparation can take up to 48 hours. The refuelling and maintenance phase typically lasts around 25 to 30 days, with another 3 to 5 days for startup and grid synchronisation. Notably, in a 2023 planned outage, Unit 2’s shutdown period was reduced from the initially planned 75 days to 63 days.

Real-world examples confirm this timeline. During a planned outage in 2015, Unit 1 at KKNPP was taken offline for refuelling and turbine inspection. The reactor was shut down in June 2015 and restarted in January 2016, resulting in an outage duration of approximately seven months. The extended timeline was influenced by inspection results and operational considerations, as detailed in the Comptroller and Auditor General’s performance report.

Factors that influence shutdown duration

Several variables affect how long a planned outage takes. The reactor design plays a central role—PHWRs, PWRs, and BWRs (Boiling Water Reactors) each have different operational characteristics. Grid stability and load-following capacity also influence how quickly power can be reduced.

Fuel condition, turbine design, and the level of automation in plant systems can shorten or extend the process. In India, logistical challenges such as the availability of trained personnel, spare parts supply chains, and regulatory inspection backlogs often introduce delays.

Even the scope of the planned work matters. A minor systems check might require only a 10-day outage, while a full turbine overhaul or control system upgrade can push the timeline beyond a month.

Emergency vs scheduled shutdowns: key differences

In emergency scenarios, Indian reactors can trip and reach subcritical conditions in seconds using automated safety systems. However, these rapid transitions impose thermal and mechanical stress on reactor vessels, turbines, and support systems. Scheduled outages, by contrast, are deliberately slower to minimise long-term wear and risk.

Controlled shutdowns are essential to protect both workers and equipment. For example, certain safety checks, like steam generator valve tests, can only be done within specific power ranges. Skipping these steps would not only be unsafe but would also violate regulatory and operational protocols.

Conclusion

Scheduled nuclear reactor outages are complex but well-orchestrated processes. In India, PHWRs benefit from on-power refuelling, but still require planned shutdowns for critical inspections and maintenance. The typical shutdown sequence for a PHWR involves 6 to 12 hours to reach cold conditions, followed by up to three days for system isolation, and several weeks for inspection and servicing.

For PWRs such as Kudankulam, the timeline aligns with global benchmarks—usually a 6 to 8 hour shutdown followed by up to 30 days of maintenance. Final ramp-up to full power is slow and controlled to protect equipment and ensure safety.

These outages demonstrate a high level of engineering discipline and operational planning. They reflect India’s commitment to maintaining nuclear safety and long-term reliability in its growing energy mix. 

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