NEK Completes an Exceptional Operating Cycle and Enters the 2025 Planned Outage Nuclear Energy as the Foundation of a Stable Future

The Krško Nuclear Power Plant (NEK), after nearly a year and a half of uninterrupted operation with outstanding results, is concluding its 34th fuel cycle, during which it delivered more than 8.51 billion kilowatt-hours of domestic low-carbon electricity to the power grid. This surpassed the planned production and played an important role in ensuring a reliable supply of daily needs and supporting economic development.

Over 511 calendar days at full power, NEK achieved 100% availability and capacity, confirming its technical excellence, reliability, and careful management. This once again demonstrates its indispensable role in ensuring the energy independence of Slovenia and Croatia.

On Sunday, 28 September 2025, NEK will begin its planned outage, which is essential for ensuring safe, stable, and reliable plant operation in the new fuel cycle and in the long term until 2043. During this period, extensive maintenance work and system upgrades will be carried out, along with the carefully planned process of replacing part of the fuel after 18 months.

Achievements Between Outages

Since the last planned outage, which ended on 2 May 2024, NEK has operated continuously at full power, maintaining grid stability and reliability even during prolonged summer heatwaves. It is important to note that NEK’s technical capabilities enable it to serve as a stabilizing force in the grid, ensuring voltage and frequency stability even in the event of sudden fluctuations.

To adapt to climate change, NEK upgraded the lightning protection down-conductor and capture system on safety-related buildings and completed the hail protection project for these structures.

Hail Protection

In 2025, due to increasingly severe hailstorms and the associated risk of damage and potential failure of safety-related equipment, NEK implemented additional hail protection for safety-related components (cooling system of diesel generators 1, 2, and 3, emergency control room cooling units, and main control room cooling units). The steel support structures are classified as safety and seismic Category I. The hail protection is designed to withstand hail impacts with a diameter of up to 20 cm.

Lightning Protection Upgrade

In response to climate change, NEK carried out upgrades to the lightning protection system in the plant’s safety-related area during 2024 and 2025. The system is now capable of withstanding lightning strikes with amplitudes of up to 400 kA, which corresponds to an event with a return period of 10,000 years. With this upgrade, NEK became the first facility in Europe with such lightning protection, which posed a unique challenge in both design and equipment manufacturing.

100% Availability and Over 8,514 GWh of Electricity

By strictly adhering to all regulatory and environmental requirements and the highest nuclear industry standards, the plant produced more than 8.51 billion kilowatt-hours of electricity in the current fuel cycle.

Both the availability factor and capacity factor were 100% throughout the operating period, meaning electricity from NEK was continuously available.  

The 2025 Planned Outage Begins on 28 September

After extensive and thorough preparations, the planned outage will begin on Sunday, 28 September 2025, with the plant’s disconnection from the grid. Resynchronization is planned for the end of October 2025.

Key Features of the 2025 Planned Outage

The outage is based on the highest safety standards, which NEK consistently applies in all phases of operation and shutdown. It is conducted 24 hours a day, 7 days a week, and represents an investment in safe and stable future operation; therefore, preparations for it have been extensive and take place throughout the entire operating cycle.

Scope of Work

In less than a month, more than 4,200 work orders and nearly 33,000 activities will be completed, including the replacement of 56 fuel assemblies, preventive maintenance of safety-related equipment, inspections of structural integrity, and pressure boundary checks. These are processes of inspection and refurbishment of equipment to ensure reliable operation during the next operating cycle. Most of the installed equipment can only be maintained when the plant is shut down and cooled.

Major Maintenance Activities Include:

Overhaul of low-pressure turbine No. 2

Inspection of all four high-pressure turbine control valves

Overhaul of all three secondary heater drain pumps (HD)

Overhaul of all three condensate pumps (CY)

Replacement of polar crane load-bearing cables in the containment building

Overhaul of motor-operated valve 10046 in the component cooling system

Service and testing of main steam safety valves

Replacement of seal packages on reactor coolant pumps

Visual inspection of steam generator secondary structures

Visual inspection of reactor head penetrations and surfaces

Visual inspection of instrumentation penetrations on the reactor vessel bottom

Replacement of 220 V battery pack

Replacement of ventilation duct segments in the reactor building

Replacement and modernization of lighting in the reactor building

Main Upgrades and Technological Improvements

This year’s outage will include 20 key modifications, among them:

Replacement of the central alarm system in the main control room and local alarm systems — these systems provide alarm notifications in case of deviations in more than 2,000 key process parameters, enabling operating personnel to respond and initiate corrective actions in accordance with alarm response procedures.

First phase of replacement and upgrade of the process computer system (to be fully replaced by the end of the next planned outage in 2027) — this system provides more than 30,000 process data points to operating personnel in real time, trends all analog and digital variables, and performs additional data processing, enabling comprehensive system and component status monitoring.

Upgrade of the demineralized water treatment system, which will meet the latest nuclear industry guidelines and standards for the quality of demineralized water required by NEK for both the primary side (reactor cooling system) and the secondary side (heat/energy removal via steam generators to turbines).

Modernization of safety-related water intake filtration systems, as Sava River water is used to ensure cooling of safety systems.

Replacement of actuation relays in output relay panels, which are part of the reactor protection system and ensure the start-up of safety equipment.

Collaboration

The planned outage will be managed by NEK personnel, with the participation of more than 1,000 external specialists from Slovenia, Croatia, and other countries.

The activities will be supervised by the Slovenian Nuclear Safety Administration (SNSA) and monitored by authorized organizations, which will provide expert assessments upon completion of the outage.

International Recognition of NEK’s Excellence

NEK’s long-term operational performance is supported by responsible management, rigorous maintenance, and well-planned modifications based on internal and international operating experience and regulatory requirements. With more than 1,000 implemented modifications, NEK has further enhanced nuclear safety, plant availability, and output, which now stands at 700 MW at the grid connection point.

These achievements are the reason why NEK is regularly visited by representatives of nuclear power plants worldwide for benchmarking purposes and the exchange of best practices.

A recent visit by William D. Magwood IV, Director-General of the OECD Nuclear Energy Agency, highlighted NEK’s high safety and operational standards, as he stated that he was
 

impressed by the plant’s condition, technical solutions, implemented upgrades, and exceptional availability.

High levels of safety and reliability have also been recognized by Dr. Rafael Grossi, Director General of the International Atomic Energy Agency (IAEA), and David Wright, Commissioner of the U.S. Nuclear Regulatory Commission (NRC). Both commended NEK’s four decades of operation without significant technical failures and with exceptional reliability.

The Minister of Natural Resources and Spatial Planning, Mr. Jože Novak, also expressed his appreciation during his summer visit to the plant accompanied by his colleagues. He was briefed on the current operational status and safety measures, and after the visit he stated that NEK is one of the safest nuclear power plants in the world.This is further confirmed by the fact that NEK has received the highest rating from the World Association of Nuclear Operators (WANO) for the third consecutive time, which is a recognition of its excellent condition and internationally acknowledged safety culture.

Such acknowledgments, including appreciation for Slovenia’s active participation in international organizations and support for the development of the nuclear sector, reflect NEK’s commitment to safe, reliable, and technologically advanced operation, as well as its important role in the international nuclear community.

International Oversight of Long-Term Operational Safety with Commendation for the Plant

International peer reviews also enable independent evaluation of various aspects of plant operation.

This spring, experts from the IAEA SALTO mission (Safety Aspects of Long-Term Operation) reviewed the safety aspects of long-term operation. They commended the plant staff for their professionalism, openness, and readiness to implement improvements aimed at meeting and exceeding IAEA safety standards.

For more than four decades, NEK has reliably, safely, and sustainably supplied Slovenia and Croatia with electricity; it has a dedicated team, and society places its trust in the operation of the only nuclear power plant in the country.

On this occasion, NEK management expresses its gratitude to all employees, owners, and business partners for their professional, dedicated, and responsible work and cooperation, which are essential for the successful operation of one of the most important societal systems.

***

NEK in Numbers – Key Data at the End of the 34th Cycle and Start of the 2025 Planned Outage

8,514.5 GWh of electricity produced in the 34th operating cycle. This equals:

Equivalent to electricity for more than 2.1 million households for one year

Over 3.4 million tons of CO₂ emissions avoided, which would otherwise be generated by fossil-based electricity production

30,000 planned activities in the 2025 outage

Over 4,200 work orders in the 2025 outage

56 fuel assemblies replaced

700 MW output at the grid connection point

511 days of continuous operation between outages

100% availability and capacity factors during the cycle

More than 1,000 workers from Slovenia, Croatia, and abroad

20 key modifications in the 2025 outage

Over 1,000 upgrades during the plant’s lifetime

Krško, 25 September 2025