Manoeuvrability of the new Paks units

In a co-operative electricity system -- the amount of electricity available at any moment should be the amount of electricity demanded by consumers. However, this demand is far from constant: at night, we generally use much less electricity than during the day, and even during the day the amount of electricity demanded is constantly changing. As electricity storage on an industrial scale is limited in our country, the task of the system operator, MAVIR, to guarantee a balance at all times is a major one.

In an imaginary balance, in this case, production is placed on one side of the scale, consumption on the other, and the frequency of the system is the balance. In the European electricity system at equilibrium, the system frequency is 50 hertz. The state of the system can be determined by measuring its frequency, which decreases in the low-source condition and increases in the high-source condition. In case the frequency deviates from the nominal value, the power plants will vary their output to some extent, either automatically or manually. Small frequency variations usually require small power variations and large frequency variations require large power variations. Small changes are handled fully automatically by the power plants, while larger changes are handled at the command of the system operator.

With the rise of weather-dependent renewables, maintaining a balance is becoming increasingly challenging, as they do not generate when consumers need electricity, but when the sun is shining or the wind is blowing. Each power plant is involved in maintaining the balance in different ways. The nuclear power plants in operation today operate primarily as base-load power plants, i.e. they operate continuously at their rated capacity. This is the most cost-effective and simplest mode of operation, so most nuclear power plants are involved only in maintaining the grid frequency, with little variation in output, most of the time.

In Western Europe, there are already nuclear power plants that are very often involved not only in maintaining the grid frequency, but also in restoring it, i.e. they execute the automatic or manual frequency restoration command of the system controller, in other words, they change their output. In addition to frequency maintenance and restoration activities, dispatching is also becoming increasingly common, mainly due to the growing number of weather-dependent generators and the new rules of the liberalised electricity market. This means that there are periods when a power plant is not able to sell part of its electricity, so a load and then a load are added to its schedule.

In Hungary, the requirements for power plants wishing to connect to the transmission grid, including this issue, are set out in the MAVIR Operating Rules and its guidelines. In addition, the International Atomic Energy Agency and the European Nuclear Operators Association also make recommendations which should be taken into account in the design of a manoeuvrable nuclear power plant.

Units can be recharged when supply is too high, especially during sunny or windy periods, and when demand is too low, or even when both conditions are combined, which is most common on weekend nights.

Newly built power plants are subject to even stricter requirements than existing plants. The new Paks units must also be able to participate in frequency holding and recovery, as well as load following. The power output of the VVER-1200 units planned for Paks can be controlled over a wide range, with a power output range of 50-100%a of nominal power, which can be run twice a day, subject to appropriate limits and cycle numbers (maximum 2 per day, 5 per week, 200 per year).

There are several solutions to enable the block type originally designed for base-load operation to be loaded and unloaded. Implementation is a complex process involving the whole power plant. The reactors of the Paks II NPP are designed to be able to track power basically by varying the steam pressure in the secondary circuit and, if necessary, by using control rods and/or boric acid. The ability of the new units to participate in frequency maintenance and restoration is a system-level service that can be sold on the electricity market and can be of economic benefit to the plant operator.