Very different to the environmental shut-downs elsewhere (france) which are about water temperature effects on the ecology: this is a pretty straightforward physical/engineering limit. Danube low, less water available.

For the first time in the history of the Paks Nuclear Power Plant, all four units must be shut down at the same time due to the near-record low water level of the Danube. At the time of writing this article, only one turbine is operational, while the system operator has implemented an emergency breakdown protocol, the government has introduced energy saving measures, and the entire country is already monitoring events. In our article, we present how the country got to this point, why the water level of the Danube became a key issue for security of supply, and what technical, economic and system management consequences a complete shutdown in Paks can have.

At the time of writing this article (3.8. 5:55 AM CET), only one of the eight turbines of the Paks Nuclear Power Plant is operational. Based on previous statements, it has become a realistic scenario that by Monday morning, the country's most important power generation facility will be completely shut down. This did not happen overnight: the events of the past few days were part of a process at the end of which Hungary's energy security was put to an unprecedented test. But how did the country get to this point? What happened in the days that eventually led to the successive shutdown of the Paks blocks?

The nominal total output of the Paks Nuclear Power Plant is 2,000 megawatts, which is provided by the four modernized units. In recent days, however, due to the drastically low water level of the Danube restrictions introduced gradually curtailed production. Although individual units are shut down from time to time during regular maintenance, since the first unit was put into operation in 1982, it has never been seen that the entire four-block power plant is forced to shut down at the same time. The current process began on Tuesday with the first performance restrictions. On Wednesday afternoon, Unit 3 was shut down, and by Thursday morning, Unit 2's performance had also been reduced to 50 percent. With this, only two of the four blocks were operational on Thursday morning, also with reduced capacity.

The energy system is now burdened by two mutually reinforcing processes.

While there is a significant contraction in available generating capacity, due to the extreme heat, the demand for electricity jumps. Under normal conditions, the Paks nuclear power plant provides a significant part of the system's continuous, so-called base load production, while the daytime solar power plant surplus is often already exported. The situation is aggravated by the fact that, for the time being, there is no indication that the water level of the Danube will rise significantly in the coming days.

In order to understand why the subsidence of the Danube causes such a big problem, it is worth briefly reviewing how the cooling system of the Paks Nuclear Power Plant works. The cooling of the power plant is based entirely on the natural water level of the river: the water of the Danube reaches the so-called cold water channel by gravity, from where it is sent to the cooling water pumps through filters. In normal operation, when all four units and their corresponding eight turbines are in operation, a difference in water level of about 30 centimetres develops on the intake side due to the continuous extraction of water from the pumps. However, as soon as each turbine stops, the corresponding pump is not needed either.

This reduces water extraction, reduces the suction effect, and the power plant can operate safely even at a water level a few centimeters lower on the Danube. That is why the gradual shutdown of the units not only reduces electricity production, but also buys time for the operators, while the water level of the river is getting closer and closer to the critical value. It is important to point out here that the safety cooling system of the Paks Nuclear Power Plant is completely separate from the normal operating cooling water system. The safety pumps are designed to be located lower than the operating pumps, so they are able to withdraw water from the Danube even when the normal cooling system has already reached the limit of its functionality.

That is why shutting down all units does not mean that the power plant will be left without cooling. On the contrary: with the shutdown, the cooling water demand is drastically reduced, which gives the operators valuable time to maintain a safe state. However, if the water level of the Danube continues to drop, another emergency solution is also available: the power plant can install floating pumps in the cold water channel in order to ensure the minimum necessary cooling water supply even in such conditions. Float pumps are not used to allow the power plant to continue generating electricity, but only to guarantee the cooling of already shut-down units under all conditions.

---- whole much longer article in link ----

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