The Chernobyl site has lost its external electricity supply, what possible consequences might this have?

 A continuous electricity supply of nuclear power plants is vital. This also applies to shut-down reactors, given that the spent fuel that has been irradiated contains radioactive material, the decay of which generates heat, and this heat must be removed continuously. If this is not ensured, the spent fuel may overheat after a period of time depending on its decay power and cooling conditions, which could damage it and release radioactive material into the environment.

Another reason for the need for a continuous supply of electricity is that it is essential to monitor the processes inside the nuclear installation, and this is only possible if the measurement systems and the visualisation of the measurements can be kept operational.

Communication channels between operators and between operators and the authorities also need to be maintained, which also requires electricity.

These aspects make it essential to have a continuous electricity supply to the installations.

On 9 March 2022, reports from the Chernobyl site indicated that the connection to the external electricity grid had been lost, and that power to the facilities could only be supplied by the emergency diesel generators. According to the Ukrainian operator, these have a reserve of 2 days of diesel fuel. Some sources have predicted that after 2 days, this could result in a large radioactive release. However, the International Atomic Energy Agency has said that there is no immediate emergency and that months are available.

What we can really expect? This can be assessed from the information released by Ukrainian Authorities in the so-called European stress test following the 2011 Fukushima accident. This Ukrainian report states that the interim storage facility for spent fuel at Chernobyl, the so-called ISF-1 facility, can 'survive' for about 2 months without active cooling - i.e. it is guaranteed to keep the fuel under water. This is a plausible figure.

Figure 1: Picture from an animation of the ISF-1 interim storage facility with the storage pool (Source: EBRD YouTube, https://www.youtube.com/watch?v=GYR3GmkRZV0)

The ISF-1 facility is a large pool in a building where some 21 000 pieces of spent fuel assemblies from RBMK reactors are stored. These fuel assemblies are 7 metres long, and they are stored in vertical protection tubes under water. Some of the assemblies have been resting and cooling for 30 years, while the "newest" assemblies have been out of operation for 21 years. The over two decades cooling time is very long, the heat output of a single assembly is probably less than 50 W. Of course, the large number of assemblies can give a high total heat output of the order of MW, but there is a large amount of water around and above the assemblies.

When the power supply is lost, the diesel generators start up first to provide power for cooling. When the diesel fuel runs out - and if not replaced - the cooling pumps shut down, after which this heating power starts to heat up the water in the pool. It can take several weeks before the water temperature is so high that evaporation becomes intense. Even more time is needed before the boiling of the water starts. According to the Ukrainian stress test report, it can take about 2 months by which time the surface temperature of the assemblies can be so high (300°C) that possible cladding damage can occur.

Figure 2: The ISF-1 interim storage building from the outside (Source: EBRD YouTube, https://www.youtube.com/watch?v=TdV0EpKICNQ)

It means that there is still time before a major radioactive release could occur as a result of a loss of power supply at Chernobyl NPP, and theoretically there are several options for the operator to intervene (e.g. application of a mobile diesel generator on site, or even fire trucks can supply water to the pool), but the availability of these devices is currently unknown.

That said, it is of course a big problem that the battles are affecting the facility and the secure electricity supply (the biggest problem is that the Russian army is at war in Ukraine). The other intervention possibilities mentioned above may also be limited by the battles.

The loss of power also makes artificial ventilation of the wet storage facility impossible, also complicating the situation in the installation.

Figure 3: Structures in the ISF-1 temporary storage pool (Source: EBRD YouTube, https://www.youtube.com/watch?v=TdV0EpKICNQ)

The loss of power supply is also a major problem because some measurements are not working and the data are not available to the nuclear safety authority.

It is also very worrying that the Ukrainian operators have been unable to leave the Chernobyl site for the 15th day due to the intervention of Russian troops, and there has been no shift change since then, with 210 people in the plant for the 15th day.

As I have written before, the Russian troops are regularly and systematically violating the basic rules of nuclear safety, which gives rise to extremely serious concerns.