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expert reaction to Russian attacks at the Zaporizhzhia Nuclear Power Plant

There has been news this morning of Russian forces in Ukraine shelling the Zaporizhzhia power plant, the largest nuclear plant of its kind in Europe.


Dr Mark Foreman, Associate Professor at Chalmers University of Technology, Sweden, said:

“The shelling of a nuclear site is an alarming development, I hold the view that it is an exceptionally irresponsible thing to do.

“One concern that some members of the public have is that a nuclear power plant could explode in the same way as a nuclear weapon. This is impossible. It is not possible for shelling of a nuclear reactor site to cause a nuclear detonation, the worst which can occur is a release of radioactivity. This could occur if a reactor or a waste store was damaged. So far I have seen no reports of any releases of radioactivity from the site.

“Serious nuclear reactor accidents can be divided into two broad classes, there are the reactivity initiated accidents (RIA) and the loss of cooling accidents (LOCA). The reactivity initiated accident is caused by an excessive rate of energy release from the nuclear fuel, examples of this class include SL1 and the Chernobyl accident. Here it is possible to induce strong mechanical effects on the reactor and its fuel.

“It is not possible for a RIA accident to result in a nuclear detonation. To create a nuclear detonation three requirements must be satisfied. These are

  1. That the fuel must go from a subcritical state (where no nuclear reaction is occurring) into a super prompt critical state, where the rate of nuclear reaction will continue to increase with time.
  2. The change from one state to the other required in requirement 1, must occur in a shorter time than the time between the random appearance of neutrons in the fuel.
  3. Neutrons must be injected into the fuel at the right moment to start the reaction.

“As an operating reactor is already critical (has a self-supporting nuclear reaction) the first of the requirements is not satisfied. Even a nuclear reactor which has been shut down cannot undergo nuclear detonation as it is impossible to increase the reactivity sufficiently quickly. The reactor type at the site is a Russian version of the pressurized water reactor (VVER-1000), light water reactors like the VVER, BWR and PWR are much safer in terms of RIA accidents than the RBMK-1000 reactor which was involved in the 1980s Chernobyl accident.

“An intense RIA in a light water reactor will tend to self terminate, as the water boils more and more in the reactor the volume of the steam voids in the reactor will increase. This will reduce the ability of the neutrons to cause further reactions (nuclear fission) so that the reactor will slow down and maybe even stop.

“The other broad class of reactor accident (LOCA) is more likely as a result of something damaging the reactor site. Examples of LOCA accidents include Three Mile Island and Fukushima.

“If a reactor is subject to an earthquake or other violent event it is normal to shut it down by inserting control rods. This is known as a SCRAM, after a reactor is SCRAMed then the rate at which heat is released by the fuel in the core will rapidly drop.

“However the heat production in a recently operated nuclear reactor is sufficient to boil water and eventually cause serious damage to the nuclear reactor. Even after the nuclear fission reaction has stopped the radioactive decay of the products of the reaction will release heat. This heating is known as decay heat and compared with the heating by the nuclear fission reaction the amount of heat energy per hour is far smaller. But if the supply of cooling water to the reactor was to be interrupted then it is possible for the reactor to boil dry and over heat. In an extreme case this could result in the fuel cladding or even the fuel melting. There are three periods of grace which can prevent a LOCA progressing into a full core melt (meltdown).

“Firstly the water have to boil to form steam, this will cause the reactor to dry out.

“After the reactor has dried out then the fuel will take some time to heat up to the temperature at which it starts to melt.

“Finally there is the time required for the molten mass in the stainless steel reactor pot (reactor pressure vessel) to melt its way through the bottom. After the fuel melts through the bottom then it will come into contact with the concrete below it. When while some of the reactors at Fukushima suffered complete melting of their cores relatively little of the radioactivity in their cores was released to the environment. The release of radioactivity from an overheated nuclear reactor is controlled largely by the boiling points of the radioactive elements, When the fuel is over heated the radioactive gases such as xenon will be liberated from the fuel, after the xenon and krypton the iodine will be released from the reactor fuel.

“As the boiling dry and other stages of the core melting process take hours or days, as long as the reactor has been shut down immediately after the event which provokes the LOCA event then the short-lived radioactive iodines will have decayed away. It is likely that these short-lived iodines are more carcinogenic to the thyroid than the medium lived iodine (131I). Anything which plant workers can do which delays the progression of the core-melting event will reduce the exposure of the public to radioactive iodine and other radionuclides. A Swedish scientist (L.E. Holm) examined the health of people who were given moderate doses of 131I, he found no evidence of these radioactive iodine exposures causing thyroid cancer. However after Chernobyl and hydrogen bomb tests in the south pacific high rates of thyroid cancer were observed among the general public. One explanation is that the shorter lived radioactive iodines such as 133I, 134I and 135I are far more carcinogenic than the 131I. So as a result delaying the meltdown for a few days can greatly reduce the health impact of the event.

“After the iodine comes out of the reactor then next main radioactive element will be cesium, the cesium will be likely to leave the fuel as it overheats as a vapor which then forms aerosol particles which can leave the reactor buildings. Both at Fukushima and Chernobyl only a small fraction of the strontium and plutonium in the reactors were released.”


Prof Robin Grimes FRS FREng, professor of materials physics at Imperial College London, said:

“The heart of a nuclear reactor is its pressure vessel.  This contains the nuclear fuel and therefore the radioactive products from the nuclear reaction. The pressure vessel is very robust and can withstand considerable damage from phenomena such as earthquakes and to an extent kinetic impacts.  It is not designed to withstand explosive ordinance such as artillery shells.  While it seems to me unlikely that such an impact would result in a Chornobyl-like nuclear event (though obviously this has never been tested and it is not impossible), a breach of the pressure vessel would be followed by the release of coolant pressure, scattering nuclear fuel debris across the vicinity of the plant and a cloud of coolant with some entrained particles reaching further.  At the very least this would make it enormously difficult to deal with the issue and the problem would continue to evolve.

“The pressure vessel also contains the coolant which removes the heat generated by the nuclear reaction.  Even after the nuclear reaction is turned off there remains decay heat which must continue to be removed for some time, up to a few weeks certainly.  It is therefore crucial that coolant continues to be circulated around the reactor core within the pressure vessel.  Following a loss of coolant (often referred to as a loss of coolant accident or a LOCA) the core will sustain massive damage resulting in the loss of radioactive products from the fuel (such as happened in Fukushima).  Coolant must be maintained and we must continue to monitor this.

“It is therefore staggering and reckless to the extreme that shells have been fired close to a nuclear plant, let alone targeting buildings within the plant.  Even if they were not aiming for the nuclear plant, artillery is notoriously inaccurate in a time of war.

“Fortunately, it is clear from IAEA measurements and from others that radiation levels in and around the plant have not risen.  We can therefore assume that for the time being the core remains intact.  But as I said, we must continue to monitor this carefully and I have no doubt the IAEA are considering this very carefully.”


Malcolm Grimston, Hon. Senior Research Fellow at the Imperial Centre for Energy Policy and Technology, Imperial College London, said:

“I agree that direct attack on the reactors is not the main threat, at least unless Russia decides to destroy Ukraine completely. Nor can a VVER ‘run away with itself’ as the RBMK could (and did at Chernobyl) so it can’t ‘explode’. Further, it seems clear that it is not in the Russian interest to cause a major radiological incident – if they want, say, to annex Ukraine and integrate it into the Russian economy and polity, then they would need power, and the nuclear plants which provide more than 50% of Ukraine’s electricity would be very valuable assets.

“Zaporozhzhia is fairly close to the Russian border and Russia operates 38 of its own reactors (and has/had a thriving plant export business) that presumably it would not wish to undermine – in any vaguely normal world anyway. The videos of yesterday’s attack clearly do not show a major artillery action designed to damage the plant but a limited attack with small-scale weaponry designed to allow the plant to be taken over; though this is still a war crime, of course.”


Dr Mark Wenman, Reader in Nuclear Materials at Nuclear Energy Futures, Imperial College London, said:

“The Zaporizhzia nuclear plant has six VVER-1000 pressurised water reactor units producing 1/5 of Ukraine’s electricity.

“The plant is a relatively modern reactor design and as such the essential reactor components are housed inside a heavily steel reinforced concrete containment building that can withstand extreme external events, both natural and man-made, such as an aircraft crash or explosions.  The reactor core is itself further housed in a sealed steel pressure vessel with 20 cm thick walls.  The design is a lot different to the Chernobyl reactor, which did not have a containment building, and hence there is no real risk, in my opinion, at the plant now the reactors have been safely shut down.”


Prof Tom Scott, Professor in Materials, University of Bristol, said:

“Shelling nuclear power plants is against the Geneva convention and this is obviously very worrying.

“The good news is that radiation levels around the plant are reportedly normal and 5 of the 6 reactors are now turned off, with one still operating.

“The reactors are all Pressurised water reactors and hence don’t have graphite cores which could set on fire as per Chernobyl. Their inherent safety design should mean they are naturally quite resilient to any external perturbations and hence I am not overly concerned that inadvertent damage could cause a major nuclear incident.

“However, it would be more concerning if the reactors were being deliberately targeted to induce a nuclear incident.”


A Nuclear Industry Association spokesperson said:

“We condemn in the strongest possible terms the Russian military attacks around the Zaporizhzhia plant that have endangered the lives of nuclear workers bravely discharging their duties. We commend the extraordinary dedication of the station’s staff and operators in what are terrible circumstances and emphatically endorse the IAEA’s call for a halt to all use of force around Ukraine’s nuclear power plants.

“We understand that the fire at the plant was not in the reactor buildings, has been extinguished and has not affected essential equipment with no reported change in radiation levels. We will continue to monitor developments and urge observers to follow an IAEA press conference scheduled for 9.30am (GMT) today.”



Declared interests

Prof Robin Grimes: “is Foreign Secretary of the Royal Society.  He was chief scientific adviser in the Ministry of Defence (MoD) for nuclear science and technology from April 2017 to December 2021 and was Foreign & Commonwealth Office Chief Scientific Adviser from February 2013 to August 2018.  He is commenting above in his academic capacity.”

None others received.

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