Chernobyl: nuclear material may be re-igniting

35 years ago the world saw one of its worst ever nuclear disasters: the explosion and meltdown of one of the reactors at the Chernobyl power station. In the aftermath, the destroyed but still smouldering and intensely radioactive reactor core was encased in a concrete sarcophagus, which has since been strengthened further with the addition of a £1B 36,000 tonne tornado-proof steel dome designed to last a century. But, from within, there are signs that the radioactive remains of the reactor may be spawning new nuclear reactions: monitors have begun picking up increasing signs of life inside that could lead to another blast. Neil Hyatt is a nuclear materials chemist from the University of Sheffield, and he explained the readings to Chris Smith...

Neil - In one particular room - 305/2, which is underneath what remains of the Chernobyl reactor in Unit 4 - the neutron counters have been picking up a steady increase in neutron production since about 2016, and the rate of neutron production has increased by about a factor of two. And this has been observed in other areas of the shelter in the 35 years since the accident also.

Chris - And why are neutrons a worry?

Neil - Well, the concern is that what's giving rise to the neutrons in the current hypothesis is some residual fission. So the breaking of uranium, plutonium atoms inside that residual fuel material.

Chris - Can you paint us a picture, then, of what the environment actually looks like, and where that material is, and what sort of form it's in? Because obviously there would have been the core of the reactor that was stuffed full of radioactive uranium that was powering the reactor; the reactor exploded; what did that ultimately lead to, that we've now got this mess there that's radioactive?

Neil - In the course of the accident, the fuel material heated up; reacted with its cladding - its corrosion proof wrapper - and the reactor internals, and then also with the concrete surrounding, and materials that were dropped onto the burning reactor, sand, some lead. That all sort of mixed up (that's perfect glass forming material if you're a glass scientist) and then flowed like lava out from the reactor room into the basement of the reactor building. It's distributed in sort of clumps, the most famous being the elephant's foot formation towards the bottom of the basement. It's intensely radioactive even still because of the presence of the highly radioactive fission products.

Chris - And then the worry is that because there is all that uranium in there, if enough pockets of it are in close enough contact with each other, they could do what they were doing in the reactor under controlled conditions. But obviously there are no controls now, and they could therefore start the chain reaction or promote the chain reaction in their new venue, which is the elephant's foot and other parts of this debris. What could happen as a result of that then?

尼尔-这就是担心,它可能是一个good idea to differentiate between this room, 305/2, and the rest of the formation. So in the rest of the formations, it's very unlikely that they could ever find themselves in a configuration with enough water, which is also crucial to sustaining a fission reaction, to be able to do that. In room 350/2, our knowledge is a bit more patchy. So what we think is that there's a lump of material which is richer in fuel material - so uranium, plutonium, and it's covered by more friable lava material - and we don't really understand in detail the configuration of that material and how much water has ingressed. So there's an uncertainty as to whether that could sustain fission in the future. We think it's highly unlikely, because the calculations suggest that there's unlikely to be enough fissile material there to have a self-sustaining fission reaction. But we don't know for sure. You also asked me what the environment was like inside the shelter, and it's pretty grim...

Chris - Have you been in there? Have you been inside the new steel dome?

Neil - I haven't been inside the New Safe Confinement, but I've been to Unit 3, which is the one next door to Unit 4. And I think when you go on that tour, you're about 20 metres from the elephant's foot at the closest point. I worked with a team who work inside the concrete sarcophagus and it's wet; in the summer it's quite humid; dark bits of wall have fallen down, concrete piles, bits of metal, trying to trick you up and stick in you. And then on top of that you've got the radiation field. So it's a very challenging environment to have to work in.

Chris - What are the prospects of an explosion then? You've said that it's unlikely, but what might make that happen? And if it did, what would be the consequences?

Neil - It's difficult to say. If you draw parallels, for example, with natural fission reactors that were observed at a place called Oklo, water migrates in, sustains the nuclear reaction. The reactor then heats up and that dries out the water, the reactivity then dies down. And then once it cools down the water migrates and again, and the fishing reaction starts. So probably the worst case scenario is some kind of oscillating low power reactor producing steam. You've got an increase in temperature and that begins to sort of degrade the surrounding structural environment. So it's undesirable. There are things that we could do to try and quench that if we wanted. So for example, spraying in a gadolinium containing solution or boron containing solution to get the reaction quenched, if that indeed is the mechanism.