Increased Neutron Levels At Chernobyl-4: How Dangerous Is Corium?

When the Chernobyl nuclear plant suffered the power output surge that would destroy its #4 reactor, a substance called ‘corium‘ was formed. This originally lava-like substance formed out of the destroyed fuel rods along with surrounding materials, like concrete, that made up the reactor. The corium ultimately cooled down and left large amounts of solid corium in the rooms where it had pooled.

Over the past few days there have been numerous reports in the media regarding a ‘sudden surge’ in neutron flux levels from this corium, with some predicting a ‘second Chernobyl disaster’. Obviously, this has quite a few people alarmed, but how dire are these neutron output changes exactly, and what do they tell us about the condition of the corium inside the ruins of the #4 reactor building?

Matter of Perspective

When it comes to translating scientific measurements and data into health and safety information, perspective is everything. For example, after the 2011 Fukushima Daiichi disaster, marine life near the wrecked plant got exposed to increased levels of radioactive cesium isotopes. Some of these, like tuna, are migratory, and are caught by fishers off the US West Coast. As explained by NOAA Fisheries, the fact that we can measure these increased levels in the caught tuna does not mean that they’re of concern to public health, but it’s still not cool.

When it comes to consuming fish, the bigger danger is heavy metals. In the case of tuna, mercury levels are generally high enough that the mercury exposure from a single 85 gram serving of tuna may surpass the (US EPA) safe levels for mercury consumption in an entire week. We have previously covered the dangers of methyl mercury in fish and its role in Minamata disease.

The radioactive beaches of Guarapari, due to monazite minerals.

Back to Chernobyl. How much have emissions increased, and is that a lot? 1 milliSievert (mSv) annually is a standard maximum dose for the public. While it’s obviously a good idea to have a conservative maximum dose, there are places where naturally occurring radiation exposes the public to a lot more.

The famous black beaches of Brazil expose tourists and inhabitants to significantly higher levels than this 1 mSv due to the presence of monazite, a phosphate mineral that contains thorium and some uranium. Monazite is found in India, South Africa and other regions as well. Despite these massive violations of what is perceived as the safe limit, studies are not finding significant harm from these high background radiation levels.

Back to Corium

The corium at Chernobyl-4 is not a homogeneous mass, but displays distinct phases depending on when different materials were added. In sub-reactor room 305/2 where the corium first pooled, it is mostly presumably the black ceramics type. All of the corium consists out of a silicate glass matrix with the other substances mixed into it. Constituents are the (non-enriched) uranium oxide fuel, the zirconium from the fuel rod cladding and serpentinite.

Serpentinite was used inside the RBMK reactor as radiation shielding at the top of the reactor, due to its ability to slow down neutrons through elastic collisions — the neutrons literally bounce off without changing the serpentinite. In the corium samples analyzed, the main constituent was silicon dioxide (SiO2), commonly known as glass. With the amount of SiO2 in the corium samples ranging from 60-70%, the long-term stability of corium hinges on the stability of this glass matrix under constant irradiation.

As the radioactive isotopes in the corium, mostly transuranics and actinides, undergo radioactive decay, the short-lived isotopes are responsible for most of the radiation. The high-energy, but low penetration, alpha decay particles can cause so-called Coulomb explosions, which could conceivably damage the crystalline SiO2 matrix to the point of collapse. Estimates are that the self-irradiation levels of the Chernobyl-4 corium are not sufficiently high to warrant such a transformation over a 100 year timespan.


A recent significant change at the Chernobyl-4 reactor has been the placement of the New Safe Confinement (NSC) structure over the ruins. This replaces the original 1980s improvised concrete sarcophagus which was originally erected around the reactor remains by the Soviet government. This old sarcophagus was anything but airtight, allowing rainwater and even small animals to enter the structure.

With the NSC in place, however, this steady supply of rainwater has now stopped, which presumably would have begun to dry out the reactor remains and the sub-subterranean rooms. As a highly effective neutron moderator and absorber, this rainwater is speculated to have reduced the reactivity of the corium. With the water content decreasing, the initial effect would be that of a void formation that decreases neutron capture, and thus increases reactivity of the fissile material.

With less water, the neutron flux has accordingly increased. However, it should also be noted that this is not a new or sudden phenomenon. The neutron flux has been gradually increasing over the past four years, with a doubling during that time in room 305/2. It and the other changes since the installation of the NSC having been a subject of constant study by the Institute of Nuclear Power Plant Safety (IPBAE) of the Ukraine Academy of Sciences.

According to the IPBAE, there are many uncertainties about what may happen next, but that so far the neutron flux has not exceeded established safety limits. With the neutron flux density still gradually increasing, the situation warrants caution, but not alarm. Their models show that the worst case situation would cause a sudden spike in thermal output due to a run-away fissile reaction with the boiling off of any remaining water in the material contained in room 305/2.

The resulting steam explosion might weaken the reactor’s ruins and the degrading sarcophagus further, but the NSC is expected to contain the radioactive dust in this scenario. Definitely a bad day for NSC operators, but unlikely to affect the area outside this containment.

Tempest in a Corium Teapot?

Considering that this is a slowly developing situation that has been constantly monitored ever since the NSC was rolled into place above Chernobyl-4’s ruins, and with even the worst-case scenario likely to remain within the confines of the NSC, it seems somewhat odd that it would get this much media attention. Especially considering that the current neutron flux density is in science lingo merely ’cause for concern’, meaning a situation that still far removed from any situation that would require immediate action.

The goal at this point is for scientists to work on further monitoring and modelling the interactions that are happening in room 305/2 and other parts of Chernobyl-4’s ruins. With Chernobyl-4’s corium being essentially unique in its severity and scope, much of this is still a learning experience. Yet it should not be a concern to the average citizen.

For example radioactive radon gas is the number one cause of lung cancer in non-smokers in the US, responsible for 21,000 lung cancer deaths each year. The reasonable approach for people who don’t work at or near the Chernobyl-4 NSC would be to pay attention to the radon gas levels in their house and mind the amount of tuna they eat.

In addition, as covered in our earlier article on the Chernobyl nuclear plant disaster, the leading cause behind the disaster was a complete lack of safety culture. This pervaded not only the design of this RBMK-style reactor, but also the way it was operated and maintained. In current day Ukraine, and under the watchful eye of the IAEA, safety culture is no longer optional at the now shuttered Chernobyl nuclear plant.

None of this is to say that accidents cannot happen, but it is important in life to keep the appropriate perspective.

60 thoughts on “Increased Neutron Levels At Chernobyl-4: How Dangerous Is Corium?

    1. But noting down the observations is what makes it interesting from a scientific standpoint.

      Though, a bit unnerving as well, but personally I don’t think it will be a major concern. And worst case, we can just start pouring water into the place again.

        1. I might have a better idea. Mount huge windmills everywhere in the exclusion zones and send in nuclear workers to maintain them, if nuclear is so harmless. Heh! Heh!

        1. I’ve just read the linked article and I’m disappointed in you.
          “oh nose, tv entertainment even based on history is made to entertain the viewer?!”
          “How could they?”

          HBOs Chernobyl never claims to be a documentary and yes the political/personal/power/etc stuff shown is not accurate – from not entirely to not at all (in varying degrees).
          The real persons portrayed didn’t act or behave in RL like in the mini series – “Who would expect that?”

          [Masha Gessen] talks about the “narrative vacuum” surrounding the Chernobyl disaster, claims it will now be filled by HBOs mini series (reasonable) but fails to explain why this is a problem with the inaccuracies she found / sees / which are there (in the series).
          Or did I miss that part?

          AFAIK most technical details of the meltdown and so on are presented as accurate as they could be.
          But I just know as far as the English Wikipedia article (read it a few years before that series “aired”).

  1. SiO2 is not glass, except in very rare cases. Glass is a generic term for any solid without a crystalline structure. Window glass, glass in bottles, i.e. everyday glass is indeed mostly SiO2 (maybe 70 percent), the remainder being sodium and calcium oxides. Actual silica glass is fairly exotic.

    1. Not really. We have an aversion to this particular kind of risk. There are huge differences in acceptance depending on what kind of danger it is. We readily climb ladders or drive a car, even though the risk is much bigger.

      An interesting question is how many years of operation per meltdown for one reactor would be acceptable ?

          1. Well, for the sake of the plant operators, nobody wants one to break on them, so I would say:

            TMI: 1 in 60 years

            Point being, if the result of a meltdown is a small fart of radioactive xenon, the biggest worry is the inconvenience of fixing the reactor. Therefore, if a reactor breaks after 60 years of use, it was due an overhaul anyways.

            Fukushima: NaN

            Cannot be estimated because the outcomes depend on public and state reaction to the disaster. If you’re going to be superstitious and panic, and drive people out of their homes and start ostracizing them, even kill them to depression, alcoholism and suicide, then we have to count you into the equation.

            Chernobyl: NaN

            Should not be operating such reactors in the first place.

          1. It is though. Since we’re not shutting the whole program down for the possibility of a meltdown tomorrow, we can define an upper bound and say that we would accept 6,200 reactor-years per meltdown and possibly less.

  2. My impression from videos shot inside the old sarcophagus was that is was more of a bike shed, made from concrete and steel bars with corrugated steel roof and walls. It looked like a starry night in there, with all the holes in the roof…

    1. You have to remember though that this sarcophagus was built within a year of the accident. They actually had to build walls around it while working on it at the sides to make it safe enough to work. At the top it was completely impossible to have any people around so those parts were lifted into place by cranes.

      Considering all those challenges it’s not too surprising it didn’t age well.

  3. > 1 milliSievert (mSv) annually is a standard maximum dose for the public.

    Normal average background radiation in the US: 3.10 mSv per year. Most of this comes from radon. Naturally occurring levels typically vary over two orders of magnitude from 0.3 to 30 mSv per year.

      1. Technically. 1 and 30 are on the same order of magnitude, but 1 and 32 are not. Likewise for 0.3 and 1 but not 0.2 and 1. It’s right up to the limits.

        If you compare the extremes however, they are on two different orders of magnitude.

          1. Just do a Carl Sagan and say “tens” of anything. It gave him a fudge factor from 20 to infinity. There are to magnitudes in the Sagan metric. Tens and Billions –> ‘tens of billions”.

    1. For comparison, the average threshold for acute radiation sickness is 1000 mSv in a short period of time. Statistically significant increase in cancer risk: 100-250 mSv in a short period of time.

      The exclusion zone at Fukushima was set according to a limit of 20 mSv estimated annual dose, resulting in a 20 km zone around the plant getting emptied of people. Later analysis showed that this area was twice over-estimated.

      According to measurements from the public: “two-thirds received an external radiation dose within the normal international limit of 1 mSv/yr, 98% were below 5 mSv/yr, and ten people were exposed to more than 10 mSv.”. The evacuation attempt itself resulted in an officially recorded number of 2308 deaths, practically none of which could be attributed to the radiation or the accident itself.

      The median thyroid equivalent dose was estimated to be 4.2 mSv and 3.5 mSv for children and adults respectively. In comparison, the absorption of radioactive iodine from the Chernobyl accident resulted in an average absorption of 490 mSv in the evacuees.

      1. Taking facts from a website, paid for by the nuclear industry to promote nuclear power, I would never fully trust. It would be like getting facts about smoking from the bat (British American Tobacco) website, information provided would be technically correct, but some pertinent information may be omitted because it did not did not fit the desired narrative.

        1. What’s the alternative, taking “facts” from Greenpeace or some other politically motivated fringe group?

          At least the nuclear industry has an incentive not to lie, because their whole problem is a PR issue and getting caught telling porkies does more damage than admitting the truth.

        2. Point being, any authoritative source that has any expertise on nuclear power, such as the IAEA, NEA, ENEA, UNSCEAR… etc. are “paid for by the nuclear industry to promote nuclear power” when they bring out facts that speak in favor of it, debunking claims of danger put out by the opposition. That is both true, and completely irrelevant – of course these organizations are paid for by the nuclear industry, and when they bring out favorable facts they are by definition promoting nuclear power.

          It’s like going to a consumer advocacy group to read about a car, and when they say, “This car is perfectly fine.”, you conclude that they’re industry shills and not to be trusted. Surely, even though they never lied to you and all their facts check out, they must have left out some detail that confirms your notion that the car is in fact a piece of junk.

          1. Not really. Firstly, the IAEA are not there to advocate for us. It is there to maximise the “peaceful” use of nuclear technology, so that analogy is poor.

            You appear to have misrepresented Truth’s perfectly reasonable point, that we should not trust vested interests to give us a full and fair picture. This is all pretty elementary, but the IAEA has form, having long ago infiltrated the WHO in a classic conflict of interest

            I think you need to grasp this: nuclear power is only there because we need nuclear weapons research so other nuclear powers can’t attack us with their nukes. Nothing wrong with that, but this technology should not be in the hands of power companies who are motivated by profit and are likely to cut corners – as did TEPCO at Fukushima.

      2. None of which has any relevance if you have inhaled airborne reactor fuel particles, as have billions of people the world over.

        Nothing so daft as folks running round with silly little Geiger counters when you have plumes of plutonium on the jet stream. It’s all pretty obvious and elementary, but sometimes your head is so full of stats you can’t see the obvious. Happens to us all.

    2. Dodging the discussion about orders of magnitude and political angles:
      I assume the “1 milliSievert (mSv) annually” is meant to be the dose with an “artificial” origin. Set deliberately low even compared to background radiation so the risk of any bad result is extremely unlikely, but unfortunately giving the public a skewed idea of what is dangerous.

    3. Once again, none of which is going to help you if you have breathed in atomised reactor fuel. Seattle residents were breathing in an estimated 25 particles per day after the accident. D’ya feel lucky?

  4. One wonders if the naturally stopping neutrons materials on the inside have become saturated and are no longer stopping neutrons.

    Shielding working by slowing neutrons sounds a bit hinky. Slow neutrons are what make reactors go.

  5. “the fact that we can measure these increased levels in the caught tuna does not mean that they’re of concern to public health, but it’s still not *cool*”

    Pun not intended? LoL :)

    1. I believe it is fair to say that increasing levels should always be a concern for several reasons. No matter how small the increase, it normally shows man-made radioactive particulates are present. These artificially enriched particles are unlike naturally occurring uranium and so on and if ingested are mistaken for nutrients and accumulated in our internal organs: iodine isotopes are accumulated in the thyroid, radioactive strontium is an analogue for calcium and is accumulated in the bones (bone cancer, leukemia), and caesium is mistaken for phosphorus and accumulated in the heart and other muscles (cancers, heart attacks). Strontium and other radionuclides also degrades into even more dangerous substances once inside the body. There is a cheery little page here in Wikipedia:

      There are around 2000 radionuclides that can enter the human body, degrade our internal organs and our immune systems, causing all sorts of other problems too, as controlled experiments showed decades ago, when they told us nuclear power plants were safe. Now they have exploded, they have no choice but to confuse the public with irrelevant radiation readings. Think about it. Does not matter what the Geiger says if your lungs are full of hot particles carried across the world by the jet stream.

    1. By the UN it is still part of Ukraine.

      ” the referendum held in the Autonomous Republic of Crimea and the city of Sevastopol on 16 March 2014, having no validity, cannot form the basis for any alteration of the status of the Autonomous Republic of Crimea or of the city of Sevastopol; 6. Calls upon all States, international organizations and specialized agencies not to recognize any alteration of the status of the Autonomous Republic of Crimea and the city of Sevastopol on the basis of the above-mentioned referendum and to refrain from any action or dealing that might be interpreted as recognizing any such altered status.”
      – Resolution adopted by the General Assembly on 27 March 2014

  6. Yawn…
    monazite? meh. i recently measured potassium chloride, salt substitute. a pretty nice 10x background or so. and people EAT that because sodium chloride (common table salt) is bad for the health :D :D :D
    (potassium chloride is still perfectly safe to use, but when people start worrying because of neutrons from the old RBMK1000, you know is time for a good size asteroid,)
    i also measured some tritium vials i bought and my GM cant read anything off them. thats for those who are fretting for the “nuclear wastewater” from fukushima, which contains traces if tritium.
    the ignorance in matters of physics in general, and the BS of the media, are a lethal mix that is costing us trillions in damages and missed economic development

  7. A small error in the article: The fuel in Chernobyl was not “non-enriched”, but “low-enriched”. This makes a big difference, as non-enriched fuel tends not to become critical (unless under special conditions, for example, if it is emplaced into very clean heavy water like in the Canadian CANDU reactors). But re-criticality in enriched fuel is easily possible. Bad thing is, that it is hard to find any data on that neutron flux in room 305/2. Everybody writes, that it has doubled. But what is the absolute number? 10^0 or 10^10 neutrons per m² per second?

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