“Nuclear barbecue”: why the Chernobyl nuclear power plant scared foreign scientists and is there a real threat?

Ukraine and the Chernobyl nuclear power plant are on the front covers of the world media again. On May 5, the journal Science published an article with the chilling headline “It’s like the embers in a barbecue pit. Nuclear reactions are smoldering again at Chernobyl” telling that nuclear reactions are smoldering in Chernobyl again. Referring to the data of Ukrainian scientists, its author stated that fission reactions are smoldering again in uranium fuel masses buried deep inside a mangled reactor hall, which could lead to a new accident with the destruction of the sarcophagus.

After the article became viral, the state specialized enterprise Chernobyl NPP and the supervisory authorities stated that there was no threat. Olha Kosharna, an expert in the field of nuclear energy, believes that foreign journalists have become a victim of the escalation of the situation by Ukrainian scientists.

Kosatka.Media explains what caused a stink, whether there is a real threat and what to expect.

What scared foreign journalists

The above-mentioned article says that fission reactions are smoldering again in uranium fuel masses buried deep inside a mangled reactor hall.

“It’s like the embers in a barbecue pit,” says Neil Hyatt, a nuclear materials chemist at the University of Sheffield.

Further, the journalist writes: “Now, Ukrainian scientists are scrambling to determine whether the reactions will wink out on their own—or require extraordinary interventions to avert another accident”.

The reason to raise alarm for journalists of Science was the words of Anatolii Doroshenko of the Institute for Safety Problems of Nuclear Power Plants (ISPNPP), who told that sensors were tracking a rising number of neutrons, a signal of fission, streaming from one inaccessible room.

His colleague ISPNPP’s Maxim Saveliev added: “There are many uncertainties. But we can’t rule out the possibility of [an] accident.”

The article says that the neutron counts are rising slowly and managers still have a few years to figure out how to stifle the threat.

“The spectre of self-sustaining fission, or criticality, in the nuclear ruins has long haunted Chernobyl,” the author writes.

The neutrons flux compression inside the destroyed reactor was linked to a decrease in the moisture content that had accumulated there since the time when the 4th power unit was covered only with a sarcophagus that let rainwater inside.

“ISPNPP modeling suggests the drying of the fuel is somehow making neutrons ricocheting through it more, rather than less, effective at splitting uranium nuclei,” the author says.

“It’s believable and plausible data,” Hyatt says. “It’s just not clear what the mechanism might be,” Hyatt says.

The explanation of Ukrainian nuclear scientists

A week after the publication of the article, SSE Chernobyl NPP and the Institute for Safety Problems of Nuclear Power Plants of the National Academy of Sciences (ISPNPP), the State Scientific and Technical Center for Nuclear and Radiation Safety (SSTC NRS) published an official statement.

They confirmed an increase in neutron activity in room 305/2 under the destroyed reactor. There is no access to it, however, sensors are tracking the changes.

“As of today, the increase does not exceed the established safety limits controlled by the standard nuclear safety control system,” the website of the Institute for Safety Problems of Nuclear Power Plants said.

The press service of the ISPNPP published a graph that shows the change in neutron activity in room 305/2 since June 2016.

ЧАЭС, Чернобыльская АЭС, Чернобыль, радиация, помещение 305/2, выросла радиация, зона отчуждения, поток нейтронов, Украина

The dynamics of neutron-flux density: (a) – near a nuclear hazardous accumulation in room 305/2 of the OU; (b) – at the remote periphery. Photo: ispnpp.kiev.ua

Fuel-containing masses (FCM) emit radiation. This is a solidified mass that was formed after the explosion of the reactor. It consists of remelted nuclear fuel, serpentite backfill, zirconium cladding of fuel elements, metal elements of building structures, concrete, and sand.

Scientists have a hypothesis that there are two accumulations of such materials in room 305/2. Initially, they contained a lot of moisture, which penetrated through the sarcophagus above the 4th power unit. After new safe confinement (NSC) was installed, the moisture began to gradually evaporate, which accelerated the fission reactions. This was the reason for the increased density of neutron fluxes.

 “Before the confinement was installed in the design position, the fuel-containing materials were completely waterlogged and the dynamics of the neutron flux density was within the seasonal trends due to regular moisture inflows from precipitation and condensate. In other words, no potentially dangerous changes in the level of subcriticality of these unsafe fissile materials were observed. After the installation of the NSC in the design position, as a result of the exclusion of atmospheric precipitation into the Shelter object, the process of evaporation and loss of moisture by the porous structure of lava-like FCM began, which led to a registered increase in the neutron flux density,” the ISPNPP explained.

ЧАЭС, Чернобыльская АЭС, Чернобыль, радиация, помещение 305/2, выросла радиация, зона отчуждения, поток нейтронов, Украина

Taking water samples on the premises of the Shelter. Photo: ispnpp.kiev.ua

The ISPNPP mentions that an increase in activity in fuel-containing materials due to their dehydration after the Arka installation was predicted back in 2015. Indeed, over the past 4 years, the neutron flux has been increasing.

Is there any threat?

The Institute for Safety Problems of Nuclear Power Plants says that the "southern" accumulation of fuel-containing materials in room 305/2 is potentially dangerous. According to available data, the clusters can have a two-layer porous structure: the upper layer is black lava-like FCM, and the lower layer is a potentially critical composition with a high concentration of unsafe fissile materials.


ЧАЭС, Чернобыльская АЭС, Чернобыль, радиация, помещение 305/2, выросла радиация, зона отчуждения, поток нейтронов, Украина

Location of the areas containing materials with high concentrations of uranium. Photo: energoatom.com.ua

ЧАЭС, Чернобыльская АЭС, Чернобыль, радиация, помещение 305/2, выросла радиация, зона отчуждения, поток нейтронов, Украина

Location of the areas containing materials with high concentrations of uranium in room 305/2. Photo: dspace.nbuv.gov.ua

This "potentially critical composition" (in crude terms, a mass in which, according to fears, a self-sustaining chain reaction could begin) frightened Science journalists.

However, Ukrainian scientists believe there is no danger at the moment.

 “Over the entire observation period since the beginning of the installation of the confinement in the design position, no excess of the limits of safe operation was observed. Currently, the neutron flux density sensors in all rooms have stable values without an upward trend, the current levels do not pose a threat of a self-sustaining fission chain reaction,” the press service of the Chernobyl NPP said.

They noted that the activity of neutrons would increase slightly until the optimal ratio of nuclear fuel to water is reached. “With further dehydration of this medium, a slight drop or an abnormally long period of constant neutron flux density is predicted,” experts suggested.

Also, the State Scientific and Technical Center for Nuclear and Radiation Safety (SSTC NRS) stated that the situation with a slight increase in the neutron flux density is not unpredictable and sudden.

 “If we take into account the data of these sensors with other indicators, then, considering the growth rates, we can say that the timing for reaching the established limits is pretty much, so there is no reason for worry so far,” the Center said.

Some believe that the problem is being escalated

Nuclear energy expert Olha Kosharna recalled that the decay of radionuclides in fuel-containing masses is ongoing and this is a natural process. Moreover, in her opinion, a self-sustaining chain reaction in a destroyed reactor is also impossible in the future.

“There are sensors for measuring the neutron flux to control the level. If the level specified in the technical regulations is exceeded, FCM is treated with solutions of neutron absorbers. This has been done for many years. The increase in the level is linked to the changes in the environment of the FCM after the establishment of the "Arch". Water does not get from atmospheric precipitation; it is also an absorber of neutrons. In order for the self-sustaining chain reaction mentioned in the article to begin, a critical mass is needed. Its formation is unlikely, in my opinion,"said Kosharna.

She added that the news about the increase in the radioactive background of the 4th power unit was taken from the report at the conference in Slavutych, which took place at the end of April. It was voiced by a researcher at the Institute for Safety Problems of NPPs of the National Academy of Sciences.

 “In my opinion, they add pressure to get funding for the study of fuel-containing masses, and the development of a strategy and technologies for their extraction. The Shelter implementation plan should have ended with this, not just with the Arch. But donors ran out of money. Therefore, they again want to create a new Fund funded by international donors to complete this project. I believe such a technique from an ethical point of view does no credit to the Institute,” added Kosharna.

By the way, the Center for Nuclear and Radiation Safety suggested that the whole question was about money.

 “It's hard to say what caused the problem. Perhaps this is only an attempt to obtain funding, because only on the 20th of April the first meeting of the Assembly of the International Cooperation Account for Chernobyl, created in November 2020, took place. It is very weird to observe the activity and panic of some scientific institutions and individual scientists in Ukraine and abroad on an issue that has been known for a long time and which is not critical and requires a solution, but in a planned manner,” the SSTC NRS said.

Tags: NPP, electricity

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