Over 32 years ago in the Chernobyl NPP the human mankind has experienced the most terrible accident in civil nuclear power. In series of many faulty steps including bad construction of the RBMK – 1000 reactor, at 26 April 1986, reactor meltdown has occurred. Almost 200 tons highly radioactive fuel started melting down throught the reactor materials including control rods, graphite and steel under the active zone of reactor.

In this short article I would like to present one of the most interesting and mysterious place in ChNPP, basements of the RBMK – 1000 reactor where “elephant foot” can be found.

Let’s start!

Corium – molten nuclear fuel with all the reactor materials that has also been melted because of extremely high temperature in active zone of reactor (control rods, reactor moderator, etc.).

Well, all this mass, which melted through the rooms located under the core of the RBMK – 1000 reactor, stopped in the lower floors of the building, often creating quite specific shapes.

Rooms.

The reactor was placed on a cross-shaped steel structure, the entire room was marked with the number 305/2 (+9 meters from the lowest floor).

8 steam channels were there, in time of the accident molten fuel mass got to the lower floors of NPP by those channels.

Under the room 304/3 there is a place that was often shown in the pictures, this is the place where the “elephant’s leg” is located. This is a room 217/2 (+6 meters).

Hardened corium in the shape of a large droplet known as the foot of an elephant, is largely composed of silicon dioxide with an admixture of other compounds, including uranium.

Spectroscopy has shown that the corium material include uranium with an average fuel burnout for the fourth reactor of about 12.5 MW*d/kg(U) [this value varies depending on where the sample was collected from].

The exposure dose measured on the surface of the elephant’s leg was equal 8000 R/h, which is equivalent to 70.16 Gy/h, calculated as the dose rate absorbed in dry air.

Metal scrap near the Kopachi village. [123.7 uSv/h]

But, the corium awaits us.

The dynamic viscosity of the corium in few places is given in the table below.

Percentage in the total mass of a given type of corium:

The radioactive activity of specific radioisotopes in a room 217/2 in MBq/g [for a day of the accident] is equal to:

But how does its look in other places under the Sarcophagus where sometimes people are working?

Exposure dose usually varies from 0.001 R/h to 25 R/h. In the above-mentioned room 305/2, the dose rate equals about 800 R/h, with maximum readings reaching up to 1200 R/h (for 2010), although lecturers had mentioned to us places where this value reaches 3500 R/h. On the turbine hall, the exposure dose usually does not exceed 8 R/h.

If we will consider a beta particles flux, the value varies from 150 to 550 000 (particles /cm^2*minutes), although it usually does not exceed 3000 in places where people operates. To approximate these measurements, you will not leave the Zone if your clothes have on the surface a Beta flux greater than 20 (particles/cm^2*minutes).

Literature:

Лагуненко А.С. Поиск и исследование скрытых скоплений топливосодержащих материалов разрушенного 4-го блока Чернобыльской АЭС

Пазухин Э.М. Лавообразные топливосодержащие массы 4-го блока Чернобыльской АЭС: топография, физико-химические свойства, сценарий образования, влияние на окружающую среду

Е. Д. Высотский, А. А. Ключников, А.В. Михайлов, В. Н. Щербин, В. Б. Шостак, Ядерно-опасные скопления топливосодержащих материалов в объекте «Укрытие»

Боровой А.А., Богатов С.А., Пазухин Э.М. Лавообразные топливосодержащие массы объекта «Укрытие»

Гончар В.В., Жидков А.В. Динамика высокотемпературного взаимодействия аварийного ядерного топлива с конструкционными материалами РБМК

Бурлаков Е.В., Былкин Б.К., Гарин Е.В. и др. Расчетный анализ радиационных характеристик кон- струкций реактора первого блока Чернобыльской АЭС после его окончательного останова

Богатов С.А., Боровой А.А., Гаврилов С.Л. и др. База данных по местонахождению

и состоянию ядерного топлива 4-го блока ЧАЭС до и после аварии.Препринт РНЦ

«Курчатовский институт»

Боровой А.А. Мой Чернобыль

А.А. Боровой, Анализ текущей безопасности объекта «Укрытие» и прогнозные оценки развития ситуации. Отчет МНТЦ «Укрытие»

Ushakov V.S., Burakov B.E., Andersen E.B. Interaction of UO2 and zircaloy during the

Chernobyl accident

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