Secondary changes in fuel-containing masses from the fourth block of the Chernobyl NPP

A review of existing data reveals that fuel-containing masses (FCM) are reaction products of nuclear fuel and construction materials of the destroyed fourth block of the Chernobyl NPP. They were formed as unique lavas during the active stage of the accident mainly in the southeastern quadrant of the bottom reactor shaft (room 305/2). From here, they took various paths into the subreactor rooms and cooled there. Thus, secondary U mineralization processes that destroy the Chernobyl lavas occur in the Shelter. These phenomena are not connected with any specific conditions of the Shelter (for example, high radiation fields) but are a natural course of geologic processes that occur continuously in the Shelter. The secondary U minerals (epijanthinite, studtite, rutherfordine, etc.) are very unstable chemical compounds. They are soluble in water. This forces a re-examination of the nuclear safety of the Shelter