Process resulting in the destruction of the No. 4 unit reactor at the Chernobyl nuclear power plant

Abstract

The results of an investigation of structural fragments of the core of the destroyed reactor in the No. 4 unit of the Chernobyl nuclear power plant are analyzed. It is shown on the basis of investigations of the fission product distribution over the cross-section of the graphite blocks and the determination of the physical properties of graphite that the temperature of the graphite blocks, including the reflector, at the moment they were ejected from the core exceeded 1000°C. The heat content of the fuel was estimated on the basis of an analysis of fragments of the dispersed uranium dioxide particles and an analysis of possible graphite dispersal mechanisms at the moment of the explosion. It is shown that energy sufficient for dispersing and partial vaporization of the fuel and for dispersing the graphite could have been introduced into the fuel during the accident process. Analysis confirms the possibility of a core destruction scenario with ejection from the shaft and ejection of part of the fuel in the form of vapor and dispersed particles into the atmosphere.In spite of the investigations conducted for more than 20 years after the accident at the Chernobyl nuclear power plant, many questions have still not been answered. According to the official version [1], the amount of fuel ejected from the reactor and distributed over the territory of Ukraine, Belarus, and Russia is 3% of the initial load. Recently, this figure has been increased to 5%, which, apparently, is a reliable estimate based on direct investigations performed over many years [2]. It has been assumed a priori that all the remaining fuel lies in the reactor building. However, estimates of the fuel distribution over the rooms in the building differ strongly from one another, making them unreliable. The possibility that dispersed (or vaporized) fuel was ejected into the upper layers of the atmosphere was considered to be fantastic, though it was obvious that "hot" particles propagated over very long distances. In addition, there is no clarity concerning graphite -the amount of graphite collected on the site and found in the building does not equal the almost the 2000 tons of graphite present in the reactor.According to the most widely accepted version, the explosion occurred in the shaft, a large part of the fuel in the form of a fused mass dropped into the bottom rooms of the reactor, the remaining fuel was dispersed in the form of dust and fragments in different rooms, and the graphite partially burned. In time, these ideas changed but the basis remained the same [3,4]. A change appeared when a new version was advanced. In this version, the entire core was ejected from the shaft and exploded in air with the fuel being vaporized and the graphite dispersed [5]. Many specialists, for example, [6], did not accept this version, but the Research and Design Institute of Electric Technology (NIKIET) -the organization of the main designer of the reactor -supported it [7].The possibility of fuel vaporization was first advanced back in 1986 on the basis of a...