Criteria for the fracture of RBMK fuel-channel tubes when accidental overheating occurs

Abstract

An analysis of the safety of channel-type reactors using modern computer codes assumes a limit of the integrity of the fuel-channel tubes for planned emergencies. The most probable reason for tube fracture is overheating (and of the fuel also) when there is a mismatch between the channel power and the coolant flowrate at high pressure. Three well-known incidents involving channel fracture in RBMK reactors were consequence of this mismatch. A correct estimate of the integrity of fuel-channel tubes when analyzing any emergency is based on a comparison between the current parameters of the state of the tube and certain fracture criteria, which, if they are reached or exceeded, the channel is assumed to be fractured. The nodalization scheme of the thermal hydraulic code for calculating the parameters of the circulation system must then be subjected to corresponding changes. Depending on the code employed, by means of which the integrity of the tubes is estimated, a certain fracture criterion is used. When only a thermal hydraulic code is employed, the integrity of the channel is determined by a temperature criterion, where the fracture of the tube is recorded but its state prior to fracture is not determined. The use of thermomechanical codes enables the stress-strain state of the tube to be calculated both before and at the instant of fracture, and here its integrity is estimated using deformation or force criteria. A Model of the Stress-Strain State of the Tube. A possible deformation of the channel tubes in the active zone of the RBMK reactor will occur by interaction with the graphite moderator. An analysis of experimental data on the fracture of tubes with graphite blocks and the results of an inspection of the channels and of the graphite stacks after incidents at the Leningrad and Chernobyl nuclear power plants show that at a pressure of more than 4 MPa the blocks and graphite stack as a whole do not exhibit a high resistance to bulging of the emergency channel. The temperature of the fracture and the fracture deformation for tubes with graphite cannot be distinguished from the overall mass of data obtained under freedeformation conditions. Hence, the criteria for channel-tube fracture, obtained primarily in experiments without graphite, can be used to estimate the possibility that the channel tubes may fracture when there is accidental overheating in the active zone of the reactor at a pressure greater than 4 MPa. In turn, at a pressure of less than 4 MPa, the strength of the graphite blocks may turn out to be sufficient to prevent channel-tube fracture. We will consider the plane and axisymmetric problem of the deformation of a defect-free channel tube loaded with an internal pressure. The tube is assumed to be thin-walled, since the ratio of the mean radius r to the wall thickness ~5 in the initial unreformed state for the fuel-channel tubes of the RBMK reactor is 10.5. When a thin-walled cylindrical tube with an internal pressure p fractures while preserving axial symmetry during deformation, the normal shear and axial stresses can be written as follows in a cylindrical system of coordinates: r r a# a o =p~;% =p-~=~-, (1)