An accident is a particular set of system and component failures in which radionuclides can be emitted into the environment. Such a sequence of events is called a scenario. The collection of all scenarios forms an event tree. Let S n (n = 1, 2, ..., N) be the probability that the nth system fails and S n = 1 -S n the probability that the same system operates without failure. Then the probability P m of each scenario is a product of the corresponding probabilities of independent events either S n or S n depending on whether or not the system n is in a failure state. To formalize the procedure, we introduce for the nth system the indicator q n , where q n = 0 if the system is in a failure state and q n = 1 if the system is in an operating state. Then the probability of the nth scenario can be calculated from the formula where Π is the product symbol. The next step in analyzing an object is to develop and construct a failure tree for individual systems. This is a structural relation between the system components from the standpoint of operational reliability and failure-free operation. The probability of failures of the systems S n is calculated by constructing a failure tree. In other words, a failure tree is a logical chain of failures of components which are connected by the logical symbols AND or OR corresponding to multiplication or addition of random failure events.To perform the calculation, it is necessary to have information about the characteristics and parameters of the systems and components of each system. The main sources of information are the working log, the technological regulations, the operating instructions, the technical certificate, the data card, and other documents which make it possible to obtain the failure statistics. Analysis of accessible materials has shown that radiation hazardous systems and objects include mainly components whose failure probability is described by the formulawhere λ j is the failure rate of the jth component (1/λ j is the average run-to-failure time of an individual component).To a make a quantitative estimate of the degree of safe operation, it is necessary to have normative requirements for the probability of incidents depending on their level. Since at the present time such norms do not exist for facilities which reprocess and condition radioactive wastes, we shall use data on chemical-technological practice [1]. Similarly to the international scale for assessment of dangerous events at nuclear power plants, the required safe-operation probability was divided into seven levels according to the consequences.Many years of operation of facilities under the conditions of the Radon Moscow Scientific and Industrial Association show that if one system of a given facility fails, then this failure can result in an incident no higher than level I. In accordance with this empirical fact, we shall assume that a level-II incident can occur as a result of the operational failure of any two sys-P S S m n q n q n n = − ∏ ( ) ,

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In the paper there is shown the topicality of theoretical researches of processes arising in a surface layer of materials at combined machining methods. The peculiarities of thermal processes development in a surface layer of grey iron at high speed heating are considered. On the basis of the solution of a thermal conductivity differential equation the influence of heat source parameters (heat flow density, coefficient of concentration and movement speed) upon characteristics of a temperature field in a surface layer of material worked. The quantitative correlations of dimensions of an area of phase changes in a surface layer at a constant value of heat flow density and at the ensuring of constancy of maximum temperature value on the surface worked are determined. The characteristics correlation of a temperature field at the change of a coefficient of concentration and flow density of a heat source is revealed.