In the work on the basis of the analysis of well-known researches of the thermal state of ventilated storage containers with SNF and analysis of the monitoring results of the thermal state of storage containers of the Zaoprizhska NPP, the nature of the thermophysical processes in ventilated containers of dry storage of spent nuclear fuel has been explored and generalized, which has allowed to identify and classify the main factors influencing on the thermal state of fuel and basic equipment during the whole storage facility operation. Among these influencing factors highlighted: anthropogenic, weather and climate, structural, physical.
In order to carry out a detailed analysis of the thermal state of spent fuel storage containers, taking into account the main factors of influence, a new universal approach for determining the thermal state of spent fuel and storage equipment elements based on the iterative solution of the direct conjugate and inverse heat transfer problems using models of different complexity levels has been created. Five complexity levels were identified for the SNF dry storage facility of the Zaporizhska NPP: a group of containers, a separate container, a basket with fuel assemblies, a fuel assembly, a fuel rod.
Using the iterative methodology, a series of computational studies were conducted that allowed obtaining new data on the thermal state of containers with spent fuel in the conditions of different design accidents. On base of the maximum temperature level of spent fuel, a classification of accidents was introduced, which allowed identifying those that require a priority elimination to provide a thermal safety criterion.
In the work for the first time, according to the results of thermal research, a classification of violations of normal operating conditions related to the blockage of ventilation channels, including design accidents, which are caused by this factor, is proposed. All possible variants of ventilation channels blockage are considered. It is determined that the highest temperatures are achieved by blockage of the upper ventilation vents; such blockage of normal operating conditions and emergency situations should be eliminated first and foremost.
The variants of improvement of the thermal state of containers with SNF during the whole operation period of the dry storage are considered. It was proposed to create a protective weather lid, which reduces the negative impact of wind on the thermal state of spent fuel; and also ways to modernization of the shape and size of the ventilation duct of the storage container were explored.
A new method of placing containers with spent nuclear fuel in the open site of a dry storage facility, which reduces their mutual thermal influence, has increased the efficiency of the system of thermal monitoring and provides the necessary level of safety of long-term operation of the storage facility in general. Placement containers containing fuel with higher decay heat generation in the middle of the containers group will reduce the effect of wind on the upper ventilation vents and, respectively, improve the quality of thermal monitoring.
The method of placing fuel assemblies in the storage basket with purpose to reduce the temperature level has been improved by setting the maximum permissible level of decay heat for stored spent fuel assemblies, which allows not to violate radiation safety standards. According to the method, assemblies with decay heat are placed in the center of the fuel assemblies group, which is 75% of the maximum allowable level of heat generation. For assemblies that surround the central ones, the level of heat can be increased to 106.25%, the heat release of the assemblies on the periphery of the group should not exceed the maximum allowable.
The methodical bases for predicting the thermal state of ventilated containers with spent nuclear fuel were first created in the work. The dependences of the temperature of the ventilation air in the outlet vent of the storage container and the maximum temperature in the storage cask with spent fuel from the level of decay heat and temperature of the atmospheric air are found.
A new method for controlling the thermal state of containers with SNF is proposed, which is based on comparison of the results of measurements and theoretical calculations of the predicted value of the temperature of the ventilation air at the exit from the storage container. The conceptual model of the information system for thermal state monitoring of spent nuclear fuel and the structure of the database, which will be included in it, are developed.
