EVALUATION OF EXPLOSIVE RISK IN THE PROCESS STORAGE OF WELDED NUCLEAR FUEL
Authors: Azarov S. , Sydorenko V. , Zadunaj O. 
Published: October 2019

At present, at the Chornobyl NPP, the main building of the general-purpose storage system for spent nuclear fuel is the "wet type" storage of spent nuclear fuel number 1 (SNF-1), designed for receiving and storing spent nuclear fuel. In the light of the post-Fukushima events, the task of increasing the explosion and fire safety is a priority direction of the Chornobyl NPP operation. SNF-1 should meet the current requirements of safety regulatory documents, both under normal operating conditions and during emergency situations. Among the emergencies that are likely to occur in the SNF-1 repository, the occurrence and development of fire in the interior space of reinforced concrete blocks should be considered. This condition is conditioned by the extraordinary importance of this radiation-hazardous object. An explosion on SNF-1 may occur, mainly due to violation of the rules of operation and fire safety, as well as in the event of malfunctions or failures of individual systems. Thus, the problem of providing explosion and fire safety on SNF-1 is extremely relevant.

The purpose of this work was to calculate the study of the formation of radionuclide hydrogen and its explosion in the premises of SNF-1.

Methods. Measurement, comparison, system analysis, physical and mathematical modeling.

Results. The main purpose of ensuring the explosion and fire danger of SNF-1 is to prevent the uncontrolled development of nuclear reactions and the spread of radiation. In the case of irradiation of water with low ionization radiation (for example, by gamma rays), the formation of radicals prevails, whereas for radiation with high ionization density (for example, α- and β-particles, splinters of division), the formation of molecules becomes more important. In a nuclear reactor where there are different types of radiation (γ-rays and high-energy protons formed during interaction with neutrons), both of these reactions take place simultaneously. Radicals H and OH are extremely reactive substances that are rapidly interconnected with the products formed as a result of the reaction. In the case of radioactive contamination of water in open containers filled with air and does not contain any active acceptors of OH radicals at appreciable concentrations, the observed hydrogen output is usually equal to 0.1-0.2 molecules/100 eV, which is significantly less the initial release of hydrogen formation. The larger the vessel in height, the greater the likelihood of this reaction compared with the likelihood of removing hydrogen from the liquid phase. But if we organize a non-equilibrium open system by forcing the removal of hydrogen from water, for example, by bubbling an inert gas, then the rate of its removal will exceed the rate of chemical decomposition. In this case, the hydrogen yield will increase and at the boundary it will be equal to the initial yield (unless, of course, there are no impurities of organic substances in the water, the radiolysis of which leads to the formation of molecular hydrogen). Therefore, to calculate the rate of formation of radio-hydrogen hydrogen in the reservoir basin water, the initial yield is used, that is, the maximum possible rate of hydrogen generation is calculated. Since the radioisotopes of water in the basin are mainly due to gamma radiation (beta and alpha radiation are delayed by the shells of the fuel assemblies and their energy is transferred to heat), the initial yield of the radiolytic formation of hydrogen is 0.45 molecules/100 eV. Under normal operation of SNF-1 in the basement area, the accumulation of radioactive hydrogen is eliminated at the expense of the work of the exhaust ventilation system of the surface area. The main factors determining the conditions for the formation of an explosive gas mixture based on hydrogen are the amount of stored spent fuel and the volume of water and air in the space where spent SNF is stored. The calculation is made from all sources of γ-radiation with a uniform distribution by source volume. For calculation, the main radionuclide composition of fuel in the lower beam of the fuel element was used in the case of holding 20 years and the burning depth of 24 MW×day/kg. The estimated value of the power absorbed by the water in terms of the total spent fuel, is I=1,098×1024 eV/(m3/h). The failures of the equipment of the complex of the storage system, caused by both external and internal events, which lead to failures of the ventilation system of the surface of the basin, can lead to the accumulation of radio-hydrogen hydrogen in the air volume of SNF-1 premises in the absence of ventilation systems. The break in the operation of the ventilation system, during which it is theoretically possible to achieve the lower limit of the explosive concentration of hydrogen, may be 12 days. In order to ensure explosion-proof safety in the pool of pools, it is necessary that the concentration of radiolytic hydrogen in the air be lower than the lower concentration limit of hydrogen explosion in a mixture with air (~ 4% of volume) with a stockrate of 10. Thus, the concentration of hydrogen should not exceed 0.4% of the permissible concentration of hydrogen in the air to exclude the formation of an explosive mixture with oxygen. For the case of placing all spent nuclear fuel (21284 units) in the compartments of the catchment pool 1-5, W(H2)=1.34 Hm3/h. Thus, the reasonable time of inactivity of the ventilation system will be no more than 7 days. In order to ensure that the concentration of radical hydrogen above the surface of the pool does not exceed 0.4% of the volume, the blowdown must be 250 times higher than the hydrogen generation rate, that is, the flow of air blown over the pool (ventilation) to ensure explosion-proof safety must be equal to g=250 W(H2), m3/year. The temperature of self-ignition of hydrogen in the presence of water vapor is 970 K, and the explosive properties of the hydrogen mixture are characterized by an outbreak of 4.12–75% of the volume. The degree of damage to building structures of the building of SNF-1 will be determined by the deformations and destruction that they received during the explosion of hydrogen. The formation of shock loads during an accidental explosion is directly determined by the number (volume) of stoichiometric ratios of hydrogen with the vapor and the limits of hydrogen ignition. Conclusion. Thus, the presented methodology for the estimation of the formation of radiolithic hydrogen and its possible explosion in the premises of SNF-1 allows, within the framework of a conservative approach, to conduct an explosion safety assessment and the consequences of an accidental explosion of radioactive hydrogen.