Calculation of the process of vacuum drying of a metal-concrete container with spent nuclear fuel

Abstract

An algorithm and results of calculation of the process of vacuum drying of a metal-concrete container intended for long-term "dry" storage of spent nuclear fuel are presented. A calculated substantiation of the initial amount of moisture in the container is given.Introduction. The problem of storage of spent nuclear fuel (SNF) has become pressing in recent times for the Russian Federation. The SNF of high-power channel-type reactors (HPCTRs), whose processing has not been found expedient because of the low residual content of fissionable nuclides, accounts for the considerable amount of SNF requiring long-term storage [1].Spent nuclear fuel is stored at present in near-reactor cooling pools (ponds) and separate spent-fuel storages located on the premises of atomic electric power plants under the conditions of the so-called "wet" storage. The overall capacities of these storages are not designed for long-term storage of the entire SNF accumulated during the years of operation of the atomic electric power plants.One method of solution of the above problem is the technology of compacted storage of spent fuel assemblies (SFAs) in pools [2], which makes it possible to double, in practice, the number of SNF-containing canisters compared to the original design. However, as calculations show, pools in the Russian Federation will soon be filled even in the case of the compacted method of SNF storage. Also, it should be noted that the compacted method of SNF storage aggravates the issues of safety of the existing storages and hinders access to the walls and bottoms of cooling pools with the aim of controlling the hermeticity of these structures.The period of stay of the SNF under water approximates the maximum permissible one (30 years) in many cooling pools. The safety and reliability of handling of the SNF under the conditions of "wet" storage of the fuel are largely determined by the preservation of the material of the fuel-element shells. During the "wet" storage, we have corrosion processes in the SFA boxes and degradation of the material of the fuel-element shells, which may finally lead to a loss of vacuum sealing by the fuel elements and to escape of radionuclides to production floor-areas and the environment.An alternative to the "wet" storage technology is the technology of "dry" storage of the SNF precooled in water to reduce radioactivity and heat release. In this case the fuel-storage conditions are improved, since water is a more aggressive medium than dry air or inert gases.In implementing the "dry" method of long-term (up to 40-50 years) storage of the HPCTR SNF, it was proposed that TUK-109 transport-package sets based on the metal-concrete container of the "Design Office for Specialized Machine Building Enterprise" (St. Petersburg) be used. These containers meet both the standards of safe storage and transportation of the SNF in the Russian Federation and the requirements of the International Atomic Energy Agency. Coincident with the development of the container structure, work on creation of the domestic...

Numerical Modeling of Heat and Mass Transfer Processes in the Transfer of Spent Nuclear Fuel from "Wet" to "Dry" Cask Storage

“…The rate of evaporation of the moisture from the FEC surface was calculated on the assumptions that the heat and mass transfer processes at the above stage are largely determined by natural convection. The surface temperature of the fuel element was determined thereby from the solution of the problem on the heating of a solid infi nite vertical cylinder with an internal volume heat source and boundary conditions of the 3rd kind on the outer surface by the fi nite-difference method [2]. In calculating the effective heat transfer coeffi cient, the radiative heat exchange with the environment was taken into account.…”

“…The volumes of this water largely determine the time of vacuum drying of the cask. We performed calculations of the initial quantity of moisture in the MCC just before vacuum drying with account for the wettability effect in the process of removing FECs from water at a given rate, the formation of meniscuses on distancing lattices, the appearance of capillary forces in gaps, the dynamic forces at impact loads, and the above evaporation of moisture from the FECs and the distancing lattices [2]. As a result of the calculations performed, we obtained a conservative estimate of the amount of moisture (35 g) that can get into the FEC-containing ampoule during loading.…”

“…To calculate the duration of the process of vacuum drying of the UKKh-109 metal-concrete cask, we developed a CVDD-109 program code. It is based on the simple engineering "hydraulic" model of vacuum drying described in [5]. This model is based on the assumption made in [2] that the temperature of the steam-air mixture is constant in the drying process.…”

“…It is based on the simple engineering "hydraulic" model of vacuum drying described in [5]. This model is based on the assumption made in [2] that the temperature of the steam-air mixture is constant in the drying process. In such an event, the intensity of the drying process is limited not by the rate of evaporation of water, but by the "hydraulic" parameters -pumps capacity, depth of the rarefaction created in the MCC, and hydraulic resistance of the channel of mixture outfl ow from ampoules and the MCC.…”