Modeling of 14C Diffusion from the Core of the Decommissioned ad Reactor

Abstract

A mathematical model of the diffusion of radionuclides through engineered safety barriers is developed for 14 C in order to determine the long-term safety conditions and characteristics of a decommissioned commercial uranium-graphite reactor. Dry and wet mixtures and concrete are used as the barrier materials. Diffusion is studied as the dominant mechanism for the transport of the radionuclides. The 14 C concentration at the boundary between the radwaste storage site and the environment is determined computationally and the effect of the modeling conditions on the specifi c activity of the radionuclide is analyzed. The decomposition sequence of the safety barriers for which the 14 C concentration is much lower than the admissible value is determined.After being shut down and put into a nuclear-safe state, the AD commercial uranium-graphite reactor located on the territory of the Mining and Chemical Combine is to be decommissioned according to the variant of radiologically safe on-site disposal. This concept provides for the creation of the requisite additional protective barriers ensuring that the requirements of radiological safety barriers are met. According to this principle of protection, the existing and newly developed safety barriers are combined. The additional protective barriers presuppose that the voids in the reactor space and extra-reactor rooms are fi lled with dry and wet mixtures based on clayey rocks and the openings of the concrete reactor shaft and sub-reactor space are fi lled with concrete mixtures [1].After the decommissioning has been completed, atmospheric water can no longer penetrate through the multilayer barrier that will be created above the reactor shaft. For this reason, the cracks in the side walls of the near-reactor rooms and base of the concrete wall of the shaft will be the most likely path for water penetration. In the case of penetration through cracks in the walls, water will start to impregnate the dry clay rock and water will start to propagate through the fi lling near a crack, followed by advancement into bottom of the fi lling to the graphite masonry. Under these conditions, the radionuclides can migrate into the adjoining rock from the localization site by diffusion through the entire surface of the graphite masonry and engineered barriers.To evaluate the conditions and characteristics of long-term safety of a decommissioned reactor, it is helpful to develop a mathematical model of diffusion of radionuclides through engineering barriers for the example of 14 C. The fi nal result of long-term prediction is the activity at the exit of the engineered barriers. Such studies have been performed before for the example of other radionuclides and with a smaller number of safety barriers [2].The objectives of the present work were to model the 14 C concentration at the radwaste disposal site-ambient environment boundary in order to evaluate the environmental safety of an object and analyze the effect of its activity on the sequence of engineered barriers, the thickness of the m...