Forecast of 241Am Migration from a System of Deep Horizontal Boreholes

Abstract

Highly radioactive materials classified as high-level nuclear waste (HLW) of atomic power engineering should be disposed of deeply underground in special geological disposal facilities (GDFs), which can be of either shaft or borehole type. The advantages of borehole-type GDFs result from smaller volumes of mining operations, a simpler construction technology, shorter construction time and cost. This allows us to consider them as an alternative to shaft-type GDFs. The parts of the boreholes in which waste containers should be placed can be both vertical and horizontal. Computer simulation of the migration of radionuclides from a group of parallel horizontal boreholes into the biosphere made it possible to conclude that horizontal GDF boreholes have significant advantages over vertical ones. We determined a forecast of 241Am migration by a method of mathematical modelling of 241Am release from vitrified HLW disposed of in several horizontal drillholes. The maximum concentrations of americium in the near-surface groundwater above the repository are calculated depending on the number of boreholes, the depth of their location and the distance between them, the permeability of rocks and the time of waste storage prior to disposal. Influence of different conditions on the safety of a GDF of borehole type is estimated. Calculations show that the heat generated by HLW causes a weaker groundwater convection near horizontal boreholes compared to vertical boreholes of the same capacity. In addition to that, at an equal thickness of the rock layer separating the HLW from the surface, the geothermal temperature of the host rocks in the near field of a horizontal borehole will be lower than the average geothermal temperature near a vertical borehole. As a result, the rate of radionuclides leaching from the waste forms by groundwaters will also be lower in the case of horizontal boreholes.

Thermal Effects and Glass Crystallization in Composite Matrices for Immobilization of the Rare-Earth Element–Minor Actinide Fraction of High-Level Radioactive Waste

“…Advantages of boreholes for high-level heat-generating waste disposal, including that of the REE-MA fraction, are as follows [106,110,115,121]: long-term safety due to the large depth of disposal; economical access to rocks with high insulating properties; low infrastructure requirements and small area of ground structures; short construction time, waste loading and sealing of the storage facility; the possibility of creating waste near the site of waste generation; low probability of unauthorized access; minimal control after the facility is closed; high water salinity, which prevents convection due to heat released by HLW; low solubility of actinides under reducing conditions; and instability of the colloids in brines.…”

“…The preferred types of HLW forms for placement into deep wells [108][109][110][111][112][113][114][115] are waste with high heat release, including chlorides and fluorides of Cs and Sr; cesium-strontium fractions; actinide-containing waste in a stable matrix, including Pu-containing waste; and the REE-actinide fraction of HLW. The advantages of the borehole disposal of HLW over mines include the following: long-term safety due to the large burial depth and extremely low solubility of actinides under reducing conditions; economical access to rocks with high insulating properties; lower infrastructure requirements and significantly smaller surface area; shorter terms of construction and placement of HLW; significantly lower cost; the possibility of creating waste in close proximity to the place of waste generation; extremely low probability of unauthorized access to radioactive materials; minimal control after HLW placement and storage-facility closure.…”

“…The advantages of the borehole disposal of HLW over mines include the following: long-term safety due to the large burial depth and extremely low solubility of actinides under reducing conditions; economical access to rocks with high insulating properties; lower infrastructure requirements and significantly smaller surface area; shorter terms of construction and placement of HLW; significantly lower cost; the possibility of creating waste in close proximity to the place of waste generation; extremely low probability of unauthorized access to radioactive materials; minimal control after HLW placement and storage-facility closure. In addition, the high salinity of the deep waters complicates the formation of a colloidal form of radionuclide transfer and the development of convection due to the heat released by HLW [115]. Field-research programs for deep-borehole disposal projects have been repeatedly discussed in the literature [111].…”

“…Promising matrices (waste forms) of REEs and MAs are considered to be the glasses of B-Al-Si, Ca-B-Al, Al-P, and Fe-P composition, as well as glass-ceramics with crystalline phases of zirconolite, britholite, and monazite 49,59,[112][113][114][115][116][117][118][119]. The content of the REE-MA fraction in such matrices can reach 30-50 wt.%, and very deep wells with placement of the HLW forms at a depth of 3 to 5 km are optimal for their disposal.…”