Simulation Calculation And Analysis For The Decay Heat And Pressure Of The Deep Borehole Disposal Model

Nuclear wastes are hazardous for tens of thousands of years. It is not denied that the nuclear waste may become a serious threat to future generations. The long-lived nuclear waste, including spent nuclear fuel, should be isolated from human beings and the environment for a very long period. Disposal of spent nuclear fuel by storing in suitable deep geologic formations is considered as the safest and most sustainable option at acceptable technical level. Many countries introduce mined and engineered repositories, which stores spent nuclear fuel underground range from 300 mto 800m. Irrespective of whether they use uranium dioxide(UO2) or mixed oxide(MOX) fuels, the higher radioactivities and heat outputs of the irradiated fuels create problems for spent fuels(SF) management and especially for disposal in mined repositories. Shallow repositories have considered more about near-surface geology and hydrology than deep borehole, because of isolation barrier. Perhaps even more of a concern is the need for protracted cooling and storage prior to disposal, possibly for over 100 years after removal from the reactors, the deep borehole disposal is less temperature sensitive, it is based on the concept of multiple barriers that operate together. Large diameter boreholes over 4km deep are sunk into a suitable host rock(usually the granitic basement of the continental crust) and waste packages are deployed in their lower reaches. With a geological barrier an order of magnitude greater than mined repositories, DBD utilizes the very low hydraulic conductivities found at such depths, even in fractured rocks. It also capitalizes on the likelihood that the highly saline intra-rock fluids at depth will have been out of physical and chemical contact with near-surface mobile ground waters for many Ma. This isolation arises from long-lived density stratifications that are likely to remain stable far into the future, unaffected by climate change, glaciations, sea-level rises and even tectonic events. Low lateral flow rates and almost non-existent vertical flow across the density stratification ensure that any radionuclides eventually escaping from the near-field containment go effectively nowhere in 1 Ma, the time needed for the SF to decay to a radiological safe level. The safety is based on the combination of greater depth with the very low hydraulic conductivities, density stratified saline groundwater and long groundwater residence times that can be found at the depths. Deep borehole disposal of radioactive waste has the added advantage of not producing as large a “thermal footprint” as a mined geologic repository, because boreholes placed more than ~200m apart are unlikely to thermally affect one another. In fact very deep boreholes is a geologically safer, more secure, less environmentally disruptive and potentially less costly repository concept for many forms of high level waste. Generally, the deep borehole disposal consists of three kinds of disposal concepts and each one is aimed to disposal one related waste form. The potential technique and economic benefits of deep borehole disposal have already been more apparent over time. With development of drilling technology for petroleum and geothermal production, more reliable construction of deep boreholes is appeared. Favourable geological condition can be found in most locations, especially on some geological stable continental cratons. A relevant system of regional deep borehole disposal facilities can help address waste management issues and reduce the concerns about transportation. Actually, selecting reasonable deep geological repositories is an underway process for many countries. There are several existed or planned nuclear fuel disposal facilities in the world, for example, Yucca Mountain, ONKALO, Konrad, Grimsel, Bure URL and so on. Furthermore, spent fuel is subject to international safeguards because of its uranium and plutonium content.This article lists the three versions of foreign deep borehole disposal, explains for deep borehole disposal which geological formation should choose, whether casing should be installed, analyzes the drilling depth and diameter of the casing, the well fluid, sealing methods and materials from the perspective of the existing technologies. The results show that the migration of fluid and radionuclide occurs, the reason is impetus, which is likely to come from the pressure due to the decay heat. To some certain extents, the fluid flow velocity can be regarded as the velocity of the radionuclides. So after deploying and sealing analysis the affects, which generated in the boreholes due to the decay heat, is very important, especially the affects that in the HDSM and Pb cladding.The decay heat affects the base-rock and sealing materials too, but the influences are very small. The influences due to the temperature are showed below: first, whether it is high enough to melt the HDSM to provide a preferable sealed environment; second, the time intervals and spaces between the packages; third, whether it can melt the cladding introduce the possibility of leaking the heated pore water transport to the biosphere; fourth, whether it needs high-temperature cement grouting or not. We use the fluent software to analysis. The temperature modeling and fluid modeling indicates: first, after deployment the temperature of the base rocks and sealed materials increases about 40℃, the HDSM increases about 140℃,this temperature is not high enough to melt the HDSM, but as the increase of waste packages, the temperature is high enough; second, in the spaces between the packages, the temperature is very low, as they are in the non-thermal zones, but it can solved by high-temperature grouting; third, the operating temperature should below 335℃,but as the increase of packages, it is possible the temperature increases to 335℃, we can solve the problems by reduce the number of fuel rods, increase cooling time and increase the space between packages. Fourth, generates upward flow gradient, but the velocity is insignificant. We have modified thermal modeling of DBD to be more appropriate for higher burn-up fuels by using smaller(0.36 m diameter)stainless steel containers and a smaller(0.56 m diameter)borehole. The results proved that the concept of deep borehole disposal is feasible, indicated that DBD is a viable option for higher burn-up spent fuel.