An analysis of underground autocatalytic criticality has been performed for vitrified high-level radioactive wastes arising from reprocessing of spent nuclear fuel that are placed in a water-saturated fractured geologic formation. We estimate by mass transport analyses the mass and ²³⁵U-enrichment of accumulated uranium in the host rock originated from multiple failed waste canisters, for which static neutronic analyses are performed. Uranium accumulation with 12% enrichment (denoted as U(12)) can be created. With 30% porosity of the host rock, the minimum critical mass of U(12) required for over-moderated criticality is 280 kg. Heterogeneous U(12) depositions can exhibit positive reactivity feedback due to medium temperature increase.

View full abstract

  Simulated and real miscellaneous nuclear wastes were treated in the induction heating melter to form the molten product. The residual ratio and the distribution ratio of radionuclides during the melting process were measured for different operational conditions and type of waste.
  Laboratory-scale and mock-up scale tests were carried out, with radioactive tracers (C-14, Co-60, Ni-63 as a simulant of Ni-59 and -63, Sr-85 as of Sr-90, Tc-99, I-131 as of I-129, and Cs-134 as of Cs-137) and non-radioactive tracers (Co, Ni, Sr, Nb as of Nb-94, Re as of Tc-99, Cs, and Ce as of alpha nuclides).
  More than 50% of Cs remained in the molten product under the appropriate operational conditions. Nearly 100% of Co, Ni, Sr, Nb and Ce, and 85-100% of Re remained in the molten product. C-14 and I-131 did not remain. Co, Ni, Tc and Re were found in the metal layer of the molten product, Cs, Sr and Ce in the ceramic layer, and Nb in both layers. All elements were distributed uniformly in each of the layers.

View full abstract

  Before the construction of high level radioactive waste repository, the implementing entity will go through three siting stages for the repository. In each of those three stages, the implementing entity will carry out geological surveys. In this report, the concept for the geological surveys is described, on the basic of "The policies for the high level radioactive waste disposal(a tentative draft)" issued by the Atomic Energy Commission in July, 1997.

View full abstract

  In Japan, the high-level radioactive wastes are in principle expected to dispose into the deep rock formations. So, it is very important to evaluate the groundwater flow in deep rocks, which is a main pathway of radionuclides released from the disposal facility. Especially, the target depth of geological disposal is basically considered about from several 100m to one 1000m below ground-surface and the crystalline or sedimentary rock is the most candidate geological formation in Japan.
  Therefore, it is most important to evaluate appropriately the characteristics of hydraulic parameters, groundwater flow and so on inmore complicated geological conditions, and it is necessary to establish a systematic technique to investigate or analyze the groundwater and it's flow behavior in deep geological formations with fracture, fault and so on.
  In this paper, the present status of techniques developed by CRIEPI for the investigation of hydraulic characteristics, the analysis methods of groundwater flow in fractured rocks, the investigation methods to evaluate the regional groundwater flow movement by using natural environmental isotopes or the direction and velocity of local groundwater flow by groundwater flow meter and so on, are summarized.

View full abstract

  Swedish Nuclear Fuel and Waste Management Company (SKB) is operating a Hard Rock Laboratory (HRL) at the Äspö island in southeastern Sweden, to develop new technologies concerning High Level Waste (HLW) disposal, and to be useful for the safety assessment and survey of the candidate sites for HLW disposal by applying and verifying the test and survey methods developed before. Many in-situ experiments in operation phase are on going or planned at the Äspö HRL. This paper describes an outline of in-situ experiments in both construction and operation phase, and geological and hydrological surveys during construction and fracture estimation method which can provide basic data for the estimations of groundwater flow and radionuclide migration.

View full abstract

  Power Reactor and Nuclear Fuel Development Corporation (PNC) has been conducting geoscientific studies which provide a scientific basis on which the generic feasibility of the geological disposal concept can be assessed. The purpose of these studies is to understand the natural state of the deep geological environment. Hydraulic and geochemical properties of rock mass and groundwater are main factors which control mass transport behavior. PNC has been therefore conducting hydrogeological and geochemical studies in and around the Tono mine, central Japan, as these properties are important to understand mass transport in the rock mass. PNC has been also developing investigating instruments and methods to acquire data of these properties, modeling methods, and methodologies to evaluate simulation results.

View full abstract

  The development of equipment for sampling and the evaluation of existing technology of in-situ physicochemical parameter for the geochemical investigation of deep groundwater carried out to understand the realistic geochemistry of deep geological environment at Tono area, Japan. The chemical and isotopic composition of deep groundwater to approx. 840m depth has been determined using existing and established techniques. The redox condition of groundwater in the deep granite at approx. 180m depth are monitored using developed equipment. The analysis of water-rock interaction in relation to hydrology, geological structure and mineral saturation indeces, and the analysis of chemical composition using mass balance are available to understand the groundwater evolution.

View full abstract

  Initial stress measurements have been performed at the Kamaishi and Tono mines using the hydraulic fracturing, overcoring deformation rate analysis, acoustic emission and differential strain curve analysis methods. The purpose of the measurements were to assess the applicability of each method in sedimentary (Tono) and crystalline rock (kamaishi) environments and to determine the three-dimensional stress state in both mines. The results indicate that it is preferable to use more than one measurement method as each has certain limitations and may be affected by local rock mass conditions in different ways. The measurement layout needs to be considered carefully and the results analyzed with regard to the local site geometry, overlying topography and regional-scale tectonic stress regime.

View full abstract