It is important to evaluate complex formation ability of dissolved organic matter (DOM) with radionuclide, in terms of long-term safety assessment of nuclear waste. In this review, the dependence of the complex stability on environmental factors such as pH, ionic strength, and characteristics of humic substances was described based on Loading Capacity (LC) method. The effects of humic concentration on actinide speciation and its solubility in a groundwater were evaluated with consideration of mixed complex formation.

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  For the geological disposal of high-level radioactive wastes, evaluation of the radionuclide migration toward the human environment associated with groundwater flow is essential in order to secure long-term safety. Therefore, Japan Atomic Energy Agency (JAEA) is implementing a program for developing methods for assessing long-term groundwater flow in regional scales. The present project is a part of the above and we conducted a study on key factors regarding changes of groundwater flow from the last glacial to the postglacial stages in the Horonobe and surrounding areas which were most likely covered by permafrost then. Specifically, geomorphological analysis was carried out using aerial photographs and a high resolution (10 m) DEM to obtain the distribution of the slopes that had been formed after the last glacial stage, and the relation between the geological conditions and the erosion depth of those slopes was analyzed.
  The results are summarized as follows:
  1) Although periglacial smooth slopes which expanded widely in Horonobe region over the last glacial stage were eroded by rainfall increase and steeper slopes had been formed in the postglacial stage, this postglacial erosion is not uniform and is strongly influenced by the geology. In other words, the drainage density is low and less than 40% of periglacial smooth slopes have been eroded to the present in the Koetoi and Wakkanai Formations while it is high and 60% or more of those slopes have already been eroded in the Yuchi and Sarabetsu Formations.
  2) The depths of valleys formed during the postglacial stage can be classified into two types with a comparatively shallow valley
  in the formations higher than Koetoi and the comparatively deep valley in the formations lower than Wakkanai. Additionally the varying processes of a river-bed altitude are different in the northern part and the southern part of the region.
  3) From the result of the above analysis, it is concluded that the factors which change the groundwater flow system when it shifted from the last glacial to the postglacial stage are disappearance of permafrost, rise of river-bed altitude of main stream and rainfall augmentation.

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  Japan currently plans to dispose of high-level radioactive wastes (vitrified HLWs) produced from the reprocessing of spent nuclear fuel in deep geological formations, in order to isolate the radioactive wastes from the human environment for tens of thousands of years. Such a geological repository must be designed to ensure operational safety and post-closure safety. Then, following the closure of the geological repository, post-closure safety will be provided by an engineered barrier system (EBS) and a natural barrier system (NBS) without relying on monitoring or institutional control. However, from a technical standpoint, monitoring has been required during backfilling in current studies. Additionally, there has been strong social pressure to continue monitoring during all the phases including post-closure. On the basis of the current situations, a monitoring strategy for geological disposal must be studied to ensure the long term safety of geological disposal.
  Focusing on decision making for geological repository closure, the authors have created a basic logical structure for the decision making process with the principles for ensuring safety and have developed a monitoring strategy based on the logical structure. The monitoring strategy is founded on three key aspects: the role of monitoring, boundary conditions of monitoring at the time of decision making, and a methodology for monitoring planning. Then, the monitoring strategy becomes a starting point of monitoring planning during site characterization, construction, operation and staged closure, as well as post-closure with institutional control, and of social science studies.

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