Michihiko Hironaga

Silicate Anion Structural Change in Calcium Silicate Hydrate Gel on Dissolution of Hydrated Cement

High pH conditions of aqueous solutions in a radioactive waste repository can be brought about by dissolution of cementitious materials. In order to clarify the mechanisms involved in maintaining this high pH for long time, we investigated the dissolution phenomena of OPC hydrate. In the present research, leaching tests on powdered cement hydrates were conducted by changing the ratio of mass of leaching water to mass of OPC hydrate (liquid/solid ratio) from 10∽2, 000 (wt/wt). Ordinary Portland Cement hydrate was contacted with deionized water and placed in a sealed bottle. After a predetermined period, the solid was separated from the solution. From the results of XRD analysis on the solid phase and the Ca concentration in the aqueous phase, it was confirmed that Ca(OH)2 was preferentially dissolved when the liquid/solid ratio was 10 or 100 (wt/wt), and that C—S—H gel as well as Ca(OH)2 were dissolved when the liquid/solid ratio was 500 (wt/wt) or larger. 29Si-NMR results showed that the silicate anion chain of the C-S-H gel became longer when the liquid/solid ratio was 500 (wt/wt) or greater. This indicates that leaching of OPC hydrate results in a structural change of C—S—H gel.

Gas Migration Mechanism of Saturated Highly-Compacted Bentonite and Its Modeling

In the current concept of repository for radioactive waste disposal, compacted bentonite will be used as an engineered barrier mainly for inhibiting migration of radioactive nuclides. Hydrogen gas can be generated inside the engineered barrier by anaerobic corrosion of metals used for containers, etc. If the gas generation rate exceeds the diffusion rate of gas molecules inside of the engineered barrier, gas will accumulate in the void space inside of the engineered barrier until its pressure becomes large enough for it to enter the bentonite as a discrete gaseous phase. It is expected to be not easy for gas to entering into the bentonite as a discrete gaseous phase because the pore of compacted bentonite is so minute. Therefore the gas migration tests are conducted in this study to investigate the mechanism of gas migration. On the basis of the experimental facts obtained through the gas migration tests, possible gas migration mechanism is proposed. A simplified method for calculating gas pressure at large breakthrough, which is defined as a sudden and sharp increase in gas flow rate out of the specimen is also proposed.Copyright

Corrosion behaviour of aluminium under simulated environmental conditions of low-level waste

ABSTRACT In Japan, low-level radioactive waste including aluminium alloys is generated from nuclear power plants. It is well known that aluminium reacts with both alkaline cement and water, generating hydrogen gas. For the safe management of radioactive waste disposal facilities, it is necessary to evaluate corrosion and hydrogen generation behaviours of aluminium corresponding to changing environmental conditions. In the present study, the corrosion behaviour of aluminium in alkaline solutions at 15 °C was evaluated. This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

Study on Mechanical Influence of Gas Generation and Migration on Engineered Barrier System in Radioactive Waste Disposal Facility

In Japan, some radioactive waste with a relatively higher radioactivity concentration from nuclear facilities is to be packaged in rectangle steel containers and disposed of in subsurface disposal facilities, where normal human intrusion rarely occurs. After the closure of a facility, its pore is saturated with groundwater. If the dissolved oxygen of the pore water is consumed by steel corrosion, hydrogen gas will be generated from the metallic waste, steel containers, and reinforcing bars of concrete mainly by anaerobic corrosion. If the generated gas accumulates and the gas pressure increases excessively in the facility, the facility’s barrier performance might be degraded by mechanical influences such as crack formation in cementitious material or deformation of bentonite material. Firstly, in this study, we assessed the time evolution of the gas pressure and the water saturation in a sub-surface disposal facility by using a multi-phase flow numerical analysis code, GETFLOWS, in which a pathway dilation model is introduced and modified in order to reproduce the gas migration mechanism through the highly compacted bentonite. Next, we calculated the stress applied to the engineered barriers of the facility from the results of the time evolution of the pressure and the saturation. Then, we conducted a mechanical stability analysis of the engineered barriers by using a nonlinear finite element code, ABAQUS, in order to evaluate their performances after the closure of the facility.Copyright

Expansive Development of a Decommissioning Program “Recycle Simulator” in Nuclear Power Station

A decommissioning program “RECYCLE SIMULATOR” should be put into practice in careful consideration of both recycle of non-radioactive wastes and reduce of radioactive wastes in the coming circulatory social system. Nevertheless current support systems for decommissioning planning mainly deal with decontamination, safety storage and dismantlement, so-called the prior part of the total decommissioning process. Authors emphasize the necessity of total planning of decommissioning including recycle or reuse of a large amount of demolition materials and are propelling the development of the multi expert system named “RECYCLE SIMULATOR”. This paper presents an algorithm of the recycling and reusing scenario of demolition materials and a summarized configuration. RECYCLE SIMULATOR for the demolished concrete was developed in 2000 and presented at 10th International Conference on Nuclear Engineering (icone10). Construction of a supporting multi expert system for the totally planning of decommissioning projects is objected by expansive development of the previous version. Main conclusions obtained from this paper are under this. (1) The previouslly developed expert system was advanced in its estimational function toward the satisfaction of decommissiong planners. (2) The applicability of the system was enlarged to all the radioactive and non-radioactive wastes, demolished metal and concrete products, in a corresponding site of decommissioning. (3) Finally decommissiong recycle simulator was completed in a harmonized unification.Copyright

Estimation of a Decommissioning Program Considering the Reuse of Demolition Materials

As for a decommissioning job in Japan decontamination and dismantling including safety store are executed during the prior period of the total project. The reprocessing and disposal of dismantling wastes as well recycling is to be practiced during the succeeding period. An expert system proposed in this paper has a faculty of furnishing motivations for decommissioning planners by correlative estimations of the project between its prior part and the succeeding recycle part of total process. This paper presents both a summarized configuration and an algorithm of the proposed model and indicates some contents of essential data bases (D/B) to be prepared as well some additional data to embody an assumed scenario for recycling. The proposed model provides the useful outputs concerning commercial cost, required procedures or licenses, regional encouragement, obstacles to be surmounted and so on. These outputs are available to explain the outline of the project both to inside and to outside of a plant corporate entity by combining each other. Simulated cases for concrete structures in non-controlled areas bring some information about both feasibility and comparisons of assumed recycling scenarios taking account of quality requirements in relevant technical standards, ordinances and the ability of reprocessing facilities.Copyright