RECENT PROGRESS OF THE HLW DISPOSAL PROGRAM IN JAPAN

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1 RECENT PROGRESS OF THE HLW DISPOSAL PROGRAM IN JAPAN Ê Toshiaki Ohe Department of Nuclear Engineering, Tokai University, 1117 Kita Kaname, Hiratsuka, Kanagawa , Japan Ê Abstract Introduction The 2 nd progress report for the research development of high-level radioactive waste disposal has been preparing and the first draft was recently announced by Japan Nuclear Cycle Development Institute which is the responsible organization for research & development of HLW disposal. In this draft, the following three major findings were addressed, 1) stable geological environment for waste isolation can be observed, 2) reliable data in deep underground are accumulated to demonstrate the acceptable conditions from mechanical, chemical and hydraulic point of view, 3) more elaborate and rationalized system of the engineered barrier is designed. Their research program is based on the so-called site generic approach and no specific data of certain candidate site were utilized. This implies the results are more general but detailed system design which can be adjusted the site specific conditions are quite limited. The methodology of the performance assessment becomes more sophisticated to account and correspond to the conditions anticipated in different sites. The advances of the assessment technology include 1) Feature Event Process / Process Influence Diagram approach for scenario determination, 2) channel network modeling for far-field nuclide migration analysis, 3) reference biosphere modeling for dose conversion. It seems the procedure becomes complicated and data uncertainty may affect very much to the final results. Consequently margin of the assessment result cannot help being set more or less to the upper boundary of the data range as long as applying the site generic approach. The situation is a paradox between highly advanced technology and robustness of the assessment. In 1994, Power Reactor and Nuclear Fuel Development Co. (PNC), which was the responsible organization of the research and development for HLW technology in Japan, established the first progress report for the HLW disposal (1). In the report, 1) very general view of the geological condition in Japan for waste isolation, 2) a similar disposal system concept as reported by NAGRA in Switzerland, and 3) a simple safety analysis by utilizing a data set of conceptual disposal site which was not really exist, were addressed as preliminary results. It has been 5years passed since the document was reported, accumulation of the progress both theoretically and substantially would be expected during these years. PNC has been recently reorganized as Japan Nuclear Cycle Development Institute (JNC) and JNC announced the second progress report as a result of the research project. The first draft version is available now and this would be revised through review process by both authorities and citizens because the document is completely opened and can be easily accessed through the home page (2) of internet. In this draft, three major findings were addressed; 1) stable geological environment for waste isolation can be observed, 2) reliable data in deep underground are accumulated to demonstrate the acceptable conditions from mechanical, chemical and hydraulic point of view, 3) more elaborate and rationalized system of the engineered barrier is designed. The performance assessment also has much progress from technical view point and In this presentation, the recent prog ress of the HLW research prog ram is informed by featuring the differences between two progress reports, and the assignments of the further extension of the research toward a final goal are extracted. Geological Environment for Waste Isolation The larger concern about HLW disposal among the public was extracted from questionnaire (3) and the results showed seismic and volcanic activities were typical awareness. The possibilities of such activities during the time period of disposal are therefore the most interesting feature and the issue to answer this has much importance. Geological, geomorphologic, and volcanological evidences historically accumulated so far imply that a similar geological environment would be expected up to 10 5 years from now by simple extrapolations of the previous tendencies, and this environment would be favorable for waste isolation. Fig.1 demonstrates an example of the uplift velocity observed at a certain mountainous area indicating that the velocity would be almost constant even during 0.7 million years. This example shows somewhat higher value and the general uplift velocity is expected much less than

2 that shown in the figure. The more the time scale is expanded, the less scientific evidences were left as shown in Fig.2. One possible judgement in the draft is that the current geological condition can be expected up to 10 5 years but the more extension of the time scale only from the current knowledge is limited and not fully acceptable.

3 Data of Deep Underground The reliable data accumulation is still underway by both compilation of references and observations carried out in some rock caverns. These efforts clarify the data range and the most probable data anticipated in a repository. Some typical data sets compiled in the database are shown in Table 1. Table 1 Data accumulated & measured Data categories Measured and/or accumulated values Mechanical Hydrological Geochemical Geothermal stress fields rock compressibility hydraulic conductivity hydraulic gradient fault, fissure distributions groundwater compositions thermal gradient rock thermal conductivity Elaborate and Rationalized Disposal System Many revisions of disposal system were reported. A typical example is shown in Table 2 for the case of the NAGRA type repository (4). Some of them are due to change of the design concept (for example, interim storage years) and some of them are caused by the revision of design data and methodology (for example, recalculation of stress field). These changes toward to the volume reduction of the total size of disposal site are more favorable both from resource and economical points of view. Two alternative designs were designed in the draft and the schematic view is shown in Fig.3. Fig.3 Cross-sectional View of Two Altanative Repository Des

4 Table 2 Major changes of the disposal system dimensions ( the case of NAGRA type repository at 1000m depth ) engineered barrier values cited in 1 st rept. 2 nd rept. notes thickness (cm) overpack buffer material disposal system storage prior to disposal (y) disposal area (m 2 /waste) disposal space(m/waste) tunnel diameter (m) recalculation of stress longer decay time reduction of thermal load improved excavation technique Performance Assessment Much progress is also found in the methodology of the performance assessment for the engineered barrier system, geosphere, and biosphere : 1) Feature Event Process / Process Influence Diagram approach for scenario determination, 2) channel network modeling for far-field nuclide migration analysis, 3) reference biosphere modeling for dose conversion. FEP technique By FEP technique scenarios can be classified into several categories : (1) scenarios eliminated through site selection (e.g. effect of volcanic activity), (2) scenarios eliminated by system design and QA program during sire construction (e.g. defect of waste package), (3) scenarios which have very low probabilities (e.g. impact of meteorite), (4) scenarios of which minor effects can logically be excluded, The above scenarios are excluded prior to analysis and most probable scenarios including underground-water intrusion are subjected to the assessment. The first draft version addressed only the underground-water intrusion scenario but the later version may refers to the extensive scenarios which should be taken into account. Assessment Codes In the first prog ress report, a very simple nuclide mig ration code in the fissured media was utilized: one dimensional single open fissure model. But this is too simple to realize the actual site condition thus more sophisticated model was developed: channel network model. As shown in Fig.4 a fissured media is presented by a set of three dimensional channel connections and the nuclide migration, sorption and diffusion into rock matrix are taken into account simultaneously in the model. The channel network has a pure probabilistic feature and one set of channel connections has certain possibility of occurrence. The multiple calculations are thus required to obtain statistical meaning. This step is rather time-consuming and a one-dimensional model was developed instead for the sake of simplicity. This is a paradox between advanced technology and actual applicability.

5 Disturbed Zone As for the assessment, the conversion from nuclide concentration to dose is necessary. In the first progress report, a very primitive dose conversion model was adopted. The nuclide behavior in the biosphere was assumed such that the contaminated effluent of groundwater flew out into a river and diluted there instantaneously then the river water was used as drinking water. This is one possible pathway and another possibilities have to be taken into account, for example, dose due to digestion of agricultural and sea products. In the second report more elaborated biosphere model was developed through the international collaboration work BIOMOVS and reference biosphere model for dose conversion was applied. Tentative Result Although the draft is in the stage of revision, a dose due to waste disposal is shown as a tentative result. Despite less than 10-5 µsv/y/waste which is equivalent to that reported in the first report. If we assume the repository contains wastes, which is the nominal values produced by the reprocessing of the spent fuels discharged during 1966 ~ 2015, and the simple multiplication suggests about 10-1 µsv/y as total contribution. There is no obvious acceptance level for HLW disposal yet but the criteria applied to the LLW disposal, 10 1 µsv/y may bring suggestion for comparison.

6 Fig.5 Tentative results of Dose for the Case of 400 Issue to be concerned Site specific approach JNSÕs research program is based on the so-called site generic approach and no specific data of certain candidate site were utilized. This implies the results are more general but detailed system design which can be adjusted the site specific conditions are quite limited. If the candidate site has much different geometrical condition, all the process for construction scheme and performance assessment must be re-evaluated. Of course, most of techniques developed so far can be adopted but re-evaluation process consumes time and resources. According to the national plan, the responsible organization will be established until 2000 and this organization would be in charge of the site selection and operation. The site-specific approach may be started afterwards. Code verification and validation The most characteristic feature of the HLW disposal is the length of time period of concern. Of course, we can not revise and adjust the system by the try-and-error scheme, which is commonly used in industry. The only procedure available in the HLW is the computer simulation and our decision may largely depend on the results of the computer work. Thus the verification and validation procedure has great importance. Despite many simulation results were presented in the draft, the above procedure is not clear. The former process is relatively simple because it needs only comparison of the results obtained with another independent simulation codes or purely mathematical solutions. On the contrary, the latter process is not easily achieved. Although the validation means the accordance between experiments and calculation as sited in the context, the accordance between man-made short experiments and the simulation does not fully support the code applicability for long-term phenomenon. Even if the simulation code reproduces the experimental findings on a computer display,

7 the fact is valid only for very short time of experiment. The accordance between calculation and experiment implies the only part or parts of phenomena in a real world was recreated. The real system may consist of piles of phenomena and continue up to more than 10 5 years. We thus need extensive breakthrough for the limitation of the validation procedure. Robustness The methodology of the performance assessment becomes more sophisticated to account and correspond to the conditions anticipated in different sites. It seems the procedure becomes complicated and many different data set are necessary for analysis. In this situation data reliability and uncertainty may affect very much to the final results. Accordingly, it cannot help achieving the assessment by using the upper boundary of the data range and the margin of the result may more or less be reduced as long as applying the site generic approach. The situation is also a paradox between highly advanced technology and robustness of the assessment. Conclusion The recent progress of the HLW research program is summarized by featuring the differences between two progress reports established by JNC (former PNC), the responsible organization of the research and development, then the assignments of the further extension of the research toward a final goal is extracted as follows: (1) imitation of site generic approach, (2) importance and difficulty of code validation, (3) robustness of the complex assessment technique. References (1) Power Reactor & Nuclear Fuel Development Co., ÒThe 1 st progress report for the research development of high-level radioactive waste disposal Ò, PCN TN , (1994). (2) (in Japanese) (3) String Committee on High-level-radioactive-waste Project, " The interim report of the high-levelradioactive-waste project ", May, (1996). (in Japanese). (4) Nagra, Nagra Project Gewahr, NGB 85-09, Nagra, Wettingen, Switzerland, (1985).