Presentation Information
[T8-O-2]Advancement of a probabilistic assessment method for long-term natural phenomena toward safety assessment of geological disposal
*Junichi GOTO1, Yusaku TAKUBO1, Hiromitsu SAEGUSA1, Hirohito INAKURA2, Hideki KAWAMURA3 (1. Nuclear Waster Management Organization of Japan, 2. West Japan Engineering Consultants Inc., 3. mcm japan)
Keywords:
geological disposal,safety assessment,volcanic/igneous activity,fault activity,deep-seated fluids,uplift/subsidence,probability
Background/aims:
The disposal site for high-level radioactive waste will be selected based on step-by-step investigations to avoid significant impacts of natural phenomena such as volcanic and igneous activity and fault activity in the future and to understand the areas having favorable geological environment characteristics. The long-term safety of the selected area, which will last for more than 100,000 years, will be evaluated considering the uncertainty associated with the long evaluation period. In the case of a natural phenomenon having a significant impact on the safety functions of the repository, even if the probability of the phenomenon occurring is extremely low, the extent of the impact on the geological environment will be estimated and the probability of occurrence and the extent of the impact will be evaluated separately [1]. NUMO has previously developed the ITM-TOPAZ methodology for such evaluations [2] [3]. However, the following issues have been addressed:
- Establishment of a basis for setting scenarios (evolution scenarios) of both regional and local events over the next 100,000 years and beyond
- Accumulation of basic information on the impacts on the geological environment around the repository due to volcanic/igneous activity, fault activity, and deep-seated fluids, and develop methods for setting scenarios (impact scenarios)
- Reaching consensus on how to assess the likelihood of these scenarios occurring probabilistically through expert elicitation.
A three-year study was launched in FY2024 to upgrade the ITM-TOPAZ methodology to resolve these issues to apply for the Preliminary Investigation stage and beyond.
Results:
In FY2024, we considered the evaluation process for the ITM-TOPAZ-2 methodology.
(1) Setting evaluation scale: For the scale of the evolution scenarios, an area of approximately 100 km square for plate movement and an area of approximately 50 km square for events around the site are set, and for the scale of the impact scenario, an area of approximately 5 km square is set.
(2) Setting assessment scenarios: Impact scenarios will be changed to impact propagation scenarios, and scenarios of the propagation of impacts on the geological environment of the repository by slow processes such as uplift/subsidence, erosion/sedimentation, and sea-level change, as well as fast events such as fault displacement, magma intrusion/eruption, and inflow of volcanic hot water/deep-seated fluids, will be set in discussion with domestic experts. Knowledge gained from the application of the SSHAC method in Japan [4] will be reflected in the discussions and consensus building with domestic experts.
(3) Calculation of probability: The BN method is applied to calculate the occurrence probability of the impact propagation scenario. The BN model associates information related to the evaluation target, formulas/models for calculating probability, and the collective opinions of experts, and is capable of highly objective and transparent evaluation [5]. In addition, the random forest method is applied to estimate the spatial probability density distribution of volcanic/igneous activity and fault activity, which are input information to the BN model [6].
(4) Comprehensive assessment: To incorporate the probability of occurrence of evolution scenarios and impact propagation scenarios into a risk-based safety assessment, the probability of occurrence of natural phenomena that may affect the safety functions of a geological disposal system will be presented.
In the future, we will work with domestic and international experts to refine the ITM-TOPAZ-2 methodology and conduct case studies to confirm and improve its applicability.
References:
[1]NUMO(2021): NUMO-TR-20-03. [2]NUMO(2016): NUMO-TR-16-04. [3]Jaquet et al. (2017): J Volcan Geotherm Res,345,pp.58–66. [4]CRIEPI(2023): NR22002. [5]Aspinall et al. (2023): J Appl Volcano,12:5. [6]INL(2024): INL/RPT-24-78997.
The disposal site for high-level radioactive waste will be selected based on step-by-step investigations to avoid significant impacts of natural phenomena such as volcanic and igneous activity and fault activity in the future and to understand the areas having favorable geological environment characteristics. The long-term safety of the selected area, which will last for more than 100,000 years, will be evaluated considering the uncertainty associated with the long evaluation period. In the case of a natural phenomenon having a significant impact on the safety functions of the repository, even if the probability of the phenomenon occurring is extremely low, the extent of the impact on the geological environment will be estimated and the probability of occurrence and the extent of the impact will be evaluated separately [1]. NUMO has previously developed the ITM-TOPAZ methodology for such evaluations [2] [3]. However, the following issues have been addressed:
- Establishment of a basis for setting scenarios (evolution scenarios) of both regional and local events over the next 100,000 years and beyond
- Accumulation of basic information on the impacts on the geological environment around the repository due to volcanic/igneous activity, fault activity, and deep-seated fluids, and develop methods for setting scenarios (impact scenarios)
- Reaching consensus on how to assess the likelihood of these scenarios occurring probabilistically through expert elicitation.
A three-year study was launched in FY2024 to upgrade the ITM-TOPAZ methodology to resolve these issues to apply for the Preliminary Investigation stage and beyond.
Results:
In FY2024, we considered the evaluation process for the ITM-TOPAZ-2 methodology.
(1) Setting evaluation scale: For the scale of the evolution scenarios, an area of approximately 100 km square for plate movement and an area of approximately 50 km square for events around the site are set, and for the scale of the impact scenario, an area of approximately 5 km square is set.
(2) Setting assessment scenarios: Impact scenarios will be changed to impact propagation scenarios, and scenarios of the propagation of impacts on the geological environment of the repository by slow processes such as uplift/subsidence, erosion/sedimentation, and sea-level change, as well as fast events such as fault displacement, magma intrusion/eruption, and inflow of volcanic hot water/deep-seated fluids, will be set in discussion with domestic experts. Knowledge gained from the application of the SSHAC method in Japan [4] will be reflected in the discussions and consensus building with domestic experts.
(3) Calculation of probability: The BN method is applied to calculate the occurrence probability of the impact propagation scenario. The BN model associates information related to the evaluation target, formulas/models for calculating probability, and the collective opinions of experts, and is capable of highly objective and transparent evaluation [5]. In addition, the random forest method is applied to estimate the spatial probability density distribution of volcanic/igneous activity and fault activity, which are input information to the BN model [6].
(4) Comprehensive assessment: To incorporate the probability of occurrence of evolution scenarios and impact propagation scenarios into a risk-based safety assessment, the probability of occurrence of natural phenomena that may affect the safety functions of a geological disposal system will be presented.
In the future, we will work with domestic and international experts to refine the ITM-TOPAZ-2 methodology and conduct case studies to confirm and improve its applicability.
References:
[1]NUMO(2021): NUMO-TR-20-03. [2]NUMO(2016): NUMO-TR-16-04. [3]Jaquet et al. (2017): J Volcan Geotherm Res,345,pp.58–66. [4]CRIEPI(2023): NR22002. [5]Aspinall et al. (2023): J Appl Volcano,12:5. [6]INL(2024): INL/RPT-24-78997.