In Finland, the disposal system for spent nuclear fuel is based on a nested barrier system where barriers are designed to confine and limit possible radionuclide release rates into the biosphere. A postulated leak in a waste canister is used to assess the performance of the individual barriers. The radionuclide migration is depicted by using a simplified solute transport model that is an alternative approach compared to numerical modelling used largely in the performance assessments.An earlier simplified model is extended by introducing both a simplified biosphere analysis and a probabilistic simulation in order to estimate the radiological consequences of the disposal. In the simplified model, the radionuclide migration is calculated by using an analogy to a branching radioactive decay chain. The time constants of the model are solute half-times and delay times in the release barriers and biosphere compartments. A dose assessment is performed based on the resulting radionuclide inventories in the biosphere compartments. With a probabilistic simulation, initial data can be distributed and confidence levels for the release rates or dose rates can be determined.A simulation with seven nuclides (C-14, Cl-36, Se-79, Mo-93, Nb-94, Ag 108m, I-129) using the simplified model and chosen parameter distributions and correlations reveal that the distributions for results (time constants, dose rates, release rates) are roughly log-normal or log-uniform. The median total dose rate (50 % confidence level) and cautious 95 % confidence level have about two orders of magnitude difference at most during the 10 millennia time scope. The parameters with the largest uncertainty do not affect the final result if they are not related to the most significant barriers of the system (canister or buffer). The simplified model performs well for the geosphere transport but overestimates the results of the biosphere analysis compared to earlier results with similar parameters.
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机译：在芬兰，乏核燃料处置系统基于嵌套式屏障系统，该屏障的设计旨在限制和限制放射性核素向生物圈的释放速率。废物罐中假定的泄漏用于评估各个屏障的性能。通过使用简化的溶质迁移模型来描述放射性核素的迁移，这是与主要用于性能评估的数值模型相比的另一种方法。通过引入简化的生物圈分析和概率模拟来扩展较早的简化模型，以估算处置的放射学后果。在简化模型中，通过使用类似于分支放射性衰变链的方法来计算放射性核素迁移。该模型的时间常数是释放壁垒和生物圈隔室中的溶质半衰期和延迟时间。根据生物圈隔室中产生的放射性核素清单进行剂量评估。通过概率模拟，可以分配初始数据并确定释放速率或剂量率的置信度。使用七个核素（C-14，Cl-36，Se-79，Mo-93，Nb-94， Ag 108m，I-129）使用简化的模型以及选择的参数分布和相关性揭示，结果的分布（时间常数，剂量率，释放率）大致为对数正态或对数均匀。在10千年的时间范围内，中值总剂量率（50％置信水平）和谨慎的95％置信水平之间最多存在大约两个数量级的差异。如果不确定性最大的参数与系统的最重要障碍（容器或缓冲区）无关，则它们不会影响最终结果。简化的模型对于地圈运输具有良好的效果，但是与具有类似参数的早期结果相比，高估了生物圈分析的结果。
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