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Coupled physical processes will take place in a multibarrier disposal system for spent nuclear fuel and high-level radioactive waste. The knowledge of these processes (thermal, hydraulic, mechanical, chemical, microbiological, etc.) as well as the scope and scale of their interactions is fundamental for the safety assessment of a disposal facility. Numerical modeling is an important component in the process of acquiring and deepening the knowledge of coupled processes, while experimental evidence isimportant for model validation. This article will present a hydro-mechanical model developed by the Lithuanian Energy Institute (LEI) in the framework of H2020 project BEACON (Bentonite Mechanical Evolution). The non-linear elastic model developed in COMSOL Multiphysics (Burlington, MA 01803, USA) was applied to predict the swelling behavior of large-scale oedometer experiments (MGR) performed by Research Centre for Energy, Environment and Technology (CIEMAT, Spain). In these experiments on bentonite hydration at isochoric conditions, a sample was made of two layers of calcium bentonite (FEBEX type) having initially different hydro-mechanical characteristics: one layer made of pellets and the other of a compacted block. Satisfactory agreement between the modeling results and the experimental data were obtained, especially for water intake and sample saturation.

In Lithuania all the spent nuclear fuel (SNF) came from operation of the Ignalina nuclear power plant with two reactors of RBMK type (RBMK is a Russian acronym for 'Channelized Large Power Reactor' which is a water-cooled graphite-moderated reactor: RBMK-1500). Approximately 22,000 SNF assemblies are due for geological disposal in Lithuania. Currently it is envisaged that SNF will be stored in dry interim storage facilities (new and existing) for at least 50 y prior to possible deep geological disposal.The decision on the final SNF management option (disposal in a national repository, disposal in regional repositories, etc.) has not yet been made but some investigations of the possibilities to dispose of the SNF in Lithuania have been initiated. With the support of Swedish experts, analysis of possible geological formations for SNF disposal was performed and the existence of potentially suitable formations agreed. The geological formations prioritized as prospective include the crystalline rocks in southern Lithuania and two clayey formations: the Lower Triassic clay formation and the Lower Cambrian Baltic Group clay formation, with priority given to the Lower Triassic clay formation.This paper presents the main aspects of the research and other activities undertaken over the past decade in the field of SNF disposal: international cooperation; current status and plans for the Lithuanian national program; further investigations required; and competence developments.

As in other nuclear countries, the operation of the Ignalina nuclear power plant in Lithuania has led to the accumulation of around 22 thousand assemblies of spent nuclear fuel (SNF). The development of geological disposal program involves an iterative assessment of the system safety supported by scientific research on radionuclides migration and related processes. This study focused on the application of Contribution to the Sample Mean (CSM) and Contribution to Sample Variance (CSV) methods to complement the uncertainty and sensitivity analyses of the time-dependent flux of I-129 from the engineered barriers of a conceptual disposal facility for RBMK-1500 SNF (RBMK is abbreviation of "High Power Channel-type Reactor" (in Russian)). The analysis was performed using a MATLAB platform (8.0.0.783 (R2012b), MathWorks, MA, USA). The mean and variance ratios derived from CSM and CSV plots were applied to estimate the effect of reduced uncertainty range on mean flux and its variance, and the uncertainty analysis was also complimented. Increasing the lower bounding value of defect size enlargement time range to 4.6 × 10 4 years would lead to a lower mean flux until 5 × 10 4 years after repository closure. Later on (up to 1 million years after repository closure), the only reduction of the upper bounding value of the SNF dissolution rate range would affect a decreased mean flux.Keywords: RBMK-1500 spent nuclear fuel; deep geological repository; uncertainty and sensitivity; CSM; CSV; radionuclide migration 3 × 10 −12 (p = 0.15) 1 × 10 −11 (p = 0.7) 3 × 10 −11 (p = 0.15) Discrete 1 Probability density function. SNF: spent nuclear fuel.Minerals 2019, 9, 521 5 of 26 Characterization of Model BehaviorThe model realized using the computer code AMBER [34] assessed time-dependent radionuclide release from the SNF matrix, dissolution, radioactive decay, and contaminant transport by diffusion through engineered barriers. The uncertainty of the main transport-related parameters, including defect size enlargement time, was characterized by probability density function for each parameter. The model output was a large number of the time-dependent flux values over an extended period (10 3 -10 6 years after repository closure), which allowed for the evaluation of the mean flux, the quantiles, and the distribution of the peak flux, as illustrated in Figure 2.

Lietuvos energetikos institutas, Branduolinės inžinerijos problemų laboratorija, Breslaujos g. 3, LT-44403 Kaunas El. paštas Povilas.Poskas@lei.ltBranduolinė energetika, kaip ir kitos pramonės šakos, teikia ne tik ekonominę naudą, bet ir sukuria atliekas. Branduolinės atliekos, nors jų santykinis tūris ir yra žymiai mažesnis nei kitų pramonės šakų atliekų, yra radioaktyvios ir todėl ypač pavojingos aplinkai bei žmonėms. Radioaktyviosiomis atliekomis vadiname naudotas ir netinkamas naudoti radioaktyviąsias medžiagas, kurių aktyvumas viršija nustatytą lygį. Joms priskiriamas panaudotas branduolinis kuras ir kitos, pakartotinai naudoti neskirtos, radionuklidais užterštos ar turinčios jų savo sudėtyje, medžiagos, kurių radionuklidų savitasis aktyvumas viršija nebekontroliuojamuosius radioaktyvumo lygius. Ne visos radioaktyviosios atliekos yra vienodai pavojingos žmogui, todėl skirstomos į tipus ir klases. Atsižvelgiant į radioaktyviųjų atliekų tipą ir klasę taikomi skirtingi jų tvarkymo, saugojimo, talpinimo į atliekynus būdai, taip pat skiriasi ir pačių atliekynų tipai (paviršinis Landfill tipo, paviršinis su gelžbetonio rūsiais ar giluminis).Nuo 1994 m. Lietuvos energetikos instituto Branduolinės inži-nerijos problemų laboratorijos mokslininkai kaupia patirtį analizuodami ir spręsdami radioaktyviųjų atliekų tvarkymo problemas Lietuvoje ir užsienyje. Šiame straipsnyje aptariamas pastarojo de-