Janez Perko

Belgian Nuclear Research Centre SCK-CEN · Institute of Environment Health and Safety

The effectiveness of sol-gel based treatments for the protection of concrete depends on their capacity to penetrate into the material pores. Optimization of sol formulation to achieve maximum penetration depth is not a straightforward process, as the influence of different physical properties of the sol varies with the pore size distribution of eac...

This paper investigates the synergistic interaction between carbonation and microcracks in hardened cement pastes. Ordinary Portland cement pastes with three different water/cement ratios of 0.4, 0.5 and 0.6, hydrated for 28 days with crack apertures ranging from 10 to 150 μm were subjected to accelerated carbonation in a climate chamber that contr...

Because of its solid structure, low permeability, slow diffusion and good chemical retention for many radionuclides, cementitious materials are often used for conditioning radioactive waste and are omnipresent in many radioactive waste disposal facility concepts as engineered barriers. Cementitious materials are not in geochemical equilibrium with...

Many environments to which concrete is exposed are highly aggressive due to various chemical components. In such environments, concrete is subjected to processes of chemical degradation, among which carbonation is one of the most frequently seen degradation processes. Though, the influence of saturation degree (or relative humidity - RH) of the spe...

Modelling of combined dissolution-precipitation at the pore scale requires the conceptualization of mineral precipitation and crystal growth, the formation of a protective diffusive layer by precipitants and slow reaction kinetics that are all coupled with alterations of the microstructure. In this work, we propose an improved approach for handling...

The performance of impregnation treatments used for protection and remediation of porous building materials relies on sufficient penetration depth. The penetration of sol–gel impregnation products into partially saturated porous material is driven by capillary suction and depends on material properties, such as pore size distribution on one hand, a...

A coupled numerical approach is used to evaluate the influence of pore connectivity and microcracks on leaching kinetics in fully saturated cement paste. The unique advantage of the numerical model is the ability to construct and evaluate a material with controlled properties, which is very difficult under experimental conditions. Our analysis is b...

In this paper, we describe a single-relaxation-time (SRT) lattice Boltzmann formulation, which can be effectively applied to anisotropic advection-dispersion equations (AADE). The formulation can be applied to space and time variable anisotropic hydrodynamic dispersion tensor. The approach utilizes diffusion velocity lattice Boltzmann formulation w...

Simulation of dissolution processes with a pore-scale reactive transport model increases insight in coupled chemical-physical-transport processes. However, modelling of dissolution process often requires a large number of time steps especially when the buffering capacity of solid phases is high. In this work we analyze the interplay between solid b...

This paper presents an overview of a research program at the Belgian Nuclear Research Centre (SCK•CEN) concerning long-term durability of cementitious materials.The need for this research originates from the Belgian nuclear waste disposal concept, which proposes ordinary portland cement as the candidate binder for engineered barriers. Three main de...

This work aims at understanding of carbonation process in unsaturated fractured cementitious materials, which influences significantly the material properties. Dissolution of cement hydration products (portlandite (CH), C-S-H) and precipitation of calcium carbonate cause changes in surface geometry. On the other hand, geometry determines the amount...

The problem of calcium leaching kinetics of ordinary Portland cement based mortars is revisited via a mesoscale approach. Based on state-of-the-art lattice-Boltzmann technique, a comprehensive suite of leaching analysis is undertaken to address a number of open questions such as (i) the competing effects of interface transition zone (ITZ) and aggre...

In this paper, a newly developed lattice Boltzmann method based reactive transport model to simulate changes in microstructure of ordinary Portland cement paste due to calcium leaching is presented. The model takes three-dimensional digitized cement paste microstructure as input and is capable to capture an evolution of microstructure due to leachi...

The role of capillary pores percolation and gel pores are investigated to explain the underlying differences between relative diffusivity obtained from different experimental techniques using microstructures generated from two different types of hydration model viz., CEMHYD3D (a voxel based approach) and HYMOSTRUC (a vector based approach). These m...

Microcracks have a potentially significant influence on geochemical processes in cement-based materials. Microcrack properties (e.g. length, aperture, roughness) in turn are changed by various reactions, such as carbonation of the material. In this work, experimental crack geometry is analyzed statistically in order to understand how the crack feat...

Durability models used for service life predictions are often based on empirical data derived from lab scale experiments. However, the parameters derived from lab scale experiments are not always valid for field applications and for many degradation mechanisms lab scale experiments are often carried out in accelerated conditions. In such a situatio...

Many degradation mechanisms of concrete structures are physico-chemical processes. These physico-chemical processes alter the cement matrix mineralogy due to dissolution/precipitation of mineral phases and consequently, the cement morphology is changed which affects the physical and mechanical properties. Due to slow nature of many of these physico...

This paper provides a comprehensive overview of existing experimental and modelling approaches to determine effective diffusion coefficients of water saturated ordinary Portland cement-based materials. A dataset for diffusivity obtained from different experimental techniques have been presented for cement paste, mortar and concrete. For cement past...

Physical and mechanical properties of cement-based materials are directly linked to their microstructure. In recent years, different microstructure modelling platforms have emerged (e.g., HMYOSTRUC, CEMHYD3D and µic) which simulate the evolution of the microstructure of cement paste during hydration. These microstructures can be utilized to obtain...

In modern science and engineering, computer modelling forms an inevitable part in the assessments of risk or engineering performance. With the increasing amount of experimental data and process knowledge also the complexity of underlying models increases. However, the increasing model complexity involves certain consequences. Firstly, increasing mo...

The benchmark problem presented in this paper deals with the leaching of calcium from hardened cement paste. The leaching of calcium results in the dissolution of the cement minerals which affects physical, chemical and mechanical properties of porous cement matrix. The dissolution of cement minerals in this case progresses heterogeneously as a con...

This paper describes the application of a single relaxation time (SRT) lattice Boltzmann scheme to the transport in porous media with large spatial variations of diffusion coefficients. Effective diffusion coefficients can vary substantially within porous media because of their dependence on porosity and tortuosity which can span over several order...

Leaching process of cement-based materials is relevant for the long-term durability of cement based barrier materials used in radioactive waste disposal systems. Ca leaching changes the properties by reducing the pH, increasing the porosity which leads to the variation of transport properties. This process is extremely slow under environmental cond...

Calcium leaching in cement paste causes dissolution of portlandite and AFm phases which results in the opening of capillary pores and decalcification of amorphous C-S-H resulting in an increase of gel porosity. These alterations in turn affect transport and mechanical properties of cement paste. The aim of this study is to analyse the influence of...

Near-surface cement-based disposal systems for hazardous materials such as radioactive waste will undergo chemical alterations due to interaction with the surrounding environment. One of the most relevant long-term geochemical alteration processes is decalcification or leaching of the cement phases by percolating water. Consequently, the cementitio...

Calcium leaching is one of the important degradation mechanisms causing dissolution of the crystal- line phases such as, AFm, portlandite increasing capillary porosity. Further it leads to decalcification of an amorphous C-S-H phase causing increase in the gel porosity and in turn degrading the long term performance of concrete structures. In thi...

The paper presents an approach that extends the flexibility of the standard lattice Boltzmann single relaxation time scheme in terms of spatial variation of dissipative terms (e.g., diffusion coefficient) and stability for high Péclet mass transfer problems. Spatial variability of diffusion coefficient in SRT is typically accommodated through the v...

Analysis of the effects of fractures in porous media on fluid flow and mass transport is of great importance in many fields including geotechnical, petroleum, hydrogeology and waste management. This paper presents sensitivity analyses examining the effect of various hypothetical fracture networks on the performance of a planned near surface disposa...

The paper aims to highlight recent developments at the Belgian Nuclear Research Centre SCK•CEN in experimental and numerical study of the coupled physical-chemical behaviour of concrete subject to chemical degradation. The discussion mainly focusses on three interlinked research projects covering novel experimental methods to study the alteration o...

ONDRAF/NIRAS, the Belgian Agency for Radioactive Waste and Enriched Fissile Materials, and its partners have developed long-term safety and performance analyses in the framework of the license application for a surface disposal facility for low level radioactive waste (category A waste) at Dessel, Belgium. This paper focusses on the methodology of...

This report summarizes the latest developments at SCK•CEN in modelling long-term evolution of cementitious materials used as engineered barriers in waste disposal. In a first section chemical degradation of concrete during leaching with rain and soil water types is discussed. The geochemical evolution of concrete thus obtained forms the basis for a...

In a large cement-based structures such as a near surface disposal facility for radioactive waste, voids and cracks are inevitable. However, the nature of cracks and crack patterns are impossible to predict. Cracks facilitate preferential water flow through the facility because their saturated hydraulic conductivity is generally higher than the con...

This paper presents a consistent approach for the development of a comprehensive data base of time-dependent hydraulic and transport parameters for concrete engineered barriers of the future Dessel near surface repository for low level waste. The parameter derivation is based on integration of selected data obtained through an extensive literature...

A Lattice Boltzmann (LB) based reactive transport model intended to capture reactions and solid phase changes occurring at the pore scale is presented. The proposed approach uses LB method to compute multi-component mass transport. The LB multi-component transport model is then coupled with the well-established geochemical reaction code PHREEQC whi...

Long-term safety of radioactive waste disposal facilities relies on the longevity of natural or engineered barriers designed to minimize the migration of contaminants from the facility into the environment. Especially near surface disposal facilities, such as planned by ONDRAF/NIRAS for the Dessel site in Belgium, long-term safety relies almost exc...

We here present model assumptions for the cementitious near field of the Belgian near surface repository for four types of leaching models. For each of these model types, the report describes upfront the purpose and scope of the model which includes approach taken, timeframe of interest, calculation endpoints such as safety indicators, complementar...

Two numerical codes, TOUGH2 with the EOS5 module and CODE_BRIGHT, were compared in a confidence building effort for multiphase flow problems in a geological repository in Boom Clay, Belgium. A model study comparison between two codes was carried out through three numerical examples, including a one-dimensional hydro-gas (HG) case, a two-dimensional...

Assessment of gas generation and transport is inevitable for evaluation of the safety of nuclear waste disposal in deep geological formations. The long-term safety of the geological disposal facility is guaranteed by several engineered and natural barriers. The reference disposal concept in Belgium consists of a concrete-based repository situated i...

Two numerical codes, TOUGH2 with the EOS5 module and CODE_BRIGHT, were compared in a confidence building effort for multiphase flow problems in a geological repository in Boom Clay, Belgium. A model study comparison between two codes was carried out through three numerical examples, including a one-dimensional hydro-gas (HG) case, a two-dimensional...

Study of presence of cracks in porous media and their effect on fluid flow and mass transport is of great interest in many fields including geotechnical, petroleum or civil engineering. In radioactive waste disposal there is considerable interest in crack behaviour in concrete structures for the following reasons: (i) open cracks are more permeable...

This paper presents a methodology to assess the spatial-temporal evolution of chemical degradation fronts in real-size concrete structures typical of a near-surface radioactive waste disposal facility. The methodology consists of the abstraction of a so-called full (complicated) model accounting for the multicom-ponent-multiscale nature of concrete...

The paper presents an application of a simplified 2D model of chemical degradation for a cracked concrete container termed monolith under near surface disposal conditions. The objective is to gain an improved understanding of the rate of degradation of the cracked monolith and its impact on transport of radionuclides under saturated conditions. Che...

The sorption and concomitant transport behaviour of major ions in saturated/unsaturated reinforced concrete is of paramount importance for estimating the service life of concrete. The present work specifically aims to assess the effect of elevated chloride concentrations originating from chloride bearing waste encapsulated in reinforced concrete co...

The objectives of this report are to provide underpinning arguments for the long term evolution of the cementitious near field of the Dessel (Belgium) repository as implemented in the reference and expected evolution scenarios. Following topics are covered: (i) initial and temporal evolution of physical-chemical variables as saturation degree of wa...

This report NIROND-TR 2010-17 provides guidance and direction on the groundwater monitoring programme for the near surface disposal facility for category A waste in Belgium, in the Dessel nuclear zone. It addresses the extent and timeframes of the potentially contaminated groundwater area(s) as a result of leaching of radionuclides and non-radioact...

This presentation gives an overview of some of the challenges in demonstrating long-term safety of radioactive waste repositories.

For various countries, the direct disposal of high level nuclear fuel wastes is a key option for the backend of the fuel cycle. For HTR/VHTR reactors this is assumed for the introductory phase of this reactor system. However, closed fuel cycles or a separation of spent coated-particles from the graphite moderator and specific treatment, conditionin...

This report documents a first benchmarking exercise carried out with two in-house, independently developed, biosphere codes – BIOSPHERE and BIOHINT – that are used at SCK•CEN to quantify the radiological impact (dose) for the most commonly treated exposure pathways in repository safety assessment.

During the current project phase of the disposal of category A waste – short-lived low and intermediate level radioactive waste – ONDRAF/NIRAS must, following the governmental decision of 23 June 2006, develop a near surface disposal facility in Dessel, which is based on the near surface facility design developed by the STOLA-Dessel partnership dur...

For many near-surface disposal facilities, concrete is an important engineered barrier. Knowledge on the durability of such concrete components and its relation to radionuclide sorption is important for a defensible safety assessment. One of the most challenging issues in safety assessment concerns the long time scales involved and the time evoluti...

When modelling complex physical problems model simplifications present an important step in a modelling procedure. One of the most common simplifications is dimensionality reduction. With dimensionality reduction the model becomes better manageable and consequently more robust. Mass transport modelling through porous media is typically characterize...

This work introduces a procedure for automated determination of weight function free parameters in moving least squares (MLS) based meshless methods for non-uniform grids. The meshless method used in present work is Diffuse Approximate Method (DAM). The DAM is structured in 2D with the one or two parameter Gaussian weigh function, 6 polynomial basi...