Assessment of Permian Zubers As the Host Rock for Deep Geological Disposal

energies

Article Assessment of Permian Zubers as the Host Rock for Deep Geological Disposal

Leszek Lankof

Department of Renewable Energy and Environmental Research, Mineral and Energy Economy Research Institute of the Polish Academy of Sciences, J. Wybickiego 7A, 31-261 Krakow, Poland; [email protected]

Received: 9 March 2020; Accepted: 21 April 2020; Published: 3 May 2020

Abstract: Together with renewable energy sources, nuclear power represents an important contribution to a sustainable energy mix in many countries and has an important impact on sustainable development. Nuclear energy production is also a source of high-level radioactive waste (HLW) and spent nuclear fuel (SNF), which require special concern. Disposal in deep geological formations is one of the solutions for the long-term management of HLW and SNF. It requires the development of a concept ensuring long-term safe isolation of waste and its validation applying the safety case methodology, which is a formal compilation of evidence, analyses and arguments that quantify and justify a claim that the repository will be safe. The results of laboratory testing of a potential repository host rock are an important component of the evidence that helps in the safety assessment of the deep geological disposal concept. This paper presents results of research focused on the physical, geomechanical and sorption properties of the Brown and Red Zuber unit rocks from the Kłodawa Salt Mine in Poland, which together with rock salt are an important component of Polish salt domes. The Brown and Red Zubers are typical evaporite lithostratigraphic units for the Polish part of the Zechstein Basin. They consist of halite (15–85%) and water-insoluble minerals, such as anhydrite, clay minerals, carbonates, quartz and feldspar, which occurred in varying proportions in the tested samples. The properties of the zuber rocks have been compared with those of rock salt, which is considered a suitable host rock for deep geological disposal of radioactive waste.

Keywords: zuber rock; rock salt; radioactive waste; deep geological disposal; radionuclide sorption; geomechanical properties; sustainable development

1. Introduction The deep geological disposal of high-level radioactive waste (HLW) and spent nuclear fuel (SNF), ensuring its long-term isolation from the environment and groundwater, relies on interdisciplinary scientiﬁc and technical issues. The majority of the HLW and SNF comes from controlled energy production during the process of nuclear ﬁssion of heavy elements in nuclear power plants. Nuclear power represents a crucial contribution to the energy mix in many countries and has an important impact on sustainable development [1–3]. At the end of the last century, the share of electricity production from nuclear energy reached its maximum, which was about 17.9% in 1996 [4]. Currently there are 443 nuclear power reactors in the world in 30 countries. Their installed capacity is around 391 GWe and the share of nuclear power plants in global electricity production is about 10% [5]. Interest in nuclear power remains strong in many regions, particularly in the developing world. Taking into account population growth, rising electricity consumption, climate change, security of energy supply and the price volatility of other fuels, one may expect an important role of nuclear energy in the energy mix in the longer run [6]. According to the latest research it is rather unclear how nuclear energy will contribute to global human well-being as part of sustainable development [7].

Energies 2020, 13, 2239; doi:10.3390/en13092239 www.mdpi.com/journal/energies Energies 2020, 13, 2239 2 of 32

The extensive production of nuclear power plants generates significant amounts of radioactive waste among which SNF and HLW are the most radiotoxic. SNF is a product of the nuclear reactors operation and its composition depends on the nuclear fuel burn-up. The main part of SNF (about 95%) consists of 238U. The remaining part (4–5%) is a mixture of actinides and nuclear fission products of 235U and 239Pu isotopes, some of which have high radiation and long half-lives, in the millions of years. HLW is mainly a product of SNF reprocessing with high concentrations of both short and long-lived alpha, beta and gamma emitters [8]. After recovering uranium and plutonium from SNF, solutions containing fission products and actinides are solidified in the calcination process and then vitrified with borosilicate glass and stored in steel containers [9–11]. Radioactive waste management depends on the fuel cycle type applied in production of nuclear energy. SNF is initially stored for at least five years in wet storage facilities located at the power plants. Then it is delivered to a reprocessing plant or to an interim storage facility [10,12,13]. SNF and HLW require interim storage for about 50 years to lower the radiation level and temperature on the surface of the containers to meet the safety requirements in terms of the protection of people and the environment during the disposal facility operational and post-closure period. One of the commonly accepted disposal options is deep geological disposal at depths between 250 m and 1000 m for mined repositories [14,15]. In theory and practice, geologic isolation of radioactive waste is a safe, environmentally sound and permanent solution [16]. Currently, several countries producing nuclear energy assume deep geological disposal as an official policy [17]. The first research on the disposal of radioactive waste in deep geological formations began in the United States in the 1960s at the Salt Mine in Lyon, Kansas [18], and in Germany in 1965 in the underground research laboratory at the Asse Salt Mine [19]. In addition to rock salt, also crystalline rocks, clays and tuffs were investigated as a host rock for underground repositories [20–22]. Many underground research laboratories have been built in all countries intending deep geological disposal of HLW and SNF. In situ tests in bedded rock salt and in salt domes were conducted in underground laboratories in France, Germany and in the United States [23,24]. Due to its physicochemical and geomechanical properties, rock salt deposits were also considered as an appropriate host rock for the underground storage of natural gas, hydrogen or compressed air [25–27], as well as also for location of underground neutrino detectors [28,29]. Many years of in situ and laboratory tests have confirmed the suitability of rock salt as a host rock for an underground radioactive waste repository. The first and only deep geological disposal facility for now, the Waste Isolation Pilot Plant in the rock salt Salado Formation (Nevada, USA), obtained the permit for disposal of transuranic waste on the basis of a safety case presented in an 80,000 pages WIPP Compliance Certification Application (CCA) to the Environmental Protection Agency [30]. In connection with the plans for constructing a nuclear power plant in Poland, this paper reports on an investigation that was carried out regarding site selection for the deep geological disposal of SNF and HLW. The site selection study included the analysis of archival geological data in terms of rock structure size, as well as their geological structure and hydrogeological conditions. The analysis allowed for the initial selection of sites within igneous and metamorphic rocks, clayey rock formations as well as bedded rock salt and dome rock salt structures. Then detailed radioactive waste disposal criteria were developed on the basis of IAEA recommendations, taking into account local geological conditions, the adopted disposal concept and the applicable regulations. The developed criteria allowed to select the three salt domes of Damasławek, Łani˛etaand Kłodawa in Central Poland and the clay rock formation in the Jarocin–Pogorzela region for deep geological repositories [31]. Mainly due to hydrogeological and surface conditions (protected nature reserve areas), pre-selected sites within igneous and metamorphic rocks were classified for further possible research. The selected salt domes consist of evaporites of four Zechstein cyclothems, namely PZ1–PZ4. Within older cyclothems, rock salts dominate. Anhydrite, carbonates, shales and potash salts are also found in smaller amounts. Within the younger cyclothems PZ3 and PZ4, in addition to rock salts, Energies 2020, 13, 2239 3 of 32 anhydrites, carbonates and potash salts, a large share is zuber rocks, i.e., rocks containing 15–85% halite and minerals such as anhydrite, clay minerals, carbonates quartz and feldspar. Within the PZ3 cyclothem, the Brown Zuber (Na3t) was distinguished as a lithostratigraphic unit of the Gwda Formation [32–34] and within the PZ3 cyclothem the Red Zuber (Na4t) as a lithostratigraphic unit of the Korytnica Formation [32,33,35]. Analysis of the geological structure of the selected salt domes has shown that at an optimal depth for SNF and HLW disposal, in addition to rock salt, zuber rocks are very common. The idea of deep geological disposal in these salt domes [36,37], taking the amount of waste generated by planned nuclear power plants into account, indicated the need to include zuber rocks in the design concept and to carry out preliminary tests on the samples of these rocks. Zuber rocks have been the subject of mineralogical, stratigraphic and tectonic studies [32–35], which helped to determine the stratigraphic position of these units within the PZ3 and PZ4 cyclothems of the Polish Zechstein basin. Further mineralogical and geochemical research [38,39] were aimed to determine the genesis and subsequent evolution of these rocks. Similar lithostratigraphic units known as Tonmittelsalz and Tonbrockensalz were identified in the Gorleben salt dome in Germany. Mineralogical–geochemical investigations of these rocks carried out on samples taken from a deep borehole in the Gorleben salt dome have shown some similarities to the equivalent zuber beds of the Polish Zechstein basin [40]. This article presents the results of the research concerning the physical, geomechanical and sorption properties of the Brown and Red Zuber rocks from the Kłodawa Salt Mine in Poland, and its comparison to the properties of rock salt. The research was conducted at the Mineral and Energy Economy Research Institute, in co-operation with the University of Technology (AGH), the Chemkop Research and Development Centre for Mining of Chemical Raw Materials, the Silesian University and Warsaw University.

2. Materials and Methods The Brown and Red Zuber rocks have not been tested as a mineral resource or in respect of their potential use for construction of underground repositories of SNF and HLW. The results of the research on these rocks presented below, with the comparison to the salt rock properties, represent the first attempt toward a comprehensive evaluation of zuber rocks as the host for the deep geological disposal of heat-generating radioactive waste. The research was carried out on core samples, taken from the Brown and Red Zuber units in the Kłodawa Salt Mine. The research was carried out in two phases. In the first phase (on behalf of the National Atomic Energy Agency in 1999–2001), zuber samples with a water-insoluble minerals content amounting to ca. 20–30% were selected for the laboratory tests. The comparative material consisted of the samples collected from the units of Older Halite (Na2) and the Youngest Halite (Na4a). The aim of the research was to determine the mineralogical composition, loss of mass and fracture propagation rate at increased temperatures, physical properties (porosity, density, permeability, gas content), sorption properties and mechanical properties of the zuber rocks. During the second phase (under Grant No. 5T12B01425) the scope of the research was similar. However, the gas content tests were replaced by an extended scope of geomechanical tests. The tests were conducted on samples showing a higher variability in water-insoluble minerals content. In the first phase of the research, cores of the Brown Zuber unit were taken from the SW gallery III at a depth of 600 m below surface and cores of the Red Zuber unit were taken in the vicinity of the Barbara Shaft and in the S gallery (12), at depths of 600 m and 450 m below surface, respectively. A total of 60 samples of the Brown Zuber unit rocks and 31 samples of the Red Zuber unit rocks were cut out of the cores, in the form of cylinders of 90 mm and 50 mm in height and of 90 mm and 110 mm in diameter. Rock salt (PZ2 and PZ4) samples for comparative testing from were taken from galleries at a depth of 630 m below surface. In the second phase of the research, the Brown Zuber cores were Energies 2020, 13, 2239 4 of 32 taken in the SW gallery III and NE gallery III and the Red Zuber cores were taken in the west transport Energies 2020, 13, x FOR PEER REVIEW 4 of 34 galleryEnergies at 20 a20 depth, 13, x FOR of 600 PEER m REVIEW below surface. A total of 132 samples of the Brown Zuber and 100 samples4 of 34 of the Red Zuber were cut out of the cores in the form of cylinders with similar dimensions as in the and 100 samples of thethe Red Zuber were cut out of the cores in the form of cylinders with similar firstdimensions phase. Samples as in the for firs eachtt phase.phase. tests SamplesSamples were then forfor prepared eacheach teststests from werewere cylindrical thenthen preparedprepared core fromfrom fragments. cylindricalcylindrical Sampling corecore locationsfragments.fragments. are presentedSampling locations in Figure are1. presented in Figure 1.

Figure 1. Study area and sampling locations in the Kłodawa Salt Mine on the background of the local FigureFigure 1. 1.Study Study area area and and sampling sampling locationslocations in the Kłodawa Salt Salt Mine Mine on on the the background background of ofthe the local local geological structure (according to [33,41]). geologicalgeological structure structure (according (according to to [ 33[33,41],41]).). The Red Zuber is one of the youngest lithostratigraphic units of the fourth Zechstein cyclothem TheThe Red Red Zuber Zuber is is one one of of the the youngest youngest lithostratigraphiclithostratigraphic units units of of the the fourth fourth Zechstein Zechstein cyclothem cyclothem (PZ4), and the Brown Zuber lithostratigraphic unit was deposited during the third cyclothem (PZ3) (PZ4),(PZ4) and, and the the Brown Brown Zuber Zuber lithostratigraphic lithostratigraphic unit unit was was deposited deposited during during thethe thirdthird cyclothemcyclothem (PZ3)(PZ3) in in the gradually shallowing Zechstein basin. Both the Brown and the Red Zubers are units of thein gradually the gradually shallowing shallowing Zechstein Zechstein basin. basin. Both theBoth Brown the Brown and the and Red the Zubers Red areZuber unitss are of signiﬁcantunits of significant thickness within the salt structures occurring in the Central Poland region. Figure 2 shows thicknesssignificant within thickness the salt within structures the salt occurring structures in occurring the Central in the Poland Central region. Poland Figure region2 shows. Figure the 2 shows general the general lithostratigraphic column of the Zechstein deposits in Central Poland and examples of lithostratigraphicthe general lithostratigraphic column of the column Zechstein of depositsthe Zechstein in Central deposits Poland in Central and examples Poland ofand the examples core samples. of thethe core samples.

FigureFigure 2. 2.Example Example of of core corecore samples samplessamples and andand thethe stratigraphic profileprofile proﬁle ofof of thethe the Zechstein Zechstein deposits deposits in inthe the Central Central PolandPoland region. region..

The article presents selected results on thethe mineral composition, loss of mass and thermal expansion, sorption of radionuclides and geomechanical properties of zuber rocksrocks..

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The article presents selected results on the mineral composition, loss of mass and thermal expansion, sorption of radionuclides and geomechanical properties of zuber rocks. In order to determine the mineral composition of the Brown and Red Zuber unit samples, microscopic observations of thin sections were carried out using Polmi A and Nikon 120 microscopes. Chemical microanalysis was also carried out using a CAMECA SX 100 electron probe micro analyzer. Independently, Energy Dispersive X-ray microanalysis with the Scanning Electron Microscope Jeol 540 was carried out. To determine the amount of water-insoluble minerals content, samples were weighed and then dissolved in water. Halite dissolution was carried out until the reaction with 0.1 molar AgNO3 solution disappeared. Then the water-insoluble minerals were dried and weighed again. The phase composition of the water-insoluble minerals was determined using a PHILIPS PW 3710 X-ray diffractometer with a cobalt tube providing monochromatic radiation. Adsorption of the radioactive isotopes was tested by static method on granulated samples with grain sizes of 2–5 mm. Samples of 100 g were mixed with 80 cm3 of saturated brine containing 90 90 3 3 SrCl2 and YCl3. The activity of solution was 24.5 MBq/dm (12.25 MBq/dm for each isotope). 90 90 Solution containing SrCl2 and YCl3 were placed in a shaker, and at specified time intervals a sample of 2 cm3 of solution was collected and centrifuged. Then, using a Geiger–Muller counter with a mica-window detector, the β-decay frequency was measured within the centrifuged brine. After measurement, samples were poured back into the solution. A similar method was applied to study Europium isotopes sorption. In this case, the tested solution contained approximately 83% of 152Eu and 17% of 154Eu in a saturated brine. The activity of solution was 12.2 MBq/dm3. Since Eu isotopes emit γ radiation, a scintillation counter with NaJ and thallium as the detection material was used to measure the radiation in the brine samples. All counting frequency measurements were referred to the counting frequency of the 90Sr + 90Y and 152,154Eu reference sources. Before strength testing, samples were subjected to nondestructive testing, applying ultrasonic waves. The purpose of this test was to assess the samples’ heterogeneity and elastic properties (dynamic modulus of elasticity and Poisson’s ratio) by measuring the velocity of the compressional, shear and surface waves. Wave velocity measurements were conducted using an ultrasonic detector with a UMT-11 digital processor and transmitter–receiver transducer pair having a nominal frequency of 1 MHz. Deformation and strength experiments included two types of tests: tests at a constant stress rate and tests at a constant strain rate. Triaxial strength tests with a constant strain rate were carried out in the MTS-815 press with a pressure cell and a TestStar 2 electronic load control system. The constant strain rate was 1 10 4 s 1 or 1 10 5 s 1 and the confining pressure was held constant at 5 MPa. × − − × − − During the strength tests, axial and lateral strains were monitored and the volumetric strain was calculated automatically. Strength tests in the triaxial stress state allowed to determine the maximum differential compressive stress, strain modulus E, Poisson’s ratio, critical axial deformation and critical circumferential deformation. Triaxial compression experiments at constant stress rate conditions were carried out on cylindrical samples of approx. 44 mm in diameter and 88 mm in height. The samples were placed in the pressure cell and axially loaded using a VEB WPH Leipzig hydraulic press. Confining pressure was controlled using an oil pump. First, the sample was loaded equally to reach the assumed confining pressure, and then the axial stress was increased constantly at a rate of 0.1 MPa s 1. The purpose of these · − experiments was to determine the values of the elasto-plastic deformations and the strength parameters, i.e., the maximum stresses and deformations at sample failure. Triaxial creep tests were carried out in PeChem-3B machines with a triaxial pressure cell on cylindrical samples of 44.0 mm in diameter and a slenderness ratio of λ = 2. The samples were placed in flexible silane covers resistant to high temperatures. Initially they were loaded hydrostatically at a constant stress rate of 0.005 MPa/s, followed by axial stress up to the specified stress value. Total displacements were measured with dial indicator gauges of 0.001 mm accuracy. Creep tests at Energies 2020, 13, 2239 6 of 32 elevated temperatures were carried out in triaxial pressure cells equipped with resistance heating bands in which samples were heated first to the specified temperature and then loaded. Tests were carried Energies 2020, 13, x FOR PEER REVIEW 6 of 34 outEnergies at axial 2020 stress, 13, x FOR from PEER 12.5 REVIEW MPa to 25 MPa and the confining pressure was 2 or 5 MPa. The6 of di 34ff erential stressabout ranged 5 months. between In fact 7.5, tests and 20.0lasted MPa. from The 55 to minimum 889 days. test The duration lower limit was results scheduled from the for failure about 5of months. Inabout fact,one sample tests 5 months. lasted during In from factthe ,test. 55tests to The lasted 889 majority days. from The of55 the to lower 889triaxial days. limit creep The results testslower fromwere limit carried the results failure out from in of severalthe one failure sample stages of during theoneat test. different sample The majoritystressduring and the of temperature test. the triaxialThe majority conditions. creep of tests the The triaxial were initial carried creep conditions tests out were in for several carriedall tests stages out were in at similar.several different stages stress and temperatureat differentRelaxation conditions.stress experiments and temperature The consisted initial conditions. conditions of loading The for the initial all sample tests conditions in were a relativ similar. for elyall testsshort were time similar. (about 1 h) up Relaxation experiments consisted of loading the sample in a relatively short time (about 1 h) up to Relaxation30 MPa of the experiments axial stress consisted at a confining of loading pressure the of sample 5 MPa inwith a relatively simultaneous short measurement time (about 1of h) up to to 30 MPa of the axial stress at a confining pressure of 5 MPa with simultaneous measurement of 30 MPacreep ofdeformation. the axial stress Then,at by a reducing confining the pressure axial stress, of 5 deformation MPa with simultaneous was held at a constant measurement level to of creep creepdetermine deformation. the stress Then at which, by reducingthe creep theprocess axial disappears stress, deformation and to determine was held the at stressa constant drop rate.level to deformation. Then, by reducing the axial stress, deformation was held at a constant level to determine determineThe geomechanical the stress at which research the creepmethods process presented disappears above and are toin determine accordance the with stress the drop International rate. the stress at which the creep process disappears and to determine the stress drop rate. SocietyThe forgeomechanical Rock Mechanics research recommendations methods presented and above procedures are in accordance for determining with the International the strength SocietyparametersThe geomechanical for and Rock creep Mechanics characteristics research recommendations methods of rock [42 presented–44] and. Specimens procedures above arefor the in for accordancestrength determining and withrheological the the strength International tests Societyparametersare presented for Rock and in Mechanics creep Figures characteristics A1 recommendations–A4. of rock [42– and44]. proceduresSpecimens for for the determining strength and the rheological strength tests parameters andare creep presentedVarious characteristics samplingin Figures methods ofA1 rock–A4. [ 42applied–44]. Specimensin the first and for thesecond strength phases and of rheologicalthe research tests resulted are presentedin in FiguresessentialVarious A1 difference– A4sampling. in respect methods of theapplied distribution in the first of water and -secondinsoluble phases mineral of sthe content research in the resulted collected in essentialtestVarious material. difference sampling Respective in respect methods comp of arisonsthe applied distribution are in presented the of firstwater in and- insoluble Fig secondures 3mineral and phases 4s, content showing of the in researchthe the empiricalcollected resulted in test material. Respective comparisons are presented in Figures 3 and 4, showing the empirical essentialdistribution difference of the in water respect insoluble of the distribution minerals content of water-insoluble (histograms) minerals and the matching content in probability the collected test distributiondistribution. of the water insoluble minerals content (histograms) and the matching probability material.distribution. Respective comparisons are presented in Figures3 and4, showing the empirical distribution of the water insoluble minerals content (histograms) and the matching probability distribution.

(a) (b) (a) (b) Figure 3. Distribution of the water-insoluble minerals content distribution in the Brown Zuber unit Figure 3. Distribution of the water-insoluble minerals content distribution in the Brown Zuber unit Figuresamples 3. takenDistribution in the first of the (a) waterand the-insoluble second ( bmineral) phases ofcontent the research distribution. in the Brown Zuber unit samplessamples taken taken in inthe the ﬁrstfirst ( a)) and and the the second second (b) ( bphase) phase of the of theresearch research..

(a) (b) (a) (b) FigureFigure 4. 4.Distribution Distribution of the water water-insoluble-insoluble mineral mineralss content content distribution distribution in the in Red the Zuber Red Zuberunit unit samplesFiguresamples taken 4. taken Distribution in in the the ﬁrstfirst of (( thea) and water the the- insolublesecond second (b (mineral)b phase) phase sof content the of theresearch research. distribution. in the Red Zuber unit samples taken in the first (a) and the second (b) phase of the research. InIn three three cases cases concerning concerning the water water-insoluble-insoluble mineral mineralss content content distribution distribution,, satisfactory satisfactory results results werewere obtainedIn obtained three cases by by normal concerning normal distributiondistribution the water - matchinsoluble matching.ing. mineral However, However,s content in the in distribution case the case of the of, satisfactoryBrown the Brown Zuber results Zuberunit unit were obtained by normal distribution matching. However, in the case of the Brown Zuber unit samplesample tests tests conducted conducted in in thethe second phase phase (Figure (Figure 3b3),b), the the best best match match was wasobtained obtained by inverse by inverse Gauss Gauss sampledistribution. tests conduc When analyzingted in the secondthe results, phase one (Figure should 3b notice), the bestthe different match was range obtaineds of th eby water inverse-insoluble Gauss distribution.minerals content When in analyzing the samples the taken results, in onethe shouldtwo research notice phases.the different ranges of the water-insoluble mineral s content in the samples taken in the two research phases.

Energies 2020, 13, 2239 7 of 32 distribution. When analyzing the results, one should notice the diﬀerent ranges of the water-insoluble minerals content in the samples taken in the two research phases.

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3.1. Mineral3. Results Composition

The3.1. Mineral Brown Composition and Red Zuber lithostratigraphic units are typical for the Polish part of the Zechstein basin andThe they Brown consist and mainly Red Zuber of halite lithostratigraphic and water-insoluble units are typical minerals, for the such Polish as anhydrite, part of the Zechstein clay minerals, carbonates,basin and quartz they and consist feldspar, mainly which of halit occure and in varying water-insoluble proportions. minerals, such as anhydrite, clay Haliteminerals, is mainlycarbonates, found quartz in the and form feldspar, of large, which often occur deformed in varying crystals proportions. (Figure 5a). Locally it occurs in the form ofHalite smaller is mainly idiomorphic found in crystals the form (Figure of large,5b). often There deformed are gas–liquid crystals ( inclusionsFigure 5a). inLocally the halite it occurs crystals, whichinare the frequentlyform of smaller irregularly idiomorphic distributed crystals (Figure(Figure 55c).b). There It may are indicate gas–liquid that inclusions a substantial in the part halite of the crystalscrystals, wasformed which are as afrequently result of theirregularly recrystallization distributed and (Figure deformation 5c). It may of theindicate primary that halitea substantial crystals. Anhydritepart of the crystals was found was informed various as a forms.result of The themost recrys commontallization is and a fine-crystalline deformation of the anhydrite primary often mixedhalite with crystals. other minerals, such as clay minerals or calcite, and larger secondary anhydrite crystallizing Anhydrite was found in various forms. The most common is a fine-crystalline anhydrite often in fractures (Figure5d). Clay minerals occur mainly in the form of clay–anhydrite and clay–quartz mixed with other minerals, such as clay minerals or calcite, and larger secondary anhydrite aggregatescrystallizin (Figureg in 5fracturese). The most(Figure common 5d). Clay minerals minerals withinoccur mainly this group in the areform chlorites of clay– andanhydrite illites, and which occurclay in varying–quartz aggregates proportions. (Figure Research 5e). The carried most common out by [45 minerals] confirm within the dominancethis group are of chlorites magnesium and and iron chloritesillites, which and occur micas, in mainlyvarying illiteproportions. and muscovite. Research Thecarried presence out by of[45] expansive confirm the clay dominance minerals of of the smectitemagnesium group was and notiron noted.chlorites In and many micas tested, mainly samples, illite and carbonates muscovite. andThe gypsumpresence of were expansive also identified. clay Carbonatesminerals occur of the bothsmectite in halitegroup (Figurewas not 5noted.f) mainly In many as idiomorphictested samples crystals, carbonates and and in gypsum the fine-grained were anhydrite–clayalso identified. matrix. Carbonates Occurrence occur both of secondary in halite (Figure gypsum 5f) mainly also indicates as idiomorphic the presence crystals and of waterin the that allowsfin thee-grained recrystallization anhydrite–clay of the matrix anhydrite.. Occurrence Gypsum of secondary was identified gypsum within also indicates the fine-crystalline the presence of matrix as wellwater as in that the allows single the crystals recrystallization within the of halite. the anhydrite. Quartz identified Gypsum was in thin identified sections within occurs the both fine- in its crystalline matrix as well as in the single crystals within the halite. Quartz identified in thin sections detrital form, which indicates that it may be associated with the original sedimentation conditions in occurs both in its detrital form, which indicates that it may be associated with the original the Zechsteinsedimentation basin condi andtions as a secondaryin the Zechstein idiomorphic basin and quartzas a secondary (Figure 5idiomorphicg). Quartz quartz is often (Figure accompanied 5g). by plagioclaseQuartz is often occurring accompanied mainly by in plagioclase the Red Zuber occurring unit mainly rocks in (Figure the Red5h). Zuber unit rocks (Figure 5h).

Figure 5. Cont.

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FigureFigure 5. Mineral 5. Mineral composition composition of of the the zuber zuberrocks rocks (according to to [41,45,46] [41,45,46). ]).(a) (Faractures) Fractures in halite in halite filled filled with fine-grainedwith fine-grained anhydrite anhydrite (Anh); (Anh) (b; )(b secondary) secondary crystallized crystallized fine-grainedfine-grained halite halite;; (c ()c )fluid fluid inclusions inclusions in halite;in (halited) idiomorphic; (d) idiomorphic crystals crystals of anhydrite of anhydrite (Anh) (Anh) in in a a fine-grainedfine-grained matrix matrix;; (e) ( teypical) typical clayey clayey-quartz-quartz fine-grainedfine-grained concentration concentration with with parallel parallelgrains grains orientation;orientation; (f ()f )coarse coarse-grained-grained calcite calcite concentration concentration;; (g) clayey-anhydrite(g) clayey-anhydrite (clay-anh) (clay-anh) concentration concentration with with isomorphic isomorphic quartz quartz crystals; crystals; ( h(h)) plagioclase plagioclase (plag)(plag) with with xenomorphic crystals of quartz (qtz). Image (a)—partially cross polarized light; images (b,c)— xenomorphic crystals of quartz (qtz). Image (a)—partially cross polarized light; images (b,c)—plane plane polarized light; images (d–h)—cross polarized light. polarized light; images (d–h)—cross polarized light.

3.2. Loss3.2. Loss of Mass of Mass and and Thermal Thermal Expansion Expansion as as a a Result Result of Heating Loss of mass during heating is strictly connected to the loss of water occurring in zuber rocks. Loss of mass during heating is strictly connected to the loss of water occurring in zuber rocks. Generally, rock salt and the accompanying minerals contain small quantities of water. Water occurs Generally,in the rockintergranular salt and pores, the accompanying liquid inclusions minerals and in hydrated contain small and clay quantities minerals. of Z water.uber rock Water samples occurs in the intergranularwere heated in pores, the temperature liquid inclusions range of and 80–140 in hydrated°C to determine and clay the mass minerals. loss and Zuber expansion rock samples changes were heatedas ina result the temperature of the gradual range heating of of 80–140 the rocks.◦C to determine the mass loss and expansion changes as a result of theThe gradual results were heating compared of the to rocks. the results of the mineralogical study and chemical tests to learn Theabout results the mechanism were compared of those phenomena. to the results The of results the mineralogical of the tests indicate study that and the chemical average mass tests loss to learn aboutin the the mechanism Brown Zuber of unit those samples phenomena. was about The 1.1% results at 140 of°C, the and tests in the indicate Red Zuber that unit the samples average it mass was loss about 1.9%. The maximum mass loss of 2.5% was recorded for the Red Zuber unit sample and in the Brown Zuber unit samples was about 1.1% at 140 ◦C, and in the Red Zuber unit samples it was aboutminimum 1.9%. mass The loss maximum of 0.7% was mass recorded loss offor 2.5% the sample was recorded of the Brown for theZuber Red unit Zuber. In the unit casesample of rock and salt, the mass loss was very low at around 0.1% at 140 °C [46]. The relative sample mass changes with minimum mass loss of 0.7% was recorded for the sample of the Brown Zuber unit. In the case of rock respect to the original mass, depending on temperature, are presented in Figure 6. salt, the mass loss was very low at around 0.1% at 140 ◦C[46]. The relative sample mass changes with respect to the original mass, depending on temperature, are presented in Figure6.

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Figure 6. Zuber rock and rock salt mass loss dependence on temperature (according to [46]). Figure 6. Zuber rock and rock salt mass loss dependence on temperature (according to [46]). Figure 6. Zuber rock and rock salt mass loss dependence on temperature (according to [46]). The results of the respective tests indicated that in addition to water-insoluble minerals content, heatingThe resultstime is of the the second respective important tests indicated factor influencing that in addition the samples to water-insoluble mass loss [34,35]. minerals The content, volume The results of the respective tests indicated that in addition to water-insoluble minerals content, heatingchanges time exceeding is the second 1% occurred important during factor 24 inﬂuencingh of the experiment. the samples Later, mass in lossthis [range34,35 ].of Thetemperature, volume heating time is the second important factor influencing the samples mass loss [34,35]. The volume changesthe loss exceeding of mass was1% occurred insignificant. during Research 24 h of the on experiment.the Brown Zuber Later, unit in this rocks range carried of temperature, out by [41] changes exceeding 1% occurred during 24 h of the experiment. Later, in this range of temperature, theindicates loss of mass an increase was insigniﬁcant. in mass loss Research with an on increase the Brown in the Zuber content unit rocksof the carried water- outinsoluble by [41 ]minerals indicates to the loss of mass was insignificant. Research on the Brown Zuber unit rocks carried out by [41] anabout increase 50% in–60 mass%, at loss which with it an reaches increase maximum in the content values. of In the samples water-insoluble with higher minerals water to-insoluble about indicates an increase in mass loss with an increase in the content of the water-insoluble minerals to 50–60%,minerals at whichcontent, it reachesthe mass maximum loss value values. decreases In samples (Figure with 7). higher water-insoluble minerals content, about 50%–60%, at which it reaches maximum values. In samples with higher water-insoluble the mass loss value decreases (Figure7). minerals content, the mass loss value decreases (Figure 7).

Figure 7. Mass loss of the Brown Zuber unit samples, depending on temperature and on the water-insoluble minerals content. Figure 7. Mass loss of the Brown Zuber unit samples, depending on temperature and on the water- insoluble minerals content. DueFigure to 7. the Mass temperature loss of the Brown range Zuber applied unit in samples the experiment,, depending only on temperature intergranular and water on the could water- have beeninsoluble released. minerals Water occurring content. in ﬂuid inclusions and bound in hydrated minerals could be released Due to the temperature range applied in the experiment, only intergranular water could have in higher temperatures [47]. Microscopic observations of halite crystals also indicated that liquid been released. Water occurring in fluid inclusions and bound in hydrated minerals could be released inclusionsDue to were the nottemperature destructed range at temperatures applied in ofthe up experiment to 120 C, in only the majorityintergranular of cases. wate Consequently,r could have in higher temperatures [47]. Microscopic observations of◦ halite crystals also indicated that liquid theybeen werereleased. not theWater sources occurring of evaporated in fluid inclusions water. It isand not bound possible in hydrated to exclude, minerals however, could that be long-term released inclusions were not destructed at temperatures of up to 120 C in the majority of cases. Consequently, exposurein higher to temperatures high temperatures [47]. Microscopic could lead to observations partial destruction of halite of inclusions. crystals also The indicated released water that liquidcould they were not the sources of evaporated water. It is not possible to exclude, however, that long-term enhanceinclusions the were pressure–solution not destructed creepat temperatu in associationres of up with to 120 the  dislocationC in the majority creep of [48 cases.–50] and Consequently, lead to the they were not the sources of evaporated water. It is not possible to exclude, however, that long-term

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Energiesexposure2020 ,to13 ,high 2239 temperatures could lead to partial destruction of inclusions. The released10 water of 32 could enhance the pressure–solution creep in association with the dislocation creep [48–50] and lead to the decrease in strength at high strain rates [51]. Water occurring in rock salt structures has the decreaseform of in saturated strength brine. at high Therefore, strain rates it does[51]. Water not have occurring any leaching in rock properties salt structures and has does the not form cause of saturateddissolution brine. of halite Therefore, or any itother does easily not have soluble any salts leaching accompanying properties zuber and does rocks not, except cause for dissolution the grain ofboundary halite or migration any other process easily soluble [52,53] salts. It should accompanying be emphasize zuberd that rocks, the except loss offor the the intergranular grain boundary water migrationof the zuber process rock [samples52,53]. It being should tested be emphasized in the laboratory that the conditions loss of the could intergranular not exactly water reflect of theprocesses zuber rockoccurring samples in beingsitu in tested the in salt the dome. laboratory The phenomenon conditions could of notthermally exactly induced reflect processes mass loss occurring and brine in situmigration in the salt should dome. be Theconsidered phenomenon in the oftime thermally aspect, inducedsince zuber mass rock loss heating, and brine caused migration by the should presence be consideredof heat-generating in the time nuclear aspect, waste since, would zuber continue rock heating, for a very caused long by period. the presence of heat-generating nuclearThermal waste, expansion would continue measurements for a very were long period.intended to determine the rock expansion coefficients and Thermal to check expansion whether measurements cracks appear were in zuber intended rocks to determineunder variable the rock temperature expansion coeconditionfficientss and[46]. toSamples check whether were heated cracks at appear temperatures in zuber of rocks 80 °C, under 100 variable°C, 120 °C temperature and 140 °C conditions during 24 [ 46h.]. After Samples each wereheating heated phase at temperatures, rock observations of 80 ◦ wereC, 100 performed,◦C, 120 ◦C using and 140 reflected◦C during light 24 microscopy. h. After each heating phase, rock observationsThe samples were heated performed, to 100 C using were reflected also observed light microscopy. in thin sections applying transmitted light microscopy.The samples Special heated attention to 100 was◦C paid were to also the observedboundaries in between thin sections the particular applying mineral transmitted phases, light the microscopy.internal structure Special of attentionthe halite wasand the paid detrital to the materials boundaries. between the particular mineral phases, the internalThe tested structure samples of the did halite not andshow the direction detritalal materials. textures. Consequently, one can assume that the sampleThe size tested changes samples during did notheating show w directionalere similar in textures. all directions. Consequently, The relative one canthermal assume expansion that the is sampleexpressed size by changes Formula during (1): heating were similar in all directions. The relative thermal expansion is expressed by Formula (1): ℎ − ℎ0 h h0 ∙ 100% (1) − ℎ0 100% (1) h0 · where:where: 0 hh0—initial—initial samplesample length;length; hh—sample—sample length length after after heating. heating. TheThe resultsresults ofof thethe teststests indicateindicate thatthat thethe averageaverage relativerelative expansionexpansion ofof thethe BrownBrownZuber Zuber unitunit samplessamples was was about about 0.4% 0.4% at at 140 140◦ °C,C, and and of of the the Red Red Zuber Zuber unit unit samples samples was was about about 0.2%. 0.2%. The The maximum maximum relativerelative expansion expansion ofof 0.7%0.7% waswas recordedrecorded inin case case of of the the Brown Brown Zuber Zuber unit unit sample sample and and minimum minimum ofof −00.3%.3% forfor thethe samplesample ofof thethe RedRedZuber Zuber unit unit [ 46[46,54,55],54,55].. TheThe extremeextreme andand averageaverage relativerelative thermalthermal − expansionexpansion values values of of the the tested tested samples samples are are presented presented in in Figure Figure8. 8.

FigureFigure 8. 8.ZuberZuber rock rock relative relative thermal thermal expansion expansion depen dependingding on on temperature temperature (according (according toto [46]) [46.]).

HaliteHalite foundfound in the the Brown Brown and and Red Red Zuber Zuber unit unit samples samples was wasrepresented represented by the by crystals the crystals of several of severalgenerations. generations. Those were Those cracked were cracked locally to locally various to variousextent. Observations extent. Observations of the contact of the zones contact between zones between various halite crystals indicated that either there were no cracks, or they may have existed

Energies 2020, 13, x FOR PEER REVIEW 11 of 34

Energies 2020, 13, 2239 11 of 32 various halite crystals indicated that either there were no cracks, or they may have existed in submicroscopic dimensions. Cracks were most frequently propagating in the zones of occurrence of thein submicroscopic ingrowths of other dimensions. minerals Cracks or gaseous were– mostliquid frequently inclusions, propagating where halite in thecrystal zones structures of occurrence were weakened.of the ingrowths Small of fractures other minerals in detrital or gaseous–liquid mass accompanying inclusions, halite where were halite also observed.crystal structures Those were were foundweakened. especially Small in fracturessamples with in detrital a largemass shareaccompanying of clay minerals. halite The cracks were alsoin the observed. halite crystal Those showed were afound different especially nature in than samples those with in the a large detrital share mass. of clay They minerals. were straight The cracks-line in formations the halite crystal repeating showed the systema different of natural nature cleavage than those planes in the of detritalthat mineral. mass. However, They were the straight-line crack propagation formations was irregular repeating and the diminishingsystem of natural in the cleavage surrounding planes detrital of that minerals. mineral. However, the crack propagation was irregular and diminishingHeating in of the samples surrounding did not detrital cause minerals. any essential structural or textural changes. Even at temperatureHeatings ofof samples 140 C, didno notsignificant cause any cracking essential system structural changes or textural were changes.recorded Even in the at temperaturesmicroscopic images.of 140 ◦C, Heating no significant at temperatures cracking systemof up to changes 140 C were leads recorded to small in volumetric the microscopic changes images. of ca. Heating 1% mass at loss,temperatures mainly owin of upg to 140loss ◦ofC leadswater tofrom small the volumetric detrital part changes and the of ca.occurrence 1% mass of loss, sporadic mainly and owing small to cracksloss of mainly water from in halite. the detrital No thermal part andcracking the occurrence was observed of sporadic on the contact and small zone cracks between mainly the in detrital halite. substanceNo thermal and cracking the halite, was despite observed the on differences the contact in zonetheir betweenthermal propert the detritalies [46] substance. and the halite, despite the differences in their thermal properties [46]. 3.3. Sorption Properties 3.3. Sorption Properties The sorption tests were conducted in the Isotopic Laboratory of the Department of Radiometry of theThe Faculty sorption of Physics tests were and conductedNuclear Technology in the Isotopic of AGH. Laboratory Laboratory of the experiments, Department with of Radiometry the use of radioactiveof the Faculty isotopes of Physics 90Sr + and 90Y Nuclearand 152,154 TechnologyEu were carried of AGH. out Laboratory on monolithic experiments, and granulated with the (grain use diameterof radioactive of 2–5 isotopesmm) samples90Sr +of 90theY Brown and 152,154 andEu Red were Zuber carried units, out as well on monolithicas on the rock and salt granulated [56]. (grainThe diameter isotopes of of 2–5 90Sr mm) + 90 samplesY were selected of the Brown owing and to Redtheir Zuber high share units, in as wellSNF, as while on the the rock isotope salt [s56 of]. 152,154EuThe are isotopes generally of 90consideredSr + 90Y were to be selected a homologue owing of to 241 theirAm, high representing share in SNF,transuranians. while the isotopes of 152,154TheEu arefraction generally F of the considered isotope adsorbed to be a homologue from brine of containing241Am, representing radioactive transuranians. isotopes is determined by theThe following fraction equation: F of the isotope adsorbed from brine containing radioactive isotopes is determined by the following equation: 퐼 − 퐼0 퐹 = |I I0 | (2) F = −퐼0 (2) I0 wwhere:here: I0—initial frequency of the decay counts in the solution of radioactive isotopes in brine; I0—initial frequency of the decay counts in the solution of radioactive isotopes in brine; II——frequencyfrequency of the decay counts in specific specific time after granulated sample addition. TheThe fraction fraction FF reachesreaches the the value value of of 1 1 for for complete complete sorption sorption of of radioactive radioactive isotopes isotopes by by the the sample sample 152, 154 90 90 (the(the frequency ofof thethe decaydecay countscountsI I= =0). 0). TheThe FF values values of of isotopes isotopes152, 154Eu andand 90Sr + 90YY obtained obtained in in thethe tested tested rock rock samples samples at room temperature are presented in FiguresFigures9 9 and and 10 10,, respectively. respectively.

Figure 9. Sorption of the 152,154Eu isotope of the Brown and Red Zuber unit samples and rock salt samples (according to [56]).

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Figure 9. Sorption of the 152,154Eu isotope of the Brown and Red Zuber unit samples and rock salt Energiessamples2020, 13 (according, 2239 to [56]). 12 of 32

Figure 10. Sorption of the 9090Sr + 9090YY isotope isotope of of the the Brown Brown and and Red Red Zuber Zuber unit unit samples and rock salt samples (according to [[56])56])..

The obtainedobtained results demonstrated that fractionfraction of water-insolublewater-insoluble minerals occurring in zuber rocks inin contrast contrast to rock to rock salt has salt high has sorption high sorption capability capability to immobilize to immobilizepermanently permanently such radionuclides such 90 152,154 90 asradionuclidesSr and as 90Eu.Sr and It was152,154 foundEu. It was that, found in the that, case in of the the case isotope of the Sr,isotope probably 90Sr, probably three diff erentthree mechanismsdifferent mechanisms could be responsiblecould be responsible for immobilization: for immobilization: sorption, exchangesorption, ofexchange strontium of ionsstrontium with those ions ofwith calcium those andof calcium precipitation and precipitation of strontium sulphateof strontium (SrSO sulphate4) resulting (SrSO from4) resulting the chemical from composition the chemical of 2- zubercomposition rocks containing of zuber rocks SO4 containingions. The SO specified42- ions. mechanismsThe specified depend mechanisms on temperature: depend on thetemperature: capability ofthe sorption capability decreases of sorption and decreases the strontium and sulphatethe strontium solubility sulphate increases solubility with theincreases increase with in temperature.the increase Noin temperature. significant influence No significant of brine influence composition of onbrine the composition course of sorption on the was course found of (tests sorption were was conducted found with(tests the were use conducted of sodium-chloride with the use and of magnesium-chloride sodium-chloride and brines). magnesium In the case-chloride of the brines). tests conducted In the case on monolithicof the tests samples,conducted it was on monolithic found that thesamples, radionuclides it was found were penetratingthat the radionuclides into the host were rock. penetrating The related autoradiogramsinto the host rock. indicate The thatrelated the autoradiogramsradionuclides migrated indicate a small that the distance radionuclides through the migrated clay fraction a small in whichdistance they through were retained the clay [ 56fraction]. The abovein which conclusions they were confirm retained that [56] both. The the above Brown conclusions and Red Zuber confirm unit rocksthat both could the be Brown a geological and Red barrier Zuber that unit would rocks ecouldfficiently be a immobilize geological radionuclidesbarrier that would in the efficiently case of a HLWimmobilize or SNF radionuclides canister failure. in the case of a HLW or SNF canister failure.

3.4. Geomechanical Properties The tests of thethe geomechanicalgeomechanical properties included three typestypes ofof experiments:experiments: (1) ultrasound testing, (2) short short-time-time tests of strength and deformation and (3) creep and relaxation tests concerning rheological properties [[25,3325,33–3636].]. 3.4.1. Ultrasound Testing 3.4.1. Ultrasound Testing Ultrasound tests were conducted in the ﬁrst phase on the samples of 2.5 cm in diameter and they Ultrasound tests were conducted in the first phase on the samples of 2.5 cm in diameter and they consisted of the measurements of compressional P-wave velocity (VP) and surface wave velocity (VR) consisted of the measurements of compressional P-wave velocity (VP) and surface wave velocity (VR) in parallel to the sample axis. Because of strong attenuation of the shear S-wave energy, its velocity, in parallel to the sample axis. Because of strong attenuation of the shear S-wave energy, its velocity, necessary for the determination of dynamic moduli of elasticity, was determined on the basis of the necessary for the determination of dynamic moduli of elasticity, was determined on the basis of the surface wave velocity (VR). In the second phase of the research, tests were conducted on the samples surface wave velocity (VR). In the second phase of the research, tests were conducted on the samples of 5 cm in diameter and the P-wave velocity was determined in perpendicular and parallel directions of 5 cm in diameter and the P-wave velocity was determined in perpendicular and parallel directions to the sample axis. The test results are presented in Table1. to the sample axis. The test results are presented in Table 1.

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Table 1. Results of the ultrasound testing of the zuber rock samples (according to [57]). Table 1. Results of the ultrasound testing of the zuber rock samples (according to [57]). Brown Zuber Red Zuber Brown Zuber Red Zuber VelocityVelocity VelocityVelocity Average VelocityVelocity vVelocityelocity TypeType of Phase MeasuredMeasured measuredMeasured AnisotropAverage MeasuredMeasured measuredMeasured AAverageverage Waveof Phase Parallel to the Perpendicular Anisotropy Parallel to the Perpendicular Anisotropy Parallel to Perpendicular y Parallel to Perpendicular Anisotropy Wave Sample Axis to the Sample Coeﬃcient Sample Axis to the Sample Coeﬃcient the Sample to the Sample Coefficien the Sample to the Sample Coefficient (m/s) Axis (m/s) (m/s) Axis (m/s) Axis (m/s) Axis (m/s) t Axis (m/s) Axis (m/s) I 3551–4882 3857–5238 VP I 3551–4882 3857–5238 VP II 3562–4500 3403–4633 0.97 3062–4137 1857–4800 0.75 II 3562–4500 3403–4633 0.97 3062–4137 1857–4800 0.75 I I1828 1828–2130–2130 VR II II1962 1962–2198–2198 1794 1794–2206–2206 1.01 1.01 1470 1470–2071–2071 1144 1144–2109–2109 1.01

TheThe datadata presentedpresented inin TableTable1 1indicate indicate that that the the elasticity elasticity parameters parameters of of zuber zuber rocks rocks are are strongly strongly diversiﬁed,diversified, andand thethe RedRed ZuberZuber unitunit samplessamples displaydisplay higherhigher anisotropyanisotropy andand aa largerlarger dispersiondispersion ofof d parameterparameter values.values. The The tests tests allowed allowed to to determine determine the the value value of ofthe the modulus modulus of ofelasticity elasticity (E ()E andd) and the d thePoisson Poisson’s’s ratio. ratio. The Thevalue value of the of dynamic the dynamic modulus modulus of elasticity of elasticity (E ) changes (Ed) changes in the inrange the rangefrom 16.9 from to 16.954.7 toGPa, 54.7 while GPa, whilethat of that the of dynamic the dynamic Poisson Poisson’s’s ratio ratio from from 0.09 0.09 to to0.38 0.38 and and indicate indicate the the poor elasticelastic propertiesproperties ofof zuberzuberrocks. rocks.

3.4.2.3.4.2. DeformationDeformation andand StrengthStrength TestsTests TheThe purposepurpose ofof thethe deformationdeformation andand strengthstrength teststests waswas toto determinedetermine thethe valuesvalues ofof elasto-plasticelasto-plastic strain,strain, with with the the identification identification of the of elastic the elastic components components and the and determination the determination of the strength of the indicators, strength i.e.,indicators, ultimate i.e. stresses, ultimate and stresses strains atand sample strains failure. at sample The failure experiments. The experiments included two included types of two tests: types tests of withtests: a te constantsts with stressa constant rate and stress tests rate with and a tests constant with strain a constant rate. strain rate. EighteenEighteen zuber zuber rock rock samples samples (nine (nine for for each each type type of zuber of zuber units) units in each) in of each the two of the research two research phases werephases tested were at constanttested at stressconstant rate conditions.stress rate conditions The sample. The diameter sample was diameter 44 mm with was a 44 slenderness mm with of a λslenderness= 2. Three diofff erent = 2. valuesThree different of the confining values pressureof the confining of σ3 = 2,pressure 5, and 10 of MPa σ3 = were2, 5, and applied. 10 MPa The were rate ofapplied axial stress. The wasrate 0.1of axial MPa stresss 1. Similar was 0.1 results MPa·s were−1. Similar obtained results in both were phases obtained (Table in2 ).both The phases stress–strain (Table · − characteristics2). The stress– obtainedstrain characteristics in the second obtained phase for in thethe Brown second and phase Red for Zuber the Brown unit samples and Red are Zuber presented unit insamples Figures are 11 presentedand 12, respectively. in Figures 11 and 12, respectively.

FigureFigure 11. 11. Stress–strainStress–strain characteristics of the the BrownBrown ZuberZuber unitunit samples—conventionalsamples—conventional triaxial triaxial compressioncompression teststestsat at aa constantconstant stress stress rate rate (according (according to to [ 58[58]]).).

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FigureFigure 12. 12. StressStress–strain–strain characteristics of the Red Red Zuber Zuber unitunit samples—conventionalsamples—conventional triaxial triaxial compressioncompression tests tests at at a a constant constant stress stress rate rate (according (according to to [ 58[58]]).). The samples tested in constant strain-rate conditions had a diameter of 2.5 cm in the first phase of The samples tested in constant strain-rate conditions had a diameter of 2.5 cm in the first phase research and 5 cm in the second. In both phases, the slenderness of the samples was λ = 2 and the of research and 5 cm in the second. In both phases, the slenderness of the samples was  = 2 and the confining pressure was σ3 = 5 MPa. In the first phase, 12 samples were tested, with the strain rate confining pressure was σ3 = 5 MPa. In the first phase, 12 samples were tested, with the strain rate 6 1 5 1 dε1/dt of 5 10− s− or 1 10− s− . In the second phase, 18 compression tests were conducted with dε1/dt of 5× × 10−6 s−1 or 1 ××10−5 s−1. In the second phase, 18 compression tests were conducted with the the strain rate of 5 10 6 s 1, 1 10 5 s 1 or 1 10 4 s 1. The triaxial compression tests at a constant strain rate of 5 ×10×−6 s−−1, 1− ×10×−5 s−1− or −1 ×10−4× s−1. −The− triaxial compression tests at a constant stress stress rate and constant strain rate were conducted on samples very similar in dimensions, to obtain rate and constant strain rate were conducted on samples very similar in dimensions, to obtain the the comparable results and avoid specimens size effect on the triaxial compressive strength. comparable results and avoid specimens size effect on the triaxial compressive strength. The stress–strain characteristics of the Brown and Red Zuber unit samples obtained in the second EnergiesThe stress 2020, –13strain, x FOR characterisPEER REVIEWtics of the Brown and Red Zuber unit samples obtained in15 the of 34second testing phase are presented in Figures 13 and 14, respectively. Figure 13a,b show the axial and testing phase are presented in Figures 13 and 14, respectively. Figure 13a,b show the axial and volumetric strain curves of the Brown Zuber unit samples, respectively, and Figure 14a,b show the volumetric strain curves of the Brown Zuber unit samples, respectively, and Figure 14a,b show the axial and volumetric strain curves of the Red Zuber unit samples. axial and volumetric strain curves of the Red Zuber unit samples.

Figure 13. Cont.

Figure 13. Stress–strain characteristics of the Brown Zuber unit samples in the tests at a constant strain rate. (A)—axial strain; (B)—volumetric strain (according to [57]).

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Energies 2020, 13, 2239 15 of 32

Energies 2020, 13, x FOR PEER REVIEW 16 of 34 FigureEnergiesFigure 13.2020Stress–strain ,13. 13 ,Stress–strain x FOR PEER characteristics REVIEW characteristics of of the the BrownBrown Zuber unit unit samples samples in inthe the tests tests at a atconstant a constant strain16 strain of 34 rate.rate. (A)—axial (A)—axial strain; strain; (B )—volumetric(B)—volumetric strain strain (according(according to to [57]). [57]).

FigureFigure 14. Stress–strain 14. Stress–strain characteristics characteristics of of the theRed Red ZuberZuber unit unit samples samples in inthe the tests tests at a at constant a constant strain strain rate. (rate.FigureA)—axial (A )—axial14. Stress–strain strain; strain; (B)—volumetric (B characteristics)—volumetric strain ofstrain the (according Red(according Zuber tounit to [57]). [57 samples]). in the tests at a constant strain rate. (A)—axial strain; (B)—volumetric strain (according to [57]). The maximum values of differential stress and axial strain are presented in Table 2. The detailed resultsThe of maximumthe short-term values compression of differential tests stress are presented and axial strainin Table ar eA1. presented in Table 2. The detailed results of the short-term compression tests are presented in Table A1.

Table 2. Maximum values of differential stress (σ1–σ3) and axial strain (ε1) obtained in the conventional σ ε triaxialTable 2. compression Maximum values tests. of differential stress ( 1–σ3) and axial strain ( 1) obtained in the conventional triaxial compression tests. Constant Rate of Stress Constant Rate of Strain Type of Test Constantσ1–σ 3Rate of Stress ε1 dε1/dt Constantσ1–σ Rate3 of Strain ε1 σ3 (MPa) ε ε Type of Test (MPa)σ1–σ3 (%)1 d(sε1−/dt1) (MPa)σ1–σ3 (%)1 σ3 (MPa) −1 2 37.8–41.4(MPa) 1.6–4.0(%) 1 ×(s 10)− 6 14.7–54.0(MPa) 5.8–9.7(%) Brown Zuber 2 37.8–41.4 1.6–4.0 1 × 10−−56 14.7–54.0 5.8–9.7 Brown Zuber 5 44.3–48.3 5.9–7.4 1 × 10 19.0–24.8 4.0–6.2 Phase I 5 44.3–48.3 5.9–7.4 1 × 10−5 19.0–24.8 4.0–6.2 Phase I 10 47.2–60.4 6.8–7.3 102 22.7–40.147.2–60.4 1.6–2.2 6.8–7.3 1 × 10 −6 39.9 6.1 Brown Zuber 52 29.8–48.4 22.7–40.1 1.6–5.9 1.6–2.2 1 1 ×× 1010−−56 18.1–45.939.9 0.6–8.2 6.1 BrownPhase Zuber II 5 29.8–48.4 1.6–5.9 1 × 10−−45 18.1–45.9 0.6–8.2 Phase II 10 42.0–60.7 1.5–7.6 1 × 10 27.0–54.5 1.0–9.6 −4 102 39.4–43.4 42.0–60.7 1.4–3.4 1.5–7.6 1 × 10 27.0–54.5 1.0–9.6 Red Zuber 2 39.4–43.4 1.4–3.4 −5 Red Zuber 5 37.1–45.6 3.1–3.8 1 × 10 9.5–22.3 1.2–4.9 Phase I 5 37.1–45.6 3.1–3.8 1 × 10−5 9.5–22.3 1.2–4.9 Phase I 10 53.8–68.3 2.6–5.9 Red Zuber 10 2 35.3–45.753.8–68.3 1.4–3.1 2.6–5.9 1 × 10−6 45.9 7.8 Red Zuber 2 35.3–45.7 1.4–3.1 1 × 10−6 45.9 7.8

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The maximum values of diﬀerential stress and axial strain are presented in Table2. The detailed results of the short-term compression tests are presented in Table A1.

Table 2. Maximum values of diﬀerential stress (σ1–σ3) and axial strain (ε1) obtained in the conventional triaxial compression tests.

Constant Rate of Stress Constant Rate of Strain Type of Test 1 σ3 (MPa) σ1–σ3 (MPa) ε1 (%) dε1/dt (s− ) σ1–σ3 (MPa) ε1 (%) Brown 2 37.8–41.4 1.6–4.0 1 10 6 14.7–54.0 5.8–9.7 × − Zuber 5 44.3–48.3 5.9–7.4 1 10 5 19.0–24.8 4.0–6.2 × − Phase I 10 47.2–60.4 6.8–7.3 Brown 2 22.7–40.1 1.6–2.2 1 10 6 39.9 6.1 × − Zuber 5 29.8–48.4 1.6–5.9 1 10 5 18.1–45.9 0.6–8.2 × − Phase II 10 42.0–60.7 1.5–7.6 1 10 4 27.0–54.5 1.0–9.6 × − 2 39.4–43.4 1.4–3.4 Red Zuber 5 37.1–45.6 3.1–3.8 1 10 5 9.5–22.3 1.2–4.9 Phase I − 10 53.8–68.3 2.6–5.9 × Energies 2020, 13, x FOR PEER REVIEW 6 17 of 34 2 35.3–45.7 1.4–3.1 1 10− 45.9 7.8 Red Zuber × 5 5 43.4–51.4 3.5–4.3 1 10− 27.6–40.4 3.8–6.7 PhasePhase II II 5 43.4–51.4 3.5–4.3 × 1 × 104 −5 27.6–40.4 3.8–6.7 10 51.1–67.6 5.2–6.3 1 10− 35.9–37.2 0.9–3.8 10 51.1–67.6 5.2–6.3 × 1 × 10−4 35.9–37.2 0.9–3.8

The teststests results results indicated indicated that that the the strength strength of the of tested the tested samples samples depended depended on the water-insoluble on the water- mineralsinsoluble content,minerals which content, is illustratedwhich is illustrated by Figures by15 Figuresand 16, 15 presenting and 16, thepres valuesenting ofthe di valuesﬀerential of stressdifferential at the stress moment at the of moment sample of failure, sample during failure, the during tests the with tests a constant with a constant stress and stress constant and constant strain rate,strain respectively. rate, respectively.

Figure 15.15. Dependence of the maximummaximum didifferentialﬀerential stressstress valuevalue ((σσ11––σσ33) on water-insolublewater-insoluble mineralsminerals content—testscontent—tests withwith aa constantconstant stressstress rate.rate.

Figure 16. Dependence of the maximum differential stress value (σ1–σ3) on water-insoluble minerals content—tests with a constant strain rate (according to [57]).

The results of the conducted tests indicate that the strength and deformational features of the zuber rocks samples depend on the water-insoluble minerals content. As the water-insoluble minerals content increases, the total axial strain and the differential stress value decrease. The increased content of water-insoluble minerals also reduces axial and volumetric strains. The load rate did not influence the strength of the tested rocks. The strength of the tested zuber rock samples was

Energies 2020, 13, x FOR PEER REVIEW 17 of 34

Phase II 5 43.4–51.4 3.5–4.3 1 × 10−5 27.6–40.4 3.8–6.7 10 51.1–67.6 5.2–6.3 1 × 10−4 35.9–37.2 0.9–3.8

The tests results indicated that the strength of the tested samples depended on the water- insoluble minerals content, which is illustrated by Figures 15 and 16, presenting the values of differential stress at the moment of sample failure, during the tests with a constant stress and constant strain rate, respectively.

Figure 15. Dependence of the maximum differential stress value (σ1–σ3) on water-insoluble minerals Energiescontent—tests2020, 13, 2239 with a constant stress rate. 17 of 32

Figure 16. Dependence of the maximum differential stress value (σ –σ ) on water-insoluble minerals Figure 16. Dependence of the maximum differential stress value (1σ1–σ33) on water-insoluble minerals content—tests with a constant strain rate (according to [57]). content—tests with a constant strain rate (according to [57]). The results of the conducted tests indicate that the strength and deformational features of the The results of the conducted tests indicate that the strength and deformational features of the zuber rocks samples depend on the water-insoluble minerals content. As the water-insoluble minerals zuber rocks samples depend on the water-insoluble minerals content. As the water-insoluble content increases, the total axial strain and the differential stress value decrease. The increased content minerals content increases, the total axial strain and the differential stress value decrease. The of water-insoluble minerals also reduces axial and volumetric strains. The load rate did not influence increased content of water-insoluble minerals also reduces axial and volumetric strains. The load rate the strength of the tested rocks. The strength of the tested zuber rock samples was slightly lower did not influence the strength of the tested rocks. The strength of the tested zuber rock samples was than the lower strength limit obtained for the rock salt samples. In the case of the tests at a confining pressure of 2 MPa, the strength values obtained were within the range of strength of weak rock salt. The value of ultimate strain of the zuber rock samples was more than two times lower than that of the rock salt and the difference was increasing with the confining pressure increase.

3.4.3. Rheological Properties Creep is the irreversible deformation that increases with time under constant load and is that part of the geomechanical behavior that has the greatest influence on the time dependent strain and stress evolution in the surrounding rock massif. The creep behavior of rock salt is of special interest for underground repositories for radioactive wastes. Creep is time-dependent deformation and it goes through there stages: primary (transient) creep, in which the strain rate is relatively high and decreases with time, followed by steady-state creep, with the constant and slow strain increase, and the final stage, in which the strain rate rapidly accelerates, leading to ultimate material failure [59–61]. The research included 40 conventional creep tests in which the axial stress was higher than the confining pressure. In 32 tests, the confining pressure value was 5 MPa, and the axial stress values were in the range from 12.5 to 25 MPa (i.e., differential stresses were changing from 7.5 to 20 MPa). The temperature of the samples was changing during the experiments from room temperature to 90 ◦C. In the remaining eight uniaxial creep tests conducted at room temperature, the axial stress value was 8 MPa. In a considerable number of tests, the stress and temperature conditions were changed during the experiment, which caused that the testing periods ranged from 55 to 889 days. An example of a zuber rock sample test, conducted at constant stress and temperature conditions, is presented in Figure 17, and an example of the course of the longest tests, in which the differential stress values were changing from 7.5 to 15 MPa and the temperature ranged from 35 to 60 ◦C, is presented in Figure 18. The detailed results of the multi-step rheological tests are presented in Tables A2 and A3. Energies 2020, 13, 2239 18 of 32 Energies 2020, 13, x FOR PEER REVIEW 19 of 34

FigureFigure 17. Long-term 17. Long-term creep creep curves curves of of the the Brown Brown andand Red Zuber Zuber unit unit samples samples at a at constant a constant pressure pressure andEnergies atand room2020 at, room13 temperature, x FOR temperature PEER REVIEW (A )(A and) and at at a temperaturea temperature ofof 6060 ◦°CC( (BB)) (according (according to to[58]). [58 ]). 20 of 34

FigureFigure 18. Long-term 18. Long-term creep creep curves curves of the of Brown the Brown Zuber unitZuber samples unit samples at changing at changing stress and stress temperature and conditionstemperature (according conditions to [58 (according]). to [58]).

Eight uniaxial creep tests were designed to determine the influence of humidity on creep rate. For that reason, four zuber rock samples were sprayed with saturated brine and protected with plastic foil. The comparison of the creep curves in both types of samples is presented in Figure 19. Tests results confirmed the strong impact of brine on the increase in creep-rate values. Test conditions led to the failure of one sample already in the transient creep phase.

Figure 19. Creep curves of dry and saturated brine-sprayed samples (σ1 = 8 MPa).

Another purpose of the conducted tests was to determine the influence of the water-insoluble content on the creep rates. Figure 20 presents the steady-state creep rates of the samples with various water-insoluble minerals contents at room temperature and at temperatures of 60 °C. The results do not indicate a clear influence of the water-insoluble minerals content on creep rate in both temperatures. That is especially significant in view of the essential influence of the water-insoluble minerals content on zuber rocks strength.

Energies 2020, 13, x FOR PEER REVIEW 20 of 34

Figure 18. Long-term creep curves of the Brown Zuber unit samples at changing stress and Energiestemperature2020, 13, 2239 conditions (according to [58]). 19 of 32

Eight uniaxial creep tests were designed to determine the influence of humidity on creep rate. Eight uniaxial creep tests were designed to determine the inﬂuence of humidity on creep rate. For that reason, four zuber rock samples were sprayed with saturated brine and protected with For that reason, four zuber rock samples were sprayed with saturated brine and protected with plastic plastic foil. The comparison of the creep curves in both types of samples is presented in Figure 19. foil. The comparison of the creep curves in both types of samples is presented in Figure 19. Tests results Tests results confirmed the strong impact of brine on the increase in creep-rate values. Test conditions conﬁrmed the strong impact of brine on the increase in creep-rate values. Test conditions led to the led to the failure of one sample already in the transient creep phase. failure of one sample already in the transient creep phase.

Figure 19. Creep curves of dry and saturated brine-sprayed samples (σ1 = 8 MPa). Figure 19. Creep curves of dry and saturated brine-sprayed samples (σ1 = 8 MPa). Another purpose of the conducted tests was to determine the influence of the water-insoluble Another purpose of the conducted tests was to determine the influence of the water-insoluble content on the creep rates. Figure 20 presents the steady-state creep rates of the samples with various content on the creep rates. Figure 20 presents the steady-state creep rates of the samples with various water-insoluble minerals contents at room temperature and at temperatures of 60 ◦C. The results do not water-insoluble minerals contents at room temperature and at temperatures of 60 °C. The results do indicate a clear influence of the water-insoluble minerals content on creep rate in both temperatures. not indicate a clear influence of the water-insoluble minerals content on creep rate in both That is especially significant in view of the essential influence of the water-insoluble minerals content temperatures. That is especially significant in view of the essential influence of the water-insoluble on zuber rocksEnergies strength. 2020, 13, x FOR PEER REVIEW 21 of 34 minerals content on zuber rocks strength.

Figure 20. Steady-state creep rate of the Brown and Red Zuber unit samples, depending on the Figure 20. Steady-state creep rate of the Brown and Red Zuber unit samples, depending on the water- water-insoluble minerals content. insoluble minerals content.

Relaxation is the reverse process of creep and it consists of the reduction of stresses in a sample at a constant strain. The main purpose of the conducted experiments was to estimate the stress values to which relaxation tends and what it depends on. These values can be considered as the limit below which the creep phenomenon does not occur. In both research phases, twenty relaxation tests, in the triaxial stress state, were conducted in total. The confining pressure value was 5 MPa during the tests and the initial axial stress values were 20 MPa, 25 MPa, or 30 MPa, with the temperatures of 22 °C, 60 °C, and 90 °C. Selected test results are presented in Figure 21.

Figure 21. The Brown and Red Zuber unit samples’ relaxation curves at the initial axial stress of 30 MPa (according to [62]).

The relaxation limit depends on both initial stresses and temperature. An increase in temperature causes that relaxation tends to achieve a steady state at lower stress conditions. However, the initial stress increase causes that relaxation tends to reach a steady state at higher stress conditions.

Energies 2020, 13, x FOR PEER REVIEW 21 of 34

EnergiesFigure2020, 1320., 2239Steady-state creep rate of the Brown and Red Zuber unit samples, depending on the water-20 of 32 insoluble minerals content.

Relaxation isis thethe reversereverse processprocess of of creep creep and and it it consists consists of of the the reduction reduction of of stresses stresses in in a samplea sample at aat constanta constant strain. strain. The The main main purpose purpose of of the the conducted conducted experiments experiments was was to to estimate estimate the the stress stress values values to whichto which relaxation relaxation tends tends and and what what it it depends depends on. on. These These values values can can bebe consideredconsidered asas thethe limit below which the creep phenomenon does not occur. In both research phases, twenty relaxationrelaxation tests, in the triaxial stress state, were conducted in total. Th Thee confining conﬁning pressure value was 5 MPa during the tests and the initial axial stress values were 20 MPa, 25 MPa, or 30 MPa, with the temperatures of 22 °C,◦C, 60 ◦°C,C, and 90 °C.◦C. Selected test results are presented in Figure 2121..

Figure 21. The BrownBrown andand Red Red Zuber Zuber unit unit samples’ samples’ relaxation relaxation curves curves at theat the initial initial axial axial stress stress of 30 of MPa 30 (accordingMPa (according to [62 to]). [62]).

The relaxationrelaxation limit limit depends depends on bothon initialboth stressesinitial stresses and temperature. and temperature. An increase An in temperatureincrease in causestemperature that relaxation causes that tends relaxati to achieveon tends a steady to achieve state at lowera steady stress state conditions. at lower However, stress conditions. the initial stressHowever, increase the initial causes stress that relaxationincrease causes tends that to reach relaxa ation steady tends state to atreach higher a steady stress state conditions. at higher stress conditions.The conducted experiments allowed to determine the Norton creep law coeﬃcients, described by the following formula:

. vs Q n = RT εe f Ae− σe f (3) where: Q—free activation energy (J mol 1); · − R—gas constant 8.3144 J mol 1 K 1 = 1.987 cal mol 1 K 1; · − · − · − · − T—temperature (in absolute scale) (K). The creep law coeﬃcients were determined analyzing individual samples during multi-step creep tests at variable stress and temperature conditions. The average values of the A, Q/R and n coeﬃcients are presented in Table3.

Table 3. Average values of the creep law coeﬃcients [62].

A (%/day) Q/R (K) n Brown Zuber 572.4 6294.3 4.1 Red Zuber 32,655.1 7351.8 3.9

The comparison of the Brown and Red Zuber unit samples’ creep rates to those of the fast-creeping salts (BGRa) and slow-creeping salts (BGRb) [63] is presented in Figure 22. Energies 2020, 13, x FOR PEER REVIEW 22 of 34

The conducted experiments allowed to determine the Norton creep law coefficients, described by the following formula: 𝜀 𝐴𝑒 𝜎 (3) where: Q—free activation energy (J·mol−1); R—gas constant 8.3144 J·mol−1·K−1 = 1.987 cal·mol−1·K−1; T—temperature (in absolute scale) (K). The creep law coefficients were determined analyzing individual samples during multi-step creep tests at variable stress and temperature conditions. The average values of the A, Q/R and n coefficients are presented in Table 3.

Table 3. Average values of the creep law coefficients [62].

A Q/R n (‰/day) (K) Brown Zuber 572.4 6294.3 4.1 Red Zuber 32655.1 7351.8 3.9

Energies 2020The, 13, 2239comparison of the Brown and Red Zuber unit samples’ creep rates to those of the fast-21 of 32 creeping salts (BGRa) and slow-creeping salts (BGRb) [63] is presented in Figure 22.

FigureFigure 22. Creep 22. Creep rates rates of theof the Brown Brown and and Red Red Zuber Zuber unitunit samples and and of of the the fast-creeping fast-creeping (BGRa) (BGRa) and and slow-creeping (BGRb) rock salt samples in the effective stress function at a temperature of 50 °C. slow-creeping (BGRb) rock salt samples in the eﬀective stress function at a temperature of 50 ◦C.

The comparisonThe comparison presented presented in in Figure Figure 22 22 indicates indicates thatthat both both the the Brown Brown Zuber Zuber and and the theRed Red Zuber Zuber unit rocks may be classified as fast creeping rocks. unit rocks may be classified as fast creeping rocks. 4. Conclusions 4. Conclusions From the standpoint of the deep geological disposal of radioactive waste, the largest differences Frombetween the rock standpoint salts and of zuber the deep rocks geological are associated disposal with ofgeomechanical radioactive waste,properties. the The largest short-time differences betweenstrength rock saltsof the and tested zuber zuber rocks rock are samples associated is divers withe geomechanicaland lower than properties.that of rock Thesalt. short-timeThe zuber rock strength of thesamples tested zuber with the rock highest samples strength is diverse values and correspond lower than to the that rock of rocksalt sample salt. The with zuber the lowest rock samplesstrength. with the highestMuch strengthlower are, values however, correspond the deformations to the rock at the salt mo samplement of withzuber the rock lowest failure. strength. Their average Much values lower are, however,are three the deformations times lower than at the those moment of rock of salt zuber sample rocks. failure. Those properties, Their average in comparison values are to three the timessteady- lower than thosestate creep of rock rate salt being samples. similar Those to that properties, of rock salt in and comparison the unclear to influence the steady-state of water-insoluble creep rate beingminerals similar to that of rock salt and the unclear influence of water-insoluble minerals content on creep rate, indicate a lower long-term strength of zuber rocks in comparison to that of rock salt. The changes in zuber rock mass resulted mainly from dehydration during the heating period. Higher water content in samples of the Brown and the Red Zuber unit rocks compared to rock salt may be responsible for enhancement of the creep mechanisms and lead to the decrease in long-term strength. Consequently, a repository may not be constructed so deeply in zuber rocks as it may be in the case of rock salts. Besides, one should also take into account the necessity to leave larger safety pillars between disposal and functional galleries. The remaining tested parameters indicated beneficial zuber rock properties from the standpoint of deep geological disposal of SNF and HLW. The changes in linear dimensions of the Brown and the Red Zuber unit samples observed during sample heating to the temperature of 140 ◦C indicated a lack of a clear dependence between the thermal expansion of the samples and their mineral content. That may confirm the assumption that zuber rocks and the accompanying rock salts had been properly heated and compressed even before they were uplifted in the form of a salt dome. It indicates that the temperatures expected to exist in a potential repository, ranging from 100 to 120 ◦C, close to the radioactive waste containers, should not cause changes or deformations, which would exclude siting of repositories in zuber rocks. The sorption properties of zuber rocks are beneficial in comparison to those of rock salt. The obtained results demonstrated that the fraction of water-insoluble minerals occurring both in the Brown and Red Zuber unit rocks, in contrast to rock salt, has a high sorption capability to permanently immobilize 90Sr and 152,154Eu radionuclides. Energies 2020, 13, 2239 22 of 32

The research presented above was conducted in laboratory conditions, using rock samples with specific volumes. For that reason, confirmation tests of the results obtained should be carried out in in situ conditions.

Funding: This research was funded by National Centre for Research and Development, grant No. 5 T12B 01425 and the APC was funded by budgetary resources of MEERI PAS. Conﬂicts of Interest: The author declares no conﬂict of interest. Energies 2020, 13, x FOR PEER REVIEW 24 of 34 Appendix A Appendix A

Figure A1. The Brown Zuber unit samples for short-term compression tests at a constant stress rate Figure A1. The Brown Zuber unit samples for short-term compression tests at a constant stress rate and for rheological tests. and for rheological tests.

Energies 2020, 13, 2239 23 of 32

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Figure A2. The Red Zuber unit samples for short-term compression tests at a constant stress rate and for rheological tests.

Energies 2020, 13, 2239 24 of 32 Energies 2020, 13, x FOR PEER REVIEW 26 of 34

Figure A3. The Brown Zuber unit samples for short-term compression tests at a constant strain rate.

Energies 2020, 13, 2239 25 of 32

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Figure A4. The Red Zuber unit samples for short-term compression tests at a constant strain rate.rate.

Energies 2020, 13, 2239 26 of 32

Table A1. Results of short-term compression tests of the Brown and Red Zuber unit samples (according to [57,58,62]).

Short-Term Compression Tests at Constant Stress Rate Short-Term Compression Tests at Constant Strain Rate Young’s Modulus Water-Insoluble Confining Maximum Maximum Water-Insoluble Strain Maximum Maximum Dynamic Poisson’s Sample (GPa) Sample Minerals(%) Pressure Effective Axial Strain Minerals (%) Rate Effective Axial Strain Modulus Of Ratio (-) 1 (MPa) Stress (MPa) (%) E1 E2 (s− ) Stress (MPa) (%) Elasticity (GPa) 4 B27 38.6 42.1 2.2 12.0 18.8 B3 24.5 10− 50.0 9.6 43.8 0.37 5 B59 42.72 32.5 1.57 8.7 19.2 B14 71.3 10− 31.8 1.1 ** 0.15 4 B72 12.1 24.7 1.74 * * B34 44.8 10− 33.0 4.0 36.2 0.12 5 B25 25.5 53.4 5.85 10.2 13.3 B41 79.7 10− 18.1 0.6 ** 0.11 4 B63 56.65 34.8 2.19 * * B80 38.7 10− 27.0 1.0 49.3 0.13 5 B75 56.2 37.0 1.64 10.6 19.0 B82 79.9 10− 29.2 0.7 ** 0.15 4 B24 38.6 57.1 3.53 18.5 24.5 B117 4.3 10− 54.5 9.4 37.5 0.44 5 B96 71.510 52.0 1.45 13.2 29.6 B118 7.3 10− 45.9 8.2 40.4 0.34 B124 31.9 70.7 7.57 18.9 21.0 B122 9.0 5 10 6 39.9 6.1 33.4 0.30 × − 4 C9 28.8 38.6 2.38 11.8 19.7 C10 65.1 10− 37.2 0.9 54.6 0.09 5 C89 24.92 47.7 3.13 11.9 13.7 C13 47.7 10− 39.7 4.1 34.3 0.23 4 C92 46.7 37.3 1.37 10.2 18.7 C33 46.6 10− 36.4 3.8 27.3 0.09 5 C14 63.0 50.0 3.81 12.2 23.8 C36 46.9 10− 27.6 3.8 30.4 0.14 4 C29 42.75 56.4 3.45 15.7 23.6 C23 38.1 10− 36.2 3.2 38.6 0.11 5 C90 45.6 48.4 4.27 10.1 15.2 C51 32.2 10− 34.5 5.4 ** 0.17 4 C48 38.0 67.5 5.98 * * C99 23.4 10− 35.9 2.8 32.6 0.21 5 C50 30.910 61.1 5.24 13.8 18.4 C93 21.9 10− 40.4 6.7 26.8 0.11 C97 39.9 77.6 6.30 15.9 24.4 C96 24.6 5 10 6 45.9 7.8 37.1 0.21 × − *—no unloading cycle performed; **—ultrasonic wave velocity could not be determined.

Table A2. Load-thermal conditions and the results of the rheological tests—the Brown Zuber unit samples (according to [58,62]).

Water-Insoluble No. of Test Eﬀective Stress Time Stationary Creep Transient Primary Sample Temp. (◦C) AQ/R n Minerals (%) Stages σef (MPa) (days) Rate (%/day) Creep (%) Strain (%) 10 60 0–92 0.108 1.01 4.41 649.2 10 35 92–371 0.0361 1006.4 4495.7 B2 27.5 5 7.5 35 371–511 0.0095 947.2 4.64 7.5 50 511–700 0.0158 609.9 3374.0 15 50 700–757 0.590 1056.5 5.22 10 60 0–87 0.0785 3.85 5.96 471.9 B12 17.2 2 10 75 87–150 0.218 5.71 580.2 7890.9 Energies 2020, 13, 2239 27 of 32

Table A2. Cont.

Water-Insoluble No. of Test Eﬀective Stress Time Stationary Creep Transient Primary Sample Temp. (◦C) AQ/R n Minerals (%) Stages σef (MPa) (days) Rate (%/day) Creep (%) Strain (%) 10 22 0–93 0.00503 0.79 3.38 345.1 B15 49.8 2 7.5 22 93–156 0.0008 196.3 6.39 10 60 0–87 0.1338 0.79 3.71 804.3 10 90 87–100 1.355 1707.9 9328.7 B19 23.6 4 7.5 90 100–175 0.2693 1214.1 5.62 7.5 60 175–203 0.0345 741.7 8279.7 B55 67.9 1 10 60 0–142 0.00488 4.54 5.81 29.3 - - 10 60 0–92 0.0508 2.69 5.21 305.4 10 35 92–371 0.01345 375.0 5452.0 B88 47.2 5 12.5 35 371–511 0.0413 428.4 5.01 12.5 50 511–688 0.0777 312.0 4191.5 15 50 688–889 0.2288 409.7 5.92 10 22 0–93 0.0084 0.88 3.31 576.3 B99 13.7 2 7.5 22 93–156 0.00108 265.0 7.13 10 60 0–87 0.0526 2.40 5.76 316.2 10 90 87–116 0.5745 724.1 9633.2 B109 17.5 4 7.5 90 116–175 0.398 1794.3 1.28 7.5 60 175–203 0.0881 1894.1 6076.1 10 22 0–93 0.00352 1.74 4.39 241.5 B112 12.4 2 20 22 93–156 0.0321 3.63 102.2 3.19 10 22 0–93 0.00505 1.37 3.45 346.5 B116 13.8 2 15 22 93–211 0.0110 3.63 125.2 1.92 10 22 0–93 0.00828 2.22 4.72 568.1 B130 9.0 2 15 22 93–211 0.0223 6.56 253.9 2.44 10 60 0–87 0.00382 1.93 6.22 23.0 B131 17.2 3 10 90 87–155 0.0112 2.23 14.1 4221.1 12.5 90 155–203 0.0125 2.46 28.0 0.62 Energies 2020, 13, 2239 28 of 32

Table A3. Load-thermal conditions and the results of the rheological tests—the Red Zuber unit samples (according to [58,62]).

Water-Insoluble No. of Test Eﬀective Stress Time Stationary Creep Transient Primary Sample Temp. (◦C) AQ/R n Minerals (%) Stages σef (MPa) (days) Rate (%/day) Creep (%) Strain (%) C11 26.3 1 12.5 75 0–114 0.0028 3.17 6.33 232.3 12.5 90 0–5 4.31 * 11,591.8 C27 64.1 3 7.5 90 5–17 0.893 **6.09 7886.9 3.08 7.5 60 17–53 0.367 ** 53,059.6 3583.0 10 60 0–85 0.0227 8.41 18,086.2 C53 38.2 2 12.38 10 75 85–148 0.04 8.64 61,125.3 4376.7 7.5 90 0–72 0.211 4.03 11,133.0 C55 27.4 2 5.58 7.5 60 72–127 0.0116 56,856.8 11,688.4 10 60 0–54 0.1197 2.34 47,418.0 C59 11.9 2 4.65 10 30 54–110 0.00245 18,559.8 13,079.5 10 22 0–371 0.00648 3.39 14,307.2 7.5 22 371–511 0.00177 67,912.3 4.51 C70 29.7 4 3.74 15 22 511–700 0.0102 4.76 32,388.7 2.53 17.5 22 700–884 0.0143 9.03 19,391.0 2.19 10 22 0–93 0.00774 4.6 31,451.1 C72 3.9 2 11.64 15 22 93–181 0.0178 5.9 50,766.5 2.05 10 22 0–371 0.00221 3.97 34,817.6 12.5 22 371–511 0.00852 4.28 3661.4 6.05 C74 44.6 4 7.7 15 22 511–700 0.0279 6.72 1927.2 6.51 17.5 22 700–885 0.0348 6.89 19,402.9 1.43 10 60 0–87 0.00717 2.13 106,167.8 C82 2.6 3 10 90 87–176 0.0234 2.92 16.74 46,614.3 4766.0 10 40 176–204 0.00927 28,516.1 2104.1 10 22 0–93 0.0121 4.24 24,434.0 C91 35.0 2 7.45 7.5 22 93–156 0.00174 20,068.7 6.74 10 22 0–93 0.00325 4.62 61,515.1 C95 38.8 2 6.04 20 22 93–211 0.041 8.67 232.3 3.66 10 60 0–87 0.0393 1.73 11,591.8 C98 8.0 2 7.96 10 90 87–150 0.7469 7886.9 11,865.1 * No stationary creep phase was recorded during the test and the value given relates to the pre-destructive phase. ** Stationary creep rate increased due to sample failure during the ﬁrst stage of the experiment. Energies 2020, 13, 2239 29 of 32

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