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Julkaisussa on yleiskuvaus turvallisuusperustelun roolista käytetyn ydinpolttoaineen loppusijoituksessa. Turvallisuusperustelu on se dokumentaatio, jolla loppusijoituksen luvanhakija haluaa lupahakemuksessaan osoittaa loppusijoituslaitoksensa turvallisuuden. Turvallisuuden todistustaakka on luvanhakijalla. Käytetyn ydinpolttoaineen huollon suunnitelmia Suomessa ja muissa maissa kuvataan lyhyesti. Suomessa käytetyn polttoaineen loppusijoituksen turvallisuusperustelu koskee pitkäaikaisturvallisuutta. Kansainvälisissä asiantuntijajärjestöissä IAEA ja NEA tehtyä turvallisuusperustelutyötä kuvataan, ja tämän työn vaikutus havaitaan myös suomalaisessa turvallisuusajattelussa. Suomessa Posiva Oy on edennyt kolmanteen turvallisuusperustelun suunnitelmaansa: suunnitelma on tarkentunut Säteilyturvakeskuksen kanssa käydyn dialogin myötä. Samaan aikaa Säteilyturvakeskus päivittää omaa ohjeistustaan. Havaitaan, että Posivan uusin turvallisuusperustelun suunnitelma on rakenteeltaan varsin lähellä STUKin uusimman ohjeluonnoksen ajattelua. Tieteellisen tutkimuksen rooli on merkittävä turvallisuusperustelulle, sillä sekä turvallisuusperustelu itse että viranomaisohjeistus perustuvat olennaisesti tieteellisesti tutkittuun tietoon, ja kummallekin on olennaista, että yhteys tutkimukseen säilyy.

The activity of 234U relative to 238U (AR) in groundwaters is controlled by isotope fractionation in water-rock interface. The fractionation is controlled by redox-conditions and rock-groundwater contact time. The measured ARs form thus an important source of information and offer an effective tool to characterise groundwater systems along with other hydrochemical data. The purpose of this paper is to elucidate the link between ARs and groundwater conditions. The paper considers the formation of AR in groundwaters and examines measured data from several study sites in the Fennoscandian Shield.

This paper aims to discuss the applicability of the classical matrix diffusion model against the integrated body of new data obtained by different methodologies on several samples of three granite boulders. The matrix diffusion model was tested against observations from the upper (most weathered in contact with air) and lower (fresh in contact with the ground) part of a boulder block. A U(VI) enrichment up to nearly 300 ppm (compared to about 10 ppm background concentration) mostly as uranophane was observed in the zone between the weathered and fresh rock. U-series disequilibrium studies indicated that most of U has been accumulated recently, about 10 000 years ago [1].High interconnected porosity (total porosity of > 1% and up to about 5.5% in altered minerals) characterizes the weathered zone (upper part), whereas the maximum porosity values in the fresh zone (lower part) of the rock are about 0.4 – 0.6%. Stable isotope studies δ18O and δ2H confirm that the mineralogical changes observed in the weathered upper part are due to old hydrothermal events. That is, the alteration is much older than uranium accumulation. Mössbauer spectroscopy showed that the Fe(III) content of the biotites from the upper to the lower part decreases from 30% in the weathered zone to 17% in the fresh rock, thus indicating possible redox control for the observed U precipitation. Fission track studies showed that secondary U(VI) also occurs within minerals grains (especially plagioclase) in the upper part.Mathematical simulations indicate that matrix diffusion alone is not enough to reconstruct the past U accumulation. The simulated concentrations derived from U concentration in pore water multiplied by Kd are clearly too small, indicating apparent insufficiency of the Kd approach. However, even with only matrix diffusion, the simulations roughly reconstruct the observation that U levels are clearly higher in the upper part of the boulder than in the lower part.

Low 234U/238U activity ratios observed in rock and mineral samples were scrutinized. U isotope fractionation leading to 234U depletion (234U/238U234U release depends on the valence contrast between the U isotopes, 238U occurring in +4 form and ingrown 234U, due to oxidizing microenvironment, in +6 form. Observed U isotopic fractionation combined with other uranium series disequilibrium measurements provides a tool for locating redox fronts formed as a result low temperature rock-groundwater interaction.

Performance assessments of geological repositories for nuclear waste require information about how possibly escaping radionuclides will be transported through the bedrock. Retardation mechanism studies provide an important part of the database required. So far Finnish performance assessments have considered sorption and matrix diffusion as retardation mechanisms. In this report we discuss how the data supplied by experimental retardation mechanism studies meet those required by performance assessments in the Finnish case. Experimental structural studies on small-scale pore network are discussed in detail. The C-14-PMMA method and helium-gas methods provide porosity, diffusivity and permeability data. The effort within the Palmottu natural analogue project that aimed to locate large-scale flow channels is described briefly. Specific retardation mechanism studies are discussed. The laboratory studies cover column experiments and anion exclusion studies. Column experiments provide data on sorption and matrix diffusion. The natural analogue studies cover work done in the Palmottu, the Hämeenlinna boulder, and the Hästholmen projects. Studies at Palmottu provide in situ data about sorption and matrix diffusion. This review indicates that data required by current migration models in performance assessments can be met by existing experimental methods. But the situation changes if more detailed modelling is wanted. In that case, a strategic question must be answered: what is a reasonable level of detail and conceptual rigour in performance assessment modelling, considering the unavoidable overall uncertainties due to long time scales and heterogeneous bedrock? The results obtained so far in natural analogue studies indicate that there is room for improvements in sorption modelling. The current Kd based approach could be supported by parallel mechanistic sorption modelling. It is acknowledged, however, that mechanistic sorption modelling is possible for neither all nuclides, nor all minerals, nor all geochemical conditions. Notwithstanding this, it would nevertheless better utilise the current state-of-the-art understanding of sorption.

The accuracy of uranium-series dating was studied quantitatively using the natural long-lived decay chains 4n+2 and 4n+3. An unusually well-bounded matrix diffusion case was used as the reference for USD simulations. The simulations applied the traditional closed and open system models, as well as detailed multistage models that incorporated the known accumulation history of the decay chain members. The results show clearly the improved accuracy and consistency in dating with detailed mass flow information.

One of the major problems in testing radionuclide transport models with a natural analogue is the potential discrepancy between experimental and modelling concepts. In the matrix diffusion studies at Palmottu the measured U-series concentration profiles and mathematical simulations showed disagreement, suggesting discrepancy in respective concepts of radionuclide attachment on the rock pores. Here, the discrepancy was approached by studying uranium fixation in more detail within the most loosely-bound fraction.

The main objective of the JYT-programme has been to provide the authorities with independent information and research results relevant for the safety of nuclear waste management. The main emphasis in this research programme has been devoted to the final disposal of spent fuel. The whole area of the research programme has been subdivided into following main topic areas: (1) bedrock structure and stability, rock investigation methods and characteristics and flow of ground water, (2) release of radionuclides from a repository and subsequent transport in the bedrock, (3) performance and safety assessment of repositories and other phases of nuclear waste management, (4) natural analogue studies, (5) waste management technology and costs and (6) socio political and other societal issues and environmental impact assessment.

Uranium-series activity ratios for U(VI) minerals from the Shinkolobwe mine in southern Zaire indicate that these minerals have not experienced significant preferential loss of uranium since their formation more than 100,000 years ago. The minerals examined include rutherfordine, UO2CO3, schoepite, [(UO2)8O2(OH)12]·12H2O, becquerelite, Ca[(UO2)6O4(OH)6]·8H2O, and uranophane, Ca[(UO2)2(SiO3OH)2]·5H2O. No correlation between mineral species and mineral age was evident. The oxidative dissolution of primary uraninite (UO2+x) has maintained ground waters supersaturated with respect to all of the secondary U(VI) minerals, providing an inexhaustible source of dissolved U6+ for mineral formation and growth. As long as uraninite persists in an oxidizing environment, the assemblage of secondary U(VI) phases is determined by local ground water chemistry (including transitory changes), but not necessarily a unidirectional reaction path towards equilibrium with U(VI) minerals of lower solubility. Thus the Shinkolobwe mine displays a complex assemblage of U(VI) minerals that reflects variations in the availability of dissolved elements besides U. Similarly, for a geologic repository exposed to oxidizing waters, the assemblage of corrosion products that will form during the corrosion of spent UO2 fuel is likely to be as complex as mineral assemblages found in natural uranium deposits under similar conditions.

A boulder sample was studied for its unusual U content. Analyses of U-series nuclides within the rock matrix perpendicular to an assumed fracture face show abrupt pulse-like concentration distributions with very low concentrations of U daughters. Both Th-230/U-234 and Pa-231/U-235 activity ratios are low, indicating recent U accumulation into the rock. Matrix diffusion is tested as a possible cause for the experimental observations. We assume that the diffusion process was triggered and controlled by rock expansion, strong mixing of different water types and rapid land uplift at the end phase of the last glaciation.

Uranium-series disequilibria data, in conjunction with petrographic analyses, indicate that the uranyl oxide hydrate becquerelite can persist for hundreds of thousands of years, possibly longer. Becquerelite probably forms continuously as ground water compositions permit and is resistant to U leaching by ground water. On the time scale of interest for the geologic disposal of spent UO2 nuclear fuel, becquerelite is a long-lived sink for uranium in oxidizing, U and Ca-bearing ground waters. Such long-term stability also supports recent solubility experiments that indicate natural becquerelite has a lower solubility product than that determined for synthetic becquerelites.

Concentration profiles in rock matrix around water-carrying fissures were measured at Palmottu U deposit. The profiles were interpreted by the classical matrix diffusion concept. Site-specific sorption studies were performed for U using standard batch experiments and surface complexation modelling; the response of sorption isotherms was also tested. Sitespecific matrix properties as well as initial and boundary conditions were used in simulations. Our results indicate that matrix diffusion alone cannot explain the observed enrichment of U and its daughters in the rock matrix.

Concentration distributions of natural decay chains 4n+2 and 4n+3 in crystalline rock intersected by a natural fracture were measured. Calcite coating on the same fracture surface was dated. Material properties of the rock matrix, and nuclide concentrations in groundwater were measured. The interpretation of the concentration distributions is based on the classical matrix diffusion concept. Although support was obtained, this calibration exercise does not yet validate the model. Besides initial and boundary conditions, matrix properties are uncertain due to the small amount of rock material. Experimental sorption data was not available, but its importance and the need for systematic studies was demonstrated.

Radioactive disequilibria, between U-238, U-234, and Th-230, in crystalline rock adjacent to a fracture, indicates mass transfer of U and Th between water in the fracture and the rock. The matrix diffusion theory was used to interpret the observed profiles of mobilized nuclides around the natural fracture. The interpretation of the profiles was based on the use of uranium series disequilibrium code, URSE, and migration code FTRANS. The model system was characterized using all available site-specific data, and the system evolution was outlined using the geology of the Palmottu site. The simulated concentration profiles, as a function of depth from the fracture surface, indicate that measured profiles can be modelled by matrix diffusion, assuming realistic initial and boundary conditions and diffusion times of 300,000 years.

Työssä on tarkasteltu luonnon uraanisarjojen nuklidien välisiä radioaktiivisia epätasapainoja ja niiden kvantitatiivista tulkintaa. Uraanisarjat pystyvät säilyttämään sisältämiensä nuklidien eripituisten puoliintumisaikojen ansiosta informaatiota pitkiä aikoja (noin 350 000 vuotta) kallioperässä tapahtuneista muutoksista. Radioaktiivinen epätasapaino aiheutuu radionuklidien erilaisesta fysikaaliskemiallisesta käyttäytymisestä luonnon olosuhteissa. Uraanisarjojen useiden keskeisten alkuaineiden geokemiallinen käyttäytyminen tunnetaan varsin hyvin, mikä antaa menetelmän käytölle hyvät lähtökohdat. On kehitetty matemaattinen malli, jonka avulla voidaan simuloida korkeintaan nelijäsenisen radioaktiivisen hajoamisketjun pitoisuuksia yksinkertaistetuissa tilanteissa. Tarkasteltavat systeemit ovat termodynaamisessa mielessä joko ajoittain avautuvia tai täysin avoimia, jolloin vastaavasti systeemin massavirta ympäristönsä kanssa on kestoltaan erittäin lyhyt tai jatkuva. Eri systeemityyppejä edustavia yksittäisiä häiriöitä voidaan yhdistellä ajallisesti jatkuvaksi häiriöskenaarioksi. Matemaattisen mallin perusteella on kehitetty tietokoneohjelma URSE (uraanisarjojen epätasapainot), joka on testattu sekä vertailulaskuin toiseen tietokoneohjelmaan että kirjallisuudessa raportoitujen keskeisten tapausten avulla. Uraanisarjojen epätasapainojen tulkinta perustuu radioaktiivisen hajoamisketjun taipumukseen pyrkiä itselleen luonteenomaisella nopeudella kohti radioaktiivista tasapainoa. Ulkopuoliset massavirrat häiritsevät tätä tasapainottumisprosessia; häiriöiden luonnetta ja kestoa voidaan päätellä siitä kuinka kaukana systeemi on tasapainotilastaan. Uraanisarjojen epätasapainojen kvantitatiivinen tulkinta on kuitenkin erittäin vaativa tehtävä, koska edellytyksenä on itse tutkittavan systeemin teoreettinen hallinta. Vastapainona sen avulla voidaan toisaalta validoida luonnon systeemin konseptuaalisia malleja satoja tuhansia vuosia kattavien ajanjaksojen yli, mihin aikaskaalaan ei mitenkään voi päästä laboratorio- tai kenttäkokein.

Three Finnish natural analogue studies are presented shortly, they deal with different aspects of disposal system in fractured rock. Native copper study touches on the long term stability of copper canister in granitic bedrock. Anion profile studydeals with penetration depth of matrix diffusion in rapakivi granite. Palmottu uranium deposit is being studied as migration analogue in fractured granitic bedrock. An attempt is made to relate natural analogue studies to the disposal system and to evaluate the applicability of the results in performance assessments.