The focus of this doctoral thesis is the redox chemistry involved in radiation induced oxidative dissolution of spent nuclear fuel and UO2 (as a model substance for spent nuclear fuel). It is shown that two electron oxidants are more efficient than one electron oxidants in oxidative dissolution of UO2 at low oxidant concentrations. Furthermore, it is shown that H2O2 is the only oxidant that has to be taken into account in radiation induced dissolution of UO2 under deep repository conditions (granite groundwater dominated by α-radiolysis). Previously determined rate constants for oxidation of UO2 by H2O2 and O2, and rate constants for dissolution of U(VI) from the surface are successfully used to reproduce numerous UO2 dissolution rates reported in the literature. The impact of reactive solutes (Fe(II)(aq), 2-propanol and chloride) and Pd-inclusions (as a model for ε-particles) in combination with H2, on radiation induced oxidative dissolution of UO2 is investigated. It is shown that both the studied reactive solutes (under oxygen free conditions) and the combination of Pd inclusions and H2 inhibit the dissolution. Calculations (based on the fuel inventory) show that 1 µM Fe(II)(aq) decreases the dissolution rate by a factor of ~50 and that 1 ppm surface coverage of ε-particles is sufficient to completely stop the dissolution of 100 year old fuel (assuming 40 bar H2).The dissolution behavior of NpO2 and PuO2 in H2O2 containing aqueous solution without complexing agent is studied and compared to UO2. Based on the measured dissolution rates, we would not expect the dissolution of the actinides to be congruent. Instead, in a system without complexing agent, the rates Np and Pu are expected to be lower than the U release rate. The effect of ionizing irradiation on the UO2 reactivity is studied in order to elucidate the effect of self-irradiation on the reactivity of the spent fuel matrix. It is shown that a threshold dose must be achieved before any effect of irradiation can be seen. Beyond the threshold the reactivity seems to increase with increasing dose. Furthermore, the effect appears to be permanent. The effect of particle size on the reactivity of UO2 powder is studied in view of proposed theories suggesting a particle size dependence of both the pre-exponential factor and the activation energy for redox reactions. The rate constant and activation energy for oxidation of UO2 by MnO4- seems to agree with the proposed equations. The radiation chemical synthesis of UO2 nanoparticles is studied. It is shown that U(VI) released by dissolution of spent nuclear fuel could be reduced to UO2 nanoparticles.These particles could, due to their high reactivity towards H2O2, act as oxidant scavenger in a future deep repository for spent nuclear fuel. / Denna doktorsavhandling behandlar redoxprocesser involverade i strålnings­inducerad oxidativ upplösning av använt kärnbränsle och UO2 (som modellsubstans för använt kärn­bränsle).Detta arbete visar att två-elektron oxidanter är mer effektiva än en-elektron oxidanter i oxidativ upplösning av UO2 vid låga oxidantkoncentrationer. Dessutom visas, på kinetiska grunder,att H2O2 är den enda oxidant som måste tas hänsyn till vid stålningsinducerad oxidativ upplösning av UO2 under djupförvarsförhållanden (granitiskt grundvatten dominerat av α-radiolys). Tidigare bestämda hastighets­konstanter för oxidation av UO2 med H2O2 och O2, samt hastighets­konstanter för upplösning av U(VI) från ytan har framgångrikt använts för att återskapa UO2 upplösningshastigheter rapporterade i litteraturen. Inverkan av reaktiva ämnen i vattenfas (Fe(II)(aq), 2-propanol och klorid) samt av Pd-inneslutningar (som modell av ε-partiklar) i UO2 matrisen i kombination med H2, på strålningsinducerad upplösning av UO2 har studerats. Studien visar att både de reaktiva ämnena i vattenfasen (under syrefria förhållanden) och Pd- inneslutningar i kombination med H2 hämmar upplösningen. Beräkningar (baserade på ett bränsle­inventarie) visar att 1 µM Fe(II)(aq) minskar upplösningshastigheten med en faktor ~50 samt att 1 ppm ytbeläggning av ε-partiklar är tillräckligt för att effektivt stoppa uppslöningen av 100 år gammalt bränsle (vid 40 bar H2).Upplösning av NpO2 och PuO2, i jämförelse med UO2,har studerats i vatten­lösning innehållande H2O2 utan komplexbildare. Baserat på de uppmätta upp­lösnings­hastigheterna förväntas upplösningen av dessa aktinider från UO2-bränsle vara inkongruent. I ett system utan komplexbildare kan NpO2 odh PuO2 upplösningshastighetern förvantas vara lägre än UO2 upplösningshastigheten. Effekten av joniserande strålning på reaktiviteten hos UO2 har studerats för att klargöra effekten av egen-bestrålning. Studien visar att dosen måste nå ett visst tröskelvärde innan någon effekt på reaktiviteten kan observeras. Vid doser över tröskelvärdet ökar reaktiviteten med ökande dos. Effekten verkar vara permanent. Partikelstorlekens inverkan på reaktiviteten hos UO2 pulver har studerats med utgångspunkt i föreslagna samband mellan partikelstorlek och pre-exponentiell faktor och mellan partikelstorlek och aktiveringsenergi. Studien visar att hastighets­konstanten och aktiveringsenergin för reaktionen mellan UO2 och MnO4- överenstämmer med de föreslagna sambanden. Stålningskemisk syntes av UO2 nanopartiklar har studerats. Studien visar att U(VI) frigjort genom upplösning av använt kärnbränsle i ett djupförvar kan reduceras till UO2 nanopartiklar. Dessa partiklar kan, på grund av sin höga reaktivitet med H2O2, fungera som infångare av oxidanter i ett framtida djupförvar för använt kärnbränsle. / QC 20100908

<p>I det svenska djupförvaret för kärnbränsle ska kompakterad bentonit användas som barriär mellan kopparkapslar med utbränt kärnbränsle och berget. I kontakt med vattenförande sprickor kan bentonitbarriären under vissa omständigheter avge montmorillonitkolloider. Förutom att barriärens egenskaper urholkas pga förlusten av material kan kolloiderna, om de är stabila, underlätta transporten av sorberade radionuklider ut mot biosfären.</p><p>Den här studien fokuserar på att undersöka vilka effekter grundvattenkemin har på montmorillonitkolloiders stabilitet. Grundvattnets sammansättning, pH och jonstyrka, kommer sannolikt att förändras under djupförvarets livslängd, delvis pga inträngandet av glactialt smältvatten. Initialt kommer omgivande berg att värmas upp pga värmealstring från det radioaktiva sönderfallet i det utbrända kärnbränslet. Effekterna av pH, jonstyrka och temperatur på montmorillonitkolloiders stabilitet har analyserats genom att följa hur kolloiderna aggregerar med tiden. Minskningen av partikelkoncentration med tiden mättes med Photon Correlation Spectroscopy (PCS).</p><p>Aggregationsexperimenten visar att, vid ett givet pH och en given temperatur, ökar hastighetskonstanten för aggregation med ökande jonstyrka. Kritiska koaguleringskoncentrationen (CCC) för NaCl och CaCl2 för Na-montmorillonit och Ca-montmorillonit beräknas utifrån ett samband mellan hastighetskonstanterna och jonstyrkan.</p><p>Hastighetskonstanten för aggregation minskar med ökande pH eftersom ytpotentialen ökar. Effekten blir tydligare vid högre jonstyrkor och högre temperaturer, men kan däremot inte observeras vid låga temperaturer.</p><p>Temperatureffekten på bentonitkolloidernas stabilitet är pH-beroende. Vid pH≤4 ökar hastighetskonstanten för kolloidaggregation med ökande temperatur, oavsett jonstyrka.Vid pH≥10 minskar hastighetskonstanten med ökande temperatur. I mellanliggande pH-område minskar hastighetskonstanten för aggregation med ökande temperatur, förutom vid den högsta jonstyrkan, där den ökade. Beräkningar baserade på DLVO-teori matchar de experimentella resultaten.</p> / <p>In deep geological repositories in Sweden, encapsulated nuclear waste will be surrounded by compacted bentonite in the host rock. In future contact with water-bearing fractures, this bentonite barrier can release montmorillonite colloids under certain conditions. This process can lead to loss of buffer material. Furthermore, these colloids, if stable, may facilitate the transport of associated radionuclides towards the biosphere. Colloid stability is determined by groundwater chemistry.</p><p>This study addresses the effects of groundwater chemistry on the stability of montmorillonite colloids. During the lifetime of the repository, the pH and ionic strength of the groundwater are expected to vary, partly due to intrusion of glacial melt water. Initially, the temperature will be higher in the surrounding host rock due to heat released from radioactive decay in the spent nuclear fuel. The effects of these parameters on the stability of montmorillonite suspensions were evaluated by studying the aggregation kinetics. The change in particle concentration with time was monitored by Photon Correlation Spectroscopy (PCS).</p><p>Aggregation kinetics experiments showed that for a given pH and temperature, the rate constant for colloid aggregation increased with increasing ionic strength. The relationship between the rate constant and the ionic strength allowed the NaCl and CaCl2 critical coagulation concentration (CCC) for Na- and Ca-montmorillonite to be determined.</p><p>The aggregation rate constant decreased with increasing pH as the surface potential increased. This effect became more pronounced at higher ionic strengths and higher temperatures but could not be observed at low temperature.</p><p>The effect of temperature on the stability of the suspensions is pH-dependent. At pH≤4, the rate constant for colloid aggregation increased with increasing temperature, regardless of ionic strength. At pH≥10, the aggregation rate constant decreased with increasing temperature. In the intermediate pH interval, the aggregation rate constant decreased with increasing temperature except at the highest ionic strength, where it increased.</p><p>The experimental results were in agreement with DLVO calculations.</p>

<p>Denna licentiatavhandling behandlar effekten av två inneboende egenskaper (fissions produkter och egenbestrålning) hos utbränt kärnbränsle på hastigheten för strålningsinducerad upplösning av bränslematris (UO2). I ett framtida djupförvar kommer det utbrända kärnbränslet att deponeras 500 meter ner i berggrunden i en reducerande miljö. Under dessa förhållanden är UO2-matrisen själv en av de skyddande barriärerna mot frigörande av radionuklider, på grund av dess låga löslighet. När bränslet kommer i kontakt med vatten kommer U(IV) att oxideras till U(VI) av radiolysprodukter från vattnet och lösligheten för bränslematrisen kommer därmed att öka betydligt. </p><p>De flesta tidigare studier har utförts på obestrålad UO2 som skiljer sig signifikant från utbränt kärnbränsle. I utbränt kärnbränsle är de flesta fissionsprodukterna och neutronaktivieringsprodukterna radioaktiva och bränslet kommer därför bli bestrålat av sig självt. Effekten av joniserande strålning på reaktiviteten för UO2(s) har undersökts här. UO2 (pulver och fragment av en kuts) bestrålades i en 60Co γ-källa eller framför en elektronaccelerator varpå reaktiviteten för UO2 studerades genom oxidation av UO2 med MnO4 -. Det visade sig att reaktiviteten för UO2 ökar när det blir bestrålat för första gången (<20 kGy). Effekten ökar med ökande dos tills den når ett maxvärde ~1.3 gånger reaktiviteten för obestrålad UO2 vid torrbestrålning. Vid våtbestrålning ökar en dos på 140 kGy reaktiviteten 2.5 gånger. Effekten verkar vara permanent. </p><p>Tidigare studier har visat att H2O2 är den viktigaste oxidanten för upplösning av utbränt kärnbränsle under djupförvarsförhållanden. I vätgasatmosfär, som förväntas i ett djupförvar, har det visat sig att upplösningshastigheten är långsammare. Det har delvis förklarats med reaktionen mellan H2O2 och H2, som är väldigt långsam utan katalysator. Den katalytiska effekten av UO2 på den reaktionen har undersökts och det visades att den inte katalyseras av UO2. </p><p>En annan möjlig katalysator för reaktionen är ε-partiklar (ädelmetallpartiklar bestående av Mo, Ru, Tc, Pd och Rh) som bildats av fissionsprodukterna. Pd är en välkänd katalysator för reduktion med H2. Den eventuella katalytiska effekten av Pd har undersökts här. Även en eventuell katalytisk effekt av Pd på reduktionen av U(VI) med H2 undersöktes, både i vattenfas och i UO2-kutsar innehållande olika mängder Pd (som en modell för ε-partiklar).</p><p> Vi fann att Pd har en katalytisk effekt på reaktionen mellan H2O2 och H2 och andra ordningens hastighetskonstant är bestämd till (2.1±0.1)x10-5 m s-1. Pd har också en katalytisk effekt på reduktionen av U(VI) med H2 både i vattenlösning, hastighetskonstant (1.5±0.1)x10-5 m s-1, och i den fasta fasen. Hastighetskonstanten för processen i fast fas är 4x10-7 m s-1 och 7x10-6 m s-1 för kutsar med 1 respektive 3 % Pd. Dessa värden är väldigt nära diffusionsgränsen för den här typen av system. Den katalytiska effekten i den fasta fasen visar att upplösningen för 100 år gammalt bränsle kan stoppas helt. Vid 40 bar H2 krävs 10-20 ppm ädelmetallpartiklar och med 1 % ädelmetallpartiklar räcker det med 0.1 bar H2 för att stoppa upplösningen. </p> / <p>This licentiate thesis deals with the influence of two inherent properties (fission products and self irradiation) of spent nuclear fuel on the rate of radiation induced fuel matrix (UO2) dissolution. In a future deep repository the spent nuclear fuel will be deposited 500 meters down in the bedrock in a reducing environment. Under these conditions the UO2-matrix itself is one of the protective barriers against release of radionuclides due to its very low solubility. When the fuel comes in contact with water, U(IV) will be oxidized to U(VI) by products from radiolysis of water and the solubility of the fuel matrix will increase significantly.</p><p>Most previous studies have been performed on unirradiated UO2 which differ significantly from spent nuclear fuel. In spent nuclear fuel most of the fission products and neutron activation products are radioactive and therefore the fuel will be irradiated by itself. The effect of ionizing radiation on the reactivity of UO2 has been investigated here. UO2 (powder and fragment of a pellet) has been exposed to irradiation in a 60Co γ-source or in an electron accelerator and then the redox reactivity was studied. The kinetics for oxidation of UO2 by MnO4 - was used as a monitoring reaction. It was shown that the reactivity of UO2 increases when being irradiated for the first time (<20kGy). The effect increases with increasing dose until reaching a maximum value ~1.3 times the reactivity of unirradiated UO2 for dry irradiation. For wet irradiation a dose of 140 kGy increases the reactivity ~2.5 times. This effect appears to be permanent.</p><p> Previous studies have shown that H2O2 is the most important oxidant for spent nuclear fuel dissolution under deep repository conditions. Under H2 atmosphere, as expected in a deep repository, it has been shown that the dissolution rate is slower. This has partly been attributed to the reaction between H2O2 and H2 which is very slow without a catalyst. The catalytic effect of UO2 on this reaction was examined showing that UO2 does not catalyze this reaction.</p><p> Another possible catalyst for this reaction is the ε-particles (noble metal particles containing Mo, Ru, Tc, Pd and Rh) formed by the fission products. Pd is a well known catalyst for reduction by H2. The possible catalytic effect of Pd on the reaction between H2O2 and H2 is examined here. The possible catalytic effect of Pd on the reduction of U(VI) by H2 is also examined, both in aqueous phase and in UO2 pellets containing different amounts of Pd (as a model for spent fuel containing ε-particles).</p><p> It was found that Pd has a catalytic effect on the reaction between H2O2 and H2, the second order rate constant is determined to (2.1±0.1)x10-5 m s-1. Pd also has a catalytic effect on the reduction of U(VI) by H2 both in aqueous solution, rate constant (1.5±0.1)x10-5 m s-1, and in the solid phase, rate constants 4x10-7 m s-1 and 7x10-6 m s-1 for pellets with 1 and 3 % Pd respectively. These values are very close to the diffusion limit for these systems. The catalytic effect in the solid phase shows that the dissolution for 100 year old fuel can be completely inhibited, at 40 bar H2 a noble metal particle content of 10-20 ppm is needed and with 1 % noble metal particle content 0.1 bar H2 is enough to stop the dissolution. </p>

The release of radionuclides is a key process in the safety assessment of a deep geological repository for spent nuclear fuel. A large fraction of the release is assumed to be a consequence of dissolution of the fuel matrix, UO2. In this doctoral thesis, the kinetics and the mechanisms behind oxidative U(IV) dissolution were studied. The eects of solid phase inclusions mimicking the presence of fission products, and solutes mimicking expected groundwater components were also evaluated. Palladium, as a model substance for noble metal particle (fission products) inclusions, was shown to catalyze surface oxidation of U(IV), as well as reduction of U(VI). The second order rate constant for the surface reduction of U(VI) by H2was found to be on the order of 10-6 m s-1 (diusion controlled). Under 40 bar H2, 1 wt.% Pd was sufficient to suppress oxidative U(IV) dissolution in 2mM H2O2 aqueous solution. During g γirradiation under 1 bar H2, 0.1 wt.% Pd were sufficient to completely suppress oxidative dissolution. Under inert conditions, where H2 is only produced radiolytically, complete inhibition is observed for 3 wt.% Pd. The presence of Y2O3 as a model substance for trivalent fission products was found to decrease U(VI) dissolution significantly under inert, as well as reducing conditions. Based on kinetic data, it was shown that pure competition kinetics cannot explain the observed decrease. From experiments using pure oxidants it was shown that Y2O3 doping of UO2 decreases the redox reactivity. In addition, from experiments where hydroxyl radical formation from the catalytic decomposition of H2O2 was monitored, it could be concluded that doping has a minor influence on this process. On the basis of numerical simulations, the H2 concentration necessary to suppressradiolytic H2O2 production was found to increase with an increase in dose rate or HCO-3 concentration. Furthermore, the steady state concentration of H2O2 was found to be inversely proportional to the H2 pressure, and proportional to the square root of the dose rate. Fe2 diers strongly from the total reaction volume, the actual dose rate should not be converted into a homogeneous dose rate in numerical simulations. / QC 20110511

QC 20110608

The topic of this doctoral thesis is the potential near field immobilization of the radionuclide 79Se after intrusion of groundwater into a spent nuclear fuel canister in a repository. 79Se is a non naturally occurring long lived selenium isotope formed as a result of fission in nuclear fuel. Given the long half life (~3 x 105 y) and that the oxyanions of selenium are expected to be highly mobile and potentially difficult toimmobilize the isotope is of interest for the long term safety assessment of high level waste repositories. In this work the near field has been limited to the study of processes at or near the UO2 surface of (simulated) spent nuclear fuel and to processes occurring at or near the surface of iron (canister material) corroding under anoxic conditions. Selenite (HSeO32-) was found to adsorb onto palladium (simulated noble metal inclusion in spent nuclear fuel). Under hydrogen atmosphere selenite was reduced to elemental selenium with a rate constant of ~2 x 10-9 m s-1 (with respect to the Pd surface, 24 bar H2) forming colloidal particles. The rate constant of selenite reduction was increased by about two orders of magnitude to ~2.5 x 10-7 m s-1 (with respect to the Pd surface, 10 bar H2) for a UO2 surface doped with Pd particles, indicating that UO2 is an efficient co-catalyst to Pd. Selenate (SeO42-) was neither adsorbed nor reduced in the presence of Pd, UO2 and hydrogen. In the iron corrosion studies selenate was found to become reduced to predominantly elemental Se in the presence of a pristine iron surface. Iron covered by a corrosion layer of magnetite did however appear inert with respect to selenate whereas selenite was reduced. The reduction of dissolved uranyl into UO2 by the corroding iron surfaces was found to significantly increase the removal rate of selenite as well as selenate. The uranyl was found to transiently transform the outer iron oxide layers on the iron, forming a reactive mixed Fe(II)/Fe(III) oxyhydroxide (Green rust). Exchanging the solution and increasing the carbonate content (from 2 mM to 20 mM NaHCO3) only resulted in a minor, transient remobilization of uranium. Addition of H2O2 did however result in a significant release of uranium as well as selenium from the iron oxide surfaces. An irradiation experiment was also performed confirming the one electron reduction barrier of selenate as an important factor in systems where selenate reduction would be thermodynamically favorable. / QC 20101208

The test production of a simulated nuclear fuel, possible to use as a calibration standard was donethru dissolving the preferred dopant-metals in a nitric-salt into weak acid. This solution wasmixed with dissolved uranium-salt to aimed concentration. This was precipitated by rising pHforming metallic-hydroxide.The precipitated salt was dried in an reduction gas, pressed to pellets and sintered in oven. Thequality of the pellets and its content was evaluated thru comparing theoretical and measureddensity, ICP-MS for which ground elements and which concentration of these forming thepellets. XRD was used to evaluate formed associated ground-elements and the lattice parameters.Finally, SEM was used to evaluate how homogenous the distribution of the doped elementsturned out.There are still a variety of tests yet to perform in order to create a high quality calibrationstandard. Our preparation of dissolved metallic salts will hopefully be usable for any furtherexperimentalist in this field. The single-doped calibration-standards are aimed to be used byStudsvik to calibrate their Lazer Ablation Inductivity coupled Plasma Mass Spectrometryinstrument.

In Sweden the encapsulated nuclear waste will be surrounded by compacted bentonite in the granitic host rock. In contact with water-bearing fractures the bentonite barrier may release montmorillonite colloids that may be further transported in groundwater. If large amounts of material are eroded from the barrier, the buffer functionality can be compromised. Furthermore, in the scenario of a leaking canister, strongly sorbing radionuclides, can be transported by montmorillonite colloids towards the biosphere. This thesis addresses the effects of groundwater chemistry on the generation, stability, sorption and transport of montmorillonite colloids in water bearing rock fractures. To be able to predict quantities of montmorillonite colloids released from the bentonite barrier in contact with groundwater of varying salinity, generation and sedimentation test were performed. The aim is first to gain understanding on the processes involved in colloid generation from the bentonite barrier. Secondly it is to test if concentration gradients of montmorillonite colloids outside the barrier determined by simple sedimentation experiments are comparable to generation tests. Identical final concentrations and colloid size distributions were achieved in both types of tests. Colloid stability is strongly correlated to the groundwater chemistry. The impact of pH, ionic strength and temperature was studied. Aggregation kinetics experiments revealed that for colloid aggregation rate increased with increasing ionic strength. The aggregation rate decreased with increasing pH. The temperature effect on montmorillonite colloid stability is pH-dependent. At pH≤4, the rate constant for colloid aggregation increased with increasing temperature, regardless of ionic strength. At pH≥10, the aggregation rate constant decreased with increasing temperature. In the intermediate pH interval, the aggregation rate constant decreased with increasing temperature except at the highest ionic strength, where it increased. The relationship between the rate constant and the ionic strength allowed the critical coagulation concentration (CCC) for Na- and Ca-montmorillonite to be determined. In order to distinguish the contribution of physical filtration and sorption to colloid retention in transport, the different retention mechanisms were quantified. Sorption on different representative minerals in granite fractures was measured for latex colloids (50, 100, 200 nm) and montmorillonite colloids as a function of ionic strength and pH. Despite of the negative charge in mineral surfaces and colloids, sorption was detected. The sorption is correlated to the mineral point of zero charge and the zeta potential of the colloids, and increases with increasing ionic strength and decreasing pH. In transport experiments with latex colloids in columns packed with fracture filling material, the retention by sorption could clearly be seen. In particular at low flow rates, when the contact time for colloids with the mineral surfaces were the longest, sorption contributed to retention of the transport significantly. The retention of latex colloids appeared to be irreversible in contrary to the reversible montmorillonite colloid retention. Generation, stability and sorption of the montmorillonite colloids are controlled by electrostatic forces; hence, the results were in qualitative agreement with DLVO.