RD&D-Programme 2004 - SKB

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Influence of hydrogen on dissolution of alpha-doped uranium dioxide Since radical-rich gamma and beta radiation have a favourable effect on hydrogen activation accompanied by consumption of radiolytic oxidants, the relevance of the results for the scenario with spent fuel in a repository can rightfully be questioned. Since virtually only alpha radiation is expected to persist for a long time, the favourable effect of gamma and beta radiation would diminish progressively. A material that resembles old fuel is alpha-doped uranium dioxide, where different quantities of an alpha-emitting isotope are homogeneously distributed in the uranium dioxide matrix. Alpha-doped uranium oxide has been synthesized in cooperation with ITU in Karlsruhe. As a part of SKB’s work in the EU project SFS, leaching of a uranium dioxide pellet doped with ten percent uranium-233 was studied in a titanium autoclave under three different hydrogen pressures. Leaching was started with a 10 mM sodium chloride solution under a hydrogen pressure of 16 bar. Samples of the solution were taken at different times and analyzed by alpha spectrometry. After four months, the estimated uranium concentration in the solution was still less than 3·10 –10 M. To dissolve all oxidized uranium from the pellet, a bicarbonate solution was added to the autoclave, but no changes were noted in the uranium concentration. No measurable dissolution of uranium could be observed, not even when the hydrogen pressure was reduced to 1.6 bar during a 178-day period. The hydrogen pressure was further reduced by a factor of ten to 0.16 bar (1.6 bar of argon with a hydrogen concentration of 6.08 percent by volume) during a 141-day period. Preliminary results from alpha radiometric analyses of uranium-233 are shown in Figure 15-6. The results show that no measurable increase in the very low uranium concentrations in the solution could be observed during the entire test period (more than 15 months). If, in this case, uranyl carbonate species were produced in a layer several tens of microns thick near the surface (range of alpha particles), some of them would diffuse away and contribute to an increase of the uranium concentration in the solution. These results are confirmed by the measured levels of radiolytic oxygen, which lie below the detection limit (10 –8 M) for the oxygen sensor that was in the autoclave solution during the entire test. The same applies to any radiolytic oxygen or hydrogen peroxide: some of this would diffuse away from the surface and cause an increase in the oxygen levels, which would eventually be recorded by the oxygen electrode, since the test lasts such a long time. There is almost no gamma or beta radiation, and published data /15-23/ with 5 MeV helium ions (alpha radiation) indicate that there is no influence of hydrogen on 100 10 mM NaCl 10 mM NaCl + 2mM HCO 3 — 10 —7 10 10 —8 P H2 (bar) 1 0.1 10 —9 10 —10 10 —11 233 U (mol/l) 0.01 10 —12 0.001 10 —13 0 200 400 600 800 1,000 Time (days) Figure 15-6. Measured hydrogen pressure and 233 U concentrations in the solution as a function of time. RD&D-Programme 2004 177
oxidant consumption in the bulk solution. At the end of the test, the uranium dioxide surface will be examined by spectroscopic methods to determine if oxidation has occurred. Published independent experiments with alpha radiolysis in hydrogen-saturated solutions and uranium dioxide surfaces show that no oxidation occurs /15-29/. Titanium surfaces will also be studied with respect to uranium. Even though titanium surfaces are not expected to influence the results, the tests will be repeated in autoclaves with quartz surfaces. Independent results from leaching of similar uranium dioxide pellets in glass vessels with only six percent hydrogen gas in argon do not show any increase in the uranium concentrations with time either /15-20/. Influence of pH, salinity and temperature The dissolution rate of spent fuel is dependent on more than just the redox potential, even though this is the most important factor. Other factors that influence the dissolution rate are the composition of the fuel and the groundwater. Experiments with leaching of fuel samples taken from different segments in a fuel rod with burnup varying from 21 to 49 MWd/kgU /15-36/ show a weak and almost linear increase of leaching (cumulative released fraction) up to 40–43 MWd/kgU, followed by a decrease. The influence on the formation of secondary phases in connection with fuel dissolution is investigated separately /15-37/. The water composition in all other studies has been chosen to avoid the formation of secondary phases, since this makes it more difficult to interpret the results /15-38/. Fuel dissolution at different pHs from 3 to 9.2 and different carbonate concentrations from 0 to 10 mM has been studied by flow-through leaching /15-39/. The tests show that low pHs increase the dissolution rate under oxidizing conditions. In hydrogen-saturated solutions, pH had no influence. The importance of very high pHs typical of concrete leaching has also been investigated /15-40/. The influence of temperature has been studied in the range 20–70°C with the aid of autoclaves filled with argon or hydrogen, and studies at higher temperature are planned, see under the heading “Programme” below. SKB has only performed a few of its own measurements of fuel dissolution at high salinities. Studies of the dissolution of alpha-doped uranium dioxide employing the isotope dilution method at low and high salinities and development of analysis methods are planned in cooperation with Posiva within the framework of the EU project NF-Pro. Radiolysis studies and radiolytic modelling Mass balance tests have been conducted where the time dependence for formation of hydrogen and oxygen has been studied in a closed system with approximately two grams of fuel fragments and with initially oxygen-free solutions. In order to permit mass balance calculations, a new leaching system has been used permitting simultaneous analysis of both gas phase and solution. Oxygen and hydrogen in the gas phase were measured at different times by means of gas phase electrode detectors. The aqueous phase was analyzed with respect to hydrogen peroxide (luminescence measurements) and dissolved radionuclides (ICP-MS). The way in which the composition of the solution influences radiolytic oxidant production and fuel dissolution has been studied and modelled /15-41/. Carbonate and chloride concentrations in the solution were varied between 0 and 10 mM. Since analysis of the gas phase during the course of the test always entails a risk of leakage and loss of radiolysis gases, particularly as their concentrations grow higher with time, long-term tests with sealed glass vials have also been carried out. In the test series, the size of the fuel fragment (surface-area-to-volume ratio) and the composition of the solution (chloride and carbonate contents) were varied. The vials were opened and the gas phase and the solution were analyzed after one, two and three years. The measured concentrations of radiolytic oxygen and hydrogen peroxide that did not react with the fuel surface, as well as the concentrations of dissolved uranium, were used to calculate a mass balance of radiolytic oxidants /15-21/. 178 RD&D-Programme 2004
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Technical Report TR-04-21 RD&D-Prog
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Preface SKB, Svensk Kärnbränsleha
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welding and nondestructive testing
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Alternative methods. SKB is followi
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5.5 Nondestructive testing of canis
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15 Fuel 163 15.1 Initial state in f
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18.1.10 Swelling pressure 229 18.1.
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Part I SKB’s programme and plan o
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Nuclear power plant Clab m/s Sigyn
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Figure 1-2. The Äspö HRL consists
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Figure 1-4. Interior from the Canis
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Siting SKB’s main alternative is
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Transport tunnel KBS-3V Deposition
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2.1 Nuclear fuel programme In Swede
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Encapsulation plant 2003 2004 2005
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2.2 LILW programme Parts of the sys
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environment. The barriers can also
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this cooperation entails that the o
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4 Overview - technology development
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4.7 Design of the deep repository T
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Diameter 1,050 Diameter 949 Diamete
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Conclusions in RD&D 2001 and its re
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Figure 5-3. Graphite shapes in cast
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Figure 5-5. Positions for test bars
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Programme The development project c
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Programme Trial fabrication of all
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The technology and procedures for c
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A method and equipment based on las
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Spacer plate Defect A Defect B Chan
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Newfound knowledge since RD&D 2001
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2004 2005 Process model welding met
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6.2.2 The welding process In parall
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Figure 6-9. Lid weld L028. Section
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Tensile test bars have been taken o
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6.3.3 The welding process The param
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A limitation in the evaluation of t
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Nondestructive testing methods such
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a b Through-transmission Reflection
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simulation program, and the body of
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SKB has devised a quality system fo
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8 Canister - encapsulation The desi
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Figure 8-2. Work stations in the en
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The filled canister transport casks
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Stages encapsulation plant 2003 200
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Figure 8-4. Possible location of a
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9 Transportation of encapsulated fu
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9.3 SKB’s transportation system -
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absorbs neutron radiation. The lid
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Applicable international and Swedis
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een specified yet. Since the size o
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• Methods exist for full-face dri
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Programme SKB keeps track of curren
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Judgement with regard to long-term
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Furthermore, it must not have an ad
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Studies of equipment will be conduc
Page 117 and 118: A third type of seal is intended to
Page 119 and 120: 10.6.1 Requirements and premises In
Page 121 and 122: the long-term safety of the concept
Page 123 and 124: Newfound knowledge since RD&D 2001
Page 125 and 126: Conclusions in RD&D 2001 and its re
Page 127 and 128: 11.3 Execution - stepwise design Si
Page 129 and 130: Programme Design step D, which incl
Page 131 and 132: 12 Deep repository - monitoring SKB
Page 133 and 134: • Trigger levels for action. •
Page 135 and 136: Part III Safety assessment and rese
Page 137 and 138: As a complement to the numerical mo
Page 139 and 140: Table 13-2. Research on the initial
Page 141 and 142: 13.2.4 Backfill Together with Posiv
Page 143 and 144: Table 13-3. Projects and experiment
Page 145 and 146: spent nuclear fuel. It was neverthe
Page 147 and 148: concepts or whose descriptions requ
Page 149 and 150: 14.2.2 Radionuclide transport and d
Page 151 and 152: At present, the computational time
Page 153 and 154: Inflow Nuclides in a soluble phase
Page 155 and 156: 15.1.2 Geometry The deep repository
Page 157 and 158: 7 6 BWR 55 MWd/tU PWR 42 MWd/tU PWR
Page 159 and 160: 15.1.11 Gas composition Conclusions
Page 161 and 162: 15.2.5 Heat transport Dealt with in
Page 163 and 164: simulating the repository environme
Page 165 and 166: 238 U 237 Np 239 Pu Concentration,
Page 167: The catalytic effect of uranium dio
Page 171 and 172: The influence of hydrogen peroxide
Page 173 and 174: A research programme to study cast
Page 177 and 178: 16.2.3 Heat transport No new knowle
Page 179 and 180: Programme The ongoing experiments w
Page 183 and 184: Programme Short-term tests aimed at
Page 185 and 186: Swelling properties The buffer must
Page 189 and 190: have been performed for some ten co
Page 191 and 192: 17.1.13 Impurity levels Bentonite i
Page 193 and 194: out when the buffer swells, but our
Page 195 and 196: These processes have been studied i
Page 199 and 200: • The gas injection phase will be
Page 203 and 204: Conclusions in RD&D 2001 and its re
Page 205 and 206: 19 9 D=205 d=175 D=172 d=170 D=168
Page 207 and 208: • Tests of the wetting process cl
Page 211 and 212: Figure 17-4. Natural redox front in
Page 213 and 214: These phenomena have been studied b
Page 215 and 216: 17.2.24 Radionuclide transport - ad
Page 217 and 218: Programme SKB does not consider sor
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18.1.6 Smectite content SKB, togeth
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Programme See section 18.2.2. 18.1.
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The wetting of the backfill from th
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ackfill must have a compression mod
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Programme See section 17.2.17. 18.2
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18.2.23 Radionuclide transport - so
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10 -5 I-129 Release rate (moles/yea
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and the rock matrix, is also crucia
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A thermal model will be constructed
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In a continuation of the super-regi
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A thorough overview has been done o
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The authorities are of the opinion
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19.2.11 Advection/mixing - groundwa
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a so-called Markov-directed Random
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Diffusion measurements in the labor
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Uranium series analyses from clay-
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19.2.18 Microbial processes Conclus
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19.2.20 Colloid formation - colloid
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19.2.23 Methane ice formation Concl
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19.2.26 Integrated modelling - radi
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development that has taken place ha
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20.2 Review of RD&D 2001 Following
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work will continue, particularly in
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2 In Sea (mol) 3 Water Sea (mol/m 3
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The above work will be pursued in c
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certain substances, such as uranium
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In connection with the Safe project
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20.9 International work and dissemi
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the underlying material are current
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These reports describe dominant fun
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Glacial Permafrost Permafrost Borea
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Shoreline displacement has an isost
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The hydromechanical modellings that
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model that includes these factors a
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The salinity of the sea, inland sea
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• Public opinion and attitudes -
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Socioeconomic impact • Local deve
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Previously existing material is bei
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• The driving forces behind the d
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• Very effective methods for tran
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• A subcritical nuclear reactor w
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out in Cadarache, France. Hindas an
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Important results since 2001 includ
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Programme The goal of SKB’s resea
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Kasam says that disposal in Very De
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24 Decommissioning The facilities c
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SKB is responsible for management a
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25 Low- and intermediate-level wast
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25.2.1 Repository for short-lived L
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Programme Work on the design of the
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observation is that isosaccharinic
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stored so that the material can be
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6-4 Claesson S, 2004. Elektronstrå
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Chapter 14 14-1 SKB, 2004. Interim
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15-42 Ekeroth E, Jonsson M, 2003. O
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17-19 Le Bell J C, 1978. Colloid ch
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19-29 Hudson J (ed), 2002. Strategy
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19-77 Bath A, Milodowski A, Ruotsal
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20-19 Kautsky U (ed), 2001. The bio
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20-73 Blomqvist P, Jansson P-E, Esp
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20-119 Berggren J, Kyläkorpi L, 20
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23-10 NEA, 2002. Accelerator-driven
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SKB’s master plan Appendix A
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A1 Introduction A1.1 Background The
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generations unnecessarily. Another
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Encapsulation plant 2003 2004 2005
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facilitated by the fact that the re
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A2.3 System design A2.3.1 Fundament
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The system analyses will be largely
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Two safety reports will therefore b
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KBS-3H Basic Design Detailed engine
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Time horizon 2017 The current phase
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2003 2004 2005 2006 2007 2008 Phase
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In the subsequent phase, verificati
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The canister development work will
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Deep repository Year 2003 2004 2005
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A4.2 Site investigations Site inves
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Already in the feasibility study, l
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In step D2, the facility descriptio
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Table 4. Current situation and prel
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2003 2004 2005 2006 2007 2008 Desig
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Table 5. Continued. Technology issu
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A4.6.1 Siting factors Before priori
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A possible alternative scenario is
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of the NPPs starts, however, long-l
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Between now and 2008, an organizati
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Appendix B Abbreviations Abaqus ACL
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GIA Gis Goldsim Hindas HMC HPF HRL
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PMMA POD Posiva Prism Proper Protot
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ISSN 1404-0344 CM Digitaltryck AB,
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