1. Dr. M. Brovchenko & Prof. E. Merle-Lucotte LPSC/IN2P3/CNRS - Grenoble INP, France EVOL Winter School 04/11/2013 in Orsay, France

2. Outline 1.Breeding of the GEN IV systems 2.Historical overview of MSRs 3.MSFR concept 4.Validation of neutronic characteristics 5.MSFR kinetic and load following

3. 1.Breeding of the GEN IV systems

4. Introduction to the Physics of the MSFR1/55

5. Nuclear Waste Fissile Nuclei Neutron Capture Neutron Current Reactors Introduction to the Physics of the MSFR2/55

6. Sustainability: - Minimize the nuclear waste - Extending the nuclear fuel supply by breeding Nuclear Waste Closed Cycle Introduction to the Physics of the MSFR3/55

7. FBR-Na MSFR LWR thermal fast U233 fission Am241 fission Am241 capture Am241 not fissile Neutron population Probability to do the nuclear reaction Example of Am241 Am241 fissile Fast neutron spectrum allows to burn the transuranics How to make Transuranics fissionable ? Introduction to the Physics of the MSFR4/55

8. 2.Historical overview of MSRs

9.  1964 – 1969: Molten Salt Reactor Experiment (MSRE) Experimental Reactor Power: 7.4 MWth Temperature: 650°C U enriched 30% (1966 - 1968) 233 U (1968 – 1969) - 239 Pu (1969) No Thorium inside  1970 - 1976: Molten Salt Breeder Reactor (MSBR) Never built Power: 2500 MWth Thermal neutron spectrum  1954 : Aircraft Reactor Experiment (ARE) Operated during 1000 hours Power = 2.5 MWth Historical studies of MSR : Oak Ridge Nat. Lab. - USA Introduction to the Physics of the MSFR5/55

10. Which constraints for a liquid fuel? Melting temperature not too high High boiling temperature Low vapor pressure Good thermal and hydraulic properties (fuel = coolant) Stability under irradiation Good solubility of fissile and fertile matters No production of radio-isotopes hardly manageable Solutions to reprocess/control the fuel salt Lithium fluorides fulfill all constraints Molten Salt Reactors Advantages of a Liquid Fuel Homogeneity of the fuel (no loading plan) Heat is produced directly in the heat transfer fluid Possibility to reconfigure passively the geometry of the fuel: - One configuration optimize the electricity production managing the criticality - An other configuration allow a long term storage with a passive cooling system Possibility to reprocess the fuel without stopping the reactor: - Better management of the fission products that damage the neutronic and physicochemical characteristics - No reactivity reserve Liquid fuelled-reactors: why “molten salt reactors” ? Introduction to the Physics of the MSFR6/55

11. Which constraints for a liquid fuel? Melting temperature not too high High boiling temperature Low vapor pressure Good thermal and hydraulic properties (fuel = coolant) Stability under irradiation Good solubility of fissile and fertile matters No production of radio-isotopes hardly manageable Solutions to reprocess/control the fuel salt Lithium fluorides fulfill all constraints Neutronic cross-sections of fluorine versus neutron economy in the fuel cycle Thorium / 233 U Fuel Cycle Molten Salt Reactors F[n,n’] Na[n,γ] Liquid fuelled-reactors: why “molten salt reactors” ? Introduction to the Physics of the MSFR7/55

12. Molten Salt Reactor (MSR) Renewal of the concept Thorims-NES5 then FUJI-AMSB in Japan since the 80’s Reactor of very low specific power fed with 233 U produced in sub-critical reactors Resumption of the MSBR’s studies by CEA and EDF since the 90’s TIER-1 project of C. Bowman in the 90’s Pu burner (LWR’s spent fuel assemblies dissolved in liquid fuel) in sub-critical reactors TASSE (CEA) project in the 90’s Plutonium burner (liquid fuel) in sub-critical reactors AMSTER (EDF) project in the 90’s Plutonium burner then breeder reactor in Thorium cycle REBUS (EDF), MOSART (Kurchakov Institute), SPHINX (Czech Republic) Projects of actinide burners MOST Network 2001-2004 European network having assessed the studies, the experiments and the state of knowledge concerning molten salt reactors Molten Salt Reactor (MSR) Renewal of the concept Introduction to the Physics of the MSFR8/55

13. Participation to the project TIER I of C. Bowman (1998) Re-evaluation of the MSBR from 1999 to 2002 Use of a probabilistic neutronic code (MCNP) Development of an in-house evolution code for materials (REM) Coupling of the neutronic code with the evolution code From the Thorium Molten Salt Reactor to the Molten Salt Fast Reactor Breeder in the Thorium fuel cycle and Actinide Burner Reactor Develop to solve the problems of the MSBR project –Bad (null to positive) feedback coefficients –Positive void coefficient –Unrealistic reprocessing –Problems specific to the graphite moderator - Lifespan - Reprocessing and storage - Fire risk Thermal vs Fast Spectrum Renewal of the concept at CNRS Introduction to the Physics of the MSFR9/55

14. Coupling of the in-house code REM for materials evolution with the probabilistic code MCNP for neutronic calculations Tools for the Simulation of Reactor Evolution Introduction to the Physics of the MSFR10/55

15. production from nucleus jdisappearance sum over all nuclei j production by nuclear reaction production by radioactive decay disappearance by radioactive decay disappearance by nuclear reaction Molten Salt Reactors: addition of a feeding term, equal to the number of nuclei added per time unit for each element (flow) Reprocessing: new terms Efficiency linked to the nucleus extraction probability Tools for the Simulation of Reactor Evolution Integration Module: Bateman Equation for nucleus i Introduction to the Physics of the MSFR11/55

16. TMSR general parameters: - total power: 2500 MWth (1000 MWe) - salt composition: 78% LiF – 22% (HN)F 4 (21.4% ThF 4 – 0.6% UF 4 ) - mean temperature: 630 °C (900 K) TMSR geometrical parameters: - core radius: 1.6 m - core shape: cylindrical (H=D) - salt volume: 20 m 3 - fertile blanket: Thorium - hexagon size (moderator): 15 cm - channel radius (fuel salt): varying Molten Salt Reactor operated in the Thorium Fuel Cycle (TMSR) Historical MSR Studies at CNRS Systematic studies (L. Mathieu) Introduction to the Physics of the MSFR12/55

17. - core volume adjusted to keep the same salt volume r = 4 cm r = 8.5 cm single channel 3 different moderation ratios: thermal fast Variation of the channel radius (moderation ratio) Introduction to the Physics of the MSFR13/55

18. no graphite moderator + Influence studied through 4 core characteristics: - Total feedback coefficient - Breeding ratio - Neutron flux / Graphite lifespan - Fissile inventory Influence of the channel radius on the core behavior Introduction to the Physics of the MSFR14/55

19. Three types of configuration: - thermal (r = 3-6 cm) - epithermal (r = 6-10 cm) - fast (r > 10 cm) Influence of the channel radius on the core behavior Introduction to the Physics of the MSFR15/55

20. Thermal spectrum configurations - low 233 U initial inventory - quite long graphite life-span - iso-breeder - positive feedback coefficient Epithermal spectrum configurations - quite low 233 U initial inventory - very short graphite life-span - quite negative feedback coefficient - iso-breeder Fast spectrum configurations (no moderator) - very negative feedback coefficients - very good breeding ratio - no problem of graphite life-span - large 233 U initial inventory Influence of the channel radius on the core behavior Introduction to the Physics of the MSFR16/55

21. 3.MSFR Concept

22. 3000 MWth Thermal power 3000 MWth Input/output operating 650-750 °C temp. 650-750 °C 565 °C Melting Temp. 565 °C 18 m 3 Fuel Salt Volume 18 m 3 3.9 s Fuel circulation time 3.9 s 3400 kg 233 U initial inventory 3400 kg 95 kg/year 233 U production 95 kg/year LiF-ThF 4 - 233 UF 4 with 77.5 % LiF Fuel Molten salt LiF-ThF 4 - 233 UF 4 with 77.5 % LiF or LiF-ThF 4 -(Transuranics)F 3 initial composition (mol%) or LiF-ThF 4 -(Transuranics)F 3 Molten Salt Fast Reactor Introduction to the Physics of the MSFR17/55

23. Physical Separation (in the core)  Gas Reprocessing Unit through bubbling extraction  Extract Kr, Xe, He and particles in suspension Chemical Separation (by batch)  Pyrochemical Reprocessing Unit  Located on-site, but outside the reactor vessel Fission Products Extraction: Motivations Control physicochemical properties of the salt (control deposit, erosion and corrosion phenomena's) Keep good neutronic properties Gas injection Reprocessing by batch of 10-40 l per day Gas extraction Fuel reprocessing Introduction to the Physics of the MSFR18/55

24. 1/ Salt Control + Fluorination to extract U, Np, Pu + few FPs - Expected efficiency of 99% for U/Np and 90% for Pu – Extracted elements re-injected in core 2/ Reductive extraction to remove actinides (except Th) from the salt – MA re-injected by anodic oxidation in the salt at the core entrance 3/ Second reductive extraction to remove all the elements other than the solvent - lanthanides transferred to a chloride salt before being precipitated 1/ 2/3/ On-site Chemical reprocessing unit Introduction to the Physics of the MSFR19/55

25. To remove all insoluble fission products (mostly noble metals) and rare gases, helium bubbles are voluntary injected in the flowing liquid salt (bottom of the core) → Separation salt / bubbles → Treatment on liquid metal and then cryogenic separation (out of core) Online noble gazes bubbling in core Introduction to the Physics of the MSFR20/55

26. Element Absorption (per fission neutron) Heavy Nuclei0.9 Alkalines< 10 -4 Metals0.0014 Lanthanides0.006 Total FPs0.0075 Fast neutron spectrum  very low capture cross-sections  low impact of the FP extraction on neutronics  Parallel studies of chemical and neutronic issues possible Batch reprocessing: On-line (bubbling) reprocessing: Influence of fuel reprocessing on the neutronics Introduction to the Physics of the MSFR21/55

27. Optimization studies: Initial fissile matter ( 233 U, Pu), salt composition, fissile inventory, reprocessing, waste management, deployment capacities, heat exchanges, structural materials, design.. Generation IV reactors: fuel reprocessing mandatory Neutronic core of the MSFR associated to reprocessing units (on-site) What is a MSFR ? Molten Salt Reactor (molten salt = liquid fuel also used as coolant) Based on the Thorium fuel cycle With no solid (i.e. moderator) matter in the core  Fast neutron spectrum MSFR: result of neutronic optimization studies Introduction to the Physics of the MSFR22/55

28. Reactor Design and Fissile Inventory Optimization = Specific Power Optimization 2 parameters: 3 limiting factors: The produced power The fuel salt volume and the core geometry The capacities of the heat exchangers in terms of heat extraction and the associated pressure drops (pumps)  large fuel salt volume and small specific power The neutronic irradiation damages to the structural materials which modify their physicochemical properties. Three effects: displacements per atom, production of Helium gas, transmutation of Tungsten in Osmium  large fuel salt volume and small specific power The neutronic characteristics of the reactor in terms of burning efficiencies  small fuel salt volume and large specific power and of deployment capacities, i.e. breeding ratio (= 233 U production) versus fissile inventory  optimum near 15m 3 and 400W/cm 3 Liquid fuel and no solid matter inside the core  possibility to reach specific power much higher than in a solid fuel MSFR: Design and Fissile Inventory Optimization Introduction to the Physics of the MSFR23/55

29. Core: No inside structure, perhaps thermo-hydraulical injection element (plate or deflector) to be added Outside structure: Upper and lower Reflectors, Fertile Blanket Wall + 16 external modules: Pipes (cold and hot region) Bubble Separator Pump Heat Exchanger Bubble Injection Fuel Salt Loop = Includes all the systems in contact with the fuel salt during normal operation MSFR: Design and Fissile Inventory Optimization Introduction to the Physics of the MSFR24/55

30. Fuel salt volume / specific power t(100 dpa)t(100 ppm He)t(-1 at% of W) 12 m 3 - 500 W/cm 3 85 years2.2 years4.7 years 18 m 3 – 330 W/cm 3 133 years3.2 years7.3 years 27 m 3 - 220 W/cm 3 211 years5.5 years10.9 years MSFR: Design and Fissile Inventory Optimization Structural material composition Ni based alloy: NiWCrMoFeTiCMnSiAlBPS 79.4329.9768.0140.7360.6320.2950.2940.2570.2520.0520.0330.0230.004 Introduction to the Physics of the MSFR25/55

31. Most irradiated area (central part of axial reflector – radius 20 cm/thickness 2 cm) MSFR: Design and Fissile Inventory Optimization Structural material composition Ni based alloy: Fast neutrons (E n > 100 keV) eject nucleus from structure material creating damages through atom displacement (evaluated with cross section MT444) DPA Displacement per atom NiWCrMoFeTiCMnSiAlBPS 79.4329.9768.0140.7360.6320.2950.2940.2570.2520.0520.0330.0230.004 Introduction to the Physics of the MSFR26/55

32. MSFR: Design and Fissile Inventory Optimization mainly on 59 Ni, ( 58 Ni) and 10 B (Boron quantity may be re-ajusted) Structural material composition Ni based alloy: He production NiWCrMoFeTiCMnSiAlBPS 79.4329.9768.0140.7360.6320.2950.2940.2570.2520.0520.0330.0230.004 Introduction to the Physics of the MSFR27/55

33. MSFR: Design and Fissile Inventory Optimization Structural material composition Ni based alloy: Transmutation Cycle of W in Re and Os (neutronic captures + decays) Tungstene transmutation NiWCrMoFeTiCMnSiAlBPS 79.4329.9768.0140.7360.6320.2950.2940.2570.2520.0520.0330.0230.004 Introduction to the Physics of the MSFR28/55

34. Optimization = Medium Fuel Salt Volumes Fuel salt volume / specific power t(100 dpa)t(100 ppm He)t(-1 at% of W) 12 m 3 - 500 W/cm 3 85 years2.2 years4.7 years 18 m 3 – 330 W/cm 3 133 years3.2 years7.3 years 27 m 3 - 220 W/cm 3 211 years5.5 years10.9 years MSFR: Design and Fissile Inventory Optimization Introduction to the Physics of the MSFR29/55

35. Reactor Design and Fissile Inventory Optimization = Specific Power Optimization 2 parameters: 3 limiting factors: The produced power The fuel salt volume and the core geometry Liquid fuel and no solid matter inside the core  possibility to reach specific power much higher than in a solid fuel  Reference MSFR configuration with 18m 3 et 330 W/cm 3 corresponding to an initial fissile inventory of 3.5 tons per GWe The capacities of the heat exchangers in terms of heat extraction and the associated pressure drops (pumps)  large fuel salt volume and small specific power The neutronic irradiation damages to the structural materials which modify their physicochemical properties. Three effects: displacements per atom, production of Helium gas, transmutation of Tungsten in Osmium  large fuel salt volume and small specific power The neutronic characteristics of the reactor in terms of burning efficiencies  small fuel salt volume and large specific power and of deployment capacities, i.e. breeding ratio (= 233 U production) versus fissile inventory  optimum near 15m 3 and 400W/cm 3 MSFR: Design and Fissile Inventory Optimization Introduction to the Physics of the MSFR30/55

36. Starting mode 233 U [kg]TRU [kg] enr U + TRU [kg] Th 23225 55320 39610 135 Pa 231 U 232 U 2333 260 U 234 U 235 1 735 U 236 U 238 11 758 Np 237 531335 Pu 238 229144 Pu 239 3 9022 464 Pu 240 1 8351 159 Pu 241 917579 Pu 242 577364 Am 241 291184 Am 243 164104 Cm 244 6944 Cm 245 64 Initial heavy nuclei inventories per Gwe: MSFR starting modes Introduction to the Physics of the MSFR31/55

37. 4.Validation of MSFR neutronic characteristics

38. EVOL objective: to propose a design of MSFR by 2012 given the best system configuration issued from physical, chemical and material studies Recommendations for the design of the core and fuel heat exchangers Definition of a safety approach dedicated to liquid-fuel reactors - Transposition of the defence in depth principle - Development of dedicated tools for transient simulations of molten salt reactors Determination of the salt composition - Determination of Pu solubility in LiF-ThF4 Control of salt potential by introducing Th metal - Evaluation of the reprocessing efficiency (based on experimental data) – FFFER project Recommendations for the composition of structural materials around the core WP2: Pre-Conceptual Design and Safety WP3: Fuel Chemistry and Reprocessing WP4: Structural Materials + Coupling to the ROSATOM project MARS (Minor Actinides Recycling in Molten Salt) European ‘EVOL’ (Evaluation and Viability Of Liquid fuel fast reactor systems) Project (7th PCRD) - EURATOM/ROSATOM cooperation Neutronic Benchmark European participant : CNRS, HZDR, KIT, POLIMI, POLITO, TU Delft + KI EVOL project Introduction to the Physics of the MSFR32/55

39. Simplified Geometry of MSFR for MCNP calculations Neutronic Benchmark definition 233 U-started MSFRTRU-started MSFR Th 233 UThActinide 38 281 kg 19.985 %mol 4 838 kg 2.515 %mol 30 619 kg 16.068 %mol Pu11 079 kg 5.628 %mol Np789 kg 0.405 %mol Am677 kg 0.341 %mol Cm116 kg 0.058 %mol Introduction to the Physics of the MSFR33/55

40. On-line bubbling extraction: Removal period T 1/2 30 seconds Extracted elements Z 1, 2, 7, 8, 10, 18, 36, 41, 42, 43, 44, 45, 46, 47, 51, 52, 54 and 86. Pyrochemical reprocessing: Rate40 l/day Extracted elements Z 30, 31, 32, 33, 34, 35, 37, 38, 39, 40, 48, 49, 50, 53, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70. Neutronic Benchmark definition Thermal power (MWth)3000 Electric power (MWe)1500 Fuel Molten salt LiF-ThF 4 - 233 UF 4 initial composition (mol%)with 77.5 % LiF or LiF-ThF 4 -(Pu-MA)F 3 Fertile Blanket Molten saltLiF-ThF 4 initial composition (mol%)(77.5%-22.5%) Melting point (°C)565 Input/output operating temp. (°C) 625-775 Fuel Salt Volume (m 3 )18 9 out of the core 9 in the core Blanket Salt Volume (m 3 )7.3 Total fuel salt cycle in the system3.9 s Introduction to the Physics of the MSFR33/55

41. CNRS/IN2P3/LPSC (LPSC) Tools used for the Neutronic Benchmark -Probabilistic code MCNP with a home-made materials evolution code REM; -Extraction of nucleus i with a specific removal constants λ chem ; -Fissile and fertile composition adjusted to control the reactivity. Helmholtz-Zentrum Dresden-Rossendorf (HZDR) -HELIOS 1.10 code system with internal 47 energy group library; -Pre- and post-processor for reprocessing and re-fuelling adapted to MSFR calculation Kurchatov Institute (KI) -MCNP-4B code coupled with ORIGEN2.1 code, solves depletion equations; -Extraction of FP and fissile and fertile adjustment simulated. Introduction to the Physics of the MSFR34/55

42. Politecnico di Torino (POLITO) Stochastic calculations: -SERPENT calculation of the core without burn-up; -3-group energy cross section used for DYNAMOSS. Deterministic calculations: -Diffusion model in cylindrical r-z geometry -For circulating fuel system Technical University of Delft (TU-Delft) -HEAT an in-house developed CFD-program; -DALTON an in-house developed diffusion code; -SCALE used to cross sections calculations. Tools used for the Neutronic Benchmark Politecnico di Milano (POLIMI) ERANOS-based EQL3D procedure and extension for the MSFR: -ERANOS 2.2-N code system for core calculation (33-group energy); -FP extraction and re-fueling adjustment simulated. SERPENT-2 extension for on-line fuel reprocessing: -SERPENT is a three-dimensional Monte Carlo code; -developed extension of SERPENT-2 code takes into account online fuel reprocessing and features a reactivity control algorithm. Introduction to the Physics of the MSFR35/55

43. Same data basis JEFF-3.1 for the calculations of different partners (probabilistic tool) Neutronic energy spectrum Introduction to the Physics of the MSFR36/55

44. Neutronic energy spectrum Cross Section [barn] Neutron energy [MeV] Neutron Flux Why this shape? Introduction to the Physics of the MSFR37/55

45. Neutronic energy spectrum Multi-group spectra of 233U-started MSFR or steady state composition Introduction to the Physics of the MSFR38/55

46. Sensibility to different nuclear data bases (LPSC and POLIMI tools) Neutronic energy spectrum Introduction to the Physics of the MSFR39/55

47. Different data bases used One of the reasons of the observed differences Neutronic energy spectrum Introduction to the Physics of the MSFR40/55

48. Sensibility to composition of the fuel salt (LPSC-MCNP tool with JEFF-3.1) Neutronic energy spectrum Introduction to the Physics of the MSFR41/55

49. Thermal feedback coefficient Initial composition Different data bases 2 calculations performed: Thermal expansion Cross sections Introduction to the Physics of the MSFR42/55

50. Thermal feedback coefficient Different data bases Introduction to the Physics of the MSFR43/55

51. Evolution calculation Different tools, same data basis : ENDF-B6 Introduction to the Physics of the MSFR44/55

52. Evolution calculation Same tool, different data bases (SERPENT - POLIMI) Introduction to the Physics of the MSFR45/55

53. Evolution calculation BG = Extra Produced Fissile Material Operation Time Breeding gain : Introduction to the Physics of the MSFR46/55

54. Introduction to the Physics of the MSFR47/55 Nuclear data bases

55. 5.MSFR kinetics and load following

56. o Model hypotheses : Homogeneous reactor One energy group Φ(r,t) = Φ(r) ˑ φ(t) No precursor transport Group i : k eff ~ k inf Fast spectrum β eff Reactivity insertion with point kinetic model Normalized fission power Time after transient begin [s] Temperature [C] Reactivity [pcm] Time after transient begin [s] Introduction to the Physics of the MSFR48/55

57. o Linear reactivity insertion of 500 pcm (ρ > β ≈ 170 pcm) o In 0,001 s and 0,01 s : Prompt criticality, Fission power x 1000 Reactivity insertion with point kinetic model Normalized fission power Reactivity [pcm] Time after transient begin [s] Temperature [C] Reactivity [pcm] Time after transient begin [s] Introduction to the Physics of the MSFR49/55

58. o Linear reactivity insertion of 500 pcm (ρ > β ≈ 170 pcm) o In 0,001 s and 0,01 s : Prompt criticality, Fission power x 1000 o In 1 s and 10 s : Fission power, No temperature peak o Reactor stabilization at T > T 0 Reactivity insertion with point kinetic model No dynamic precursor following nor fuel salt temperature in and out flow: neglected at short and long term. For t ~4s (circulation time) : These effects have to be studied Time after transient begin [s] Normalized fission power Temperature [C] Reactivity [pcm] Time after transient begin [s] Introduction to the Physics of the MSFR50/55

59. Point kinetic model with zones Temperature distribution Extension of the point kinetic model : -To follow the delayed neutron precursor -To model the heat exchanger outside the core Two meshes are defined : fix and moving Fuel salt temperature for each cell: Reactivity: Fission power: Core Delayed neutron precursor abondance for each cell: Introduction to the Physics of the MSFR51/55

60. Fertile blanket Neutronic protection Heat exchanger Pump R Z To go even further: With code COUPLE o Tool based on neutronics and thermo hydraulics o Developed at KIT (Karlsruhe) o 2D axial symmetry o Neutronics: Diffusion with 2 energy groups o Thermo hydraulics : Mass conservation Navier –Stokes Energy conservation o 112/130 cells R/Z o Pump model fixing the fluid velocity o Heat exchanger model as negative heat source Coupling of neutronic and thermalhydrulic Introduction to the Physics of the MSFR52/55

61. Comparison of 3 kinetic tools Point kinetic Point kinetic with zones COUPLE Fast load following transients: Heat extraction drop to 50% Heat extraction increase to 150% With a heat exchanger model (COUPLE + Point kinetic with zones): transient starts ~ 1 s later Point kinetic with zones : recirculation clearly observed COUPLE: not that clearly cause circulation form time dependent Time after transient begin [s] Normalized fission power Point kinetic Point kinetic with zones Temperature [K] Point kinetic Point kinetic with zones Reactivity [pcm] Point kinetic Point kinetic with zones Introduction to the Physics of the MSFR53/55

62. Slow load following transients Point kinetic with zones COUPLE Time after transient begin [s] Normalized fission power Point kinetic with zones Extracted heat Fission power: Load following : Time after transient begin [s] Temperature [K] Introduction to the Physics of the MSFR54/55

63. -MSFR Concept -Neutronic characteristics -Basis in reactor kinetics -MSFR thermal-hydraulics -Coupled neutronics and thermal hydraulics -Fuel cycle aspects and transmutation