Molten Salts Characteristics Under Irradiation in Fission Related Systems

Transcription

1 First workshop Challenges for coolants in fast spectrum system: Chemistry and materials July 5 th -7 th, 2017, IAEA, Vienna, Austria Molten Salts Characteristics Under Irradiation in Fission Related Systems Presented by Victor Ignatiev National Research Center Kurchatov Institute , Kurchatov sq., 1, Moscow, Russia Ignatev_VV@nrcki.ru 1

2 Contents Introduction: MSR in Generation IV International Forum Fission and its consequences Effects of radiation Chemical behavior of fission products (including noble gases and tritium, soluble stable compounds, noble and semi noble metals) Operational constraints (e.g. solubility of metal trifluorides, materials compatibility) Fuel maintenance (preparation of initial fuel; contamination possibilities; removal of FP s, addition of actinides, maintaining the desired Redox potential) Summary 2

3 All GEN IV reactors require higher operating temperatures and / or radiation doses. The higher operating temperatures / dose rate require materials increased high temperature strength, thermal stability and irradiation resistance dpa dpa dpa dpa dpa 3

4 Different Reactor Concepts using Molten Salt are Discussed at GIF MSR SSC Meetings 4 Within the GIF, research is performed on the MSR concepts, under the MOU signed by Euratom, France, the Russian Federation, Switzerland and USA. China, Australia, Korea, Japan and contribute as observers. Two fast spectrum MSR concepts are being studied, large power units based on homogeneous core with liquid fluoride-salt circulating fuel: MSFR design in France, Euratom and Switzerland as well as MOSART concept in the Russian Federation. R&D studies are on-going in order to verify that fast spectrum MSR systems satisfy the goals of Gen-IV reactors in terms of sustainability, nonproliferation, safety and waste management. The US is working on solid fuel FHR (Fluoride-salt-cooled High-temperature Reactor) as well as liquid fueled MCFR (Molten Chloride salt Fast Reactor) China, as observer in the pssc of the MSR, is working on FHR and TMSR (Thorium Molten fluoride Salt thermal Reactor) graphite moderated designs. 4

5 Boundaries and Interfaces Present MSR concepts homogeneous core utilizes Nibase alloy as the containment vessel and other metallic parts of the system, probably graphite as reflector, and a liquid fluoride salt fueled by TRUF 3 and ThF 4 as the fertile-fissile medium. The fertile-fissile salt will leave the reactor vessel at a temperatures > 700 C and energy will be transferred to a coolant salt which in turn is used to produce supercritical steam. Fuel circuit MOSART MSFR Fuel salt, mole% LiF-BeF 2 +1TRUF 3 LiF-BeF 2 +5ThF 4 +1UF LiF-12.9ThF 4-3.5UF 4-5TRUF LiF-6.6ThF UF 4-3.6TRUF 3 Temperature, о С Core radius/height, m 1.4 / / 2.26 Core specific power, W/cm Container material Ni-Mo alloy Ni-W alloy Removal time for soluble FP, yrs Fuel Processing Unit Fuel Circuit Intermediate Circuit Power Conversion System 5

6 Useful Salt Compositions Alkali fluorides ThF 4 -based ZrF 4 -based BeF 2 -based Fluoroborate LiF 743 o C, 20 mole % NaF 727 o C, 24 mole % LiF-ThF 4 (71-29) 555 o C 3.69 mole % LiF-ZrF 4 (51-49) 509 o C LiF-KF (50-50) 492 o C LiF-RbF (44-56) 470 o C LiF-NaF-KF ( ) 454 o C LiF-NaF-RbF ( ) 435 o C LiF-BeF 2 -ThF 4 ( ) 499 o C 1.41 mole % LiF-BeF 2 -ThF 4 ( ) 460 o C 1.21 mole % NaF-ZrF 4 ( ) 500 o C 1.8 mole % LiF-NaF-BeF 2 ( ) 479 o C 1.94 mole % LiF-NaF-ZrF 4 ( ) 460 o C LiF-BeF 2 LiF-NaF-ZrF 4 ( ) 436 o C NaF-RbF-ZrF 4 ( ) 420 o C (66-34) 458 o C 0.47 mole % LiF-BeF 2 -ZrF 4 ( ) 428 o C KF-KBF4 (25-75) 460 o C RbF-RbBF 4 (31-69) 442 o C LiF-BeF 2 -ThF 4 ( ) 360 o C RbF-ZrF 4 (58-42) 410 o C KF-ZrF4 (58-42) 390 o C NaF-KF-ZrF 4 ( ) 385 o C NaF-BeF 2 (57-43) 340 o C 0.26 mole% LiF-NaF-BeF 2 ( ) 315 o C 0.43 mole % NaF-NaBF 4 (8-92) 384 o C 6

7 In most cases the base-line fuel / coolant salt is lithium-beryllium fluoride salt as it has best properties Low neutron cross section for the solvent Thermal stability of the salt components Low vapor pressure Radiation stability Adequate solubility of fuel and FP s components Adequate transport properties Compatibility with construction materials Low fuel and processing costs Fuel addition Fission products Metallic alloy BeF 2 Impurities Combined environments Graphite AnF 4 LiF LnF 3 Molten salt Gas AnF 3 7

8 Effects of Radiation When fission occurs in а molten fluoride solution, both electromagnetic radiations and particles of very high energy and intensity originate within the fluid. Local overheating is almost certainly not important in а MSR where turbulent flow causes rapid intimate mixing. Moreover, the bonding in molten fluorides is completely ionic. Such а mixture, with neither covalent bonds to rupture nor а lattice to disrupt, should be quite resistant to radiation. Nevertheless, because there plausibly exists а radiation level sufficiently high to dissociate а molten fluoride into metal and fluorine (This would occur even though the rate of recombination of Li o and F o should be extremely rapid), а number of tests of the possibility were made. 8

9 ORNL tests strongly suggested that the F 2 generation had at the high temperature not occurred (gas was generating mainly via reaction 6 Li(n,α)T), but had occurred by radiolysis of the mixture in the solid state. F 2 evolution at 35 C corresponded to about 0,02 molecules per 100 ev absorbed, could be completely stopped by heating to 100 C or above, and could be reduced by chilling to -70 C. The F 2 evolution resumed, usually after a few hours, when temperature was returned to 35 to 50 C. KI tests Fuel salt, mole % Liquid phase T, o C G(F 2 ),10-5 mol/100ev Solid phase T, o C G(F 2 ),10-2 mol/100ev 66LiF-33BeF 2-1UF LiF-31BeF LiF-16BeF 2-12ThF 4-0.3UF LiF-34.39BeF 2-0.3UF iF ThF 4-0.5UF NaF-25.9ThF 4-0.9UF These and subsequent experiences, including operation of the 8MWe MSRE at US ORNL, strongly indicate that radiolysis of the molten fuel at reasonable power densities is not а problem. It seems unlikely, though it is possible, that MSR fuels will evolve F 2 on cooling. If they do, arrangements must be made for their storage at elevated temperature until а fraction of the decay energy is dissipated 9

10 In Reactor Li,Be,U/F Natural Convection Loop KURS-2 exposure time~ 750 hrs Т мах =750 о С; Ф = 0, neutron/(см 2 s) Helium was generating through reaction 6 LiF + 1 n He + 1/2T 2 + 1/2F 2 Measured F evaluation by radiolysis corresponded to molecule per 100 ev absorbed 10

11 Fission and its Consequences Fragments produced on fission of а heavy atom originate in energy states and with ionization levels far from those normally considered in chemical reactions. The valence states that these chemical species assume are presumably defined by the requirements that (1) cation anion equivalence be maintained in the molten-salt medium and (2) redox equilibria be established between the melt and the surface layers of the container metal. The FP cations must satisfy the FP plus the fluoride ions released by disappearance of the fissioned atom. The summation of the FP yield and stable valence for each species might be as low as three per fission event. Accordingly, fission of UF 4 (releasing 4F (Br - + I - ) per fission) would be intrinsically oxidizing to metallic alloy. Fission of TRUs trifluorides probably would be nearly neutral in this regards. Maintenance of а small fraction of the uranium in the fuel as UF 3 was successfully adopted to preclude corrosion from fission of 235 UF 4 in the MSRE. А properly maintained redox potential in the fuel salt apparently will prevent any untoward immediate consequences of the fission event and will permit grow-in of the fission products in valence states defined by the redox potential. 11

12 Soluble Fission Products Rb, Ce, Sr, Ba, Y, the lanthanides, and Zr all form quite stable fluorides that are relatively soluble in fuel salts. Bromine and iodine would be expected to appear in the fuel as soluble Br and I, particularly in the case where the fuel contains an appreciable concentration of UF 3. Analyses for 131 I showed that а large fraction of the iodine was present in the fuel and that 131 I deposited on metal or graphite surfaces in the core region. However, material balances for 131 I were generally low. It is possible that some of the precursor, 131 Te (25 min), was volatilized and sparged with the krypton and xenon. Further, 131 I produced by decay of 131 Te in complex metallic deposits (as in the heat exchanger) may not have been able to return to the salt. 12

13 The data on An (Ln) trifluoride solubility in molten salt fluorides appear to follow a linear relationship within the experimental accuracy of the measurements when plotted as log of molar concentration of AnF 3 vs. 1/T(K). If more than one such trifluoride is present, they crystallize as а solid solution of all the trifluorides on cooling of а saturated melt. If so, the total (Ln plus An) trifluorides in the reactor might possibly exceed their combined solubility. 8,0 SPuF 3, % mol 7,5 7,0 6,5 6,0 5,5 5,0 4,5 4,0 3,5 3,0 78LiF-22ThF 4 75LiF-5BeF 2-20ThF 4 77LiF-17BeF 2-6ThF 4 80LiF-20ThF 4 [1] 75LiF-5BeF 2-20ThF 4 [1] 2,5 2,0 1,5 1,0 0,5 0,0 log SPuF 3, mole% = A + B/T,K Temperature, K Note: Near the liquids temperature for molten 78LiF-7ThF 4-15UF 4 and 72.5LiF-7ThF UF 4 salts the CeF 3 significantly displace PuF 3 Temperature, K 72,5LiF-7ThF 4-20,5UF 4 78LiF-7ThF 4-15UF 4 PuF 3 CeF 3 PuF 3 CeF ,35±0,02 1,5±0,1 1,45±0,7 2,6±0, ,5±0,2 2,5±0,1 5,6±0,3 3,6±0, ,4±0,4 3,7±0,2 9,5±0,5 4,8±0, ,4±0,5 3,9±0,2 10,5±0,6 5,0±0,3 13

14 Noble and Semi Noble Fission Products Some FP metals (Ge, As, Nb, Мо, Ru, Rh, Pd, Ag, Cd, Sn, and Sb) have fluorides that are unstable toward reduction by fuel mixtures with appreciable concentrations of UF 3 ; thus, they must be expected to exist entirely in the elemental state in the MSR. Selenium and tellurium were also expected to be present as elements within the reactor circuit, and this behavior was generally confirmed during operation of the MSRE Precipitation on the metal surface (most of which is in the heat exchanger) will be quite insufficient to impede fuel flow, but radioactive decay of the deposited material contributes to heat generation during reactor shutdown Operation of the MSRE did produce one untoward effect of FPs. These studies implicated tellurium FP as responsible for the embrittlement of the metal surface exposed, and subsequent work has confirmed this. Later work strongly suggest that (1) if the molten fuels were made to contain as much as 2-5% of the uranium as UF 3, the tellurium would be present as Те 2- and (2) in that form, tellurium is much less aggressive 14

15 Noble-gas Fission Products Kr and Xe form no compounds under conditions existing in а MSR. Moreover, these gases are only very sparingly soluble in molten fluoride mixtures. This low solubility is а distinct advantage because it enables the ready removal of Xe and Kr from the reactor by sparging with helium. The transmutation of Li is essential for production of tritium. This tritium will originate in principle as 3 HF; however, with appreciable concentrations of UF 3 present, this 3 HF will be reduced largely to 3 H 2. Some of this 3 H 2 would be removed, along with krypton and xenon, by sparging with helium. However, the extraordinary ability of hydrogen isotopes to diffuse through hot metals will permit а large fraction of the 3 H 2 to penetrate the primary heat exchanger to enter the secondary coolant. Such а stripping circuit would remove an appreciable (but not а major) fraction of the tritium and а small (perhaps very small) fraction of the noble and semi noble fission products as gas-borne particulates. In addition, the stripper would remove BF 3 if leaks of secondary coolant into the fuel were to occur. None of these removals (except possibly the last) appreciably affects the chemical behavior of the fuel system. 15

16 Tritium Generation, Corrosion and Control Are Coupled. Can t Separate Tritium Generation, Corrosion and Control Corrosion - preferential attack of Cr in alloys by TF: 2TF(d) + Cr(s) CrF 2 (d) + T 2 (g) Corrosion reaction consumes TF, generates T 2 Radiological: T 2 fast diffusion through metal T 1/2 = 12.3 yr β = 5.9 kev Must control corrosion and manage tritium escape from system 16

17 Ratio [P TF ] 2 /[P T2 ] Redox Potential Dictates Relative Amounts of T 2 and TF Coolant Redox Potential (kj/mol F 2 ) 17

18 Material Temperature / Pressure Equations Activation energy in kj/mole HN80MTY Mo-13,2% mass K кpа Diffusion D=1, exp(-47,7/r T), [m 2 /s] Permeability P=3, exp(-55,0/r T), [ mole/m s Pa 1/2 ] Solubility Ks=0.41 exp(-8,85/r T) [mole/ m 3 Pa 1/2 ] HN80MTY oxidation film Same Permeability P=3, exp(-48,8/r T), [ mole/m s Pa 1/2 ] ЕМ721 W-25,2% mass 77LiF-6ThF 4-17BeF 2 Тm =560 o C (833K) Tritium Control and Capture Main strategies for mitigation include: advanced materials for the piping and heat exchangers, inert gas sparging, additional coolant lines and metal hydride addition or chemical removal Additional development of permeation-resistant coatings, including W-Si Refinement of geometric configuration of the intermediate heat exchangers, minimizing tritium flux Discovery of reusable solvents for direct tritium removal from molten salt The chemistry of sodium fluoroborate and the trapping process by which tritium is retained by the salt Out-of-Core Tritium Removal with Carbon Same К 4-70 кpа Diffusion D=4, exp(-45,7/r T), [m 2 /s] Permeability P=3, exp(-55,4/r T), [ mole/m s Pa 1/2 ] Solubility Ks=0.13 exp(-9,6/r T) [mole/ m 3 Pa 1/2 ] Diffusion D=7, exp(-393,5/r T), [m 2 /s] Permeability P=1, exp(-38,8/r T), [ mole/m s Pa 1/2 ] Solubility Ks=2,0 exp(-390,1/r T) [mole/ m 3 Pa 1/2 ] 18

19 ELECTRODE POTENTIAL vs HF/H 2,F - COUPLE Corrosion effects on container materials will be provided by oxygen-containing impurities and hydrogen fluorine found in the fuel salt and determining the level of its RedOx 2UF 4 +M (solid ) 2UF 3 + MF 2 NiF 2 +M (solid ) MF 2 + Ni THE REDOX POTENTIAL OXIDATION STATES OF ACTINIDES IN LiF-BeF 2 -ThF 4 -UF 4 2HF +M (solid ) MF 2 + H 2 NiO+ BeF 2 NiF 2 + BeO 2H 2 O+UF 4 4HF+UO 2 0 Th 4+ Pa 5+ U 4+ Pu 4+ Pu U 4+ /U 3+ 2H 2 O+ZrF 4 4HF+ZrO 2-1 Pa 4+ U UF 3 +2C UC 2 + 3UF 4-2 Th 0 Pa 0 U0 Pu UF 4 +Be 0 2UF 3 + BeF 2 Th Pa U Pu ACTINIDE ELEMENT 19

20 Redox Potential Determines Extent of Te Corrosion in Hastelloy N Li,Be,U/F U(IV)/(UIII) enlargement 160 No 760 o C Decrease of RedOx potential by metallic Be 30 without loading at 760 o C 60 without loading at 760 o C 90 without loading at 800 o C K = 3500pc μm/cm ; l = 69μm K = 4490pc μm/cm ; l = 148μm 20

21 In general to achieve fuel maintenance, (1) the fuel must be delivered to and into the reactor in а proper state of purity and homogeneity, (2) the fuel must be sufficiently protected from extraneous impurities, and (3) sound procedures must exist for addition and recycling of the actinides required and (4) provision of the required redox potential in the system continuous removal of Xe and Kr by the He sparging, addition of U and TRUs to replace that lost by burnup, production of UF 3 to keep the redox potential of the fuel at the desired level, recycling of all actinides, removal of soluble FPs (principally rare earths); they рrоbаbly also include isolating 233 Pa from the region of high neutron flux during its decay in order to hold neutron absorption in these materials to acceptably low levels, removal of inadvertent oxide contaminants from the fuel; in addition, they may include addition of ТhF 4 to replace that lost by transmutation or stored with fuel removed from the operating circuit, removal of а portion of the insoluble noble and semi noble FPs. 21

22 Preparation of Initial Fuel Initial purification procedures for the MSR present no formidable problems. Nuclear poisons (e.g., boron, cadmium, or lanthanides) are not common contaminants of the constituent raw materials. All the pertinent compounds contain at least small amounts of water, and all are readily hydrolyzed to oxides and oxyfluorides at elevated temperatures. The compounds LiF and BeF 2 generally contain а small quantity of sulfur as sulfate ion. Uranium tetrafluoride commonly contains small amounts of UO 2, UF 5, and U0 2 F 2 Purification procedures used to prepare materials in many laboratory and engineering experiments have treated the mixed materials at high temperature (usually at 600 С) with gaseous HF-H 2 mixtures and then with pure H 2 in equipment of nickel or copper. Тhе HF-H 2 treatment serves to (1) reduce the U 5+ and U 6+ to U 4+, (2) reduce sulfate to sulfide and remove it as H 2 S, (3) remove Cl - as HCl, and (4) convert the oxides and oxyfluorides to fluorides. Final treatment with H 2 serves to reduce FeF 3 and FeF 2 to insoluble iron and to remove NiF 2 that may have been produced during hydrofluorination. То date, all preparations have been performed in batch equipment, but continuous equipment has been partially developed. 22

23 Two Fluid Th MOSART Flowsheet Element Time Method Kr, Xe 50 s Sparging with He Zn, Ga, Ge, As, Se, Nb, Mo, Ru, Rh, Pd, Ag, Tc, Cd, In, Sn, Sb, Te 2-4 hrs Plating on surfaces, to off-gas system, filtering 233 U, 234 U, 235 U, 236 U, 237 U 10 d Fluorination Zr, 233 Pa 1-3 yrs Ni, Fe, Cr 1-3 yrs Pu, Am, Cm, Np 1-3 yrs Y, La, Ce, Pr, Nd, Pm, Gd, Tb, Dy, Ho, Er 1-3 yrs Sm, Eu 1-3 yrs Reductive extraction in liquid Bi Sorbents Cold trap UF 6 recycle Sorbents Cold trap Make up Li,Be,Th/F NaF 600C MgF 2 UF 6 + Volatile FP F2 70C F2 UF 6 + Volatile FP NaF 600C MgF 2 70C AnF n make up and recycle to core An removal Li,Be/F addition Fertile make up Reductive extraction FP removal Fluoride volatility 550C BLANKET Core Li,Be,An/F Li,Be,Th/F Fluoride Pu, Pa, MA UF 6 UF 4 Valence Volatility extraction reduction adjustment Zr Rare earth extraction extraction Reduced Bi Cr, Fe, removal Ni Fertile Stream Recycle Pa conversion UF 4 Waste storage LiF-BeF 2 -AnF n recycle 23

24 MSFR Fuel Salt Processing 24

25 Summary, Constraints, and Uncertainties Application: GEN IV MSR: EU-MSFR, RF-MOSART, US-FHR In most cases the base-line fuel / coolant salt is LiF, BeF 2 and / or ThF 4 based salt as it has best properties Limits of Use: Max temperature of fuel salt in the primary circuit made of special Ni-alloy is mainly limited by Te IGC depending on salt Redox potential ( C) Min temperature of fuel salt is determining its melting point and the AnF 3 solubility ( C) Major Issues and Challenges: The extraordinary ability of tritium to diffuse through hot metals will permit а large fraction of the 3 H 2 to penetrate the primary heat exchanger to enter the secondary coolant. Coolant Processing and Handling: Continuous removal by the sparging of Xe, Kr; addition of U and TRUs to replace that lost by burn up; in situ production of UF 3 to keep the Redox potential of the fuel at the desired level; recycling of all An s; removal of rare earths; isolating 233 Pa; removal of inadvertent oxide contaminants from the fuel; addition of ТhF 4 and removal of а portion of the insoluble noble FP s. Main Radiation Effects: In reactor tests strongly suggested that the F 2 generation had at the high temperature not occurred (gas was generating mainly via reaction 6 Li(n,α)T), but had occurred by radiolysis of the mixture in the solid state. F 2 evolution at 35 C corresponded to about 0,02 molecules per 100 ev absorbed, could be completely stopped by heating to 100 C, and could be reduced by chilling to -70 C. KI in-reactor tests Fuel salt, mole % Liquid phase T, o C G(F 2 ),10-5 mol/100ev Solid phase 66LiF-33BeF 2-1UF T, o C G(F 2 ),10-2 mol/100ev 69LiF-31BeF LiF-16BeF 2-12ThF 4-0.3UF LiF-34.39BeF 2-0.3UF iF ThF 4-0.5UF NaF-25.9ThF 4-0.9UF R&D Needs: (1) Experiments are necessary to estalish what fraction of the uranium may bе present as UF 3 without deleterious chemical reactions of the UF 3 with other materials within the fuel circuit; (2) Details of behavior of the noble and seminoble FP s are still poorly known; (3) The technology necessary to limit to acceptable levels the rate at which tritium is released from MSR is required. 25