Sodium quality control in Sodium Fast Reactors. By Christian Latgé 1,

Transcription

1 1 st International Topical Seminar Coolants and Innovative Reactor Technologies organized within the framework of CEA and JAEA Collaboration and GEDEPEON Sodium quality control in Sodium Fast Reactors By Christian Latgé 1, 1 CEA Cadarache, DEN/Department of Nuclear Technology, St. Paul Lez Durance (France) Christian.latge@cea.fr

2 Content Why is it necessary to control the quality of Na? Basic crystallization phenomena involved in Na Cold traps : design and operation Needs for future SFR

3 Why is it necessary to purify Na? Two main impurities : O and H, even if other impurities (radionuclides) can be considered with other specific purification systems. Primary Na : [O] is a key parameter of corrosion contamination Consequences on dosimetry necessity to decontaminate (handling, repair, ISI,..) Intermediate Na : [H] has to be maintained as low as achievable in order to detect as soon as possible a water ingresss in Na. Moreover, Na purification allows to minimize tritium release. For all the circuits : Control the risks of plugging, seizing of the rotating parts, reduction of thermal transfer coefficient

4 O and H contamination during handling operations

5 General Corrosion models used in France [O] below 5ppm, Baque s model R = 0 (T< 817,13 K) R = a. V [O]. exp(-150.5/(t-817,13)) R = rate of metal loss in kg.m -2 for one year a = coefficient function of the alloy V = sodium velocity (m/s) (below 10 m.s -1 ) [O]= oxygen content in ppm (Eichelberger law) [O] above 5ppm, Thorley s model (given for [O] ranging from 0 ppm to high concentrations but used in France above 5 ppm and T>817,13 K) If V 4 m/s, R = (V/4) log 10 [O]- 3913/(T) If V > 4 m/s, R = log 10 [O]- 3913/(T) These two semi-empirical models were introduced in the contamination Anacconda code

6 Mass Transfer, Deposition phenomena : - 54 Mn, 51 Cr,, induced by the activation of core materials, are dissolved into the sodium and mainly deposited in the pumps and the coldest parts of the reactor i.e. the Intermediate Heat Exchanger (IHX ) - Radio-cobalts such as 60 Co are also produced and a low fraction is deposited on hot surfaces in primary components or structural material. Strategy of SFR operation with clean sodium two main positive consequences: - reduction of dose rates, - easier decommissioning operations and subsequent

7 Mass Transfer, Deposition phenomena : Several suitable models to describe the mass transfer related to corrosion products already developed. Nevertheless necessity to increase the efforts on the validation of these models, by comparison of model predictions with experimental data or reactor measurements. If corrosion kinetics and associated models have been reasonably well described, strong necessity to improve the knowledge on deposition mechanisms ( 54 Mn and 60 Co). Coupling O control, corrosion, mass transfer and deposition equations : calculations should allow : To validate the O control strategy (by cold trapping) (continuous or not impact on cold trap contamination) To support studies on specific activated corrosion products control (by specific traps) For this purpose : necessity to up-date the existing codes

8 Why is it necessary to purify Na? Two main impurities : O and H, even if other impurities (radionuclides) can be considered with other specific purification systems. Primary Na : [O] is a key parameter of corrosion contamination Consequences on dosimetry necessity to decontaminate (handling, repair, ISI,..) Intermediate Na : [H] has to be maintained as low as achievable in order to detect as soon as possible a water ingresss in Na. Moreover, Na purification allows to minimize tritium release. For all the circuits : Control the risks of plugging, seizing of the rotating parts, reduction of thermal transfer coefficient

9 Intermediate Na loop : contamination sources

10 Other impurities : Tritium : from boron carbide or ternary fissions, Activated corrosion products (mainly 60 Co, 54 Mn, ) Sodium : 22 Na (2,6 year) (3700 Bq/g SPX 1990) ; 24 Na (15 hours) ; 23 Ne (cover gas) Sodium impurities : 110m Ag ; 65 Zn ; 113 Sn ; 124 Sb ; (Depending on the liquid Na grade) Activation of components coming from the steel elements (Fe, Cr, Ni, Mn...) Corrosion products present in the liquid sodium, then exposed to the neutron flux of the core Structure under neutron flux 54 Mn (312 d) by activation (n,p) of 54 Fe (< 5 Bq/g SPX 1990) 60 Co (5,3 y) by activation (n,g) of 59 Co 58 Co (71 d) by activation (n,p) of 58 Ni 51 Cr (28 d) by activation (n,g) of 50 Cr The main contamination of the core components is resulting from that activation (primary pump and IHX) Deposition on steel surfaces depending on flow rate and temperature gradient Fissile compounds and Fission compounds (if pin rupture)

11 Main processes for Na quality control - Specific methods from chemistry to obtain nuclear grade sodium - Initial cleaning of loop, components and vessels (O 2, H 2 O, moisture) - Filtering (during initial filling operations) - Cold trapping : the main process for O and H - Hot trapping : mainly for O - Adsorption : for carbon, - And of course limiting ingress of pollution by appropriate operating rules or device Cleaned argon argon inlet Argon and small part of oxygen Na K

12 Concentration-temperature diagram WITTINGHAM Solubility law for H raa log 10[ H( ppm)] = T( K) [O], ppm [H], ppm NODEN Solubility law for O 1 0,1 0, Temperature, C log [ O( ppm)] = T( K)

13 Concentration-temperature diagram O H

14 Nucleation phenomena If sodium is supersaturated, presence of agglomerates These agglomerates can turn into nuclei if free enthalpy of the system reaches a limit value (primary nucleation) Nevertheless some existing crystals can produce some small particles by attrition (secondary nucleation) The primary nucleation can be homogeneous (if it occurs in the sodium bulk) or heterogeneous if it occurs on metallic surfaces. Wetting characteristics have a large influence on heterogeneous nucleation In Na, heterogeneous nucleation for O and H : no Na2O or NaH crystals in Na bulk.

15 Growth phenomena Model of the kinematic wave (Franck-Vermilyea) : explain the formation of steps Two growth phenomena : - low supersaturation : regular growth - high supersaturation : dendritic growth

16 Nucleation and growth kinetics Nucleation Na 2 O : EN = - 60 kj/mol nn = 5 NaH : EN = kj/mol nn = 10 Growth Na 2 O : EG = - 45 kj/mol ng = 1 NaH : EG = kj/mol ng = 2 thus, different cristallization mechanisms : Possibility to obtain crystallization of NaH on cold walls (large influence of supersaturation and nucleation) Necessity to provide steel packing for Na 2 O

17 Cold Trap operation : basic principle First, Na is cooled in an integrated heat exchangereconomizer Then Na flows through a cooler where it reaches a temperature below the saturation temperature Crystals formed are retained on cold walls or stainless steel packing The outlet flow is then reheated in the heat exchanger-economizer

18 Some design options Sodium distribution in the cold trap : ring type header, direct injection, Cooling fluid : air, oil, NaK oil in static NaK, Internal exchanger-economizer : to avoid plugging of the inlet pipe (Rex : SNR 300) Cooler : one cooler or modular coolers (to adapt thermal gradient with source of impurities (i.e. : hydrogen) Support for impurities : knitted mesh, pall rings, cooled wall,.. Location of support : location and surface per volume depends of the design requirements.

19 PSICHOS Cold Trap (SPX) : basic principle

20 Evolution of the design of intermediate cold traps.

21 Primary Integrated Purification system

22 Maintenance purification or purification campaign Flow-rate, Tcp = constant Tcp = Tsat - 40 C

23 Hot trapping (getter) Several hot traps were developed in the past; among them : the ZR0.87-Ti0.13 alloy for O : allows to reach very low O concentration (below solubility), but low capacity and necessity to implement a filter downstream. R = exp(-40.3x10 3 /RT).C kg O/(h.m 2 ) Kinetics were established in the range of ppm Yttrium foils for hydrogen The flowrate does not seem to have significant effect on oxidation rate Thus the limiting step is oxygen diffusion in the alloy Capacity : g(o)/g alloy Hot traps present interest in two cases : Stable trapping in case of temperature rise up (avoiding release of pollution by dissolution) Easy to implement in small experimental circuits or vessels (can be implemented with static Na) (ie implementation in an irradiation loop in PHENIX)

24 Sodium filtering Filters made of sintered stainless steel Porosity: 20 µm 19 pipes Diameter: 20 mm Dismontable Filter made of sintered stainless steel

25 Fission and activation products removal from Na Primary Cold traps : sized for purification / O and H, nevertheless collect radio nuclides as 137 Cs, 131 I, 54 Mn, 65 Zn, some 60 Co, Traps for 137 Cs by sorption: carbonaceous materials : graphite, Reticulated Vitreous Carbon, operated between 200 C and 300 C; efficency depends on sodium velocity, T cycling,..large experience gained from various reactors (EBR2, BOR60,RAPSODIE, Future goals for research : confirmation of basic phenomena and capacities, efficiency at very low [ 137 Cs] (Keq) Traps for radioactive corrosion products (mainly 54 Mn, 65 Zn): rolled-sheet nickel trap (EBR2, FFTF, KNK2, SILOE,..) : good efficiency at relatively high T (over 300 C) and in turbulent flow. Future goals for research : necessity to obtain a better understanding of diffusion phenomena with aim to estimate trapping efficiency and define design rules

26 Operational feedback; recommandations for future (1/2) For primary Na, necessity : to define requirements for [O] To estimate the purification efficiency by means of cold trap (/corrosion), in various normal or abnormal operating conditions, to improve mass transfer modeling (with regards to corrosion of new materials and deposition phenomena) to define a validated data set including, solubilities and diffusion constants for metallic compounds, To reassess the choice of the option : continuous purification by cold trapping (taking into account purification efficiency, decommissioning constraints (size, contamination, waste management, ), To evaluate the performances of specific purification processes for activated corrosion products, To adress the needs of improved reliable chemical instrumentation for monitoring Na quality, To suggest a purification process for carbon trapping (adsorption and/or filtering) (in case of Sc CO2 Brayton cycle) To evaluate innovative trapping processes,

27 Operational feedback; recommandations for future (2/2) For intermediate Na, necessity : to define requirements for [H], with regards the water leak detection system (diffusion, electrochemical, ) to assess the potentialities of innovative and efficient H detection systems,(even if existing systems, based on permeation + mass spectrometers, are very efficient) to reassess the choice of the option : purification by cold trapping (taking into account the efficiency, the decommissioning constraints (size, contamination, waste management, ), to evaluate innovative specific purification processes, including tritium (if energy conversion system is not based on a Rankine cycle SGU) to define an efficient and safe in-situ regeneration process for cold traps,, particularly if the SGU are fmade with ferritic steels. (necessity also to confirm permeation coefficients, to investigate coatings, )

28 Tritium release Tritium issue - Distribution of hydrogen and tritium in the different media of the reactor : governs tritium activities in liquid and gaseous releases, as well as tritium activities build-up in units such as the purification units. Main objectives of the code : Assess tritium releases to the environment (gaseous and aqueous) at the design stage at the operating stage guarantee that they are below the authorised thresholds Assess tritium activities in the different media (Na, steel, ) tritium build-up in purification units New issue : tritium trapping if (gas) Brayton cycle

29 View of plate heat exchanger (courtesy of Alfa- Laval Vicarb) Na chemistry View of corrugated plates (courtesy of Alfa- Laval Vicarb) Thank You for your attention ISIR US developments Na school in CEA-Cadarache