| 1 | Unfortunately the accuracy with which an impurity dependent physical or chemical property of sodium can be measured decreases with decreasing impurity concentration . |
| 2 | To get over this difficulty Alcock has suggested that instead of measuring directly the concentration of oxygen in the flowing sodium its thermodynamic potential should be measured by a suitable galvanic cell incorporated in the circuit . |
| 3 | The principal advantages of this should be continuous monitoring of the sodium and an accuracy of monitoring which , if the sodium_-_oxygen system obeys Henry 's law , should increase with decreasing concentration of the impurity . |
| 4 | 2 . |
| 5 | Theoretical . |
| 6 | ( A ) . |
| 7 | The cell . |
| 8 | The use of solid electrolytes in galvanic cells has been described in detail by Kiukkola and Wagner . |
| 9 | In a reversible cell consisting of two metal_-_metal oxide electrodes and a solid oxide electrolyte through which current is transported solely by 0<bsup>=<ast><bsup> ions , the change in free energy dg accompanying the passage of one mole of oxygen is given by : — 2EF where E is the voltage developed across the cell and F is the Faraday . |
| 10 | If the electrodes are sodium saturated with its own oxide and unsaturated sodium the change of free energy accompanying the transfer of one mole of 0<bsup>=<ast><bsup> from the saturated to the unsaturated metal will be given by : — <ast><bcomment>formula<ast><ecomment> where <ast><bcomment>formula<ast><ecomment> , <ast><bcomment>formula<ast><ecomment> are the activities of oxygen in saturated sodium ( concentration c<bsub>0<ast><bsub> ) and in the unsaturated sodium ( concentration c < c<bsub>0<ast><bsub> ) , T the absolute temperature and R the gas_-_constant . |
| 11 | If the activity of oxygen dissolved in sodium is proportional to its concentration as is required by Henry 's law then the free energy change per mole 0<bsup>=<ast><bsup> ion may be written <ast><bcomment>formula<ast><ecomment> . |
| 12 | Thus <ast><bcomment>formula<ast><ecomment> . |
| 13 | The solubility of oxygen as Na<bsub>2<ast><bsub>0 in sodium has been determined and is given by the relationship <ast><bcomment>formula<ast><ecomment> . |
| 14 | Substitution of equation ( 3 ) in equation ( 2 ) with appropriate values for the various constants gives <ast><bcomment>formula<ast><ecomment> . |
| 15 | Values of this function between 400 <deg> and 800 <deg> C at 100 <deg> intervals and for oxygen concentrations between 0.1 and 100 p.p.m are presented in fig 1 . |
| 16 | At the present time maximum sodium coolant temperatures are around 500 <deg> C and oxygen concentrations are usually intended to be maintained in the range 1_-_10 p.p.m |
| 17 | According_to the above this cell under these conditions should give voltages ranging from 224_-_147 mv . |
| 18 | ( B ) . |
| 19 | The effect of small changes of oxygen concentration and temperature on the cell E.M.F . |
| 20 | The E.M.F of such a cell placed in a sodium circuit will be affected by fluctuations in oxygen content and temperature . |
| 21 | These may be estimated from equation ( 4 ) or the following derived equations : — <ast><bcomment>formula<ast><ecomment> <ast><bcomment>formula<ast><ecomment> . |
| 22 | Equation ( 5 ) indicates that any voltage fluctuation arising from a sudden small concentration change will be controlled principally by the original concentration . |
| 23 | Thus changes from 0.1 to 1 p.p.m 1_-_10 p.p.m 10_-_100 p.p.m would result in the same change in voltage ( 1776 mv ) . |
| 24 | For relevant reactor conditions ( 500 <deg> C , C = 1_-_10 p.p.m ) the finite change of voltage de accompanying finite concentration changes dc is plotted in fig 3 . |
| 25 | The latter as might be expected vary considerably . |
| 26 | A rise of oxygen concentration from 1_-_2 p.p.m is accompanied by a voltage drop of 1723 mv while , a rise from 9_-_10 p.p.m would produce a change of only 173 mv . |
| 27 | Changes in voltage accompanying fluctuations of coolant temperature according_to equation ( 6 ) vary only slightly with concentration and are proportional to the temperature change . |
| 28 | Values at various oxygen concentrations of <ast><bcomment>formula<ast><ecomment> together with apparent changes in oxygen level for temperature fluctuations of 14 10 <deg> C at 500 <deg> C are presented in table 1 . |
| 29 | The above figures show that a 14 10 <deg> C temperature fluctuation at oxygen levels in the range 1_-_10 p.p.m would indicate an apparent change of 1712 % in oxygen concentration . |
| 30 | Providing a cell of the above type works satisfactorily the above arguments suggest that it will be sufficiently accurate as an oxygen monitor in a hot trapped sodium coolant circuit . |
| 31 | ( C ) . |
| 32 | Contamination of the sodium circuit by oxygen from the cell . |
| 33 | Experiments with solid oxide electrolyte galvanic cells have indicated that it is difficult to obtain reproducible voltages using normal potentiometric methods at temperatures below 750 <deg> C . |
| 34 | The author has obtained reproducible results with such cells at 400 <deg> C and above by using vibrating reed voltmeters that draw current from the cell only as a result of leakage through insulation resistance of <ast><bcomment>formula<ast><ecomment> . |
| 35 | Thus if voltmeters of this type were used with the Na <sol> Na<bsub>2<ast><bsub>0 cell it is possible to estimate the contamination of the circuit sodium from oxygen continuously diffusing through the electrolyte . |
| 36 | If it is assumed that in practise the maximum voltage developed by the cell at 500 <deg> C will be around 300 mv ( see fig 1 ) then in the case of the instrument with the lower resistance the current will be : — 3 x 10<bsup>_-_14<ast><bsup> coulombs <sol> sec . |
| 37 | The charge on 0<bsup>=<ast><bsup> ion <ast>?183.2 x 10<bsup>_-_19<ast><bsup> coulombs . |
| 38 | Thus the number of 0<bsup>=<ast><bsup> ions travelling through the electrolyte per second <ast>?1810<bsup>5<ast><bsup> . |
| 39 | The mass of oxygen per year at this rate would be approximately 8 x 10<bsup>_-_1<ast><bsup> g <sol> year which is a quite insignificant quantity . |
| 40 | ( D ) . |
| 41 | The use of the cell as a corrosion meter . |
| 42 | With the cell electrodes consisting of sodium with oxygen at different activities a voltage will be developed that is a function of the difference in the oxygen potential at the two electrodes . |
| 43 | Unless it is known at what oxygen potential a given material in the sodium coolant circuit will start to oxidise the cell can only be used as has been suggested above , as an oxygen concentration monitor . |
| 44 | However , if a material oxidizes in sodium at a given oxygen potential the reference electrode could be held at that potential and oxidizing or reducing conditions in the coolant circuit for that material would be indicated by a negative or positive potential at the reference electrode . |
| 45 | Thus for the specific case of niobium in a sodium circuit a corrosion indicator could be a reference electrode of sodium saturated and equilibrated with niobium separated from the coolant by a solid anionic electrolyte . |
| 46 | A negative voltage from the reference electrode would mean oxidizing conditions for niobium and positive voltage , non_-_oxidizing conditions . |
| 47 | 3 . |
| 48 | Practical . |
| 49 | The practical application of the above idea will involve considerable experimentation before it can be realised . |
| 50 | The first requirement is for an anionic electrolyte , which can be fabricated into suitable shapes impervious to gases and liquid sodium and which is neither corroded by sodium nor by sodium monoxide . |
| 51 | Possible materials are zirconia stabilised with lime and thoria doped with rare earth oxides . |
| 52 | If such a material can be made with these properties a possible way in which the cell may be incorporated in a sodium circuit is depicted in fig 4 . |
| 53 | The electrolyte A is made in the form of a thin walled closed off round end tube or probe fitting vertically into the sodium coolant circuit B . |
| 54 | The +ve electrode consisting of a small quantity of sodium saturated with sodium monoxide C is situated at the bottom of the tube . |
| 55 | The potential acquired by this pool of sodium is transmitted to the voltmeter V by a nickel conductor D , nickel being resistant to corrosive attack by oxide saturated sodium at 500 <deg> C . |
| 56 | The -ve electrode which is the coolant stream , is joined to the voltmeter by an earthed nickel conductor attached to the bottom of a well E in the coolant stream . |
| 57 | Provided the temperatures at C and E are the same , thermoelectric contributions to the voltage should be zero . |
| 58 | The probe extends out of the sodium stream through a close fitting thin walled T_-_junction F and passes into the open via a water_-_cooled O ring seal G . |
| 59 | The open end of the probe is sealed with a vacuum coupling H which also positions the +ve nickel conductor with respect to the sodium by circlips on either side of the seal I . |
| 60 | Evaporation of sodium from the pool C is minimised by a close fitting cylindrical block of electrolyte J attached to the +ve nickel conductor by nickel circlips . |
| 61 | Fixing and positioning of the probe relative to the coolant stream is effected by tie_-_bars of insulating material K joining the vacuum coupling H to the water cooled flange G . |
| 62 | The probe can be evacuated and filled with inert gas via the tube L which must of_course be electrically isolated after this has been carried out . |
| 63 | 4 . |
| 64 | Discussion . |
| 65 | It is not suggested that the above proposal will be successful but rather that it is worth a trial in the event of the inadequacy of some simpler method of monitoring the oxygen in a sodium circuit . |
| 66 | The principal difficulty encountered by the author , in determining partial molal free energies by solid electrolyte cells of very stable oxides such_as UO<bsub>2<ast><bsub> , MnO etc was vapour phase transfer of oxygen by carbonaceous impurities in the blanket gas . |
| 67 | This resulted in the oxidation of the -ve electrode and reduction of the +ve electrode which of_course led to a loss in E.M.F from the cell . |
| 68 | In the above design the two electrodes are completely separated from one another so_that this major source of trouble should not be present . |
| 69 | However , the stability of the system may be adversely affected by the thermal gradient up the probe and this can only be tested by experiment . |
| 70 | Whether such an apparatus can be incorporated in a reactor circuit in a manner that will satisfy safety requirements will need further study . |
| 71 | On the face of it however , there seems to be no reason why the cell should not be double_-_contained to prevent loss of sodium in the event of the ceramic tube being fractured . |
| 72 | Such containment however , will be complicated by the necessity of providing suitable insulating seals through its walls . |
| 73 | 5 . |
| 74 | Conclusions . |
| 75 | If other monitoring methods for oxygen in sodium in the concentration range 1_-_10 p.p.m are found to be inadequate then this galvanic cell may be worth investigating . |
| 76 | However , it will require development of a suitable electrolyte and even_then it will only be useful if the activity of the dissolved oxygen varies sufficiently with changes in its concentration . |
| 77 | A . |
| 78 | Outline of method . |
| 79 | To a measured portion of the sample , niobium and zirconium carriers are added together with hydrofluoric acid to ensure complete isotopic interchange . |
| 80 | Rare earth elements are co_-_precipitated with lanthanum as fluorides . |
| 81 | Niobium is precipitated with ammonia , partially separating it from zirconium . |
| 82 | The niobium precipitate is dissolved in a mixture of oxalic and nitric acids , and niobic acid precipitated by boiling and adding potassium bromate . |
| 83 | The niobic acid is dissolved in acid ammonium fluoride and the cycle from the ammonia precipitation repeated . |
| 84 | The niobic acid is washed , ignited to niobium pentoxide , which is mounted on a tared counting tray and weighed . |
| 85 | The g_-_activity is measured through a lead <sol> aluminium sandwich using standard gamma scintillation equipment , which has been calibrated with known amounts of niobium_-_95 . |
| 86 | B . |
| 87 | Reagents required . |
| 88 | All reagents are analytical reagent quality where available . |
| 89 | 1 . |
| 90 | Standard niobium carrier solution ( <ast><bcomment>formula<ast><ecomment> ) . |
| 91 | Fuse 20 g of pure niobium pentoxide with 72 g of potassium carbonate in a platinum dish . |
| 92 | Cool and dissolve the solidified melt in about 400 ml of hot water . |
| 93 | Transfer the solution and any undissolved solid to a glass beaker , stir thoroughly and add 16 M nitric acid until the solution is strongly acid to litmus . |
| 94 | Stand the beaker on a hot plate and keep the solution warm for 30 minutes to coagulate the precipitate . |
| 95 | Transfer to four 200 ml polythene bottles , centrifuge , decant and discard each supernate . |
| 96 | Wash each portion of the precipitate three times by stirring with 100 ml of 2 % ammonium nitrate . |
| 97 | Use a glass rod for stirring . |
| 98 | Centrifuge and discard the supernates after each wash . |
| 99 | Dissolve each portion of the precipitate in 25 ml of 30 % ammonium fluoride and 15 ml of 16 M nitric acid . |
| 100 | Combine the solutions from each of the 200 ml polythene bottles , and dilute to 2 litres with distilled water in a polythene bottle . |
| 101 | Standardize as follows : — |
| 102 | Pipette 10 ml of the solution into a 400 ml polythene beaker and add 100 ml of a saturated solution of ammonium chloride . |
| 103 | Heat the solution nearly to boiling , by placing the polythene beaker in a glass beaker of water , heated on a hot plate , and add to the solution 1 g of tannic acid dissolved in hot water . |