Unfortunately the accuracy with which an impurity dependent physical or chemical property of sodium can be measured decreases with decreasing impurity concentration .sx 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 .sx 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 .sx .sx Theoretical .sx ( a ) The Cell .sx The use of solid electrolytes in galvanic cells has been described in detail by Kiukkola and Wagner .sx In a reversible cell consisting of two metal-metal oxide electrodes and a solid oxide electrolyte through which current is transported solely by 0 :sx =: ions , the change in free energy 15DG accompanying the passage of one mole of oxygen is given by :sx - EF where E is the voltage developed across the cell and F is the Faraday .sx 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 :sx =: from the saturated to the unsaturated metal will be given by :sx - where [FORMULA] , [FORMULA] are the activities of oxygen in saturated sodium ( concentration c;0; ) and in the unsaturated sodium ( concentration c c;0; ) , T the absolute temperature and R the gas-constant .sx 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 :sx =: ion may be written Thus The solubility of oxygen as Na;2;0 in sodium has been determined and is given by the relationship Substitution of equation ( 3 ) in equation ( 2 ) with appropriate values for the various constants gives Values of this function between 400@ and 800@C at 100@ intervals and for oxygen concentrations between 0.1 and 100 p.p.m. are presented in Fig. 1 .sx At the present time maximum sodium coolant temperatures are around 500@C and oxygen concentrations are usually intended to be maintained in the range 1-10 p.p.m. According to the above this cell under these conditions should give voltages ranging from 224-147 mv .sx ( b ) The effect of small changes of oxygen concentration and temperature on the cell E.M.F. .sx The E.M.F. of such a cell placed in a sodium circuit will be affected by fluctuations in oxygen content and temperature .sx These may be estimated from equation ( 4 ) or the following derived equations :sx - Equation ( 5 ) indicates that any voltage fluctuation arising from a sudden small concentration change will be controlled principally by the original concentration .sx 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 ) .sx For relevant reactor conditions ( 500@C , C = 1-10 p.p.m. ) the finite change of voltage 15DE accompanying finite concentration changes 15DC is plotted in Fig. 3 .sx The latter as might be expected vary considerably .sx A rise of oxygen concentration from 1-2 p.p.m. is accompanied by a voltage drop of @1723 mv .sx while , a rise from 9-10 p.p.m. would produce a change of only @173 mv .sx Changes in voltage accompanying fluctuations of coolant temperature according to equation ( 6 ) vary only slightly with concentration and are proportional to the temperature change .sx Values at various oxygen concentrations of [FORMULA] together with apparent changes in oxygen level for temperature fluctuations of @14 10@C at 500@C are presented in Table =1 .sx The above figures show that a @14 10@C temperature fluctuation at oxygen levels in the range 1-10 p.p.m. would indicate an apparent change of @1712% in oxygen concentration .sx 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 .sx ( c ) Contamination of the sodium circuit by oxygen from the cell .sx 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@C .sx The author has obtained reproducible results with such cells at 400@C and above by using vibrating reed voltmeters that draw current from the cell only as a result of leakage through insulation resistance of [FORMULA] .sx Thus if voltmeters of this type were used with the Na/ Na;2;0 cell it is possible to estimate the contamination of the circuit sodium from oxygen continuously diffusing through the electrolyte .sx If it is assumed that in practise [SIC] the maximum voltage developed by the cell at 500@C will be around 300 mv .sx ( see Fig. 1 ) then in the case of the instrument with the lower resistance the current will be :sx - 3 x 10 :sx -14: coulombs/ sec .sx The charge on 0 :sx =: ion @183 .sx 2 x 10 :sx -19: coulombs .sx Thus the number of 0 :sx =: ions travelling through the electrolyte per second @1810 :sx 5:. The mass of oxygen per year at this rate would be approximately 8 x 10 :sx -1: g./ year which is a quite insignificant quantity .sx ( d ) The use of the cell as a corrosion meter .sx 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 .sx 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 .sx 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 .sx 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 .sx A negative voltage from the reference electrode would mean oxidizing conditions for niobium and positive voltage , non-oxidizing conditions .sx .sx Practical .sx The practical application of the above idea will involve considerable experimentation before it can be realised .sx 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 .sx Possible materials are zirconia stabilised with lime and thoria doped with rare earth oxides .sx 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 .sx 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. The +ve electrode consisting of a small quantity of sodium saturated with sodium monoxide C is situated at the bottom of the tube .sx 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@C .sx 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 .sx Provided the temperatures at C and E are the same , thermoelectric contributions to the voltage should be zero .sx 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. 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. 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 .sx 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. 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 .sx .sx Discussion .sx 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 .sx The principal difficulty encountered by the author , in determining partial molal free energies by solid electrolyte cells of very stable oxides such as UO;2 ; , MnO etc. was vapour phase transfer of oxygen by carbonaceous impurities in the blanket gas .sx 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 .sx In the above design the two electrodes are completely separated from one another so that this major source of trouble should not be present .sx 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 .sx Whether such an apparatus can be incorporated in a reactor circuit in a manner that will satisfy safety requirements will need further study .sx 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 .sx Such containment however , will be complicated by the necessity of providing suitable insulating seals through its walls .sx .sx Conclusions .sx 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 .sx 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 .sx A. OUTLINE OF METHOD .sx To a measured portion of the sample , niobium and zirconium carriers are added together with hydrofluoric acid to ensure complete isotopic interchange .sx Rare earth elements are co-precipitated with lanthanum as fluorides .sx Niobium is precipitated with ammonia , partially separating it from zirconium .sx The niobium precipitate is dissolved in a mixture of oxalic and nitric acids , and niobic acid precipitated by boiling and adding potassium bromate .sx The niobic acid is dissolved in acid ammonium fluoride and the cycle from the ammonia precipitation repeated .sx The niobic acid is washed , ignited to niobium pentoxide , which is mounted on a tared counting tray and weighed .sx The 15g-activity is measured through a lead/ aluminium sandwich using standard gamma scintillation equipment , which has been calibrated with known amounts of niobium-95 .sx B. REAGENTS REQUIRED .sx All reagents are Analytical Reagent Quality where available .sx .sx Standard niobium carrier solution ( [FORMULA] ) .sx Fuse 20 g of pure niobium pentoxide with 72 g of potassium carbonate in a platinum dish .sx Cool and dissolve the solidified melt in about 400 ml of hot water .sx Transfer the solution and any undissolved solid to a glass beaker , stir thoroughly and add 16M nitric acid until the solution is strongly acid to litmus .sx Stand the beaker on a hot plate and keep the solution warm for 30 minutes to coagulate the precipitate .sx Transfer to four 200 ml polythene bottles , centrifuge , decant and discard each supernate .sx Wash each portion of the precipitate three times by stirring with 100 ml of 2% ammonium nitrate .sx Use a glass rod for stirring .sx Centrifuge and discard the supernates after each wash .sx Dissolve each portion of the precipitate in 25 ml of 30% ammonium fluoride and 15 ml of 16M nitric acid .sx Combine the solutions from each of the 200 ml polythene bottles , and dilute to 2 litres with distilled water in a polythene bottle .sx Standardize as follows :sx - Pipette 10 ml of the solution into a 400 ml polythene beaker and add 100 ml of a saturated solution of ammonium chloride .sx 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 .sx