J - carbon flow through sensor membrane 83 MEASUREMENT OF CARBON THERMODYNAMIC 2 ACTIVITY IN SODIUM g , cm" , min" ; t - time, s; GJT - sodium flow rate irr/hour, L/hour; F.A, KOZLOV, Yu.I. ZAGORULKO, Yu.P. KOVALEV, H_ - sensor signal, vol. % CH ; V.V. ALEKSEEV g 4 Institute of Physics and Power Engineering, Gr - decarburizing gas flow rate through sensor ; Obninsk, o Union of Soviet Socialist Republics T,t - t emperature. C; t - sensor t emperature; Gg - sodium flow rate through sensor. INTRODUCTION ABSTRACT Continuous detection of carbon thermodynaraic activity in The report presents the brief outline on system of carbon sodium coolant of energy installations with fast neutron reac- activity detecting system in sodium(SCD), operating on the tors presupposes conducting the following main functions; : carbon - permeable membrane, of the methods and the results of - Control of carburization sodium potential with reference to testing it under the experimental circulating loop conditions. construction materials; The results of carbon activity sensor calibration with the u,s,e - Control of sodium coolant accidental contamination by carbon- of equilibrium samples of XI8H9, Fe -8Ni, Pe ~12Mn materials bearing impurities, for example, as a result of oil leakage from are listed. The behawiour of carbon activity sensor signals in a centrifuginal pump cooling system. sogium under various transitional conditions and hydrodynamic Performing these functions under the monisothermal loop condi- perturbation in the circulating loop, containing carbon bearing tions has specific features which on the one hand depend on impurities in the sodium flow and their deposits on the surfa- construction and operating parameters of detectors and on the ces flushed by sodium, are described. other hand - on the condition and behaviour of carbon-bearing impurities in sodium. CONVENTIONAL SIGNS In the constructions of basic types of devices of continuous carbon activity detecting in sodium, being developed in various - carbon activity in sodium, activity unit; countries, carbon-permeable membranes are used, which either M - carbon activity in equilibrium samples materials, act as the main sensor (diffusion cells) or are the barriers >< separating an electrolite from sodium (electrochemical cells) o activity unit; ro /1-2/. o o - carbon concentration in equilibrium samples In both cases for higher carbon flows through the sensor CD material, weight %; membrane to be brought about, they must be installed in special N_ - mole fraction of carbon dissolved in sodium; zones with a sodium t/emperature 7OO-75O,S°C. In this connection the question arises: how we can determine the carbon activity in N - carbon mole fraction conforming to solubility loop zones with a temperature below 700°C, using high-temperature c,s limit in sodium; sensor reading. The interpretation of detector reading is yet more complicated in the case when the content of carbon sodium passing through the powder layer was carried out 84 in sodium exeeds its solubility limit at loop nominal tempe- from the bottom upwoids. Behind the souree the samples of ratures (350-400°C). the Pe-8Ni, Pe-|2Mn, XI8H9 foil were placed. The analogous Even with the carbon content levels being of the order of type samples were installed also in a special chamber, pla- ppm's, its basic mass is contained in sodium in the form of a ced directly behind the sensor. dispersed phase. The local levels of carbon activity under the The mixture of 5...10 yol.% HU in Ar or He before enter- nonisothermal loop conditions would be determined in this case ing the sensor line is moistened up to 0.5-1$ VOl» H-O by the physical-chemical processes conjunction (Particle solu- (vapour) in the humidifier filled with oxalic acid dihydrate. tion, conversion of some forms of carbon-containing impurities In the sensor water vapour reacts with the carbon dissolved in into others, mass-exchange of suspension - earring flow with the membrane forming carbon monoxide. Further the gas enters the loop walls, solutions of deposits on the walls). the catalytic converter, in wMch in the presence of hydrogen The detailed consideration of the mentioned problems is in the carbon monoxide conversion to methane on nickel takes place a special report. at t=420°C, the degree of conversion is 98$. The concentration Here much attention is given to the dependence of carbon acti- of methane is measured on the gas chromatography flame-ioniza- vity on its concentration in sodium. Along with this some illu- tion detector both continuously and discretely. Plow rate and strations of the carbon activity sensor signals character under pressure of the gas in the sensor lines are stabilized by the the condition of transient temperature - and hydroflynamic regi- corresponding regulators. Purification of the initial gaseous mes in the sodium loop, containing deposits on the walls and mixture fed into the sensor, is conducted on the freezing-out particles in the flow are quoted. trap, cooled by liquid nitrogen. CARBON ACTIVITY DETECTING SYSTEM CALIBRATION (SCD) DISCUSSION OP EXPERIMENTAL INSTALLATION CONSTRUCTION Carbon transfer from the sodium, flushing the sensor mem- The sensor of the carbon activity detecting system in brane outside into the decarburizing gas medium is multistage. sodium (SCD) was installed on the bypass of the main experimen- The carbon transfer rate is determined by the full resistance, tal sodium loopSID heater, its diagram and detailed description in which reactive resistances on membrane surfaces may play a being given in reference /3/. significant part. A carbon flow through the sensor membrane The diagram of SCD in showm in Fig.1. being calculated in the assumpt/ion that a transport resistance The SCD sensor is made of armco-iron with a wall 0.5 mm corresponds to the carbon diffusion stage in the membrane mate- thick. The membranes surface is 150 cm . In some experiments rial at its zero concentration on the surfase, flushed by gas, in front of the sensor the source was placed, representing a is a rough characteristic of carbon activity in sodium. The cylindrical vessel, filled with graphite powder (the remains above circumstances require the use of some "absolute" mBthods of graphite fragments being screened through a 50-mesh grid). of achieving correlation of the values of the flows being To prevent the powder sweeping out into the loop it was isola- measured and carbon activity in sodium. The equilibrium sample- ted with 50-mesh grid in the inlet and outlet of the source standard approach is such a method , which is itsef of special i th a metal-tissue gasket over the powder layer. The interest for the purposes of measuring carbon activity in sodium. In calibrating the SCD sensor its stationary signal was The constants of this equation*; were obtained on the basis J5 compared to the values of carbon activity in sodium, which of the concentrations experimental values in the XI8H9 and were determined with the use of the samples of the foil Fe - 12 Mn samples and the activities, estimated values (from XI8H9, Fe-12Mn, Fe-8Ni, exposed under the conditions identical equation (I)) in the XI8H9 samples for two test series, in to the conditions of the flow flushing the sensor membranes which the XI8H9 and Fe -12 Mn sample were kept under the iden- (identical temperatures, roughly identical sodium rates). tical conditions. After being exposed during the time sufficient for the carbon The conditions of conducting the experiments in calibra- equilibrium distribution between the sodium and samples materi ting SCD and the chemical analysis results are listed in al ( 30 hours) to be established, the samples were quenched Table I. for "freezing" the equilibrium passing through the chambers Experiments 1-2 at low levels of carbon activity (without increased flow of cooled sodium (300-400°C). A Typical curve the source) were conducted on the condition of continuous of samples cooling is represented in Pig.3. Inac attaining sodium purification in the loop by means of a cold trap opera- the temperature of 35O-4OO°C in the chambers, the sodium was ting at 12O-13O°C. Experiments 3-5 were conducted at the pre- drained out of them into the loop dump tank. The chambers sence of the dissolved carbon source were cooled for 3-4 hours up to room temperature after which at the entry into the SOD sensor. their opening and samplesextraction was carried out. The In experiment 4 sodium fiow rate through the source was samples primary processing was performed with ethyl alcohol, somewhat lower than in experiment 3, but in test 5 - it was the final prossesing flushing in the solution of 5% nitric higher. acid, in ethyl alcohol, then in distilled water, alcohol and again in distilled water. The calculation of carbon activity The calibration results are listed in Table 2. The depen- in the samples material, based on the determined by the dence of the sensor signal on the activity as it follows from its chemical analysis methods carbon concentrations, was performed graphical representation (Fig.4-) is close to linear. The for XI8H9 and Fe - 8Ni alloys according to Nateson-Kassner equation /42. For the temperature 75O°C taking into account calibration coefficient value Kg = A /J is equal to 1.6*10 the material composition, it was reduced to the form: activity unit-cm *min/g. that is roughly an order lower than For XI8H9 the Kp = L/D*Co coefficient value, occuring in the assumption that the carbon diffusion through the sensor membrane material, M 0.0480c1 • exp (0.232GM + 0.713) (I) c at the zero carbon concentration on the surfase flushad by Fof Fe -8Ni gas, is the limiting stage. M = 0.04-8 Cc - (2) The difference between the carbon flow estimated values For calculating carbon activity in the Fe- 12Mn samples through the membrane and the experimentally measured ones is was used the equation; accounted for by the non-fulfilment of the suggestions lying in the basis of the assumption that carbon concentration on the membrane surfase flushed in gas, and resistances on (sodium- (3) armco)/(armco-gas) boundary surfaces are not equal to zero.