Purification of sodium and cover gas

At the present time, sodium purification technique with the use of cold traps based on decreasing solubility of the major part of impurities in sodium with temperature decrease has found the most wide practical application [5.3, 5.4]. In the cold trap sodium is cooled within the settling tank and the section upstream the filter. This results in reduction of oxygen and hydrogen concentrations respectively to 1 and 0.05 ppm. Cold trap is capable of retaining impurities in amount up to -30% of its volume [5.5]. 

Carbon is confined in the cold trap as suspension, the purification process being 10 times longer than that for oxygen. In order to catch caesium, graphite based purification technique was developed [5.6, 5.7]. Upon 10 years of the BN-350 reactor operation capacity of the secondary traps was exhausted and their hydrogenation was carried out. The basic amount of sodium transformed to caustic phase was removed at 420°C. Thus, normal operation of traps was restored [5.3]. 

As regards the cover gas, it is necessary to purify source argon eliminating impermissible release to the environment of Xe and Kr, short-term total activity of which in the BN-600 reactor reached 6.7 10 Bq/L value. This problem was solved using special filters and activated coal as absorbent [5.1, 5.3]. 

Control of the content of non-metal impurities in the coolant (O2, H2 and C) and cover 

Oxygen control. In order to make measurements of oxygen activity in sodium the electrochemical control technique based on galvanic cell has been mastered with sodium flowing over electrolytic pellet of thorium and yttrium sealed into the metal tube. Reference electrode is located inside the tube. E.m. F. generated depends on the temperature and oxygen concentration e.m. F. = f (T, Co2). Service life of such device is over 10 hours. 

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