The determination of radionuclides in the primary coolant loop of sodium-cooled nuclear reactors

There are further long-lived contaminations in addition to the Na-24 activity produced in the primary cooling circuit during the operation of sodium-cooled reactors. These are 1) activated impurities in the coolant itself, 2) activated corrosion products from materials of the plant, and 3) fission products which were set free in the coolant by defective fuel elements. By means of activation analysis, impurities in the ppm region in commercial sodium can be well determined. The completely non-uniform distribution of these impurities found in a sample shows the importance of taking a sample for the investigation of coolant contamination. A special device was developed for this which is brought to the main Na pipe of the cooling circuit by a 'by-pass' and thus makes the removal of a representative Na sample possible. The material of the sample removing container is also important. The deciding factors for the share of corrosion products in the coolant are temperature, flow rate and oxygen content. The transport of activated corrosion products through sodium can be quite considerable as shown by calculations. The most important factor in the contamination of the system, however, are the cladding tube defects of fuel elements which not only lead to the release of fission products into the coolant but also of fuel particles, as is known from the examination of disassembled test radiation loops. Some experiences gained at the KNK in the Nuclear Research Centre, Karlsruhe are given. (RB/LH)