Transfer and deposition of radioactive nuclides in a convection flow of sodium

Modeling Transport of Radioactive Products of Corrosion in Loops with Sodium Coolant

“…3) the chemical parameter B = K a /K d decreases with increasing sodium temperature, which results in growth of the wash-off coeffi cient; at temperatures T ≤ 420-440°C the chemical parameters of 54 Mn, 60 Co, 58 Co, and 51 Cr are close and the total wash-off coeffi cient in order of magnitude equals K w ~ 10 -10 1/sec; at higher temperatures, the wash-off coeffi cient reaches ~10 -5 1/sec for 54 Mn and ~10 -8 1/sec for the isotopes of cobalt and chromium; 4) analysis of the experiments at low sodium fl ow velocities in the convection loop [7] as well as high velocities performed on the stands MTL [8], AMTL [3], RTL [9], STCL's [10] and in the reactors BR-5, -10 [11], BN-350 [11], and BOR-60 [12] shows that the distribution of the 60 Co deposits along the pipeline can be described satisfactorily if it is assumed that the rate of adsorption on the surface is signifi cantly greater than the rate of diffusion through the laminar layer, i.e., in the relation (2) K a >> Kʹ and therefore K = Kʹ; to describe the 54 Mn distribution along the pipeline, it is necessary to assume that at temperatures above ~420°C the manganese adsorption coeffi cient K a ~ 0.05 cm/sec; in this case, the calculation agrees satisfactorily with experiment; BR-10 and BN-350 are exceptions -they operated at low sodium temperature at the exit from the core (see Table 1) and in these reactors 54 Mn behaves similarly to 60 Co (this is taken into account in Al'fa-M); 5) as shown in the experiments on the BR-10 reactor and the RTL stand, 54 Mn, 60 Co, and 137 Cs penetrate to depth 50-100 μm from sodium into the wall of the pipeline; but most of the deposits are contained in a thin surface layer 1-10 μm thick; 6) it is known from experiments in which the corrosion of chromium-nickel steel in sodium [1-3] that a ferrite zone where the diffusion mobility of the components of steel, including radioactive impurities, is approximately 100 times higher Ag(n, γʹ) 110m Ag 249.9 days than in the austenitic matrix, forms as a result of selective leaching of nickel and chromium; the thickness of the ferrite zone in the 'cold' part of the pipeline is much smaller than in the 'hot' zone; an empirical temperature dependence of the thickness h of the corrosion zone, entering in the expression for the wash-off coeffi cient was obtained on the basis of the work in [1-3]; 7) the calculations show that an increase of the sodium fl ow velocity in the pipeline affects differently the behavior of the transported particles of different size and mass; there exists a critical velocity at which the contribution of the inertial effects becomes comparable to the contribution of Brownian diffusion; when the fl ow velocity of sodium exceeds its critical value, the transport coeffi cient and, therefore, the fl ow of particles from the sodium onto the wall increase sharply; the critical fl ow velocity equals 2 m/sec for particles with diameter of about 1 μm and 0.2 m/sec for 10 μm; the larger the particles, the lower the critical velocity; the critical velocity effect also occurs for a polydisperse system.…”