Behavior of Radioactive Iodine Isotopes in the Multiple Forced Circulation Loop in an RBMK Reactor

Abstract

The flow of iodine, including 131 I, into the coolant water in a nuclear power plant with an RBMK-1000 reactor under normal operating conditions and during transient regimes is analyzed. It is shown that under normal operating conditions the specific activity of 131 I in the coolant is correlated with the iron concentration. During shutdown, its content increases by factors of 30-200. The emission of 131 I into the coolant can be decreased by factors of 10-15 and the degree of unsealing of fuel elements can be decreased if before shutdown the reactor is held for 2-5 days at 50% of the nominal power level. Recommendations are made for decreasing 131 I emissions into the atmosphere. The adoption of these recommendations at the Leningrad nuclear power plant has reduced the 131 I emissions into the atomsphere by a factor of 17.The iodine concentration in the water coolant is correlated with the change in the reactor power and increases sharply at the moment a reactor is shut down. The specific activity of 131 I in RBMK-1000 coolant is observed to increase by factors of 30-200 during the first few days after shutdown (Fig. 1, Table 1).The yield of the ith fission product from unsealed fuel assemblies is described by the relationwhere K is the coefficient of proportionality, which depends on the reactor power; Y i is the total yield on fissioning, arb. units; λ i is the decay constant of the radionuclide, sec -1 ; b is an exponent which assumes values from 0 to 1, depending on the degree of defectiveness of the fuel elements: • b = 1 for an equilibrium mechanism of emission of fission products (the constant characterizing the emission of a radionuclide into the coolant is somewhat less than the decay constant of the radionuclide; a constant or equilibrium content of fission products is established in the fuel); point defect; no correlations between emission and power; • b = 0.5 with a diffusion mechanism of emission (the emission constant into the coolant is much greater than the decay constant); slit-shaped defects; exponential power dependence of the emission of the radionuclides; • b = 0 in the absence of emission; no defects; the radionuclides enter the coolant as a result of surface contamination of the fuel elements by uranium; the emission of fission products is proportional to the reactor power [1]. Once stationary equilibrium is reached, the rate of entry of fission products into the loop becomes equal to the sum of their rates of decay and removal from the loop. The equilibrium specific activity of the ith radionuclide in the coolant can be expressed by the formula (2) A KY M i i i i = + λ λ λ 0 5 0 . ,