Interrelation of the Iodine-Activity Limit in RBMK Coolant and the Number of Defective Fuel Pins
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INTERRELATION OF THE IODINE-ACTIVITY LIMIT IN RBMK COOLANT AND THE NUMBER OF DEFECTIVE FUEL PINS
A. V. Krayushkin and A. K. Smirnova
UDC 621.039.58
A quantitative assessment of the 131I activity in RBMK coolant as a function of the number of defective fuel pins and the distribution over a fuel run is presented. The 131I yield fraction is determined using a model based on the solution of the diffusion equation with empirical coefficients. This investigation was performed for a core-average FA whose power is a function of the burnup. It is shown that in order to prevent the number of defective fuel pins from exceeding a limit it is not enough to take into account their uniform distribution over a run because the burnup strongly impacts the activity and egress of 131I to beneath the fuel-pin cladding and into the coolant.
It is well known that nuclear safety rules are used to establish the safe-operation limits for reactors on the basis of the number of defective fuel pins. The adherence to these limits is controlled by means of the benchmark radionuclide 131I, whose specific activity in the coolant is determined periodically. The safe-operating limit is also established on the basis of this specific activity. In RBMK it is taken to be 3.7·105 Bq/L. The supposition that the relation between the specific activity of 131I and the number of depressurized fuel pins is not rigorous enough has been stated before and for VVER is discussed in, for example, [1], but without any quantitative assessments. A quantitative analysis of this relation for RBMK is of interest. Radioactive 131I enters the coolant from defective fuel pins and is removed primarily by means of purification and precipitation on the surfaces of the loop. The change in the iodine activity in the coolant is determined from the relation dA/dt = Q – λA – λ0A, where A is the activity; Q is the rate of egress of 131I from defective fuel pins; and λ and λ0 are, respectively, the radioactive decay constant and the rate of removal of iodine from the coolant. The purpose of the present communication is to make a quantitative assessment of 131I activity in coolant as a function of the number of defective fuel pins. A detailed analysis of different components of iodine removal from coolant is a quite difficult problem and is not the subject of the present article. In the stationary state, Q – λA – λ0A = 0; A = Q/(λ + λ0). Correspondingly, the specific activity a=
q1 N d A Q = = . M M ( + 0 ) M ( + 0 )
(1)
where M is the mass of the coolant in the loop; q1 is the average flow rate of 131I from one fuel pin into the coolant; and Nd is the number of defective fuel pins.
National Research Center Kurchatov Institute, Moscow, Russia; e-mail: [email protected]. Translated from Atomnaya Énergiya, Vol. 128, No. 3, pp. 173–175, March, 2020. Original article submitted July 3, 2019.
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1063-4258/20/12803-0188 ©2020 Springer Science+Business Media, LLC
As specified in the nuclear safety rules, let 131I flow into the coolant from only 0.1% of the total number of
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