Radionuclide Concentrations in the Steam Condensate Supply System Due to Steam Generator Tube Chronic Leakage

Abstract

In the event that a small steam generator tube leak develops in a steam generator, heavy water from the heat transport system will pass into the steam-condensate supply system. Such leakage introduces radioactivity into the "secondary side" of the steam generator by carrying with it Tritium. Iodine and other radionuclides from the heat transport system.

Heavy water. when entering the secondary side of the steam generator via a tube leak, mixes with light water within the steam generator. The radionuclides so introduced are subsequently transported throughout the steam supply system via continuous steam flow. The concentration of radionuclides in the steam-condensate supply system may build up if the plant operates in this mode for a prolonged per1od of time. This has the potential foe radiological consequences when water/steam release from the steam-condensate supply system occurs. This paper analyses the transient build-up of radionuclide concentration in the steam-condensate supply system. In addition. the evaluation establishes the effect a number of important parameters (such as the concentration in the heat transport system. leakage rates from the heat transport system and from the s team-condensate supply system) on the radionuclide concentration in the steam-condensate supply system.

Among many constituents. Tritium and Iodine-131 are selected for modelling purposes since they are the two most important radionuclides in determining radiological consequences. Tritium is carried over by steam to the rest of the steam-condensate supply system: however, Iodine-131 remains preferentially in the liquid. Therefore, a special provision is made to model Iodine concentration buildup in the steam-condensate supply system, assuming that Iodine transport from the affected steam generator to the rest of the steam- condensate supply system is due primarily to moisture carryover. The analysis indicates that for a given leakage from a steam generator tube and normal fresh feedwater makeup, quasi-steady equilibrium activity levels (for both Tritium and I-131) will be reached after a period of time. The equilibrium activity level of Iodine- 131 is significantly higher in the affected steam generator than the rest of the steam-condensate supply system, its magnitude being sensitive to parameters such as moisture carryover.