| Radioiodine release to the environment following a PWR steam generator tube rupture accident is studied. The objective is to develop a realistic model which may be used to examine the rupture flow of primary coolant and the transport process of leaked primary coolant in the steam generator.;The droplet size is determined from a boiling breakup model. Calculations indicate that droplets have a mean size of around two microns under the pressure drop of tube rupture.;Variation of droplet size is controlled by condensation and evaporation. Calculations show that droplets tend to grow by condensation due to high boron content because of low vapor pressure on droplet surfaces. The droplets can reach their equilibrium sizes within 4 seconds.;The capture of droplets in the steam generator is calculated by considering the following four mechanisms: inertial impaction, Brownian motion, gravity settling, and interception. However, bubble scrubbing is the most important mechanism which can eliminate 50% of flashed droplets for ruptures occurring near the tube sheet.;The flashing flow which occurs in tube rupture is examined by a modified isentropic streamchannel (streamtube) model. Calculations show that the primary coolant discharges to the secondary side with an average flow rate of around 70 lb/sec in the early stage of accident.;Estimated radioiodine release from the model is first compared with past accident data. The result indicates that NRC staff and the licensee may have underestimated the actual releases in the Prairie Island steam generator tube rupture accident. Results also show that a tube rupture without offsite power available may yields a radioiodine release of 8 times larger than the value given in the SAR analysis.;Possible modification of recovery operation and equipment design are recommended in order to mitigate the radioiodine release. They are (1) trip reactor as soon as possible, (2) maintain reactor coolant pumps operation, (3) depressurize primary system as quickly as possible, (4) maintain feedwater flow properly, (5) actuate safety injection to draw borated water sooner, (6) do not isolate the faulty loop too early, and (7) modify the steam dryer design to include horizontal zig-zag plates. |
