Computational modeling of accelerating corrosion in primary coolants of a typical PWR
High temperature and pressure environment along with large neutron and gamma-ray flux lead to significantly large corrosion in the stainless steel used in the primary coolant circuit of pressurized water reactors (PWRs). The corrosion products become activated in the reactor core upon circulation. Corrosion rate depends on reactor power and it can be a linear as well as a nonlinear function of time. Computer simulation offers a novel approach to study the effect of corrosion acceleration on coolant activity under transient conditions. We present two related mathematical models for corrosion acceleration under high temperature and flow rate transient conditions. In the first part of the work, we have formulated a mathematical model based on a system of coupled first order ordinary differential equations governing the isotopic concentration. In the second part, flow rate transients have been forced through non-homogeneous conditions. Numerical simulations show that for Na-24, Mn-56, Fe-59, Co-60 and Mo-99, the values of specific activities approach the equilibrium value fairly rapidly under normal operating conditions. Spikes are observed in the specific activity values of the primary coolant when corrosion acceleration and flow rate transients are employed in the model. Both the height and time of these spikes has been found to depend on the value of acceleration in corrosion.
Measurements of radioiodine species in samples of pressurized water reactor coolant
Samples of primary coolant at two pressurized water reactors in the United States have been collected and analyzed to determine the fraction of the radioiodine present in volatile forms. The volatile species, I{sup 2} and organic iodides, would be available for prompt release following a steam generator tube rupture (SGTR) accident, which overpressurizes the secondary coolant system and causes venting to the atmosphere. Coolant samples were collected at full power, during power reduction at the start of an outage, and up to 48 h after shutdown. Radioiodine concentrations spiked as power was decreased, and an increase in the volatile species fraction to {approximately}20% was seen with 2 h of shutdown. A second peak of 30 to 40% volatile species occurred at {approximately} 1.5 days after shutdown, but is not significant for SGTR accidents.
Effect of iodine concentration on the efficiency of activated charcoal adsorbers.
Activated charcoal adsorbers are widely used in the nuclear energy industry to remove halogen vapors from gaseous effluents. Because efficiency tests conducted on several setli of adsorbers installed in the emergency ventilation system of the SAFARI-I reactor yielded unsatisfactory results, a rig for testing individual units was constructed. Using 1311-labelled iodine vapour, several independent efficiency tests were conducted on sixteen different adsorber units. Only six of seventy-four results exceeded 99% and efficiency apjpeared to decrease sharply as upstream iodine concentration decreased. Therefore, intensive tests over an upstream iodine concentration range of 0.3-300 pg I,{m3 air were conducted on one unit only. Within the range of variables tested, only the concentration seemed to affect the decontamination factor. Further tests followed and attempts to detect methyl iodide either before or after the adsorber units by chemical analysis have so far failed. Tests generating methyl iodide in concentrations from 0.1 to 2000 pg/m3 showed that the efficiency of both impregnated and unimpregnated adsorbers was independent of concentration. Tests conducted on twelve new KI impregnated adsorben displayed the same concentration dependence as the non-iodized units. 'The implications of this important result are discussed in relation to adsorber testing and the iodine concentrations that are likely to arise following an accidental iodine release. INTRODUCTION ACTIVATED charcoal adsorbers are widely used in the nuclear energy industry to remove halogen vapors from gaseous effluents. They were found to be more satisfactory than any of the other available iodine removal systems tested at Hanford.'l) Because of their importance in the event of an emergency, the characteristics of commercially available activated charcoal adsorbers have been extensively investigated by Adams and co-workers at Oak Ridge(2-4) and I t has also become standard practice for each installation to carry out in situ efficiency tests on its high efficiency particulate (so-called absolute) filters and activated charcoal adsorbers, mainly to check the installation of these units against their frames and gaskets. Usually, !some predetermined minimum acceptable value for the efficiency is set, say 99 % or a decontamination factor of 100. The ORR-type Safari-1 research reactor of * Present address : Biology Department, BattelleNorthwest, P.O. Box 999, Richland, Washington 99352. the South African Atomic Energy Board at Pelindaba had been operating at a power of 6$ MW since March, 1965. No in situ filter or adsorber tests were conducted since no appropriate sampling points had been provided in the filter pits. These were installed when the reactor was shut down in March, 1968, for its power level to be raised to 20 MW (Fig. 1). Using a standard method involving a TDA Associates DOP smoke generator and detector, the absolute filters were tested and found to more than meet specifications (>99.97 % efficient for 0.3 pm diameter spheres), thereby proving that their seating was adequate. The activated charcoal adsorber tests took a quite different course, leading to the results presented in this paper. EXPERIMENTAL METHODS Eficiency calculations If C, , = concentration before filter C,, = concentration after filter