Iodine condensing-steam conditions

Upon contact of gas-phase I2 with stainless steel surfaces, a deposited amount of iodine on the order of 0.1 mg /cm was measured, whereas comparable experiments with liquid I2 solution did not result in a measurable deposition of iodine, apparently due to the rapid dissolution of the metal iodides formed (Deane and Marsh, 1990). From the earlier desorption experiments performed by Rosenberg et al. (1969) it was assumed that FeU is the main primary product of this reaction, but that parallel formation of NiU and/or Cris cannot be ruled out all these compounds are readily soluble in water. Experiments using tubes made of steel 1.4541 (Funke et al., 1994) showed an I2 deposition rate constant in dry air at 120 °C of about 6 10" m/s. Under condensing steam conditions at the same temperature a considerably higher value of 1.4- 10 m/s was measured, indicating that under such conditions the U-U conversion on steel surfaces is a fast process. Analysis of the surfaces after completion of the test yielded only a small iodine retention in the condensed steam no iodate could be detected, indicating that no unreacted I2 had passed through the reaction tube and that only metal iodides had been... 

The benefits of containment spray systems regarding removal of airborne radionuclides from the atmosphere are not undisputed. Condensing steam in the atmosphere as well as reactions with the walls ensure an effective removal of elemental iodine, whereas organoiodides, as far as they are produced under the prevailing conditions, are decomposed by ionizing radiation. Thus, even without the action of containment spray systems, radioiodine is removed from the containment atmosphere rapidly and effectively by passive reactions. 

Due to its different chemical forms present in the containment atmosphere, fission product iodine shows a complex behavior in these areas as well. Csl carried by water droplets or by aerosol particles will behave in the same manner as do the other aerosols. The volatile iodine species I2 and CHjI will be distributed between the newly formed sump water and the atmosphere of the annuli, as is schematically shown in Fig. 7.45. Experiments have demonstrated that the extent of I2 plate-out depends on the degree of steam condensation, amounting to about 90% at a condensation temperature of about 40 °C (see Section 7.3.3.4.2.). The newly formed sumps in the annuli and in the auxiliary building show a pH of about 7 (since they consist solely of condensed steam), and temperatures between 50 and 80 °C this means that I2 plated out into these liquid phases is not only instantaneously hydrolyzed to I and HOI, but will also disproportionate rather quickly under formation of lOs . As can be seen from Fig. 7.24., under such conditions the disproportionation equilibrium will be reached within a comparatively short time. If one assumes very conservatively an initial I2 concentration in the sump water of about 1 mg/1 and a temperature of 100 C, then it can be derived from Fig. 7.24. that the... 

Lin (1983) also reported on a detailed investigation of the distribution of iodine volatilized from the reactor water in BWR water—steam circuits with forward-pumped heater drains. In this plant design, only the main condensate is directed to the condensate polishing system, whereas the different drains from the turbine are fed directly back to the feedwater tank. Under such plant conditions, about 75% of the iodine carried by the main steam flow is precipitated together with the condensate in the moisture separator downstream from the high-pressure part of... 

According to experiments performed under appropriate conditions, about 1% of the fission product iodine present in the flashed primary coolant volume is transported with the steam phase, about half of it in form of aerosols (Aim and Dreyer, 1980). This figure agrees well with that reported by Morell et al. (1985) and Hellmann et al. (1991), obtained in flashing experiments from small-diameter pipes (see Section 6.2.2.). In the experiments of Aim and Dreyer (1980), which were carried out in an uncoated steel vessel, only particulate iodide and elemental h were detected in the atmosphere of the vessel. The rate of plate-out of iodine from the atmosphere was found to depend on the specific geometric conditions of the experimental setup. In a comparatively small vessel, deposition halftimes of 8.5 hours for h and of 0.9 hours for aerosol iodide were measured within the first two hours after blowdown these values increased to 9.9 and 5.7 hours, respectively, during the following 22 hours. From other experiments markedly different values have been reported (e. g. CSE tests, see Section 7.3.S.3.8.). The reasons for these differences are not only due to the dimensions and true surface areas present in the respective experimental facility, but also to various other parameters, such as initial concentrations, turbulences in the atmosphere, rate of water droplet plate-out and of steam condensation. 

Detailed investigations of the reaction of Csl with boric acid in the condensed phase over the temperature range 400-1000 C under Ar, Ar-H2, and Ar—Ha-steam atmospheres were performed by Bowsher and Nichols (1985). The results showed that Csl decomposition in these reactions starts at temperatures above 400 °C and increases considerably beyond 700 °C, with the HI produced being partly converted to I2 (and/or iodine atoms) by thermal dissociation. Under such conditions, HI as well as I2 may react with the iron and nickel content of stainless steels under formation of the corresponding iodides (see below). Up to 960 °C, Csl and molten boric acid or boron oxide react in a diffusion-controlled reaction the rate of which is determined by the diffusion of the partners to the reaction zone. The reaction data measured in these experiments were consistent with Arrhenius law, showing an activation energy of 190 30kJ/mol. 

Tentative boundary conditions for the containment involve two successive phases. During the first one the containment has an atmosphere temperature around llO C, a humidity ratio near 60 % and with steam condensation onto the condenser surfaces. For good scaling of the volume to surface ratio, the vessel walls are rendered "neutral", as far as condensation is concerned. Condensation, aerosol diffusiophoresis and FP-paint interaction take place on suspended structures called "condenser". V/hen the bundle and circuit transient are over, the circuit is disconnected aerosol deposition and subsequent operation of a washing system transfer FPs to me sump water. This second period duration is 3 days with an atmosphere temperature increase up to 150°C and superheated conditions. This period is devoted to Iodine radiochemistry. 

---