Containment sump water

The purpose of this question was to find out whether any additional iodine mitigation measures have been planned or already implemented other than using additives in the spray and containment sump water and using controlled containment venting with a filter. Other than these measures, if already implemented, there are no other mitigation measures reported by the participating organizations. 

In PWR plants equipped with both a cold-leg and a hot-leg injection of the emergency coolant, a fraction of the fission products released from the failed fuel rods will be washed down by the downward water flow. Thus, it will be transported back to the water phase inside the reactor pressure vessel and, finally, to the containment sump water. Since the extent of this type of retention of fission products depends strongly on the contact time between the steam flow and the downward flow of the liquid emergency coolant, it is only difficult to quantify. It can be assumed that Csl (and other iodides) will be trapped almost completely in the water phase for this reason, a 90% retention of the halogens and alkalis and a 99% retention of the so-called solid fission products has been assumed in the German Storfall-Berechnungsgrundlagen . For the h fraction in the steam flow a similar degree of washout can be expected experiments performed under conditions similar to those in the relevant LOCA period have yielded h washout fractions of about 92% at 25 C and about 96% at 85 °C water temperature (Kabat, 1980). 

According to the qualitative results obtained from informal Sascha experiments (Albrecht, 1987 b), silver is largely vaporized under the conditions of the molten core — concrete interaction. This means that any silver not volatilized during the in-vessel phase would become airborne in this late stage of the accident and be transported to the containment sump water phase, i. e. at a moment when volatilized iodine would have been largely plated out in the containment sump water. 

In general, it can be assumed that the reaction between silver and iodine species in the gas phase, as well as the reaction of iodine vapor with silver aerosol or with silver deposited on the primary circuit surfaces, is only of minor significance for iodine behavior in the course of a severe accident. The main reasons are the rather short residence time of the silver aerosols in the gas phase, the fact that iodine and silver volatilization from the reactor core may differ considerably over time and, finally, the small proportion of elemental I2 and of HI (compared with the Csl fraction) assumed to be present in the gas phase during transport through the primary circuit. In contrast, Agl formation is expected to proceed to a significant extent later on in the containment sump water (see Section 7.3.3.3.3.). 

In the preceding section, iodine chemistry and the iodine partition coefficient in a pure iodine solution have been discussed. The containment sump water which is to be expected in a severe reactor accident, however, may contain a number of other substances that have a potential impact on the chemical reactions and on the resulting reaction products. It is not possible to give detailed and trustworthy information on the nature and the concentrations of all these substances, which in addition might be different in different accident sequences and in different plants therefore, only some of them can be treated here exemplarily. The particular case of the presence of an excess of 1 in the I2—H2O system and its implications for the iodine species distribution and for the partition coefficient have been discussed in the preceding section. 

In addition, the radiolytic I2 yield depends on the radiation dose applied. As can be seen from Fig. 7.34. (Bums et al., 1990), the I2 yield increases with increasing radiation dose until it reaches an equiUbrium value, at which I" oxidation by the OH- radical is compensated for by I2 reduction due to the action of Caq, Hand H2O2. With increasing temperature of the solution, the I2 equilibrium level decreases considerably, presumably due to accelerated I2 hydrolysis and disproportionation. At still higher radiation doses (which, however, will be reached in the containment sump water within a rather short time) the I2 equilibrium level decreases again, due to the reduction of the oxidizing OH- radical by H2 to the reducing H- radical (see Fig. 7.35.). 

In the experiments reported by Lucas (1985) the influence of additives to the solution and the gas phase on radiolytic oxidation of 1 was studied. At a composition of the test solution which was claimed to be quite similar to that assumed for the containment sump water, irradiation was carried out using a dose rate of 4 kGy/h. The results showed that in a closed vessel an h saturation concentration is reached at an integrated radiation dose of about 60 kGy the presence of CO2 in the gas phase resulted in an increase of the saturation level to about 20% I2 formation, while the presence of H2 did not significantly affect the results. By contrast, in an open system where I2 evolved from the test solution was trapped in a NaHCOs solution, no saturation I2 yield was obtained under such conditions, the I2 yield increased linearily with the dose rate, reaching about 55% at an integrated radiation dose of 0.24 MGy. 

Trace contaminants present in the solution may also intensify or reduce the extent of h production considerably by reaction with the intermediate products of water radiolysis. Such traces of metals (e. g. iron, copper) may be unintentionally present in the test solutions of laboratory experiments as weU as, at even higher concentrations, in real containment sump water. The widely unknown nature and concentration of trace ingredients in the sump water is one of the main problems in the application of the laboratory results on radiation-induced iodine reactions to the situation prevailing in a severe reactor accident. 

Control of the pH in the containment sump water post-accident is achieved through the use of four pH adjustment baskets containing granulated trisodium phosphate. The baskets are located below the minimum post-accident flood up level, and chemical addition is initiated passively when the water reaches the baskets. The baskets are placed at least a foot above the floor to reduce the chance that water spills in containment will dissolve the trisodium phosphate. These baskets are sized such that they provide the correct quantity of trisodium phosphate to maintain the pH of the containment sump water in a range from 7.0 to 9.5. 

The pH of the aqueous solution collected in the containment sump after completion of injection of containment spray and ECCS water, and all additives for reactivity control, fission product removal, or other purposes, should be maintained at a level sufficiently high to provide assurance that significant long-term iodine re-evolution does not occur. Long-term iodine retention is calculated on the basis of the expected long-term partition coefficient. Long-term iodine retention may be assumed only when the equilibrium sump solution pH, after mixing and dilution with the primary coolant and ECCS injection, is above 7 (Reference...). This pH value should be achieved by the onset of the spray recirculation mode. 

The production of the key reactant, the hydroxyl radical ( OH), increases with increases in the radiation dose rate to the water. The dose rate to the water depends, of course, on the total amount of fission products and other radionuclides that escape the core and enter the containment. Dose rates to sump waters will vary depending on the type of reactor, the type of accident and with time following an accident. Typical values will be in the range of 1 to 20 kGy hr. ... 

A metal with great affinity for iodine is silver. The importance of the reaction of metallic silver with iodine was first demonstrated in the LOFT FPT-2 test. Renewed attention to the reaction of molecular iodine with silver has arisen based on the results of the PHEBUS FPTO and FPTl tests in which silver-indium-cadmium control rod materials were released into the containment model of the test facility. Iodine appeared to react with this silver to form water-insoluble silver iodide (Agl). Precipitation of Agl appeared to control the behaviour of iodine in the containment model. Though details of the interactions of molecular iodine and iodide ion with metallic silver are still being investigated, it appears that silver in contact with water can be a very effective adsorber of iodine even at low values of pH. Deliberate addition of silver into containment sumps to augment any silver released to the containment by accident processes might be used to manage the iodine souree term. For silver to retain iodine, the surface must remain immersed in water. Silver iodide on a silver surface exposed to air is rapidly oxidised to form silver oxide with the release of moleeular iodine. Also, silver iodide may react widi sulphur compoimds in the atmosphere to form silver sulphide and release iodine. Finally, there is evidence that silver iodide is not stable to the beta radiation produeed by radioactive iodine. 

E. Krausmann, Y. Drossinos, A model of silver-iodine reactions in a light water reactor containment sump under severe accident conditions , J. Nuclear Materials, 264, 113, (1999). 

Plant name Type PWR Designer Op. date Thermal power (MWth) (core/total) Containment type Free volume (min/max) Sprays (borated water +-2500 ppm, except for Tihange 1 2700ppm) Qualified containment cooling (Doel) Containment sump chemical additives... 

Flow capacity (Injection phase/recirculatlon phase), chemical additive (if yes, what, concentration) BWRs contain UOH to maintain pH at 8-8.5, this will act as a chemical additive tc the sump water. PWRs trisodlumphosphate is added during the recirculation phase to increase pH and increase absorption of iodine... 

Two principal systems used to mitigate the effects of a 1/3CA are the safety injection system (SIS) [see CESSAR-DC, Section 6.3], and the containment spray system (CSS) [see CESSAR-DC, Section 6.5]. These systems utilize an In-containment Refueling Water Storage Tank (IRWST) as their source of water, which is the equivalent of the refueling water storage tank (RWST) and containment sump of a pre-ALWR plant. 

The IRWST performs additional functions beyond those of the conventional containment sump. The IRWST for System 80+ Standard Design provides a single source of water for both the safety injection and containment spray pumps. The IRWST is toroidal in shape and utilizes the lower section of the spherical containment as its outer boundary. The IRWST is enclosed to prevent contamination and excessive containment humidity. 

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