Containment sump additives

The containment sump should be designed to permit mixing of emergency core cooling system (ECCS) and spray solutions. Drains to the engineered safety features sump should be provided for all regions of the containment which would collect a significant quantity of the spray solution. Alternatively, allowance should be made for dead volumes in the determination of the pH of the sump solution and the quantities of additives injected. 

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. 

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. 

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. 

Plant Name Type Designer Operational date Thermal power (MWt) Containment Type > Free volume (m=) Sprays " Containment Sump chemical Additives Filters" Containment Venting filters... 

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... 

Plant Name Type Designer Operational Date Thermal Power MW(t) per reactor Containment Type Free Volume (m ) Sprays Containment Sump Chemical Additives Filters... 

The interfacing system LOCA (ISL) is presumed to result from exposing low pressure piping (design pressure 400-700 psi) of the interfacing system to high primary system pressure (about 2250 psi). The initial plant response to an ISL is the same as the response to an equivalent sized LOCA inside containment. However, RCS inventory is discharged outside containment and is not returned to the containment sump for recirculation. In addition, an ISL will provide a path, which bypasses the containment, for release of radioactive materials. 

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. 

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. 

In addition to the auxiliary boration system, alternative injection with the operational volume control system from the RWST or the containment sump into the primary circuit ... 

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. 

As the accident continued, the water and steam mixture leaving the open PORV had been collecting in the Pressurizer drain tank. About 15 min into the accident, the rupture disc on the pressurizer drain tank, which was designed to limit tank overpressure, blew and allowed additional radioactive water from this tank to run down to the containment sump (Kemeny, 1979). The water was periodically pumped from the sump into a storage tank of the auxiliary building, adjacent to the containment. 

A hydraulic system must have a reserve of fluid in addition to that contained in the pumps, actuators, pipes and other components of the system. This reserve fluid must be readily available to make up losses of fluid from the system, to make up for compression of fluid under pressure, and to compensate for the loss of volume as the fluid cools. This extra fluid is contained in a tank usually called a reservoir. A reservoir may sometimes be referred to as a sump tank, service tank, operating tank, supply tank or base tank. 

The process-control scheme contains loops for maintaining concentration of the absorbent (by manipulating the fresh FeEDTA2- feed), the pH and the Redox potential in the bioreactor (by addition of acid/base and ethanol, respectively). Other control loops maintain the liquid level in the sump of the absorber, and the level in the bioreactor. 

Sump Tank. The sump tank has a capacity of 100,000 gal. The primary functions of the sump tank are to receive the process water from the flash evaporators and to supply it to the process-water pumps. The tank normally contains a 3-min holdup of about 60,000 gal. This holdup is necessary for satisfactory operation of the process-water pumps. The additional 40,000 gal capacity of the tank provides for the accumulation of water in the event of failure of the process—water pumps. The sump tank also receives process water via an overflow from the seal tank. 

The equilibrium partitioning of iodine between the sump liquid and the eontainment atmosphere is examined for the extreme additive concentrations determined in Seetion ni.f.a.(2), in combination with the range of temperatures possible in the containment atmosphere and the sump solution. The reviewer should eonsider all known sources and sinks of acids and bases (e.g. alkaline earth and alkali metal oxides, nitric acid generated by radiolysis of nitrogen and water, alkaline salts or lye additives) in a post-accident containment environment. The minimum iodine partition coefficient determined for these eonditions forms the basis of the ultimate iodine decontamination factor in the staff s analysis described in subsection III.4.d. 

The reaction conditions used in the first phase (sump phase) are generally temperatures of 400 to 500 °C and pressures ranging from 100 to 700 bar. Molybdenum and tungsten oxides are commonly used as catalysts, together with iron compounds. In the /G-hydrogenation process, Bayer-mass ( red mud ), a by-product of bauxite processing, was used as an iron catalyst. Coals which contain mineral compounds with the necessary catalytic activity can be hydrogenated without the addition of catalysts. 

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