Sump water basis accident

The low-pressure scenario which is initiated by a large-sized break in the primary circuit, that is an event similar to that of the loss-of-coolant design basis accident described in Section 6.2.1.. In the severe accident scenario it is additionally postulated that, after the action of the accumulators and the borated water storage tanks, the sump water recirculation pumps will fail to operate. Thus, the decay heat cannot be removed from the reactor core vnth the consequence that the water volume present inside the reactor pressure vessel (RPV) begins to boil off at about atmospheric pressure. The AB sequence of WASH-1400 describes such a large-break scenario. In this low-pressure scenario, the treatment of fission product behavior inside the primary circuit is comparatively simple the probability of occurrence of such an accident, however, is extremely small. 

One of the most important parameters controlling iodine volatility is sump water pH not only will the I2 hydrolysis equilibrium and the iodine partition coefficient be affected by this parameter, but the product yields of radiolytic reactions and the extent of formation of organoiodine compounds as well. Because of the lack of practical experience, the sump water pH to be expected under severe accident conditions has to be calculated on the basis of assumed concentrations of potential sump water ingredients. In Table 7.17. (according to Beahm et al., 1992) an overview of substances to be expected in the sump water, which would effect a shift in solution pH either to lower or to higher values, is given. Besides these chemical substances, radiation may also affect sump water pH irradiation of trisodium phosphate solution (5.3 kGy/h) was reported to decrease the pH from an initial value of 9.0 to about 4.0 after 60 hours of irradiation (Beahm et al., 1992). It is obvious that in such a complicated system definition of the sump water pH to be expected in a real severe reactor accident is a difficult task. Nonetheless, a model for calculation has been developed by Weber et al. (1992). 

Energy management features are incorporated that limit the internal pressures and temperatures within the containment envelope to values that are below the design limits for the containment system and the equipment that is needed inside the containment when a design basis accident occurs. Examples of energy management features include pressure suppression pools, ice condensers, pressure-relief vacuum-chamber systems, structural heat sinks, the free volume of the containment envelope, spray systems, air coolers, a sump or a suppression pool recirculation water-cooUng system, and the air extraction system for the annulus in double-containment systems. 

The purpose of the contaminated water removal and disposal system (CWRDS), i.e., the sump water removal system, is to remove and retain the emergency coolant or cooling water following a design basis loss of coolant accident (LOCA) or seismic event. This system operates in conjunction with the Airborne Activity Confinement System (AACS) to control the release of liquid and airborne contaminants. 

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 ice beds are more than adequate to limit the peak pressure from a design-basis loss-of-coolant accident. However, in a long-term accident, the ice will eventually melt and containment heat removal will be required. Thus, containment sprays are provided in the upper compartment of the containment. Water from the sprays drains through sump drain lines down into the lower compartment sump, where it can be recirculated for long-term heat removal. It is noteworthy that, because of the melting ice, there will be more water in the lower compartment during many accidents than would be present in a large dry containment. 

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