Steam accident sequence

In the different reactor risk studies performed up to now, a number of potential accident sequences have been analyzed in detail. The conditions prevailing in the different stages of such an accident have been described in numerous publications, with regard to KWU-designed plants, for example, by Hassmann et al. (1987). These physical and chemical conditions (such as temperature, pressure, flow characteristics, hydrogen-to-steam ratio) vary widely between the different postulated accident sequences, influencing the release and transport behavior of the fission products. In the following sections only those parameters will be discussed which are of importance for fission product chemistry and behavior. 

The hydrogen produced is transported together with the steam to the primary circuit and, finally, to the containment atmosphere. The total amount of hydrogen released to the containment in the course of a low-pressure accident sequence is schematically shown in Fig. 7.3. (GRS, 1989). As an important consequence, reducing conditions prevail along the entire path of fission product transport from the overheated reactor core to the containment, the consequences of which for fission product chemistry will be discussed later on. 

In case the steam contains significant amounts of hydrogen (which applies to all LWR accident sequences), this element will rapidly react with elemental iodine to form hydrogen iodide HI. According to Sullivan (1967), in the iodine concentration range from 10 to 10 g-atom/1, the reaction mainly follows the equation... 

In severe accidents in PWR s, the availability of boric acid vapor and aerosols for reactions with CsOH and Csl depends on the particular accident sequence (see Section 7.3.2.3.I.). These reactions are of greater signiflcance in the high-pressure accident sequence, since in this case the boric acid inventory of the primary coolant and the emergency coolant solutions becomes concentrated in the residual water volume inside the reactor pressure vessel because of the considerable volatility of boric acid at a pressure of 16 MPa, this compound will be partly volatilized with the steam. In contrast, in low-pressure accident sequences most of the primary coolant boric acid inventory will be ejected during blowdown and, thus, not be... 

Not only does the fraction of the fission product core inventory reaching the containment depend on the particular accident sequence, the same is true for the chemical forms of the fission products, which result in part from reactions within the primary system, as has been discussed in Section 7.3.2. However, when fission products are transported from the high-temperature reducing conditions of the primary system to the lower-temperature, predominantly oxidizing and condensing steam conditions of the containment, their chemical forms may change again simultaneously, fission products deposited on the surfaces of bulk material aerosols could be resuspended due to the formation of more volatile species. 

In the gas—steam flow which enters the containment in the low-pressure accident sequence, maximum aerosol densities in the range of 20 g/m may occur. When the core — concrete interaction begins, about 1 to 3 Mg of aerosols in total are assumed to be present in the containment atmosphere, according to corresponding calculations. 

Concerning the transport of fission products out of the containment, the relative locations of the break in the primary system and of the open penetration in the containment steel shell are of great importance. It can always be assumed that these two positions are in different compartments so that the fission products have to travel a certain distance over which deposition may occur moreover, the time required to cover this distance provides an opportunity for chemical reactions to occur, in particular with respect to iodine. This time delay depends highly on the specific accident sequence, in particular on the rate of pressure increase within the containment and, thus, on the steam mass flow from the primary system to the containment. With a high degree of probability it can be assumed that the open penetration can be locked (e. g. by manual action) within a comparatively short time. 

The heat transport from the core melt into the sump water, as well as the production of permanent gases in the core melt - concrete interaction, will result in a pressure increase inside the closed containment. Provided that no measures for controlled depressurization are undertaken (see next section), then, according to the results of thermodynamic calculations, the steam-gas pressure within the containment would reach the postulated failure value of the containment steel shell of about 0.9 MPa after 5 to 10 days, depending on the accident sequence. Such an overpressure failure will not be a catastrophic burst of the shell, but rather the enlargement of the operational leaks to a size permitting the escape of gas and steam at a rate high enough to keep the pressure inside the containment at a... 

Because the plutonium-burning reactor proposed in this report is assumed to use a metal or oxide fuel, (such as Pu-Al, Pu-Zr02, or Pu-ZrH).6) the potential for an energetic steam explosion is of some concern, provided an accident sequence can be identified that leads to large quantities of molten fuel and cladding. The purpose of this section is to discuss some of the steam explosion concerns involving aluminum-water and zirconium-water in relation to the proposed low power density, low flow plutonium-burning reactor. 

Development of accident sequence models for SLP PSA requires a close co-operation between plant personnel who are familiar with an outage and PSA analysts to assure that the all possible scenarios are appropriately modelled. Available accident mitigation measures may be much broader that for the power PSAs. The systems and the plant features which have been credited in power PSA may not be available for shutdown mode (as an example, heat removal using steam generators). The development of sequences should be an iterative process to adequately model sequences which represent actual plant configuration. 

In the Zion station blackout accident sequence, steam is discharged from the primary system at the relief valve set point of 2500 psig. The active core height is 12 ft. The area of the core occupied by water is 53.4 The core decay power during boiloff is approximately 32.5 MW. Estimate the time required for the water level to decrease from the top of the active core to the core midplane. 

Results from MELCOR (Version 1.8.0) calculations of three accident sequences in a W-PWR 900 Mwe three loop plant are presented. The scenarios considered include an AB sequence and two V type events a rupture of the Low Pressure Coolant Injection System in the auxiliary building, and the rupture of ten steam generator tubes in all cases without the intervention of the active emergency core cooling systems. Emphasis is put on the release and transport of core materials. It has been found that deposition of vapors from the most volatile species is high within the core structures. Later in the accident, revaporization induced by decay heat takes place, at times in coincidence with the production of steam due to core slumping, what may change the nature and composition of source terms. 

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