Large accident sequence

In any case, like frequency analysis, examining the uncertainties and sensitivities of the results to changes in boundary conditions and assumptions provides greater perspective. The level of effort required for a consequence analysis will be a function of the number of different accident scenarios being analyzed the number of effects the accident sequence produces and the detail with which the release, dispersion, and effects on the targets of interest is estimated. The cost of the consequence analysis can typically be 25% to 50% of the total cost of a large QRA. 

Despite the large number of possible sequences, as suggested by Fig. 10.1, modelling a limited number of phenomena is sufficient. In order to assess accident consequences the models are used with different initial and boundary conditions and combined differently. This enables one to simulate a large variety of accident sequences. 

From the plethora of potential accident sequences a few are selected which determine to a large extent the accident sequences. In particular the following models are used ... 

It is surprising that this issue does not receive much attention in the field of safety studies. Probably, this is due to the fact that a limited overcoming (even 10 times the specifications value) has a small effect on the result of the risk analyses (usually dominated by very unlikely but very catastrophic accident sequences, involving a large break in the containment). 

Event-tree models in a level 2 PSA mainly account for physical/chemical phenomena. Details of the phenomena and their consequences are reduced to a rather coarse grid of time (i.e. early , late or before , after ), position (i.e. top , bottom ), magnitudes (i.e. small , medium , large ), etc. This simphfi-cation involves the risk, on the one side, that accident sequences characterized by specific details in the timing or in process conditions remain unknown, and. 

INSAG-3 Rev. 1 [4], rather than probabihstic safety criteria, states the following objective for future nuclear power plants Another objective for these future plants is the practical elimination of accident sequences that could lead to large early radioactive release, whereas severe accidents that could imply late containment failure would be considered in the design process with realistic assumptions and best estimate analysis so that their consequences would necessitate only protective measures limited in area and in time. ... 

These papers provide the calculated results of source terms for a large number of accident sequences and illustrate the dependencies of the severe accident source terms on the accident sequence. 

Such interactions can only occur, however, when the volatile fission products and the primary aerosols appear simultaneously in the primary system, in spite of the large differences in their volatilization behavior. As was discussed above, uniform thermal-hydraulic conditions do not prevail within the reactor core during a severe accident (for example, the peripheral fuel rods may fail relatively late in the accident sequence, at a point when a large part of the central rods may already be molten) and it can be assumed that the broad time-envelope of significant release of structural aerosols will encompass the release of the volatile fission products. However, as was mentioned in Section 7.3.1.2., the amount of primary aerosols formed and the timing of their formation depend highly on the specific accident sequence this is particularly true for the control rod materials. 

The end-state of each path on the containment event tree describes the effectiveness of the containment to mitigate offsite doses for that accident sequence. The radiological consequences of the core-melt accident are largely determined by three major considerations ... 

Use an advanced control room to simplify construction, maintenance and operations. Improved human factors considerations reduce the chance of operator error during an event or accident sequence compared to the large control rooms used in currently operating plants. 

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. 

This model has had a very large influence on the development of schemes for the classification of accidents applied in many countries. Classification is used to standardise the collection of data on accidents and to reduce the complexity of the data to a manageable level for statistical purposes. An early and important example is the American National Standards Institute s system for the classification of accidents, ANSI Z16.2. In this system, the following facts are recorded about the accident sequence ... 

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