Reactor accidents, general

The risk to an average individual in the vicinity of a nuclear power plant of prompt fatalities that might result from reactor accidents should not exceed 0.1% of the sum of prompt fatality ri.sks from other accidents to which members of the U.S. population are generally exposed. ... 

This article does not cover the extensive research carried out to delineate the cause and severity of RPTs possible in nuclear reactor accidents. Although there is not universal agreement, the general consensus is that the superheated liquid concept can explain many of these events and, more importantly, can indicate when RPTs are unlikely to occur in hypothetical reactor accidents. 

Large-scale experimental releases of fission product activity are clearly ruled out because of the implications on the safety of the public, described in Section V,F, as are also the smaller scale releases referred to earlier in Section V. Therefore, for verification of our conclusions we have to rely on limited experience from those few accidents that have occurred and that have released fission products (see Section I,D), but much more on our store of knowledge of all the factors involved, i.e., types of reactor accidents (Section II) through fission product release (Section III) and dispersion of a release in the atmosphere (Section IV), to analysis of the radiation and radiobiological hazards and risks to exposed members of the population. In view of these several steps involved in the estimation of hazard, it is reassuring that the many different authors who have written on the topie reach conclusions which are generally similar and differ only in limited areas. 

A violent vapor explosion can result when a cold volatile liquid is suddenly brought into contact with a hot liquid. The explosion is due to the rapid vaporization of the cold liquid from heat transfer from the hot liquid. Such explosions are referred to as vapor, steam, physical, or thermal explosions rapid-phase-transitions (RPTs) (typically when referring to explosions involving cryogenic liquids) and molten fuel-coolant interactions (FCIs) when applied to nuclear reactor accidents. Accidental vapor explosions are frequent occurrences in the metallurgical, pulp and paper, and cryogenic industries. General reviews of the various aspects of vapor explosions can be found in Reid [1] and Corradini et al. [2]. 

This example demonstrates that any unnoticed drift of a system parameter may cause unpredictable changes of the reactor state which leads to increasing temperatures in the above example, but generally might cause a loss of production or even reactor accidents. Thus, a fault detection procedure is required to recognize a steady reactor state approaching a bifurcation point. 

A comparatively slow reaction between CsOH vapor and Inconel has been reported by Elrick et al. (1984), with the formation of a cesium silicate or aluminate as a reaction product being assumed. CsOH can also react with various other metals and metal oxides at high temperature, for example with zirconia at 1900 K to form cesium zirconate CS2Z1O3. These reactions indicate that significant retention of cesium can be expected to occur in the primary circuit during a severe reactor accident, since the compounds formed are generally less volatile than CsOH. 

The code Iode has been developed to calculate iodine behavior in a reactor containment and the auxiliary building it is part of the French computer system Escadre which, similar to the US Source Term Code Package, describes the whole sequence of a severe reactor accident. The general philosophy of Iode is to model the main phenomena which may influence the behavior of iodine in the reactor containment IS chemical reactions are modelled, concerning both the water phase and the gas phase (Gauvain et al., 1991). Similar to Impair, radiolysis is not described in detail but is taken into account over its global effect on iodine species. The kinetic data of the reactions were taken from the literature as far as inorganic iodine thermal reactions are concerned other kinetic data were compiled from the elaboration of the Impair 2 code. 

In many accidents, water will be present underneath the reactor vessel when the molten material exits the vessel at the time of failure. In other cases, water may be added on top of the molten material subsequent to vessel failure. It is generally considered axiomatic that water addition is always a good thing in a reactor accident. While current guidance to operators is always to add water, it is important to note that there are several different possible outcomes when molten core debris contacts water, and only some of these outcomes are desirable ... 

From the beginning of the conceptual study on supercritical water cooled reactors, several plant transient analysis codes have been developed, modified, and applied to them [1-9]. The general name of these codes is Supercritical Pressure Reactor Accident and Transient analysis code (SPRAT). SPRAT mainly calculates mass and energy conservations, fuel rod heat conduction, and point kinetics. The relation among these calculations is shown in Fig. 4.1. SPRAT can deal with flow, pressure, and reactivity induced transients and accidents at supercritical pressure. The flow chart is shown in Fig. 4.2. 

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