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Regional Basic Professional Training Course in Korea

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<strong>Regional</strong> <strong>Basic</strong> <strong>Professional</strong> <strong>Tra<strong>in</strong><strong>in</strong>g</strong> <strong>Course</strong> (BPTC) on Nuclear Safety<br />

8.4. SELECTION OF INITIAL AND BOUNDARY<br />

CONDITIONS<br />

There are a number of <strong>in</strong>put parameters for analysis, correspond<strong>in</strong>g to the NPP status<br />

prior to the accident, which have a major <strong>in</strong>fluence on the results; they are often called<br />

key parameters. Among them, plant <strong>in</strong>itial conditions and possible bound<strong>in</strong>g values for<br />

some process variables play an important role. Initial and boundary conditions have to be<br />

selected, either realistically or conservatively (depend<strong>in</strong>g on the approach selected), from<br />

a range depend<strong>in</strong>g on the plant operational mode and on the parameter uncerta<strong>in</strong>ties.<br />

These uncerta<strong>in</strong>ties <strong>in</strong>clude technically acceptable tolerances, calculation uncerta<strong>in</strong>ties or<br />

measurement <strong>in</strong>accuracies.<br />

A typical set of <strong>in</strong>itial conditions, to be specified for thermohydraulic and neutronic<br />

calculations, is shown <strong>in</strong> Table 8‐3. For each parameter, the conservative direction<br />

(maximum/m<strong>in</strong>imum) is <strong>in</strong>dicated for two basic types of analysis: for conservative<br />

analysis of core cool<strong>in</strong>g (lead<strong>in</strong>g to a low value of the DNBR) and for conservative<br />

analysis of pressure <strong>in</strong> the primary system (lead<strong>in</strong>g to a high value of the system<br />

pressure).<br />

The results of an analysis depend on the neutronic characteristics of the core, which<br />

determ<strong>in</strong>e the reactor power behavior dur<strong>in</strong>g the course of accidents. The conservative<br />

approach typically aims to overestimate the reactor thermal power, which is strongly<br />

dependent on reactivity feedback coefficients and on the change of core parameters.<br />

Reactivity feedback depends on the direction of the change (<strong>in</strong>crease or decrease) of the<br />

parameter under consideration. The direction may change dur<strong>in</strong>g the course of the accident,<br />

and therefore the <strong>in</strong>fluence of feedback coefficients may also vary dur<strong>in</strong>g the process.<br />

❙ 482 ❙

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