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Regional Basic Professional Training Course in Korea

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❙ 939 ❙<br />

16. In‐Plant Accident Management<br />

whereas the low volatile elements exhibit a more complex behaviour depend<strong>in</strong>g on the<br />

temperature and surround<strong>in</strong>g atmosphere <strong>in</strong> the reactor vessel.<br />

The low volatile groups are not radiologically significant <strong>in</strong> general. The most significant<br />

element is Ruthenium that is allocated to the Molybdenum group.<br />

Fission products transport and retention <strong>in</strong> the primary system:<br />

With regard to the second macroscopic phenomenon, fission product transport by gas or<br />

liquid, the <strong>in</strong>fluenc<strong>in</strong>g parameters are obviously the follow<strong>in</strong>g: thermal‐hydraulics, the<br />

route taken, the traps and obstacles met <strong>in</strong> the primary system (and conta<strong>in</strong>ment). The<br />

determ<strong>in</strong><strong>in</strong>g factors are the retention fraction <strong>in</strong>side the <strong>in</strong>stallation, the transit time to the<br />

environment and the time scale of the source term. This phenomenon is generally<br />

considered to have an important impact on the source term. Concern<strong>in</strong>g the fission<br />

products <strong>in</strong> a solid particle state, called aerosol form, high retention factors are expected<br />

<strong>in</strong> the primary circuit zones with a high surface to volume ratio such as steam generators.<br />

Plant calculations and available experiments <strong>in</strong>dicate that significant retention fractions<br />

are expected due to the retention mechanisms (50% to 70% of the <strong>in</strong>itial release from the<br />

core) that are pr<strong>in</strong>cipally diffusiophoresis, thermophoresis, gravitational settl<strong>in</strong>g. The rare<br />

gases and gas form of other elements are not reta<strong>in</strong>ed at all.<br />

Fission products transport and retention <strong>in</strong> the conta<strong>in</strong>ment:<br />

The "<strong>in</strong>‐conta<strong>in</strong>ment accident source term" will normally be a function of time and will<br />

<strong>in</strong>volve consideration of fission products be<strong>in</strong>g released from the core <strong>in</strong>to the<br />

conta<strong>in</strong>ment as well as removal of fission products by plant features <strong>in</strong>tended to do so<br />

(e.g., spray systems) or by natural removal processes.[5]<br />

The route taken depends on the accident sequence be<strong>in</strong>g considered. For example, the<br />

reactor conta<strong>in</strong>ment can be totally by‐passed <strong>in</strong> the case of an SGTR – and this has an<br />

important <strong>in</strong>fluence on the source term <strong>in</strong> the environment. Likewise, the radiological

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