Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
the beginning of the corium‐concrete interaction ‐ owing to the very large amount of
turbulence of the corium‐concrete mixture – fission products can still be released, by
boiling or vaporization effect.
16.5.2.2. Fission product behaviour in the primary system and containment
After a core or corium release, fission products (in gas or liquid form) travel through the
reactor coolant system and the reactor containment or escape paths, such as the nuclear
auxiliary building, where they can either remain for some time before escaping to the
environment, or can become temporarily or permanently deposited. The fission products
behaviour along their path to the environment is governed by important physical
phenomena that are specifically related to the thermal hydraulics conditions in the
primary system and the thermodynamics of the containment.
Release from fuel:
The first important macroscopic phenomenon is obviously the release of core or corium
fission products. The important factors are the kinetic release rates and the physical and
chemical species formed. The latter depend on the atmosphere prevailing in the vicinity
of the core (temperature, hydrogen formed by the zirconium‐water reaction and reactions
with products resulting from structure vaporization, especially silver).
Modern severe accident fission products release models include empirically based fission
product inventory escape rate and fractional release models. These models introduced in
computer codes are validated against available experimental programs. Generally, at high
temperatures (above 2000°C), the release from the fuel for the volatile elements depends
almost entirely on the temperature and the time spent at those temperatures. The
surrounding atmosphere, oxidizing or reducing, are of some importance for the chemical
form but not so much for the released fraction. The volatile elements, that are mostly rare
gases, iodine, caesium, tellurium and strontium, are completely released from the fuel,
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