Thermohydraulic consequences of a power excursion resulting from a hypothetical reactivity accident
Conference
·
OSTI ID:400854
- Forschungszentrum Juelich GmbH, Juelich (Germany)
For power control and shutdown, the German research reactor (FRJ-2) is equipped with six coarse control arms, which are raised and lowered by angular movement around a pivot. A failure of the support mechanism would allow an absorber arm to swing out of the core causing a severe reactivity accident, if the safety instrumentation or two independent scram systems fail to operate in time. The frequency of such an event would be in the order of 10{sup {minus}8} per reaction year. The consequences of the accident with a reactivity insertion of 2.27% dk/k (at 23 MW operating condition) and +2.70% dk/k (at startup condition) without reactor scram were studied using the CATHENA code. The analysis shows that as a result of power excursion the flow regime is changed to inverted annular flow within a short period of time causing a rapid rise of fuel temperature. Because of the limited vapor generation, the feedback mechanism is not able to stop melting of the fuel, which occurs in all fuel elements with a maximum time difference of 63 ms between the elements. In the first case, the rate of temperature rise before reaching the power peak (989 MW) amounts of 10.9 C/ms which remains significantly below the destructive SPERT experiment. Due to the lower initial coolant temperature at the startup condition, the evaporation process does not take place so that heat generation, melting and damage to the fuel elements occur faster than in the previous case. As a result of the interaction of molten fuel and coolant a steam explosion cannot be excluded. However, the violence and the damaging effect may be smaller than in the case of the destructive SPERT experiment.
- OSTI ID:
- 400854
- Report Number(s):
- CONF-950828--
- Country of Publication:
- United States
- Language:
- English
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