Environmental Engineering Reference
In-Depth Information
action of the personnel leading reactor core meltdown. For example, in ES3
ET LLOCA PO/CL (leak from a cold loop, reactor at power), the following
factors were most significant in the basic calculation variant;
• failure to open the reverse valves in the injection lines into the cold
loops (21% in the total risk of the core damage frequency);
• common cause failures of safety injection pumps (7% in the total
risk);
• failure of the control system in automatic transition to recirculation
through the sumps because of erroneous personal actions when
setting four relays (5% in the total risk of the core damage
frequency).
In the calculations taking into account ageing, these failures remain the
dominant contributors to the frequency of melting of the reactor up to the
operating life of the unit of 30 years. In this case, if the contribution of
failure due to malfunction in opening of the reverse valves in the injection
lines after the operating life of 10 years was 41%, common cause failure
of the emergency injection pumps, 90%, and the failure of the system of
controlling automatic transition to recirculation 10%, then after operation
for 30 years the contribution of these failures to the total risk equals 21,
18 and 5%, respectively.
However, after operation for 40 years, the list of the main contributors
greatly changes. The dominant contributors are the failures of the following
elements:
• failure of the electric drives of safety injection pumps (39% in the
total risk);
• failure of the electric drive combined with failure of the safety
injection pumps (13%);
• common cause failures of safety injection pumps (9%).
The same change in the main contributors to the total risk of melting
of the reactor core is also found for other dominant failure consequences.
The time dependence of the relative contribution of the failures of the
components to the core damage frequency was analysed using the factors
of significance of risk according to Fussell and Vesely (FV) and the factors
of the reduction and increase of the risk. Figure 6.6 shows the variation of
FV (fractional contribution).
According to the calculations, the most significant changes of FV took
place in the last decade, i.e., between 30 and 40 years in operation. These
changes are associated with the rapid increase of the probability of failure
of electric drives of the pumps.
Comparison of the values of the relative contribution to the risk, obtained
at the end of the operating life, with the values for the basic variant shows
different dynamics of FV for the components sensitive to ageing, and the
reduction of this parameter for the components not sensitive to ageing.
For example, for the RWST sensors (the components sensitive to ageing),
the value of this parameter, calculated for the operating life of 40 years,
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