Environmental Engineering Reference
In-Depth Information
Time, years
6.5
Time dependence core damage frequency for the case of 'large-
break LOCA. 1) CDF as a function of time; 2) CDF (basic variant).
6.2.2 The effect of ageing on core damage frequency
(CDF)
Figure 6.5 shows the effect of ageing of the components and the core damage
frequency. Calculations, carried out using the reliability models, dependent
on time, show the monotonic increase of the core damage frequency from
the value of 6.58 · 10
-8
1/(reactor · year) in the initial operation period (10
years) up to 6.19·10
-7
1/(reactor · year) to the end of the operating life (40
years). In comparison with the basic calculation variant (7.18 · 10
-8
), the
cored damage frequency 8.6 times up to 40 years of operating life.
Table 6.5 shows the results of calculation of the failure consequences and
the distribution of risk in relation to the position of the leak and the state of
the reactor. As in the basic calculation variant (section 6.1), the dominant
failure sequence in all ESs is the scenario with the failure of the systems
for low-pressure safety injection ES3. However, its the contribution to the
total core damage frequency in the presence of large leaks decreases with
time. For example, after operation for 40 years, the given AS contributes
approximately 56% of the total core damage frequency, whereas in the basic
calculation variant it is more than 75%. The contribution of the failure
consequences with the failure of the spray system (ES2) at the end of the
operating life increases to 60% (33% in the basic variant).
Since the components characterised by the highest sensitivity to ageing
(the electric drives of the pumps and level sensors) are included in the
minimum sections of the dominant failure sequences (section 6.1), the
distribution of risk in relation to the state of the reactor and the position
of the leak changes only slightly with time.
The analysis of the minimum sections shows the time dependence of
the significance of the failure of components of the systems and erroneous
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