vgbe energy journal 5 (2022) - International Journal for Generation and Storage of Electricity and Heat

Operating experience from ageing events occurred at nuclear power plants
Tab. 2. Number of events/times per family.
Plant status N. events N. events Root cause N. events
Power operation 54 Absent Ageing Managemet Programme 12
Startup 7 Deficiencies in Ageing Management Programme 25
Hot standby 2 Deficiencies in maintenance or surveillance 55
Hot shutdown 2 Wrong material selection 15
Cold shutdown 12 Equipment specification, manufacture, storage and installation 18
Refuelling 20 Deficiencies in design 30
Other or Unknown 16 Other or unknown 4
Dectection of events N. events Ageing mechanism N. events
Periodic test / In service inspection 29 Swelling 1
Fault report in control room 65 Relaxation 3
Work surveillance 6 Fatigue 28
Supplementary inspection 4 Thermal ageing 13
Other or Unknown 9 Irradiation damage 2
Affected system N. events Corrosion 38
Primary reactor systems 41 Wear 15
Reactor auxiliary systems 12 Erosion 4
Essential service systems 1 Electrical ageing 15
Essential auxiliary systems 15 Creep 1
Electrical systems 24 Chemical ageing 1
Feed water, steam and power conversion systems 5 Other 13
I&C systems 5 Unknown 3
Service auxiliary systems 3 Consequences N. events
Structural systems 5 Degradation (damage) 28
Other 2 Deformation 12
Affected comp. N. events Embrittlement and cracking 36
Passive mechanical components 43 Material loss 30
Active mechanical components 38 Other or Unknown 7
Active mechanical components 10 Corrective Actions N. events
Active mechanical components 18 Equipment replacement or repair 112
Structural components 4 Monitoring and/or inspection improvement 39
Direct cause N. events Changes in operation modes 8
Mechanical failure 83 Changes in maintenance programme 50
Electrical failure 17 Changes in ageing management programme 18
I&C failure 9 Design modification 25
Structural failure 3 Other 2
Other 1
The analysis provides an Average Age of 28
years (331 months) with a large standard
deviation of 10 years (123 months) and a
median of 30 years (357 months). In other
words, on average, ageing related events occur
after 28 years from the start of reactor
operation.
Selected event reports have been characterised
according to the criteria defined for this
study: plant status, detection means, affected
system, affected component, direct
cause, root cause, ageing mechanism, consequences
and corrective actions. The most
relevant findings are highlighted below.
Plant status and detection
means
F i g u r e 2 (left) shows the event distribution
related to plant status. Nearly half of
the events took place during power operation.
Figure 2 (right) indicates that the
major part of the events were detected
by “fault report in control room” (58 %)
followed by “periodic test / in-service inspection”
(26 %). The fact that one of four
ageing events were detected in periodic
tests or in-service inspections highlights
the importance of having sound inspection
and maintenance programmes to
avoid sudden failures during power operation
with greater implications on nuclear
safety.
Systems and components
affected
The distribution of events per system affected
is presented in F i g u r e 3 . The largest
percentage (36 %) cor responds to the primary
reactor systems, followed by electrical
systems (21 %) and essential auxiliary systems
(13 %).
The distribution of events per component
affected is given in F i g u r e 4 . Passive and
active mechanical com ponents are the
most affected com ponents (38 % and
34 %, respectively), followed by electrical
(16 %), I&C (9 %) and structural components
(3 %).
Direct and root causes
F i g u r e 5 indicates that the main direct
cause was mechanical failure. The distribution
of root causes is given in F i g -
u r e 6 . A maximum of three different root
causes was attributed to each event. Deficiencies
in maintenance or surveillance is
the most important root cause, followed
42 | vgbe energy journal 5 · 2022