Environmental Engineering Reference
In-Depth Information
continuous improvement of safety and are ready to meet the future chal-
lenges in this respect.
One of the issues related to the current licensing basis at VVER plants out-
side of Russia was the inadequate knowledge of the design basis. The design
of VVER-440/213 and the older VVER-1000 plants was generally based on
the former USSR regulations of the early 1970s, the General Requirements
on Safety of NPP Design, Construction and Operation (OPB-73) and the
General Safety Rules for Atomic Power Plants (PBYa-74). OPB-73 marked
the beginning of a transition to the generally accepted international practice
in nuclear safety (e.g. defence in depth, single failure criterion). Knowledge
of the design base is absolutely critical for the preparation of LTO and
licence renewal, especially for the review of time-limited ageing analyses.
Operators of VVER-440/213 units have to perform a specifi c project for
design base reconstitution. In many countries, the design base has to be
entirely recreated, taking into account all essential changes in the licensing
requirements. For example, in the case of the Paks NPP, seismic loads had
not been considered in its design. The current design/licensing base includes
safe shutdown during an earthquake with 0.25 g horizontal acceleration.
Availability of a state-of-the-art Final Safety Analysis Report (FSAR), and
regular updating thereof is required for the control of compliance with the
current licensing basis and confi guration management.
The condition of the plant and appropriate plant programmes are also
preconditions for LTO, especially surveillance of reactor pressure vessel
(RPV) embrittlement and monitoring the condition of long-lived pas-
sive structures and components. The most important ageing management
(AM) activities are performed at the VVER plants from the very begin-
ning of their operation. The early AM activity was focused on known
degradation of the main systems, structures and components (SSCs), like
the RPV embrittlement, or on the early recognized issues, for example
leaking of the confi nement due to the liner degradation, outer surface
corrosion of the steam generator heat-exchange tubes. Most of the early
AM programmes were state-of-the-art, for example the RPV surveillance
programme. In the course of the fi rst periodic safety reviews, the defi -
nition of the most critical SSCs for operational lifetime and the domi-
nating ageing mechanisms were explained. Adequate assessment of the
aged condition and forecast of safe lifetime of structures and components
(SCs) can only be performed if the ageing process is monitored properly
from the very beginning of the operation. The operational history of SCs
has to be documented in suffi cient detail for the trends in ageing to be
discovered.
There are several non-technical conditions which affected the strategy of
VVER operators and can be considered as motivation for the decisions on
LTO. The positive international tendencies, with regard to LTO of existing
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