atw 2018-05v6

atw Vol. 63 (2018) | Issue 5 ı May
OPERATION AND NEW BUILD 300
(currently revised due to new
Atomic Law).
• UJV, Proposal of Methodological
Procedure for Performing of Safety
Analysis of Beyond Design Basis
Accident, UJV Rez, 2010.
Analyses of DEC-A scenarios use the
best estimate computer codes with
combination of realistic initial and
conservative (or realistic) boundary
conditions. The robust design of VVER
reactors and their safety features
enable to fulfil DBA acceptance
criteria in most DEC-A cases including
radiological consequences. For the
most severe conditions comprising
multiple failures of safety systems or
safety groups providing protection in
the level 3a of Defense in Depth (like
SBO), the new measures imple mented
after post-Fukushima Stress tests in
the level 3b of the DiD provide an
additional robust protection against
the evolution of these scenarios into
the DEC-B category (severe accident).
The acceptance criteria applied to
DEC-A analyses are identical to those
applied to DBA analysis with exception
of criterion on primary and
secondary pressure and radiological
consequences.
The computer code used for NPP
safety analyses in the Czech Republic
must be approved by the regulatory
body according to the SUJB directive
VDS-030.
2.2 Selection of DEC-A events
to be analysed and
documented in SAR
The basic set of DEC-A (BDBA) events
to be analyzed is specified in
BN-JB-1.7 0. Supplemental events
and scenarios could be specified by
PSA outcomes and engineering
judgement.
It is important to mention that in
analyses of DEC (which are often
complex sequences or combinations
of events and failures) it is logical to
transfer from “frequency of initial
events” to “frequency of occurrence of
scenarios”.
The SUJB directive BN-JB-1.7 0
requires besides the standard set of
ATWS analyses, the following DEC-A
(BDBA) events to be analyzed:
• Total long-term loss of inner and
outer AC power sources;
• Total long-term loss of feed water
(„feed-and-bleed„ procedure);
• LOCA combined with the loss of
ECCS;
• Uncontrolled reactor level drop or
loss of circulation in regime with
open reactor or during refueling;
• Total loss of the component cooling
water system;
• Loss of residual heat removal
system;
• Loss of cooling of spent fuel pool;
• Loss of ultimate heat sink (from
secondary circuit);
• Uncontrolled boron dilution;
• Multiple steam generator tube
rupture;
• Steam generator tube ruptures
induced by main steam line break
(MSLB);
• Loss of required safety systems in
the long term after a design basis
accident.
The whole set of prescribed DEC-A
analyses was already performed both
for Dukovany NPP (VVER-440) and
for Temelín NPP (VVER-1000).
Analyses of DEC-A events for the
Czech NPP’s have been performed
with the RELAP5 computer code. It is
worth noting that the RELAP5 has
been in the UJV Rez validated against
experimental data from more than 20
tests carried out at various integral
test facilities (ITF) and that approximately
half of these tests were modelling
events of the DEC-A type.
2.3 Example of DEC-A analysis:
SBLOCA in VVER-1000 with
failure of ECCS and operator
start of HPSI at 30 min
The analysis of a small break loss
of coolant accident (SBLOCA) with
the break D50 mm in the cold leg
and with a failure of the start of
emergency core cooling systems
(ECCS) and operator manual start of
high pressure safety injection (HPSI)
at 30 min was performed for the
| | Fig. 1.
Nodalization scheme of VVER-1000 for RELAP5 (only primary circuit and 1 of 4 modeled loops depicted).
Operation and New Build
Continuous Process of Safety Enhancement in Operation of Czech VVER Units ı J. Duspiva, E. Hofmann, J. Holy, P. Kral and M. Patrik