atw 2018-05v6


atw Vol. 63 (2018) | Issue 5 ı May


(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


The computer code used for NPP

safety analyses in the Czech Republic

must be approved by the regulatory

body according to the SUJB directive


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


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


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


• 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


• Loss of cooling of spent fuel pool;

• Loss of ultimate heat sink (from

secondary circuit);

• Uncontrolled boron dilution;

• Multiple steam generator tube


• Steam generator tube ruptures

induced by main steam line break


• Loss of required safety systems in

the long term after a design basis


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

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