atw 2018-05v6


atw Vol. 63 (2018) | Issue 5 ı May

VVER-1000. The nodalization scheme

of the VVER­ 1000 unit for the RELAP5

used and graphical courses of main

parameters are shown in Figure 1 and

Figure 2.

Loss of primary coolant through

the break D50mm in cold leg and

without an automatic actuation of

ECCS leads to a depletion of a primary

inventory and if not mitigated by

operator, the core uncovery and

overheating start. However with

respect to high “water volume to

power ratio” in the VVER-1000, there

is sufficient time for the operator

intervention. In the presented case,

the operator starts one HPSI pump at

30 min and soon after it the core is

quenched and its cooling restored and


2.4 Introduction of DEC-A

analyses into Safety

Analysis Report of

Dukovany and Temelín


The whole set of prescribed DEC-A

analyses was already performed both

for Dukovany NPP (VVER-440) and for

Temelín NPP (VVER-1000). It means

that from 15 to 20 DEC-A analyses for

each plant was elaborated.

As for the documentation of DEC-A

analyses in Safety Analysis Report, the

temporary solution was creation of a

new subchapter 15.9.1 which contains

basic results of all DEC-A (BDBA)

analyses required by BN-JB-1.7.

Beside that the ATWS analyses are

documented in subchapter 15.8 of the

SAR as usually.

The final foreseen solution is the

introduction of the new SAR chapter

19, that would contain both DEC-A

(BDBA without core melt) and DEC-B

(severe accident) analyses presented

in systematic and integrated way.

Then the Chapter 15 will be again

intended for analyses of events up to

DBA only.

3 Implementation of

severe accident

strategies and particular


The solution of severe accident topics

for both Czech NPPs was initiated

at the end of 1980ies of last century

as the response to the Chernobyl

accident. The activities were supported

by IAEA via. regional projects

at the beginning and after the political

changes in the Czechoslovakia, the

main source of experience was shifted

to the CSARP program. All those

activities were performed by the

group on severe accident in the UJV

| | Fig. 2.

Reactor core temperatures (SBLOCA D50mm in VVER-1000 with failure of ECCS).

Rez. Later the group started to be

partner in international activities and

recently it plays an important role in

the safety enhancement of Czech

NPPs in the relation to the severe

accident management. Generally the

group is oriented on the analytical

activities, the experimental ones were

related only to the small scale test on

the corium properties using the cold

crucible facility. Recently the new

program on the reactor pressure coolability

during the in-vessel retention

strategy is under preparation and the

large scale facility under construction.

Historically the activities were at

the beginning focused on the plant

vulnerability studies, an identification

of typical timing of a severe accident

progression and the first basic source

term estimations. Later the activities

were extended to the identification of

potential plant modifications related

to mitigation of important severe

accident phenomena like hydrogen

issue and so on. The recent key activity

for the Czech NPPs is related to a

solution of the corium localization for

the Temelín NPP (VVER-1000/320),

but also other activities are on-going.

Any analytical activities in area of

severe accident must be done on

appropriate qualitative level. The

systematic approach in the UJV

consists of nine pre-conditions which

must be continuously filled up to be

granted that analytical results are

credible. The list of preconditions is


• Knowledge of SA phenomenology

• Knowledge of analyzed facility

(power plant unit or experimental

facility in a case of validation


• Validated and state of the art

analytical code

• Knowledge of analytical tool by


• Close collaboration between code

users and developers

• Exchange of user experience,

recommendations and best


• Code qualification for specific

design feature (or for specific

reactor design like in the

VVER-440/213 case)

• Plant specific input development

and testing

• Results evaluation and interpretation

taking into account the

objective and assumptions of the

analysis as well as the code limitations

– code must not be used as

black box

Extensive validation activities are very

important for the filling up of some of

above mentioned preconditions and

the UJV is very active. As mentioned

above the group is analytical and

the access to the experimental results

is possible only via. international

cooperation. The UJV participated in

several international projects during

last more than 15 years like Phebus FP,

projects of the 5th EC FWP (ARVI,

ICHEMM, LPP, OPTSAM), later in

SARNET and SARNET2, but also

within the OECD/NEA projects like


usually including their follow up.

Another important activity is a

participation in the international

benchmarks or exercises like the


Quench-06 test, ISP-46 on Phebus

FPT-1, ISP-41 on RTF, ISP-44 on


tests), SARNTET benchmarks on

Quench-11 test or Phebus FPT3 tests

or benchmarks within Analytical

Working Group of OECD THAI project


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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