atw 2018-05v6

inforum

atw Vol. 63 (2018) | Issue 5 ı May

Continuous Process of Safety Enhancement

in Operation of Czech VVER Units

J. Duspiva, E. Hofmann, J. Holy, P. Kral and M. Patrik

A continuous process of a safety enhancement of VVER units in the Czech Republic is briefly described

including a presentation of important milestones and examples of particular safety measures already implemented.

A special attention is given to the evaluation and implementation of safety measures following stress tests recommendations

and R&D activities supporting this process. As examples an implementation of the „design extension condition

without core melt“ concept and various activities related to severe accident mitigation strategies are presented in the

more detailed way.

1 Introduction

A safety enhancement process is a

continuous effort which has been

taking place in both VVER sites at the

Czech Republic since their commissioning.

Within this systematic and

focused process, checked regularly by

periodic safety reviews, international

missions and independent inspections,

a level of safety and safety culture has

been significantly increased within

the last decades. Despite the high

level of safety reached by the preventive

means mainly at both VVER sites

in the context of the Fukushima accident,

a new period of enhancement

process has been initiated following a

stress tests exercise. Use of advanced

deterministic analytical approaches

and implementation of new preventive

safety measures has then taken

place together with a special attention

to the mitigative part of potential accidents

and relevant strategies and

measures.

The important attributes for new

potential safety measures have been

diversity and use of alternate means.

As examples of such approach could

be mentioned the diverse power

sources, the diverse RCS/SFP/CNTM

(reactor cooling system/spent fuel

pool/containment) make up, the

alternate secondary heat sink, the

alternate key parameters monitoring,

or the new communication means,

etc.

Within the systematic approach to

enhance safety a PSA has been recognized

as a very useful tool too. The

PSA has been mainly used for identification

of weak points in design

and operation and for an evaluation

and prioritization of potential safety

measures. On the other hand also a

use of PSA monitoring system helps to

manage an operational risk level

particularly during outage periods.

The other PSA applications, which

took place during the last decade, can

be represented by various case studies

devoted to modifications of plant

tech- specs, modification of test and

maintenance intervals, event analysis

etc. Besides that, the methods of risk

analysis are individually and indirectly

applied for development of procedures

related to plant safety (symptom

based procedures for emergency

scenarios, procedures for operation

in plant abnormal status, severe

accident management guidelines,

plant Tech-Specs), for the support of

plant crew training, including training

of operators at full scope simulator,

for improvement of MMI in the main

control room as well as in local control

rooms, and in other areas of the

direct manipulation with the plant

equipment, including planning and

timing of actions carried out under

non-standard conditions.

As an example of using the PSA

for identification and evaluation of

safety measures, additional cooling

towers at the Dukovany site could be

mentioned. The ultimate heat sink

problem as a consequence of some

external events of high intensity was

identified in 2008, long time before

Fukushima event, where the extreme

wind dominated the external hazards

risk. In the first version of external

events PSA, very high risk contribution

of this external hazard was

related to unsatisfactory resistance

of “big” plant passive cooling towers

(the active cooling towers – so called

fume cooling towers – had been made

the elements of original plant design,

but were not built during plant construction).

The important general

conclusions from the PSA (missing

ultimate heat sink for external winds

with return time period of 10,000

years) were doubted in the discussions

before the Fukushima event, but

a significant change in the opinion

after the Fukushima event led to an

installation of active cooling towers

with significant positive impact on the

external events’ plant risk. A full effect

of this modification was reached as

soon as the modification in the design

was supported by changes in procedures.

In the following chapters more

attention is paid to advanced deterministic

analytical approaches and an

implementation of severe accident

management strategies.

2 Implementation of

design extension condition

without core melt

(DEC-A)

The work on DEC-A (previously

BDBA) safety analyses for Czech NPPs

was initiated in 2009 as a consequence

of the Periodical Safety Review (PSR)

after 20 year of the operation of the

Dukovany NPP. This effort has been

influenced also by initiatives and

suggestions from European Utility

Requirements (EUR), WENRA safety

reference levels and by the IAEA

introduction of the DEC term and

concept into the safety standards

series.

2.1 Methodology basis for

DEC-A assessment in the

Czech Republic

Aside from the IAEA recommendations

and the Czech Atomic Law,

the following regulations, directives

and reports constitute the legislative

and methodological basis for deterministic

analyses of DEC-A (BDBA) in

the Czech Republic:

• SUJB regulation 195/1999, Requirements

on Nuclear Facilities

for Ensuring of Nuclear Safety,

Radiation Protection and Emergency

Preparedness, 1995.

• SUJB directive BN-JB-1.6, Probabilistic

Assessment of Safety, 2010

(currently revised due to new

Atomic Law).

• SUJB directive BN-JB-1.7, Selection

and Assessment of Design and

Beyond Design Events and Risks

for Nuclear Power Plants, 2010

299

OPERATION AND NEW BUILD

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

More magazines by this user
Similar magazines