1. International Topical Conferences on Nuclear Safety, IAEA, June 6-9, 2017, Vienna
Workshop 2: An Introduction and Further Explanation on Design Extension Conditions Introductory remarks: ►Evolution of the DEC Concept ►Implementation of DEC in Czech Republic
Pavel KRAL
UJV Rez, Czech Republic

2. Introduction
The concept of “design extension condition” (DEC) is relatively new and still under development
The major contributors to the devolopment of DEC concept have been the EUR, WENRA and IAEA
The DEC are usually divided to DEC without core melt (relevant to previous BDBA) and DEC with core melt (more or less equal to severe accident)
The concept of DEC has number of aspects:
Relation between DEC and DiD,
Safety analyses of DEC and relevant methodology,
NPP design modifications and other mitigation measures,
Major int’l documents focused on DEC and harmonization of approach to DEC.

3. Evolution of DEC concept
The term “design extension condition” (DEC) was firstly formally introduced in the European Utility Requirements EUR Version A in 1994 to define some selected sequences due to multiple failures with the intent to improve the safety of the plant extending the DB.
In 2008 WENRA published document “Reactor Safety Reference Levels”and used the term “design extension”.
IAEA SSR 2/1 introduced “Design Extension Conditions“ (DEC) into system of IAEA Standards in 2012.
The 2014 update of WENRA Reference Levels and 2016 Revision-1 of IAEA SSR-2/1 added clear differentiation between DEC-A (without core melt) and DEC-B (with core melt), respectively.

4. Evolution of DEC concept
EUR approach to DEC:
The DEC are in EUR divided to “complex sequences” and “severe accidents”:
Complex Sequences involve failures beyond those considered in the deterministic design basis but do not involve core melt
Severe Accidents considered in the design, both to prevent early and delayed containment failure and to minimize radioactive releases for the remaining conditions.
Selection of DEC by plant designer and then by use of probabilistic methods.
Assessment by best estimate methods (no need for application of SFC etc.).

5. Evolution of DEC concept
WENRA approach to DEC:
The Reactor Harmonization Working Group of WENRA (Western European Nuclear Regulators’ Association) published in 2008 document “Reactor Safety Reference Levels for Existing Reactors” and introduced the term “design extension” into the area of nuclear regulators.
2014 update of WENRA “Safety Reference Levels (SRL)” added clear differentiation between:
DEC A (without core melt)
DEC B (with core melt = severe accidents).

6. Evolution of DEC concept
IAEA publications relevant to DEC:
IAEA SSR 2/1 Safety of Nuclear Power Plants: Design, orig 2012 / Rev1 2016
IAEA SSG-30 Safety Classification of Structures, Systems and Components in Nuclear Power Plants. 2014
IAEA-TECDOC-1791 Considerations on the Application of the IAEA Safety Requirements for the Desigh of Nuclar Power Plant. 2016
prepared update of safety guide SSG-2 Deterministic Safety Analysis for Nuclear Power Plants (to be published in 2017)
prepared update of safety guide GS-G-4.1 Format and Content of the Safety Analysis Report for Nuclear Power Plants (to be published in 2017 or 2018)
Definition of DEC according to IAEA Glossary:
design extension conditions - postulated accident conditions that are not considered for design basis accidents, but that are considered in the design process of the facility in accordance with best estimate methodology, and for which releases of radioactive material are kept within acceptable limits.

7. Evolution of DEC concept (cont’d)
AOO
DBA
(safety ystems)
Operational States
Accident Conditions
Design Basis
Beyond Design Basis
(Accident Management)
Severe Accidents
(core melting)
DECs
DBA / DBC
(safety systems)
Plant Design Envelope
Beyond PDE
SSR-2/1, 2012
BDBA
Earlier Concept
Safety features for sequences without fuel degradation
Safety features for accident with core melting
Conditions practically eliminated

8. Evolution of DEC concept

9. Evolution of DEC concept
US NRC approach to DEC:
US NRC has not so far officially adopted the term “design extension conditions” and stays with “beyond design basis accident (BDBA)” and “severe accidents (SA)”.
But there are technical discussions dealing with this issue. Especially for new advanced reactors design.
US NRC position to DEC/BDBA events:
(1)  For operating reactors, the Commission voted not to move to the DEC approach.  We continue to have selected beyond-design basis events (BDBEs) such as SBO, ATWS, and post-Fukushima external events.   
(2)  For new reactors, we somewhat address the issue through the regulatory treatment of non-safety systems (RTNSS) and we also require measures to address severe accidents (DEC-B).
(3)  For advanced reactors, we are still developing the approach but in general still do not use the DEC terminology – although the approaches look at a broader set of events and so we expect that will end up effectively very close to those using DEC.
See Secy and related documents are the guidance for LWRs and ALWRs.

10. Evolution of DEC concept
Overview of major references dealing with DEC concept:
EUR, European Utility Requirements for LWR Nuclear Power Plants, Revision E, (the first Version A published in 1992)
WENRA, Reactor Safety Reference Levels for Existing Reactors, (the first version published in 2008)
WENRA, Guidance Document Issue F: Design Extension of Existing Reactors, 2014
IAEA, Safety of NPPs: Design, Specific Safety Requirements, SSR-2/1, Revision 1, (the previous version containing already DEC published in 2012)
IAEA, General Safety Requirements GSR Part 4, Revision 1, 2016
IAEA-TECDOC-1791, Considerations for the application of the IAEA Safety Requirements on Design, 2016

11. Implementation of DEC in Czech Republic
Methodology basis:
Czech regulatory body SUJB directive “BN-JB-1.7 “Selection and Evaluation of Design and Beyond Design Events and Risks for NPP”, 2010
Krhounkova, Kral, Macek: Proposal of Methodology Procedure of Performing BDBA Analyses of NPP Dukovany, 2009 (Rev. 2013)
Approach to analyses:
Selection of DEC-A (BDBA) events to be analyzed based on requirements of BN-JB-1.7 plus supplements according to PSA
Realistic computer code, model and analysis assumptions
Note: The computer codes used for safety analyses in the Czech Republic must be “licensed” (approved) by the Czech regulatory body according to SUJB Directive VDS The license is reevaluated and renewed every 3 years.

12. Implementation of DEC in Czech Republic
The whole set of DEC-A analyses prescribed by BN-JB-1.6 (plus some additional ones) was already performed both for Dukovany NPP (VVER-440) and for Temelin NPP (VVER-1000):
20 DEC-A analyses done for Dukovany NPP
18 DEC-A analyses done for Temelin NPP
As for the documentation of DEC-A analyses in Safety Analysis Report, the temporary solution was the creation of a new SAR subchapter 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 future foreseen solution is introduction of new SAR charter 19, that would contain both DEC-A (without core melt) and DEC-B (severe accident) analyses presented in systematic and integrated way. Then the Chapter 15 will contain only analyses of events ranging from normal operation (NO) to design basis accident (DBA).

13. Summary
The concept of “design extension condition” (DEC) is relatively new and still under development
Major contributors to introduction and evolution of DEC have been the EUR, WENRA and IAEA.
The division of DEC events according to IAEA:
DEC without core melt
DEC with core melt
The concept of DEC has number of aspects and links:
Relations to DiD and its enhancement,
Safety assessment of DEC events and development of methodologies,
NPP design modifications and structuring of safety systems
Other DEC mitigation measures,
International cooperation and harmonization of approach to DEC.
Implementation of DEC concept started in the Czech Republic in as an requirement of the Periodical Safety Review (PSR) after 20 years at NPP Dukovany. The IAEA, WENRA, EUR and other int’l documents are important basis and background for DEC implementation in the Czech Republic.