Based on the IAEA INSAG-12  , the Defence in Depth (DiD) concept   provides an overall strategy for safety measures and features of Nuclear Power Plants (NPPs) (para 47). Two corollary principles follow the general statement of DiD―accident prevention and accident mitigation. Although a continuous effort to increase the scope of the severe accidents that have been taken into consideration and to reduce their off-site consequences, a further reduction the potential radiological consequences is an important goal for future NPPs (para 124). An important advantage of future plants is their ability to incorporate corrections to deficiencies identified in the past (para 125). In that context, the concept of “practical elimination” of early or large releases  is also defined, but the attention to it extremely increases after the accident in Fukushima NPP in 2011.
2. The Term “Practical Elimination”
The concept of “practical elimination” of early or large releases is an issue, defined in details in the IAEA “Safety of NPPs: Design”, SSR2/1  , and in the WENRA Safety of new NPP design  . According to the updated in 2016 IAEA SSR-2/1, Rev.1, paragraph 2.11  :
“The design for safety of a nuclear power plant applies the safety principle that practical measures must be taken to mitigate the consequences for human life and health and for the environment of nuclear or radiation accidents (Principle 8 of the Fundamental Safety Principles). Plant event sequences that could result in high radiation doses or in a large radioactive release have to be “practically eliminated” and plant event sequences with a significant frequency of occurrence have to have no, or only minor, potential radiological consequences. An essential objective is that the necessity for off-site protective actions to mitigate radiological consequences be limited or even eliminated in technical terms, although such measures might still be required by the responsible authorities.”
Requirements concerning “practical elimination” can be found in the revised Regulation on Ensuring the Safety of Nuclear Power Plants  . This regulation was issued in 2016 by the Bulgarian Regulatory Agency (BNRA). It is developed based on the WENRA safety goals for new NPP designs  and the updated after the Fukushima NPP accident reference levels for safety harmonisation of NPPs in operation  , as well as the latest IAEA safety standards regarding: site selection for nuclear facilities, design, construction, commissioning and operation. Furthermore, this regulation introduces the provisions of the Council Directive 2014/87/EURATOM, of 8 July 2014, establishing a Community framework for the nuclear safety of nuclear installations.
The regulation  states that:
Art.4 (4) “1) accidents with the melting of nuclear fuel that lead to early or large radioactive releases into the environment should practically eliminated;
2) for the accidents with the melting of nuclear fuel that cannot practically eliminated, design provisions have to be taken so that only limited protective measures in area and time are needed for the public (no permanent relocation, no need for emergency evacuation outside the immediate vicinity of the plant, limited sheltering, no long term restrictions in food consumption) and that sufficient time is available to implement these measures.”
Art.72 (6) “With the safety analysis demonstrated that accidents with the melting of nuclear fuel, leading to large or early releases of radioactive substances into the environment are practically eliminated.”
In respect to the practical elimination, another BNRA document, the safety guide “Deterministic Safety Assessment”  defines in the section 4.18, p.12 that;
“For new plant designs the first priority is to prevent by a robust design (“practically eliminate”) the severe accidents that could lead to large or early releases to the environment, including high pressure core melt scenarios, if they cannot be excluded as physically impossible. Each representative or limiting accident sequence should be assessed for the purpose of practical elimination. The prevention of a particular sequence should be demonstrated primarily by deterministic arguments complemented with probabilistic, where appropriate, taking into account the uncertainties resulting from the limited knowledge about particular physical phenomena. Important fact is that the practical elimination of accident sequences should not be demonstrated solely by very low frequency of occurrence, below an established threshold value.”
2.2. Achievement of “Practical Elimination”
The “practical elimination” is achieved by prevention of the conditions that could lead to an early radioactive release or a large radioactive release  .
2.3. Demonstration of “Practical Elimination”
Accident sequences with early or large releases could be considered to have been practically eliminated  :
• 1) if it is physically impossible for the accident sequence to occur or
• 2) if the accident sequence can be considered with a high degree of confidence to be extremely unlikely to arise (IAEA SSR-2/1).
The demonstration should show sufficient knowledge of the accident condition analyzed and of the phenomena involved, substantiated by relevant evidence.
Demonstration of practical elimination should be mainly based on the criterion of physical impossibility. The criterion for extreme unlikelihood with high confidence should be used with lower priority. Such approach should minimize uncertainties and increase the robustness of the NPP safety case.
The demonstration of “practical elimination” has to be achieved by deterministic considerations supported by probabilistic considerations, taking into account the uncertainties due to the limited knowledge of some physical phenomena   .
The IAEA TECDOC-1791  recommends to pay attention of practical eliminating of the following severe accident conditions which could:
• damage the containment in an early phase as a result of direct containment heating, some steam explosions or large hydrogen detonation;
• damage the containment in a late phase as a result of basemat melt-through or containment excessive pressure;
• occur during an open containment―notably in shutdown states;
• bypass the containment (e.g. Steam Generator (SG) tube rupture or an Interfacing System Loss of Coolant Accident (ISLOCA).
Moreover, the aforementioned document  proposes the following categories of accident conditions that should be addressed for “practical elimination”:
• Events that could lead to prompt reactor core damage and consequent early containment failure:
- Failure of a large component in the reactor coolant system (RCS);
- Uncontrolled reactivity accidents.
• Severe accident phenomena which could lead to early containment failure:
- Direct containment heating;
- Large steam explosion;
- Hydrogen detonation.
• Severe accident phenomena which could lead to late containment failure:
- Molten core concrete interaction (MCCI);
- Loss of containment heat removal.
• Severe accident with containment bypass;
• Significant fuel degradation in a storage pool.
3. Design Features and Prevention and Mitigation Measures for Severe Accidents Management
The Kozloduy NPP is used as an example for providing a discussion on “practical elimination” of early or large releases for NPP with a WWER-1000/V320 reactor types. A number of significant modifications have been made to the existing design of the units 5 and 6 of Kozloduy NPP (WWER-1000/V320). Some new equipment has been put in place to prevent the occurrence of severe accidents or mitigate their consequences. The more significant modernization includes: containment hydrogen reducing system; after accident containment pressure reducing system; alternative steam generators feed water system; detectors for steam under reactor vessel cap; wide range thermo-couples; Post-Accident Monitoring System (PAMS); Safety Parameter Display System (SPDS); electrical power supply; high temperatures safety devices; water supply for the spent fuel pools; new center for accident management.
Recently issued publication  “Status of Severe Accident Management Guidelines (SAMGs) at KNPP” contains an overview of the above mentioned technical provisions and its implementations. Furthermore, the results from researches dedicated on some issues concerning the management of severe accidents are discussed in    . References    explain the status of Level 2 Probabilistic safety Assessment (PSA) and current studies in this area.
In the following table, the summarized design features of WWER-1000/V320 and prevention and mitigation measures to eliminate main phenomena during severe accident are presented (Table 1).
Some results of sensitivity analyses of Large Early Release Frequency (LERF) based on Level 2 PSA, are presented below.
Table 1. Measures for severe accident management.
Figure 1. Base case.
Figure 2. Sensitivity analyses.
Figure 3. Distribution of LERF by Reactor states and SFP (Source:  ).
Figure 3  shows LERF distribution for different reactor states (open and closed reactor) and for the spent fuel pool (SFP). The closed reactor is a main contributor to the risk of radioactive releases into the environment.
For the interpretation of the results presented in Table 2, the values defined in the safety guide “Probabilistic Safety Analysis of Nuclear Power Plants”  was taken into consideration. This safety guide contains quantitative criteria for severe accidents―limits of a large release requiring immediate protective measures. Furthermore, these criteria are specified for new and for existing NPPs.
According to Section 1.3 from the current guide, regarding the existing NPPs:
“b) the frequency of large radioactive releases into the environment that require implementation of urgent protective measures for the public shall be lower than 10-5 events per NPP per year.”
According to the same guide, Section 1.4, for new NPPs:
“b) the frequency of large radioactive releases into the environment that requires undertaking of immediate protective measures for the population shall not exceed 1.10-6 events per NPP per year.”
It should be stressed that threshold values for a large radioactive release frequency were used in the frame of the interpretation of Level 2 PSA results.
Table 2. Isolation of Containment (some results of sensitivity analyses).
Results in the Table 2 demonstrated that LERF values are significantly less that threshold value (LERF < 10 - 5 1/y) defined in the normative document.
The concept of “practical elimination” is defined as part of the Defence in Depth (DiD) of NPP in the IAEA document INSAG-12 in 1999. But, the special attention to it was paid after the accident in Fukushima NPP in 2011. The “practical elimination” is achieved by prevention of the conditions that could lead to early or large radioactive releases  . From this point of view, the mechanisms for the containment failure of reactor WWER-1000/V320 are presented. The summarized design features and preventing and mitigation measures already implemented at Kozloduy NPP to extend the design basis and beyond design basis envelop are presented. Issues related to external steam explosion are underlined for further study.
The authors acknowledge to the EC supported ASAMPSA_E project (Advanced Safety Assessment: Extended PSA) under contract 605001 for funding this work.
 Xing, J. and Wang, H. (2017) Practical Elimination on Large Release of Radioactive Materials and Safety Performance Research on HPR1000. In: Jiang, H., Ed., Proceedings of the 20th Pacific Basin Nuclear Conference, Springer Science and Business Media, Singapore, 385-397.
 BNRA (2016) Regulation on Ensuring the Safety of NPPs, Published SG, No. 66 of 30 July 2004, Amended SG No. 46 of 12 June 2007, Amended SG No. 53 of 10 June 2008, Amended SG No. 5 of 19 January 2010, and Published in Force from 30 September 2016 (In Bulgarian).
 Groudev, P., Andreeva, M., Mladenova, S. and Topalov, T. (2016) Status of Severe Accident Management Guidelines at Kozloduy Nuclear Power Plant. Journal of Power and Energy Engineering, 4, 1-8.
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 Chatterjee, B., Mukhopadhyay, D., Lele, H.G., Atanasova, B. and Groudev, P. (2011) Severe Accident Management Strategy Verification for VVER-1000 (V320) Reactor. Nuclear Engineering and Design, 241, 3977-3984.
 Groudev, P., Kichev, E., Mancheva, K. and Petrova, P. (2015) PSA Contribution in Development and Application of Severe Accident Guidelines. ESREL2015: The 25th ESRA Conference, Zurich, 7-10 September 2015, 399-404.
 Groudev, P., Kichev, E., Mancheva, K. and Petrova, P. (2017) Use of Level 2 PSA to Support NPP Operators Training on Severe Accident. ESREL2016, Conference Proceedings on USB, Glasgow, 25-29 September 2016, 134-138.
 Mancheva, K., Tzenkov, K. and Borisov E. (2014) Main Aspect and Results of Level 2 PSA for KNPP, Proceedings of the 10th International Conference on Nuclear Option in Countries with Small and Medium Electricity Grids, Zadar, Croatia, 1-4 June 2014, 146-1-146-15.