The «practical elimination» approach for pressurized water reactors V. TIBERI K.HERVIOU International Conference on Topical Issues in Nuclear Installation Safety: Safety Demonstration of Advanced Water Cooled Nuclear Power Plants 6-9 June 2017 Vienna
Outline 1. Background 2. The «practical elimination» approach 3. Justification of «practical elimination» 2
1. Background 1/3 Three Mile Island and Chernobyl accidents: need of significant improvement of safety Improvement achievable for water-cooled reactors, taking into account R&D works performed on core melt accidents Provisions aiming to limit the releases outside the plant in case of severe accident Impossibility to implement realistic measures to reduce radiological consequences at an acceptable level for some core melt situations (highenergy phenomena) Introduction of the practical elimination concept for reactors to be built at the beginning of the XXI century 3
1. Background 2/3 INSAG 10, INSAG 12 Severe accident (plausible situations) Early containment failure Practical elimination practical elimination of accident sequences that could lead to Late large containment early radioactive failure releases, whereas severe accidents that could imply late containment Implementation failure of would provisions be to considered limit the in the releases design process Justification that the provisions taken to prevent the situation are sufficient to exclude the situation, i.e. can be considered very unlikely with a high level of confidence Radiological consequences are not assessed Demonstration that the consequences of such accident meet the safety objectives Radiological consequences are presented in the safety analysis report 4
1. Background 3/3 WENRA, AIEA Severe accident (plausible situations) Early containment failure Late containment failure Practical elimination of early or large releases 5
2. The «practical elimination» approach 1/3 5 th level 4 th level Limitation of consequences for the populations (on-site and off-site emergency response) Control of severe accident conditions (Complementary measures for accident management) Organizational provisions to protect the population 3 rd level 2 nd level 1 st level Control of accidents within the design basis and multiple failures accidents (Engineered safety features and accident procedures) Control of abnormal operation and detection of failures (Control, limiting and protection systems and other surveillance features) Prevention of abnormal operation and failures (conservative design and high quality in construction and operation) Physical, human and organizational provisions defined by the operator Levels considered in the design - Aim to limit off-site releases as far as possible 6
2. The «practical elimination» approach 1/3 5 th level 4 th level Limitation of consequences for the populations (on-site and off-site emergency response) Control of severe accident conditions (Complementary measures for accident management) Organizational provisions to protect the population 3 rd level 2 nd level 1 st level Control of accidents within the design basis and multiple failures accidents (Engineered safety features and accident procedures) Control of abnormal operation and detection of failures (Control, limiting and protection systems and other surveillance features) Prevention of abnormal operation and failures (conservative design and high quality in construction and operation) Physical, human and organizational provisions defined by the operator Levels considered in the design - Aim to limit off-site releases as far as possible 7
2. The «practical elimination» approach 2/3 Situations likely to lead to large releases (simultaneous or successive loss of integrity of all the containment barriers, bypass): provisions allowing to significantly limit their consequences practical elimination where it appears to be impossible to define such provisions with the knowledge and techniques available at the time of the design orientations The integrity of the containment can be affected: Suddenly: high-energy phenomenon (e.g. massive and rapid reactivity insertion, H detonation, etc.) More gradually: loss of the containment heat removal function (slow increase of temperature and pressure in the containment, erosion of the basemat by the corium, etc.) Smaller uncertainties Time to act before large releases occur Possibility to define measures to limit the consequences of these situations and to demonstrate their adequacy. 8
2. The «practical elimination» approach 2/3 Situations likely to lead to large releases (simultaneous or successive loss of integrity of all the containment barriers, bypass): provisions allowing to significantly limit their consequences practical elimination where it appears to be impossible to define such provisions with the knowledge and techniques available at the time of the design orientations Only severe accident situations that could lead to large early releases have to be practically eliminated. Generally, for other situations, measures have to be taken by the operator to limit their consequences. 9
2. The «practical elimination» approach 3/3 Examples of practically eliminated situations in French PWRs: Core melt with loss of containment integrity : Massive and rapid reactivity insertion Hydrogen detonation Steam explosion «Practical elimination» approach primarily applicable to high energetic phenomena that could challenge the containment in case of severe accident By extension: core melt with containment bypass Melt of spent fuel assemblies being handled or stored in the spent fuel pool 10
4. Justification of «practical elimination» 1/3 An accidental situation can be considered as practically eliminated if: a. It is physically impossible for the accidental situation to occur b. The accidental situation can be considered with a high degree of confidence to be extremely unlike to arise Priority has to be given to demonstrations of physical impossibility Case-by-case analysis based on deterministic and probabilistic considerations 11
4. Justification of «practical elimination» 2/3 a. Physically impossible situations Intrinsic characteristics that guarantee the non-occurrence of certain dreaded phenomena (e.g. neutron feedback) Design choices limiting the quantities of substances likely to produce energetic phenomena (e.g. limitation of the capacity of deborated water tanks in order to prevent heterogeneous dilutions) Passive static systems that cannot fail (e.g. construction provisions preventing a heavy load drop from seriously damaging the structural integrity of the spent fuel pool) The demonstration of physical impossibility must not, under any circumstances, be based on measures requiring active components, since any active component has a non-zero probability of failure. 12
4. Justification of «practical elimination» 3/3 b. Situations extremely unlikely with a high degree of confidence Sufficient number of robust and independent lines of defense (deterministic approach) Requirements in terms of design, manufacturing and operation. The more a provision or a set of provisions contributes to reduce the probability of a situation occurring, the more the requirements are stringent. Provisions tolerant to: human actions and errors internal and external hazards. The occurrence of rare and severe external hazards shall not call into question a practical elimination justification. PSA: exhaustiveness of the measures taken to avoid some accidental situations (multiple failures not identified deterministically: support system failures, common cause failures, human errors, etc.). PSA should be used with care: impact of the models used, assumptions taken, large uncertainties, etc. 13
Conclusion The practical elimination is a particular approach for the design developed to enhance the prevention of situations involving a severe accident and a risk regarding the containment where it appears to be impossible to define realistic and demonstrable measures to limit their consequences. The decision to consider or not in the design such situations is structuring for a new concept of rector: discussions by vendors and safety authorities from the initial design stages of a new reactor type. Practical elimination should not stifle continuous efforts to improve safety: potential safety improvements after the initial design phase (periodic safety reviews). 14