Basic Safety Concepts in Nuclear Engineering

1. Basic Safety Concepts in
Nuclear Engineering
Dr. Gernot Thuma, GRS
3rd International Disaster and Risk Conference (IDRC),
30 May - 3 June 2010, Davos, Switzerland

2. Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH
Subordinate Federal and Authorised Scientific
Authorities State Authorities Experts Institutions
BfS
Federal Office for RSK/SSK Universities
BMU
Radiation Protection
Federal Minister for
the Environment,
Nature Conservation
and Nuclear Safety
GRS
Technical Safety
State Authorities States Organisations Research Institut
(e.g. TÜV)
Utilities/Licensees
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3. Outline
 Nuclear Energy Production
 Protection Goals (Nuclear Safety Goals)
 Safety Concepts
• (Example for the Combined Effect of Safety Measures)
 Application to Conventional Industrial Facilities
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4. Nuclear Energy Production
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5. Protection Goals (Nuclear Safety Goals)
 Reactivity control
 Fuel cooling
 Confinement of
radioactive materials
 Limitation of radiation
exposure
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6. Safety Concepts
 Defence in depth
 Multiple barriers
 Fail Safe Design
 Single failure concept
 Redundancy
+ Physical Separation
 Diversity
 …
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7. Defence in Depth
The concept of defence in depth, as applied to all safety activities, whether
organizational, behavioural or design related, ensures that they are subject to
overlapping provisions, so that if a failure were to occur, it would be detected and
compensated for or corrected by appropriate measures […] Application of the
concept of defence in depth throughout design and operation provides a graded
protection against a wide variety of transients, anticipated operational
occurrences and accidents, including those resulting from equipment failure or
human action within the plant, and events that originate outside the plant.
[IAEA Safety Requirements, NS-R-1, Safety of Nuclear Power Plants: Design]
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8. Multiple Barriers
There are barriers for various
purposes:
 Containment of radioactive
materials
 Radiation protection
 Fire protection
 Limitation of effects of component
failures
• Missiles
• Flooding
 Physical protection (security)
 …
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9. Fail Safe Design
Definition:
 Design ensuring that in the event
of a failure the system behaves in
a way that will cause no harm
Example:
 To shutdown the reactor the
control rods have to be inserted
into the reactor core
• Normally the control rods are
held and moved by electric
drives
• In the event of a power failure,
the control rods fall into the
core under gravity
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10. Single Failure Concept
Aim:
 Safety function available (100 %)
Assumption:
 Failure of a safety installation due to a random single failure
with the most unfavourable effect
 Unavailability of a safety installation due to maintenance measures
with the most unfavourable effect
Solutions:
 3 sub-systems á 100 %
 4 sub-systems á 50 %
Advantage of the 4 x 50 % solution:
 In some situations 50 % are enough to accomplish the task
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11. Redundancy
Definition:
 Duplication of critical structures,
sub-systems, or components
Aim:
 Backup for random failures,
maintenance,…
Design:
 Realization depends on the
safety function that has to be
performed
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12. Diversity
Definition:
 Different technical
implementations of a
given safety function
Aim:
 Prevention of
common cause failures
Caveat:
 Not everything that looks like a
different implementation is a
different implementation
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13. What else?
 Design basis accidents
• 30 minutes criterion
No operator action required during the first 30 minutes of an accident
 Internal and external hazards
 Appropriate instructions
• Operating and maintenance instruction
(normal operation and operational occurrences)
• For incidents and accidents:
Event sequence based workflow instruction
(operational occurrences and design basis accidents)
Protection goal oriented instructions (other accidents)
• Internal accident management measures (severe accidents)
• Off-site emergency response measures (severe accidents)
 Evaluation of the operating experience
 Systematic safety assessments (on a regular basis, e.g. every 10 years)
• Deterministic safety assessments + probabilistic safety assessments
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14. Internal and External Hazards
Internal hazards External Hazards
 Fire Natural Hazards
 Explosion  Earthquake
 Flooding  Flooding
 Missiles  Storm
(e.g. from high energy components)  Lightning
 Heavy load drop  Other meteorological hazards
(e.g. from structural failures Man-made Hazards
or crane failures)
 Explosion (off-site)
 Fire (off-site)
 Aviation accidents
Typical exceedance probabilities for the
design basis events: 10-4 - 10-5 per year
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15. Probabilistic Safety Assessment (PSA)
Aims:
 Quantification of the risk
 Identification of vulnerabilities and
particularly risky initiating events
 Basis for risk-informed planning,
maintenance measures,
retrofitting, and design
modifications
Scope:
 Level 1 - Sequences that could
lead to core damage states
 Level 2 - Release of radioactive
material to the environment
 Level 3 - Dispersion of
radionuclides outside the plant
including potential environmental
and health effects
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16. Summary of Safety Concepts
 Defence in Depth
 Multiple Barriers
 Fail Safe Design
 Single Failure Concept
 Redundancy
 Diversity
 Design basis accidents
 Internal and external hazards
 Appropriate instructions
 Evaluation of the operating experience
 Systematic safety assessments
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17. Application to Conventional Industrial Facilities
 Application of these safety concepts not limited to nuclear installations
 Adaptation requires only minor changes
• Definition of suitable safety goals
• Specification of a target safety level
proportionate to the complexity and potential hazard of the installation
 Some safety concepts already applied to high-risk industrial facilities
• But implementation in conventional industrial facilities not yet as common
and stringent as in nuclear engineering
 Consequent application of these safety concepts
to industrial facilities would significantly reduce
the risk of industrial accidents with severe consequences
for the public and the environment
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18. For further information please contact:
Dr. Gernot Thuma
Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH
Schwertnergasse 1
50667 Köln
Germany
phone: +49-(0)221-2068-607
fax: +49-(0)221-2068-10607
email: Gernot.Thuma@grs.de
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