safety concepts of plant design
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Welcome To my Presentation on
“An Approach to Assess the Safety Aspects of a Nuclear Power Plant with Respect to Design Basis
Parameters”
Nuclear Safety• Nuclear safety had been the central issue of nuclear reactor design since the
inception of nuclear power.
• The term “Safety” in the context of nuclear technology means the status and the ability of a nuclear installation to prevent uncontrolled development of fission chain reaction or unauthorized release of radioactive substances or ionizing radiation into the environment and to mitigate the consequences of incidents and accidents at nuclear installations.
• A nuclear power plant is assumed to be safe when its radiation impact in all operational states is kept at a reasonably achievable low level and is maintained below the regulatory prescribed dose limits for internal and external exposure of the personnel and population and when in case of any accident including those of very low frequency of occurrence, the radiation consequences are mitigated.
Safety Objectives and Concepts
The nuclear safety objectives and concepts:• establish the mandatory safety requirements that define the elements necessary to ensure nuclear
safety. • are applicable to the design and operation of the associated structures, systems and components as
well as to procedures important to safety in nuclear power plants.
Safety Objectives
General Nuclear Safety Objective
Technical Safety Objective
Radiation Protection Objective
Safety Concepts
The Concept of Defense-
in-Depth
Consideration of Physical
Barriers
Operational Limits and Conditions
The Concept of Defense-in-Depth• Defense-in-Depth is an element of the safety philosophy that employs successive
compensatory measures to prevent accidents or mitigate damage if a malfunction, accident or naturally caused event occurs at a nuclear facility.
• Application of the concept of defense in depth throughout design, construction and operation will provide a graded protection against a wide variety of transients, anticipated operational occurrences and accidents.
• The concept is applied in practice through the following procedures:
Prevention of Failures
Limiting The Effect of Failures
Limiting Design Basis Accidents
Severe Accident Control
Mitigation of Consequences of Significant Release
Design Phase
• Conservative design approach plays a prominent role in ensuring the safety and integrity of a nuclear
power plant throughout its life cycle.
• Design phase is the transformation of a thought to a reflection of the soon
to be built plant. • Assessment of safety is
carried out in each and every step of the process to
ensure the safest plant design as practicable.
Design Authorit
y
General Design Criteria
Design Methods
Proven Engineer
ing Practices
Requirement
Specifications
Quality Plans
Operational
Experience and Safety
Research
Safety Analysis
Design Documentation
Qualification or Quality
AssuranceVerificati
on of Design
Independent
Verification
Design Basis• The main basis for the design of a nuclear
power plant is that the possibility of an accident causing significant radioactive release is eliminated .
• A necessary and adequate condition for meeting this safety objective is that three fundamental safety functions are provided.
• To ensure a safety level as high as reasonably achievable through design, the following six categories are taken into account to ensure optimum safety of the plant.
Safety Functions
Control of Reactivity
Decay Heat Removal
Containment of Radioactive
ReleaseSpecific Requirements
Multiple
Protective
Barriers
Protection and Reactivi
ty Control Systems
Fluid systems
Reactor Contain
ment
Fuel and
Reactivity
control
Design Rules and Limits
The design authority will specify the engineering design rules and limits for all SSCs. These will comply with appropriate accepted engineering practices. The design will also identify SSCs to which design limits will be applicable. These design limits will be specified for normal operation, AOOs and DBAs. The design limits will include:
• Radiological and other technical acceptance criteria for all operational states and accident conditions;
• Criteria on protection of fuel cladding and maximum allowable fuel damage during any operational state and design basis accidents;
• Criteria on protection of the coolant pressure boundary;• Criteria on protection of the containment in case of extreme external events, severe
accidents and combinations of initiating events.
Categories of PIEs
• Postulated initiating events can lead to AOO or accident conditions and include credible failures or malfunctions of SSCs as well as operator errors, common-cause internal hazards and external hazards. Postulated initiating events will be grouped into different categories depending on their frequency of occurrence per calendar year.
• Category 1: steady and transient states during normal operation; • Category 2: anticipated operational occurrences, with frequency of 10-2 events per year; • Category 3: accidents of low frequency of occurrence, in the range between 10-2 and 10-4
events per year; • Category 4: design basis accident of very low frequency of occurrence, in the range
between10-4 and 10-6 events per year.
The Postulated Initiating Events (In Detail)
C1 (NO)• Start up• Power operation• Hot standby• Hot shutdown• Cold shutdown• Refueling• Operation with an inactive
loop• Temperature increase and
decrease at a maximum admissible rate
• Step load increase and decrease (by 10 %)
• Load increase and decrease (at a rate of 5 % load/minute) within the range between 15 and100 % full power
• Switch-over to house load operation from 100 % power with steam dump
• Limiting conditions allowed by the OLCs.
C2 (AOO)
•Inadvertent withdrawal of a control rod group with reactor subcritical•Inadvertent withdrawal of a control rod group with reactor at power•Static misalignment of control rod or drop of a control rod group•Inadvertent boric acid dilution, partial loss of core coolant flow•Total loss of load or turbine trip•Loss of main feed water flow to steam generators•Malfunction of the main feed water system of steam generators•Total loss of off-site power (up to 2 hours)•Excess increase in turbine load•Very small loss of reactor coolant
C3 (DBA)• Loss of reactor coolant (small
pipe break)• Small secondary pipe break• Forced reduction in reactor
coolant flow• Mispositioning of a fuel
assembly in the core with consequent operation
• Withdrawal of a single control rod in power operation
• Inadvertent opening and sticking open of a pressurizer safety valve
• Rupture of volume control tank
• Rupture of gaseous radioactive waste hold-up tank
• Failure of liquid radioactive waste effluent tank
• One steam-generator tube break without previous iodine spiking
• Total loss of off-site power (up to 72 hours).
C4 (BDBA)
• Main steam line break• Main feed water line break• Ejection of any single control
rod• Loss of reactor coolant and
double-ended guillotine break of the largest pipe
• Fuel handling accidents• One steam generator tube
break with previous iodine spiking.
Common Cause Failures• Common-cause failures occur when multiple components of the same type fail at the
same time.• Failure of a number of devices or components to perform their functions may occur as a
result of a single specific event or cause.• The event or cause may be a design deficiency, a manufacturing deficiency, an operating
or maintenance error, a natural phenomenon, a human-induced event, or an unintended cascading effect from any other operation or failure within the plant.
• The design will provide the following remedies against common cause failures-
Physical Separation
Diversity
Safety Class
• For the purpose of classification, the nuclear power plant shall be divided into structural or operational units called systems.
• Every system that is a structural or operational entity shall be assigned to a safety class.• When safety classification is established and applied attention shall be paid to the fact that
the ensuring of safety functions sets different requirements on equipment of different types.
Safety Class 1
Safety Class 2
Safety Class 3
Safety Class 4
Nuclear Power in Bangladesh• Bangladesh is venturing into uncharted territory by opting for nuclear power to
meet growing electricity demands.• The first ever nuclear power plant of the country will be built at Rooppur for
producing 2000 MW(e) from two units of power. • The Bangladesh government has signed with the Russian Government to
construct the power plant using the advanced VVER designs.• Existing VVER nuclear power plants have demonstrated around 1500 reactor
years of safe and effective operation.• New VVER designs are the evolution of proven VVER technology by improving
plant performance and increasing plant safety.• The viability of new passive systems implemented in new VVER design is
confirmed by extensive R&D works.
Safety Concept of VVER Designs• The safety philosophy embodied in the new VVER designs is unique among reactors on the
market deploying a full range of both active and passive systems to provide fundamental safety functions. Its safety systems can thus handle complicated situations that go beyond the traditional design basis accidents.
Main principles of new VVER designs
• Maximum use of proven technologies.• Minimum cost and construction times.• Balanced combination of active and passive systems.• Reduction in influence of human factors.
Concept of safety systems
• Passivity• Multiple train redundancy• Diversity• Physical separation
Safety Systems
Active Safety Systems
Pressurizing System
Emergency Boron
Injection System
Emergency Feed Water
System
Residual Heat
Removal System
Double Containmen
tSpray
System
Emergency Power Supply System
Passive Safety SystemsEmergency
Core Cooling System
(Passive Part)
Passive Containment Heat Removal
System
Passive SG Heat Removal
System
Passive Hydrogen Removal System
Passive Reactor Scram
SystemPassive Corium
Catcher
Advanced Features• The following safety systems are provided in the
design as additional facilities aimed at severe accident management
Severe Accident Management System
Core Melt Localizing Facility
Passive System of Heat Removal from Containment
Passive System of Heat Removal from Steam Generators
Spray System
Power Supply Systems
Advanced Safety
Features
Overview of Site Specific External Hazards
• The influence of Tsunami wave and Tornado at the specific site is practically zero with no occurrence till date and not projected for a lengthy return period. Also there has been no incident of any aircraft crash or major external explosion at the proposed site.
• Maximum Magnitude of Earthquake: 7.6 Mw in 1918 (Epicenter Distance - 203 km)
• Magnitude of Nearest Earthquake: 4.7 Mw (Epicenter Distance - 39 km)• Probabilistic PGA: 0.18g-0.20g (for a return period of 2475 years)
Seismic Events
• Maximum Water Level: 15.19 m (1998)• Predicted Maximum Water Level : 18.44m (1 of 1000 years cycles)Flooding
• Basic Wind Speed: 200 km/hWind Speed
Structural Solutions for Enhancing Protection
Seismic• Weak soils to be avoided or
compacted .• Length of a block be
restricted to three times of its width.
• Safety related main buildings be designed as Seismic category-1.
• Plant components belonging to Seismic Category-1.
• Diverse and spatially separated safety systems.
• Seismic detectors be installed onto the base mat.
• Consideration of gravitational cooling water supply or cooling with natural circulation.
Flooding
•Platforms of safety classified equipment be at a level at least equal to the MDFL (19m).•Elevated arrangement (>9m) of electrical switchgears and fuel tanks for the backup diesel generators.•Flood safe enclosures , Seals against water load, Water-tight design of penetrations and emergency core cooling systems, Adequate drainage system.•Water tight doors for the supplementary control room and the four diesel generator - safety train rooms.•Mobile flood barriers and bilge pumps.
Wind Speed
• Increasing the thickness of outer containment wall or using Modular wall barrier system.
• Plant components and safety systems designed to withstand Maximum Design Load.
Aircraft Crash
• Change of construction technique for the Shield building from reinforced concrete to a plate and concrete sandwich structure.
• Separation of external fencing structures with contraction joint and annulus from the building internal structures.
• Separation of safety systems with fire-proof physical barriers along their whole length.
Comparison of Probable RNPP and VVER Design Basis Safety
RNPP Design Basis Safety
Seismic: OBE-0.12g, SSE-0.22g.
Flooding: DBFL: 19m
Wind Speed: Design Wind Velocity > 55 m/s.
Aircraft Crash: Design Basis Aircraft Weight- Large Passenger Airplane.
Tsunami: Influence of Tsunami Wave at the site is practically zero.
Comparison of VVER-1000 and VVER-1200
VVER 1000 VVER 1200
Ideal Design Characteristics of RNPP• OBE: 0.12g, SSE: 0.22g. • DBFL: 19m and availability of flood protection measures.• Maximum design wind load > 200 km/hr. • Generating units with double containment shell. • Increased thickness of the housing building of the four trains of safety systems.• Combination of active and passive safety systems (boron injection system, passive heat
removal systems and a molten core catcher).• Elevated backup water tanks and large decantation ponds.• Cooling towers.• Outfitting of power units with hydrogen explosion, steam explosion and direct containment
heating protection systems.• Mobile diesel generators to ensure long term safe conditions of power units in case of NPP
blackout. • Diversity of all systems of AC emergency power.• Separation of I&C systems.
Thank YouFor Your Attention
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