Safety Concepts of Plant Design
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This presentation discusses the safety aspects of nuclear power plant design with respect to design basis parameters. It introduces concepts of nuclear safety and defense-in-depth. The document outlines safety objectives, requirements for multiple barriers and redundancy. It describes categories of postulated initiating events and how common cause failures are addressed. Safety classification and detailed design rules are also summarized. Finally, the presentation provides an overview of site-specific external hazards for the Rooppur Nuclear Power Plant in Bangladesh and compares its design basis safety to that of VVER reactor designs.
Safety Concepts of Plant Design
- 1. Welcome To my Presentation on “An Approach to Assess the Safety Aspects of a Nuclear Power Plant with Respect to Design Basis Parameters”
- 2. 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.
- 3. 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
- 4. 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
- 5. 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 Authority General Design Criteria Design Methods Proven Engineering Practices Requirement Specifications Quality Plans Operational Experience and Safety Research Safety Analysis Design Documentatio n Qualification or Quality Assurance Verification of Design Independent Verification
- 6. 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 Release Specific Requireme nts Multiple Protective Barriers Protection and Reactivity Control Systems Fluid systems Reactor Containme nt Fuel and Reactivity control
- 7. 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.
- 8. 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.
- 9. 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.
- 10. 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
- 11. 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
- 12. 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.
- 13. 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. •Maximum use of proven technologies. •Minimum cost and construction times. •Balanced combination of active and passive systems. •Reduction in influence of human factors. Main principles of new VVER designs •Passivity •Multiple train redundancy •Diversity •Physical separation Concept of safety systems
- 14. Safety Systems Active Safety Systems Pressurizing System Emergency Boron Injection System Emergency Feed Water System Residual Heat Removal System Double Containment Spray System Emergency Power Supply System Passive Safety Systems Emergency Core Cooling System (Passive Part) Passive Containment Heat Removal System Passive SG Heat Removal System Passive Hydrogen Removal System Passive Reactor Scram System Passive Corium Catcher
- 15. 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 Seal Structure of Circulation Pumps Spray System Fire Safety Power Supply Systems Advanced Safety Features
- 16. 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/h Wind Speed
- 17. 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.
- 18. 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. VVER Design Basis Safety Seismic: OBE-0.12g,SSE-0.25g Flooding: Mobile systems for removal of heat to the ultimate sink (engine driven pumps, fast to assembly piping). Wind Speed: Maximum Design Wind Velocity 56m/s. Aircraft Crash: DBAW – 5t at a Fall Rate-120 m/s,VVER-1000 (AES-92); DBAW- 5.7t at a Fall Rate-100 m/s,VVER-1200 (AES-2006); DBAW-20t at a Fall Rate-215 m/s and BDBAW- up to 400t at a Fall Rate-150 m/s, VVER-TOI. Tsunami: Able to Withstand Impact of Tidal Waves as High as 14m.
- 19. Comparison of VVER-1000 and VVER-1200 VVER 1000 VVER 1200
- 20. 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.
- 21. Thank You For Your Attention