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