NUCLEAR SAFETY

DR. Ahmed A. El Kady

INTRODUCTION For a nuclear facility to be built, adequate assurance shall be provided to the society that the facility will be safe .

The centeral safety goal in any nuclear installation is to assure that the designe is such that, insofar as is reasonable or practical, the radioactive material remain safely confined at all times-during operation & refueling of the plant and preparation and shipping of spent fuel and radio-active waste.

Safety should whenever possible be based on elemenating or minimzing the prime cause of danger rather than relying on secondary measures of protection.

Is Nuclear Power Safe? It is physically impossible for a Commercial Nuclear

Power Reactor to explode like a nuclear bomb

• There are at least three barriers against the dispersion of fission products (F.P);

• There are many automatic safety systems;

• Nuclear Reactors are strictly regulated by Regulatory Body, both during Sitting construction and during operation;

• Nuclear Power is as safe as, or safer, than other energy technologies;

• New reactor types(AP-1000, ESBWR=1000, CANDU-9)

have many characteristics which enhance, inherent Safety.

Multiple Barriers To block the passage of radioactive atoms from

the fuel to the surrounding population.

1- The Fuel

F.P are highly ionized particles and except for those originated near the surface of the fuel, they all come to rest within the fuel, The F.P gases I, Xe, Kr undergo diffusion and may escape from the fuel. In PWR, BWR these gases are held in the pellet- cladding gap and collected in a small plenum at the end of each fuel.

2 - Cladding

3- Closed -Coolant System

Multiple Barriers Continue; 4- Reactor Vessel

5- Containment, Confinement

6- Site Location

7 – Evacuation

In order to assure that none of the barriers is compromised as a result of abnormal ocurrencies as equipment failure, human error…, the nuclear industry has established some key design and operation principles.

Some Key Design Principles • Plant sensitivity to faults

• Defence in depth

• Equipment qualification

• Safety categorisation

• Special case procedure

• Diversity & redundancy

• Single failure criterion

• Common cause failure

Plant Sensitivity To Faults - A failure or maloperation should produce no significant operational response, or should produce a change in the plant state towards a safer condition;- Following a failure or maloperation, the plant should be rendered safe by the action; of passive features or engineered safeguards which are continuously available in the state required to control the fault;- Following a failure or maloperation the plant should be rendered safe by the action of active engineered safeguards which need to be brought into service in response to the fault.

Defence In Depth Principle: To compensate for potential human and mechanical failures, a defence in depth concept is implemented, centred on several levels of protection including successive barriers preventing the release of radioactive material to the environment. The concept includes protection of the barriers by averting damage to the plant and to the barriers themselves. It includes further measures to protect the public and the environment from harm in case these barriers are not fully effective.

Defence In Depth Continue;

The defence in depth concept provides an overall strategy for safety measures and features of nuclear power plants. When properly applied, it ensures that no single human or mechanical failure would lead to injury to the public, and even combinations of failures that are only remotely possible would lead to little or no injury. Defence in depth helps to establish that the three basic safety functions (controlling the power, cooling the fuel and confining the radioactive material) are preserved, and that radioactive materials do not reach people or the environment.

First Level 1. The reactor should have a prompt negative

temperature coefficient and a negative void coefficient.

2. Only materials whose properties are known to be stable under the operating conditions of the plant, including radiation exposure, should be used for the fuel, coolant, and safety-related structures.

3. Instrumentation and controls should be provided so that the plant operators know and have control over the status of the plant at all times. Sufficient redundancy must be included that loss of key instruments or controls does not deprive operators of needed information or prevent shutdown of the plant.

First Level Continue;

4. The plant must be built, equipment installed in a manner that satisfies the highest standards of engineering practice.

5. Continual or periodic monitoring and inspection for signs of wear and incipient failure, and to permit periodic testing of the components

Second level :1. The reactor must be provided with an

emergency core cooling system (ECCS) to prevent meltdown of the fuel and release of fission products due to fission product heating following a loss-of-coolant accident.

2. The reactor must have redundant capability for fast shutdown in the event that some of the control rods cannot be inserted, either because they are physically stuck, or because of a malfunction of electrical circuitry.

Second level Continue ;

3. The plant must be furnished with sources of power that are independent of the operation of the reactor to operate the ECCS if this becomes necessary, to provide power for the continued operation of instrumentation, and for other emergency uses in the plant. Such emergency power includes off-site power supplied by two physically separated access circuits, and on-, site power from generators driven by fast-starting, physically separated, and redundant -in- number diesel engines. On-site DC power for instrumentation is normally also supplemented with batteries.

Third level   This third level of safety supplements the first two, by

adding a margin of safety in the event of extremely unlikely or unforeseen events. The need for additional engineered safety features is determined by analytically evaluating the effect on the plant, its associated personnel, and the public, of severe incidents arising from the simultaneous failure of various components of the facility and some of the redundant systems. Such events, used in this way to evaluate the overall safety of a plant and to point up the need for supplementary safety systems, are called design basis accidents (DBA) .The analysis of DBAs, plays an important role in the design and licensing of nuclear power plant.

Third level Continue; Example of this Safety systems are :

Emergency core cooling system, ECCS;

 Containment cooling system CCS;

Light pressure injection cooling system

Redundant shut down system

Decreasing power density.Incoming coolant inventory

Fourth LevelPrevention of Accident Beyond DBAThe use of equipment and administrative procedure for;• Prevention of deviation from normal operation,• Prevention of anticipated operational occurrences which would lead to accident conditions;• Control and mitigation of accident conditions , and consequences;. Oxygen recombines or deletion.. Pressure vessel cooling,. Fuel molten catcher

Fifth Level Provisions for accident mitigation

extend the defence in depth concept beyond accident prevention.

The accident mitigation provisions are of three kinds, namely;

• Accident management; • Engineering safety features; and

cooling of vessel and containment. • Offsite counter measures, (emergency

plane).

Single Failure Criterion No single random failure assumed to occur Anywhere within the safety systems provided to perform a safety function should prevent that function being performed during any normally permissible state of plant availability. Consequential failures resulting from the assumed single failure should be considered as an integral part of the single failure.

Common cause failureThe failure of a number of devices or components to perform their functions as a result of a single specific event or cause (maintenance errors, national phenomenon, a new induced event, manufactures deficiency...).

Diversity & Redundancy The design should make the best use of diversity, redundancy and segregation in the structures, systems and components which are important to safety.Diversity: the existence of redundant components or systems to perform an identified function, where such components or system incorporate one or more different attributes (diff. operating conditions, diff. Sizes, diff. manufactures, diff. types of equip) that use diff. physical methods or principles).Redundancy: provision of more than the minimum number of (identical or diverse ) elements or systems, so that the loss of any one does not result in the loss of the required function of the whole.

Organization of the Regulatory Authority

The establishment of a specialized regulatory authority is of primary importance for the effective discharge of the national responsibilities in ensuring public health and safety with respect to nuclear power

plants and other nuclear facilities .

The IAEA has published and has under preparation a series of publications within the framework of the NUSS ( Nuclear Safety Standards ) programme. The fundamental objective of the regulatory authority are:

(a ) Establishment of regulatory, standards codes and criteria

which will govern , , design construction and operation

of nuclear power plants.

(b ) Review and evaluation of the safety analysis and

environmental reports submitted by ; the owner issue of licences;

( c ) Conduct of a programme of inspections to ensure compliance

with established rules andregulations.

The regulatory body may be organized into units which perform the activities corresponding to each of the above-listed

objectives .The organization of the regulatory body will necessarily depend upon the governmental structure, the legal system and the administrative practices of the country.

In setting up the organizational structure, the regulatory authority should be:

( a ) Vested by enabling legislation with a broad statutory authority and

, functional autonomy to carry out its functions independently of

, , applicants manufacturers suppliers and other interested parties both the

public and private sectors (b ) Staffed by highly qualified

personnel.

Responsibility of The Operating Organization Principle: the ultimate responsibility for the safety of a nuclear plant rests with the operating organization, this is in no way diluted by the separate activities and responsibilities of designers, suppliers, constructors and regulators.

Objectives of The Review And Assessment Process “A primary basis for the review and assessment is the information submitted by the operator. A thorough review and assessment of the operator's technical submission shall be performed by the regulatory body in order to determine whether the facility or activity complies with the safety objectives, principles and criteria. In doing this the regulatory body shall acquire an understanding of the design of the facility or equipment, the safety concept on which the design is based, and the operating principles proposed by the operator, in order to satisfy itself that:

Objectives of The Review Continue;

(1) The available information demonstrates the safety of the facility or proposed activity;

(2) The information contained in the operator's submissions is accurate and sufficient to enable verification of compliance with regulatory requirements; and

(3) The technical solutions, and in particular any novel ones, are proven or qualified by experience or testing or both, and are capable of achieving the required level of safety”.

Quality Assurance Principle: Quality assurance is applied throughout activities at a nuclear plant as part of a comprehensive system to ensure with high confidence that all items delivered and services and tasks performed meet specified requirements.The comprehensive system referred to in the principle begins with analysis and design in accordance with the preceding principle on proven engineering, and it continues into the use of quality assurance methods. Other fundamental technical safety principles are also important in this respect, particularly those on safety assessment and verification and on operating experience and safety research.

Assessment criteria, objectives and principles.To accomplish its tasks, the regulatory body, (RB), shall establish safety criteria, objectives, principles, guidance and regulations upon which to base its regulatory action.

1 . Acceptance criteria, In case where both the operating organization and the regulatory body develop acceptance criteria to reflect the respective philosophies, the set of acceptance criteria agreed upon by both organizations must be satisfactory to the RB. Such criteria may include consideration such as:

Acceptance criteria Continue;

1- Radiological criteria such as:

a) ALARA levels.

b)Dose limits for facility staff, including workers at the reactor site and the general public;

c)Release limits to the environment; and

d) Risk criteria (where applicable).

2-Performance criteria, including:a) limits to fuel cladding damage;

b) limits to damage of the primary coolant system boundary.

c) limits to containment systems damage;

d) Maintenance of core cooling; and

e) Frequency limits for certain anticipated operational occurences and for particular accident conditions, including frequency limits for significant fuel cladding damage

External factors affecting the plantPrinciple: The choice of site takes into account the results of investigations of local factors which could adversely affect the safety of the plant.Local factors include natural factors and man made hazards. Natural factors to be considered include geological and seismological characteristics and the potential for hydrological and meteorological disturbances. Man made hazards include those arising from chemical installations, the release of toxic and flammable gases, and aircraft impact.

External factors Continue; The investigations required give information on the likelihood of significant external events and their possible effects on nuclear power plant safety. This is developed in the form of quantified probabilities when possible. The corresponding risk evaluation takes into account the safety features provided by the design to cope with these events. Special attention is given to the potential for extreme external events and to the feasibility of installing compensating safety features.EarthquakesDBE - plant should "withstand safely” the ground motionsOBE - bring to safe state and not restart until shown safe

Radioactive Waste Management, and DecommissioningThe generation of radioactive waste, both in activity and volume, shall be kept to the minimum practicable by appropriate design measures and operating practices. Waste treatment and interim storage shall be strictly controlled in a manner consistent with the requirements for safe final disposal.The design of an installation and the decommissioning programme shall take into account the need to limit exposures during decommissioning to as low as is reasonably achievable. Prior to the initiation of decommissioning activities, the decommissioning programme shall be approved by the regulatory body.

Verification of safetyThe operating organization shall verify by analysis, surveillance, testing and inspection that the physical state of the installation and its operation continue in accordance with operational limits and conditions, safety requirements and the safety analysis. Systematic safety reassessments of the installation in accordance with the regulatory requirements, shall be performed throughout its operational lifetime, taking into account operating experience and significant new safety information from all relevant sources.

Human factorsPrinciple: Personnel engaged in activities bearing on nuclear power plant safety are trained and qualified to perform their duties. The possibility of human error in nuclear power plant operation is taken into account by facilitating correct decisions by operators and inhibiting wrong decisions, and by providing means for detecting and correcting or compensating for error.