3. Textos literarios en la clase de ELE
3.2.1. Los géneros literarios en ELE
18.3.1
Prevention of Accidents
The basic safety objective of the CANDU design is to make sure that the risk to public health is limited. Radioactive material trapped in the fuel can only be released to the public if the barriers, discussed above, are accidentally breached. The first line of defence is to prevent accidents. Accident prevention is incorporated into the CANDU design by providing:
COMMITMENT TO QUALITY
• high level of quality during all aspects of the project • strict quality control during manufacturing and installation • use of proven components
• use of well-trained staff
• periodic inspection and testing of components and systems • safe and efficient operation within the operating envelope • high level of automation to reduce the risk of operator errors
REDUNDANCY
Redundancy is the use of two or more components or systems when each are capable of performing the necessary functions. System redundancy is achieved by having independent systems (such as two shutdown systems) to perform equivalent functions, and by satisfying the “two group” design concept. Two groups of safety-related systems are provided in the station to maintain the station in a safe state even if a failure of one of the groups occurs. Group seperation provides inherent protection against common-cause failures from disturbances such as fires and third party acts that can influence a limited area of the station.
The two groups are:
• Group 1 systems
Systems that provide a safety function to mitigate an event, and that also perform a safety function or power production function during normal station operation. Group 1 includes:
÷ the power production systems ÷ one group of special safety systems ÷ a set of safety support systems
• Group 2 systems
Systems that provide a safety function to mitigate an event, but perform no function during normal station operation, are allocated as Group 2 systems wherever possible. Group 2 includes:
÷ the second group of special safety systems ÷ a second set of safety-support systems
Component redundancy is built-in for the special safety systems (the two shutdown systems, emergency core cooling system, and the containment system) that makes sure that the single failure criterion is satisfied. special safety systems satisfy an
unavailability target of 10-3, which effectively requires redundancy of all critical components. The availability of these systems is verified during operating by regular safety system component tests.
Process systems also make extensive use of redundancy to improve the station availability in the production of electrical power. This redundancy minimizes the frequency with which serious process failures occur.
DIVERSITY
Diversity is the use of two physically or functionally different means of performing the same function. It provides protection against certain types of common-mode failures, such as those arising from design or maintenance errors.
Providing two shutdown systems for CANDU reactors is good example for diversity. The design concept of system diversity is also used in the design of independent emergency cooling water and power systems provided via the two-group approach which perform support services. In addition, CANDU nuclear power stations are required to design for dual failures that consist of a design-basis initiating event with an assumed coincident unavailability impairment of one safety system. This means that the station is designed, for example, to mitigate a Loss Of Coolant Accident (LOCA) combined with loss of ECCS injection and the moderator system is shown to be an adequate means of fuel cooling for this event.
SEPARATION
Separation refers to the use of barriers or distance to separate components or systems that perform similar safety functions. Therefore, if a failure or localized event occurs in or near one system or component, it is unlikely to affect the other. Separation provides protection against common-mode or cross-linked effects such as fires and missiles.
Physical and functional system separation is designed into CANDU nuclear power stations to satisfy the two-group concept. The components of special safety systems
that perform similar functions are separated to the maximum practical extent. Redundant components within systems are physically separated according to their susceptibility and common hazards. Specific requirements are applied to the triplicated instrument cables and the duplicated power and control cables for safety-related systems. The odd and even concept of on-site power distribution is applied to equipment, the raceway system and junction boxes, to maintain physical separation between the odd and even systems. This results in maximum reliability under normal and abnormal conditions.
The separation of special safety systems from the systems used for power production (process systems) is one of the fundamental safety principles and a regulatory requirement in Canadian practice. Its objective is to make sure that events affecting a limited area of the station and functional interconnections between systems do not impair the capability to perform the required safety functions under accident conditions.
18.3.2
Mitigation of Accidents
Mitigation of the consequences of accidents is achieved by design provisions and operating procedures. These include:
• measures to prevent fuel failure following a serious process failure; • provisions to contain radioactive materials in the event of fuel failure.
Accident mitigation is also achieved by incorporating reliable and effective special safety systems that are capable of:
• limiting the consequences of accidents;
• incorporating multiple barriers as described in Article 18.2.2;
• incorporating measures to protect these barriers from damage due to accidents.
Mitigation of accidents also includes building redundancy and diversity as described in Article 18.3.1 in order to continue to provide important safety functions, such as electric power and heat removal, even after some components have failed as a result of an accident.