Design of Safety Systems

The necessary protective measures have to be implemented by safety equipment, which in turn has to be dimensioned on the basis of the expected technical demands. 

In nuclear installations the so-called design basis accidents are used for this purpose [19]. For example, the complete failure of the main coolant pipe of a reactor ( 2-F — rupture because the entire cross section is open on both sides) or the failure of the electric supply [19]. The design basis accidents serve to determine the type and dimensions (e.g. capacity, temperatures, cooling power...) of the corresponding safety systems, for example the emergency cooling system for counteracting the breach of the main coolant pipe. 

In contrast only general requirements are formulated in the field of process plants. This is explained by fact that we have to deal with a great variety of different plants and equipment, which impedes the formulation of specific requirements. Following these general requirements potential hazards are identified for different equipments by systematic investigations (vid. Chap. 9) and the necessary protective equipments are conceived. The basis is given by the following classification of potential accidents (cf. [20]). 

Major accidents against which preventative measures have to be taken ( zu verhindemde Storfalle ) result from operational malfunctions in a plant 

Major accidents occurring despite preventative measures ( Dennoch Storfaiie ) stem from a progression of operational malfunctions which cause a serious danger despite the existence of accident preventing measures. They result from hazard sources which can reasonably be discarded or from the simultaneous impact of several independent sources of hazards. In order to hmit the consequences of such accidents plant specific measures and specific measures of hazard defence have to be taken. 

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Balls, B. W., A. B. Rentcome, and J. A. Wilkenson. 1987. Specification and Design of Safety Systems for the Process Industries, 8th International System Safety Conference, New Orleans, EA. 

The design of safety systems supported by PSA provides efficient designs at a lower cost than those based solely on engineering criteria, guaranteeing a priori an adequate level of safety with a predetermined risk acceptance. It is also a useful tool for appropriate location of isolation valves. 

The design of safety systems for weUs has been and is still very prescriptive in nature based on current recommended practices like API RP 14C (Ref. 1). Many companies today are also reviewing the design of the safety system for the weUs for compliance with the performance-based requirements of lEC 61508 (Ref. 2) and ANSI/ISA-84.00.01-2004 (lEC 61511 Mod) (Ref. 3) and comparing the differences. Most are discovering that the new standards allow higher levels of safety at a lower cost, thus the enthusiasm for the new standards. 

Accurate response times are essential in the design of safety systems for the column. Rapid increases in pressmes and temperatures can occur in seconds, and accurately determining the rates of increase in these important variables and the time period to reach critical limits (safety response time) permits the engineer to quantitatively design effective safety systems. 

The main criteria used in the design of safety systems were simplicity, reliability, redundancy... 

The main criteria used in the design of safety systems were simplicity, reliability, redundancy and passivity. Special emphasis has been put on minimizing the dependence on active components and operators actions (figure 3). 

Since the release of EN 61508, there have been rafts of other standards developed that address specific industries and other disciplines. Two standards that are commonly asked to comply with in designing hoisting are EN 62061 Safety of machinery Functional safety of electrical, electronic and programmable electronic control systems, and EN ISO 13849-1 Safety of machinery Safety-related parts of control systems. The latter standard caters for design of safety systems irrespective of the technology used, whether it is electrical, mechanical, hydraulic, pneumatic etc. 

The Concept of defence-in-depth is applied in designing safety systems to achieve functional diversity, i.e. by providing diversely functioning systems that can perform same safety function (e.g. two shutdown systems), multi barriers to prevent release of radioactivity, multi-defence system, using physical separation of systems and components which serve as back-up (in safety functions) to each other, and procuring components for different systems from different suppliers, to the extent possible. Such an approach leads to a design of safety systems which will be tolerant to a wide range of human errors and equipment failures. 

Specifically, during the design of safety systems or components for which snubbers are to be used, sufficient consideration should be given as to their unique application, i.e., their response to normal, upset, and faulted conditions and the effect of these responses on the associated system and/or component. 

Safety systems must satisfy the recommendations for future reactors. The design of safety systems is in progress. 

Gd203 is used as burnable poison in specific fuel rods, while movable silver-indium-cadmium absorber rods control core reactivity. The control rod drives placed inside the RPV are hydraulically driven. Liquid boron is not used for reactivity control during normal operation. The design of safety systems meets the regulatory requirements of nuclear industry as for redundancy, independence, physical separation, diversification, and failure into a safe state. CAREM safety systems are designed to eliminate the need of active intervention in accidents within a long period of time. 

The KLT-40S power plant [13] was developed on the basis of a standard KLT-40 type nuclear propulsion plant that has the experience of more than 250 reactor-years of failure-free operation. Components of the original plant have been modernized to increase plant reliability, to extend its service life and to improve the conditions of maintenance. The design of safety systems is based on safety regulations for marine reactors and was updated to meet the requirements of the Russian Regulatory Authority - GAN RF - for nuclear power plants. 

Regarding the new design of safety systems it is proposed to reduce significantly the number of sensors and to allow multipurpose usage of sensors. An additional reduction of sensors will be achieved by integration of the reactor trip and ESFAS systems. Proven isolation devices will be used for the isolation of process systems from safety systems. 

Simplified design of safety systems, broad implementation of inherent safety features,... 

Inherent safety features. HTR-PM incorporates inherent safety features, such as the intrinsic reactor shutdown under temperature increase and passive decay heat removal. Such features make it possible to simplify the design of safety systems, but need a demonstration to be rated as sound and convincing. Safety demonstration experiments are being conducted within the HTR-10 test reactor facility. 

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