System Safety Assessment process conditions

This publication establishes design requirements for stractures, systems and components important to safety that must be met for safe operation of a nnclear power plant, and for preventing or mitigating the consequences of events that could jeopardize safety. It also establishes requirements for a comprehensive safety assessment, which is carried out in order to identify the potential hazards that may arise from the operation of the plant, under the various plant states (operational states and accident conditions). The safety assessment process includes the complementary techniqnes of deterministic safety analysis and probabilistic safety analysis. These analyses necessitate consideration of postulated initiating events (PIEs), which include mat r factors that, singly or in combination, may affect safety and which may ... 

The term DAL comes from Radio Technical Commission for Aeronautics (RTCA)/DO-254, Design Assurance Guidance for Airborne Electronic Hardware, 2000. In this document hardware is classified into five levels based on a set of criteria for each level. The derived software level (SL) is based on the contribution of the software to potential failure conditions as determined by the system safety assessment (SSA) process. 

The DAL is an index number ranking the safety-criticality of the system functions. This ranking implies that in order to make the system safe, greater development rigor must be applied to each successively critical level. Table 2.3 correlates the hardware DALs to the five classes of failure conditions and provides definitions of hardware failure conditions and their respective DALs. Initially, the hardware DAL for each hardware function is determined by the SSA process using a functional hazard analysis (FHA) to identify potential hazards and then the preliminary system safety assessment (PSSA) process allocates the safety requirements and associated failure conditions to the function implemented in the hardware. 

In 1985, Uie Cimadian Chemical Producers Association (CCPA) released a pampWet entitled, "Essential Components of Safety Assessment Systems . Modifications to process or plant was one of the topics discussed in Uiis pamplilet. CCPA recommended a 12 element program (listed below) to formally e.xamine and approve process conditions whellier permanent or temporary prior to implementation. 

The RC1 is an automated laboratory batch/semi-batch reactor for calorimetric studies which has proven precision. The calorimetric principle used and the physical design of the system are sound. The application of the RC1 extends from process safety assessments including calorimetric measurements, to chemical research, to process development, and to optimization. The ability of the RC1 to generate accurate and reproducible data under simulated plant scale operating conditions may result in considerably reduced testing time and fewer small scale pilot plant runs. 

The flow chart showing the iterative safety assessment procedure for a chemical process under normal operating conditions (c.f. Section 2.1) has its central step in the evaluation of an adequate thermal design of the process. This is shown in a simplified form as the comparison of the chemically produced heat and the heat removal capacity of the system. A necessary prerequisite to this assessment of the suitability of the design is the knowledge of the time course of the heat production rate, which itself is directly proportional to the chemical reaction rate. This explains the pivotal significance of the identification of a reaction rate law that describes the investigated process with sufficient accuracy, and its parameters. 

Before discussing the safety assessment of chemical processes under normal as well as under upset conditions in detail, the classical heat explosion theory shall be treated. The first scientists to investigate the so-called runaway of an exothermic chemical reaction were Semenov and Frank-Kamenetzidi [18,19]. They were the pioneers in investigating and describing the self-heating process of reacting systems up to an explosion-like temperature rise in its dependence on different heat loss conditions to the environment. The criteria they derived are still valid today and form the basis of any safety assessment. 

In an additional step the safety assessment has to include the evaluation of the system s response to the normal variability of all process defining variables. This shall help to exclude the occurrence of severe process deviations leading to uncontrollable upset conditions. Such considerations are also called sensitivity analyses. 

Criticality. The process plant is designed on the principle of safe geometry under all conditions (e.g., the use of HARP tanks). Continuous monitoring is provided by an approved criticality detection and alarm system. Where additional operational control is required to maintain safe conditions, this is specified in the appropriate nuclear safety assessments and Criticality Clearance Certificate. The latter specifies limits and conditions that need to be complied with during operations for example, in respect of limited tap density, moisture content, isotopic inventory and mass. It identifies any systems or instrumentation that demonstrate that compliance is maintained during operation. In addition key points of the plant are monitored by neutron monitors to give early warning of the unanticipated build-up of solid plutonium which could lead to the development of unsafe conditions. 

The time-dependent evolution of the system and process dynamics in interaction with the stochastic failure behaviour of safety systems and hiunan actions is reduced to static cause-effect models which operate with fixed probabilistic assessments for the stochastic behaviour. The order of events is predetermined by the expert and may possibly represent the chronological order of some reference sequences, but the question is, whether it is apphcable to all sequences. What is the consequence, if specific process conditions induce another order of different events ... 

Early reactor safety assessments [S-1] hypothesised that severe accidents would entail the prompt release of a significant fraction of a bounding radionuclide (t5q)ically iodine) to the reactor containment. Safety systems were designed, then, for massive, immediate response to this release. Now, it is understood that radionuclide releases will take place by multiple processes over protracted periods and will involve many different radionuclides in different chemical and physical forms. Mitigation methods will have to operate for long periods and may have to change as the sources of radionuclides vary. The inventories of radionuclides available for release from reactor fuel under accident conditions and the processes that lead to releases of these radionuclides are discussed in the next subsections of this report. 

The veirious types of measurement technologies for assessment of corrosion may be summarized as shown in Tables 2 to 5. These techniques cover both laboratory Jind field use. However, many of the direct methods, particularly the electrochemical methods of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) are generally more suited to laboratoiy evaluation. In the laboratory, test conditions are clean and more controlled. Consequently, more sophisticated measurement electrode systems can be used that take advantage of their more sophisticated measurements technologies. In the field, practicalities of changing process conditions, high flow rates, debris, electrical noise, and electrical safety limit their use. 

The starting point of the safety management process is usually taken to be the risk assessment. More often than not, when doing risk assessment and analysis, the knowledge of experts in the particular field is exploited if not essential. Nowadays even apparently simple systems consist of parts designed by experts of different disciplines electronics, mechanical, software and ergonomics come to mind first. On top of this, modem products are used in a wide variety of applications with different operational conditions. 

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