Events and Risks

Effect of intensive diabetes management on macrovascular events and risk factors in the Diabetes Control and Complications Trial. AmJ Cardiol 1995 75 894-903. 

If the commonly used therapy has a high profile of adverse events and risk greater than benefits. 

Generally efficacious, however adverse events and risk of tolerance mean that they should only be considered when other pharmacological approaches have failed and should only be prescribed for veiy short periods of time. 

Inform event and risk Admit own error Express apology ... 

Table 3.1 The relation between threat, adverse event and risk...

Top Event and Risk (fat./yr) THR Apportionment to Signalling (contribution) and Notes ... 

The risk for tsunamis is in some parts of the world higher than in others. In general, the frequency and intensity of tsunamis in the Indian and Pacific Oceans and the Mediterranean Sea is much higher than in other regions of the world. A global overview of tsunami events and risks is for example presented in the following link http //www.nerc-bas.ac.uk/tsunami-risks/. At present, however, no accurate site-specific information is available with respect to the chance of occurrence and intensities of tsunamis. 

Methods for performing hazard analysis and risk assessment include safety review, checkhsts, Dow Fire and Explosion Index, what-if analysis, hazard and operabihty analysis (HAZOP), failure modes and effects analysis (FMEA), fault tree analysis, and event tree analysis. Other methods are also available, but those given are used most often. 

An important part of hazard analysis and risk assessment is the identification of the scenario, or design basis by which hazards result in accidents. Hazards are constandy present in any chemical faciUty. It is the scenario, or sequence of initiating and propagating events, which makes the hazard result in an accident. Many accidents have been the result of an improper identification of the scenario. 

A technique called probabiUstic safety assessment (PSA) has been developed to analy2e complex systems and to aid in assuring safe nuclear power plant operation. PSA, which had its origin in a project sponsored by the U.S. Atomic Energy Commission, is a formali2ed identification of potential events and consequences lea ding to an estimate of risk of accident. Discovery of weaknesses in the plant allows for corrective action. 

Mitigation Reducing the risk of an accident event sequence by taking protective measures to reduce the likelihood of occurrence of the event, and/or reduce the magnitude of the event and/or minimize the exposure of people or property to the event. 

A common risk evaluation and presentation method is simply to multiply the frequency of each event by consequence of each event and then sum these products for all situations considered in the analysis. In insurance terms, this is the expected loss per year. The results of an uncertainty analysis, if performed, can be presented as a range defined by upper and lower confidence bounds that contain the best estimates. If the total risk represented by the best estimate or by the range estimate is... 

Quantitative risk analysis is a forecast concerning the degree of belief associated with the occurrence of future events. It normally focuses on those classes of events that are rarely expected to occur at a facility. However, because the potential consequences of such events may be so great, the possibility that the events could occur at all gives rise to concern. When a QRA generates results that reflect a very small likelihood of an event and confirm the suspicion that the event could have a severe impact, these questions inevitably arise What does it all mean What should I do about it ... 

If a situation occurs which involves more than one fire risk area simultaneously (such as an entire Refinery or Chemical complex), it would be classed as a remote contingency event, and the 1.5 Time Design Pressure Rule may be applied. 

Mitigation The lessening of die risk of an accidental event. A sequence of action on the source in a preventive manner by reducing the likelihood of occurrence of the event, or in a protective manner by reducing the magnitude of the event and for the exposure of local persons or property. 

Identify initiating events and event sequences that might contribute significantly to risk,... 

This section reflects on the limitations of the PSA process and draws extensively from NUREG-1050. These subjects are discussed as plant modeling and evaluation, data, human errors, accident processes, containment, fission product transport, consequence analysis, external events, and a perspective on the meaning of risk. 

Two studies resolved the Unresolved Safety Issue A-44, "Station Blackout." The first siudy, The Reliability of Emergency AC Power Systems in Nuclear Power Plants," when combined uh die lelevant loss-oToffsite-power frequency, provides estimates of station-blackout frequencies lor 18 nuclear power plants and 10 generic designs. The study also identified the design and operational features most important to the reliability of AC power systems. The second study, "Station Blackout Accident Analysis" (NUREG/CR-3226), focused on the relative importance to risk of laiion blackout events and the plant design and operational features that would reduce this risk. 

A Safety Case is a narrative that literally makes the case that an adequate level of safety has been reached for an installation. It requires looking at all potential hazards which could lead to a loss of the installation, a loss of life, or a major pollution event. A risk analysis is performed on each hazard evaluating the probability of the event occurring and describing the magnitude of the consequences. A discussion is then given of the measure undertaken to lower the probability of occurrence or to mitigate the consequences and a case is made that the risk for the installation meets the ALARP safety criteria. 

Societal Risk - This represents a measure of the risk to a group of people, including tlie risk of incidents potentially affecting more tlian one person. Individual risk (see above) is generally not significantly affected by the number of people involved in an incident. The risk to a person at a particular location depends on tlie probability of occurrence of the luizardous event, and on the probability of an adverse imptict at that location should the event occur. 

Hazard, risk, failure, and reliability are interrelated concepts concerned witli uncertain events and tlierefore amenable to quantitative measurement via probability. "Hazard" is defined as a potentially dangerous event. For example, tlie release of toxic fumes, a power outage, or pump failure. Actualization of the potential danger represented by a hazard results in undesirable consequences associated with risk. 

Risk is defined as tlie product of two factors (1) tlie probability of an undesirable event and (2) tlie measured consequences of the undesirable event. Measured consequences may be stated in terms of financial loss, injuries, deatlis, or Ollier variables. Failure represents an inability to perform some required function. Reliability is the probability that a system or one of its components will perform its intended function mider certain conditions for a specified period. Tlie reliability of a system and its probability of failure are complementary in tlie sense tliat the sum of these two probabilities is unity. This cluipler considers basic concepts and llieorenis of probability tliat find application in tlie estimation of risk and reliability. 

Hazard, risk, failure, and reliability are interrelated concepts concerned with uncertain events and tlierefore amenable to quantitative measurement via probability. 

Tliis cliapter is concerned willi special probability distributions and tecliniques used in calculations of reliability and risk. Tlieorems and basic concepts of probability presented in Cliapter 19 are applied to llie determination of llie reliability of complex systems in terms of tlie reliabilities of their components. Tlie relationship between reliability and failure rate is explored in detail. Special probability distributions for failure time are discussed. Tlie chapter concludes with a consideration of fault tree analysis and event tree analysis, two special teclmiques lliat figure prominently in hazard analysis and llie evaluation of risk. 

In Section 20.2, equations for tlie reliability of series and parallel systems are established. Various reliability relations are developed in Section 20.3. Sections 20.4 and 20.5 introduce several probability distribution models lliat are extensively used in reliability calculations in hazard and risk analysis. Section 20.6 deals witli tlie Monte Carlo teclinique of mimicking observations on a random variable. Sections 20.7 and 20.8 are devoted to fault tree and event tree analyses, respectively. 

A fault tree is a grapliic teclmique used to analyze complex systems. The objective is to spotlight conditions tliat cause a system to fail. Fault tree analysis attempts to describe how and why an accident or otlier undesirable event lias occurred. It may also be used to describe how and why an accident or otlier undesirable event could take place. Thus fault tree analysis finds wide application in hazard analysis and risk assessment of process and plant systems. ... 

Figure 21.1.3 depicts tlie risk to society in tenns of the annual probability of N or more deaths as a result of tlie occurrence of incident I or incident 11. Note tliat tlie scales in Fig. 21.1.3 are logaritlunic. Tlie plotted probabilities are obtained by summing tlie probabilities of the events resulting in N or more deatlis for N = 0, 3, 6, 13. Table 21.3.1 lists these events and probabilities. 

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