Risk Acceptance Criteria

There is no zero risk situations. All actions, decisions or situations involve some level of risk, though in most cases the risk is very low. Very low or reasonable risk is considered to be acceptable. Many regulatory frameworks require the management of risk to a level that is reasonable but fall short of defining the specific criteria for major unwanted events. In many risk assessments it may be necessary to determine the level of acceptable risk during the Scoping process. The criteria must be established prior to performing quantitative risk assessment to enable comparison against the desired safety level [83]. The study uses the risk acceptance criteria called "ALARP" (as shown in Fig. 4.22) proposed by the European Integrated Hydrogen project phase 2 (EIHP2) [83, 243], as well as described by the German accident commission for risk management [172].

The ALARP (As Low As Reasonably Practicable) principle was developed by the UK authority. It is based on general risk for society. This choice also satisfies the general criteria of assuring that the risk level associated with hydrogen applications should be similar to or smaller than the risk associated with comparable non hydrogen systems. The ALARP principle is that the residual risk shall be as low as reasonably practicable. It means that a risk is low enough that attempting to make it lower would actually be more costly than any cost likely to come from the risk itself. This is called a tolerable risk.

The upper line of this figure represents the risk acceptance curve. The region between this line and the lower line denotes the ALARP area. For scenarios with risk level (that lie) between these lines the risk should be reduced if practical, typically subject to cost benefit analysis. For scenarios with risk levels above the upper curve, measures to reduce the risk must be implemented.

Curve Risk Aversion

10° 101 102 10s 104 Number of fatalities(N)

Fig. 4.22 Societal risk curve, FN curve with ALARP region [83, 172, 243]

10° 101 102 10s 104 Number of fatalities(N)

Fig. 4.22 Societal risk curve, FN curve with ALARP region [83, 172, 243]

The risk acceptance criteria are in general developed based on the mathematical expression:

where, "F" is likelihood of N or more fatalities, "N" is number of fatalities, "a" is the aversion factor ( >1, often 2), and "k" is constant. The slope of the societal risk (as plotted as a log-log basis) is simply "a". It is designed to reflect the society's aversion to a single accident with multiple fatalities as opposed to several accidents with few fatalities. The Netherlands used a value of 2. This is usually interpreted to mean that the Dutch authorities have built in degree of "risk aversion" to a lager accident. Meanwhile, the UK set the value of "a" as one, which is termed as "risk neutral", i.e. no aversion. The Dutch slope and more severe than the U.K. ones

The Dutch approach [246] to the development of the criteria may be summarised as follow: (1) Start from the premise that "the risk from a hazardous activity to a member of the public should be significant when compared with the risk of "everyday life"; (2) Identify age group of lowest risk (10-14 years old) and the "everyday" risk level for this group is 1 x 10-4 /year (3) Based individual criterion on 1% of lowest everyday risk - i.e. 1 x 10-6 /year; (4) Translate into a societal risk anchor of 10-5 /year for 10 or more fatalities; (5) Apply an aversion slope of -2, as a heavier weight must be assigned to a larger consequences; (6) Apply a factor of 100 to both individual and societal risk criteria to generate a "negligible" risk value. The resultant F-N curves are illustrated in Fig. 4.22 (solid lines), and may be characterised as follows:

Application: Zones:

Anchor points:

Consequence cut-off: Frequency cut off:

Those existing hazards facilities, Netherlands

3 - Unacceptable, Reduction desired, Acceptable

1x10-5/yr for 10 or more fatalities (upper limit of "unacceptable");

1x10-7/yr for 10 or more fatalities (lower limit of "acceptable")

1000 (efficiently)

For the UK approach [246], the development of risk criteria can be summarized as follows: (1) Start from the premise that "risk of death of one in a thousand per year is about the most that is ordinarily accepted under modern conditions in the UK; (2) suggest that 1/10 of this should be tolerable for risk associated with 3rd party activities (i.e. 1 x 10-4 /yr). (3) for societal risk, use the Canvey study risk to provide an "anchor" for the lower limit intolerable (2 x 10-4 /yr for 500 or more fatalities); (4) apply an aversion slope of -1 which is "risk neutral"; (5) insert a corresponding "negligible" line 1000 times lower. The resultant F-N curves are illustrated in Fig. 4.22 (dotted lines), and may be characterised as follows:

Application: Zones:

Anchor points:

Consequence cut-off: Frequency cut off:

Identifiable community, UK 3 - Intolerable, ALARP, Negligible

2x10-4/yr for 500 or more fatalities (lower limit of "intolerable");

Upper limit of "negligible" 1000 times lower none

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