Stage 4. Analysis of conditions and assessment of the accident probability

The development of various uncontrolled processes may lead to different directions of the accident development depending on what means of prevention and control of accidents have been implemented.

At this stage risk analysis based on the assessment of probability of protection means triggering the probability of the various accident outcomes is defined. For these purposes the following approaches are used:

- fault trees;

- analysis of types and outcomes of faults;

- expert evaluation of event probabilities;

- statistical data processing activities;

- other valid techniques.

Possible conditions of realization (emission indices or other criteria of events like meteorological conditions, masses of pollutants, duration of exhaust etc.) are assessed for every outcome via modelling of accidents and levels of damaging factors, zones and possible consequences.

Stage 5. Definition of scale of accident and its consequences. This stage includes analysis of possible impacts on human beings, property and environment.

Stage 6. For assessment of possible outcomes and risks it is necessary to model accidents for every possible outcome, defined during accident development analysis.

For example, during modelling of hazardous and damaging emissions into atmosphere meteorological conditions, atmospheric stability, parameters of sources of exhaust should be considered.

In the process of analysis additional dangerous factors can be identified. If there are few dangerous and damaging factors at the facilities, the analyst should assess all spectrums of possible outcomes and summarize risks of undesirable consequences.

The impact of damaging factors on the object of concern doesn’t mean unavoidable consequences. At different stage of accident the probability of possible outcomes should be considered taking into account advantages of implementation of protection and control means.

In the long term the total risk of unwanted outcomes should be assessed for the defined object of concern. If it is necessary the effects from alternative control means should be taken into account to decrease risk to acceptable levels.

All assumptions should be oriented to the worst types of conditions and outcomes.

Stage 7. Acceptable risk levels for the objects of concern should be assigned by local authorities taking into account current standard acts, agreements and interested parties, economic and social conditions of the region, expert opinions and other circumstances.

If the local authorities have not defined the borders of acceptable risk levels, for the declaration of safety at high-risk facility, the risk borders which were approved by the Methodology should be used.

Thus, after the stage of risk assessment we should compare the calculated risk with the defined levels. There are three basic levels of

risks named negligible, acceptable and unacceptable (or catastrophic).

Negligible risk is extremely low level of danger.

Acceptable risk is accepted in a given context based on the current values of society. In other words, a risk is acceptable when it is acceptable to the general public.

Acceptable riskis a risk that is understood and tolerated usually because the cost or difficulty of implementing an effective countermeasure for the associated vulnerability exceeds the expectation of loss.

Unacceptable risk is the level of risk which leads to unwanted consequences in most cases. For the current level of human development, it is impossible to provide absolute safety. Unfortunately it is impossible to provide negligible level of risk in all fields of our life.

According to the concept of acceptable risk the level of the individual risk of human death is equal to 10^-6. Thus, in the modern society it is normal that 1 person per million could die due certain reason: 10^-6=1/1 000 000.

The modern concept of acceptable risk states that safety is freedom from unacceptable risk. However this level of the acceptable risk has been reached only in the developed countries.For most areas of industry in Ukraine it is unachievable due to the low level of social, economic and technological development.

The Methodology defines acceptable borders of the individual (Fig. 1.2), societal and territorial risks (Table 1.1).

Fig.1.2. Borders of acceptable individual risk

Table 1.1

Acceptable borders of the individual, societal and territorial risks

Risk borders	Acceptable	Unacceptable
Territorial	Rt ³10^-7	Rt > 10^-5
Individual	Ri ³ 10^-8	Ri > 10^-6
Societal	Rs ³ 10^-7	Rs > 10^-5

Local authorities can determine other borders taking into account regional features. However they are not allowed to exceed the listed levels.

For decision-making on the authority providing for operation, building or reconstruction of the high-risk facilities the listed types of borders of acceptable risk (territorial, individual or societal) can be used depending on the character of facilities.

Stage 8. Risk management and decision-making are based on the following principles:

- the level of risk can’t exceed the acceptable level;

- any type of human activity independent of profits with risk levels which are higher than acceptable levels is prohibited;

- expenses of achievement of acceptable risk should be minimal;

In the case when the risk level defined (in accordance with the Methodology) lays within lower and higher levels of acceptable range the interested parties may insist on implementation of additional protection and control means for risk decreasing.

Risk management is applied to reduce dangers by means of the following principles: legislative, technical, operational and others.

Example 1.1

Assess the level of territorial and individual risk in the vicinity of the power plant. Input data (Table A.1) for variant № 30.

To assess risk level R the formulas (1.2) and (1.3) should be used and transformed in accordance with the task of the calculations.

Territorial risk:

+0.008×0.3×6×10^-5×1.2×10^-2+0.002×0.3×7×10^-5×1.3×10^-4+0.07×0.6×5×10^-5×5×

×10^-4+0.09×0.6×6×10^-5×1.2×10^-2+0.01×0.6×9×10^-5×1.3×10^-4=4.5×10^-11+1.73×

×10^-9+5.46×10^-12+1.05×10^-9+3.89×10^-8+7.02×10^-11=4.18×10^-8.

Individual risk: P_i^k = P_t^k × P_n^k = 4.18×10^-8×0.09=3.75×10^-9.

Conclusions. Territorial risk R_t^k =4.18×10^-8 is lower than acceptable level 10^-7 (Table 1.1), thus the level of calculated territorial risk is acceptable. Individual risk R_i^k =3.75×10^-9 is lower than acceptable level 10^-8, thus the level of calculated individual risk is acceptable.

Дата добавления: 2015-11-16