Technological risk and environmental risk
Two independent barriers that each fail once in a hundred fillings let one overflow through every ten thousand. If they share the sensor, one in a hundred: the redundancy disappears.
01The risk that leaves the establishment
Occupational risk affects those who work. Technological risk is broader: it is the possibility that an industrial installation, a transport system or an infrastructure causes harm to the population and the environment, well beyond its boundaries. A leak from an acid tank at a printed circuit plant can injure an operator, but it can also contaminate a watercourse and force the evacuation of a neighborhood. That is why technological and environmental risk are managed together.
| Accident | Year | What happened | What changed |
|---|---|---|---|
| Seveso (Italy) | 1976 | A dioxin cloud from a chemical plant contaminated an inhabited area | The European major-accident directive, which bears its name |
| Bhopal (India) | 1984 | A leak of methyl isocyanate from a pesticide plant killed thousands of people | Emergency planning with the neighboring community |
| Chernobyl (Ukraine) | 1986 | A poorly conducted test destroyed a nuclear reactor | The notion of safety culture |
The Seveso directives of the European Union, the latest of 2012, require plants holding hazardous substances above certain quantities to have a prevention plan, a safety report and an off-site emergency plan, and to inform the public.
02Integral risk: chained hazards
Risks do not arrive alone. A natural event can trigger a technological one: a flood that reaches a chemical plant or an earthquake that damages a power station. These natech (natural–technological) events make it necessary to look at the whole territory, not the isolated plant. The integral view combines hazard, exposure and vulnerability: the same tank is a different risk next to a school than in a remote industrial park, and that is why land-use planning and environmental impact assessment are part of technological risk management.
For installations with many potential victims two measures are used: individual risk, the annual probability of death of a person at a point, and societal risk, the frequency of accidents with N or more victims, which is plotted in F–N curves with a stricter limit the larger N is.
03Fault trees and event trees
To quantify the probability of a major accident, the fault tree starts from the undesired event and breaks it down with logic gates until it reaches basic failures with known probability. With independent events:
Independence is the critical assumption. If two barriers depend on the same component, the same supply or the same maintenance error, they fail together: this is a common-cause failure, and it can cancel the advantage of having two barriers. The event tree works the other way around: it starts from an initiating event and follows the success or failure branches of each barrier to its consequences. Joined at the central event they form the bow-tie diagram.
An acid tank is filled with a pump. It overflows if the automatic cutoff fails (the level sensor or the controller) and the high-level protection also fails (the alarm or the operator who has to respond). The probabilities are per demand, that is, per filling.
04In the laboratory
Locate on a map the installations with hazardous substances near the university, the watercourses and the schools, and discuss which natech events would be possible.
Build the bow-tie diagram of a fire in the battery room of a data center, with its causes, preventive barriers, mitigation barriers and consequences.
Review the electrical diagram of a redundant system in the laboratory and look for components common to both branches.
05Common mistakes
- Multiplying the probabilities of barriers that are not independent. A common cause can make the risk a hundred times greater than calculated.
- Looking only inside the plant fence. Technological risk includes the neighboring population and environment.
- Forgetting natural events as triggers. A flood can disable several barriers at once.
- Taking the result of a tree as exact. It depends on the input probabilities, which often carry large uncertainties.
06Self-assessment
Two independent barriers each fail with probability 0.01. With what probability do both fail?
\( 10^{-4} \).
If both depend on the same sensor, which fails with 0.01, what is the minimum value of that probability?
0.01: the failure of the sensor makes both fail.
What is a natech event?
A technological accident triggered by a natural phenomenon.
What is the difference between individual risk and societal risk?
Individual risk is the probability of death of a person at a location; societal risk is the frequency of accidents with many victims.
07Further reading
- Charles Perrow. Normal Accidents: Living with High-Risk Technologies. Princeton University Press, 1999. Why complex, tightly coupled systems produce inevitable accidents.
- Frank P. Lees. Lees' Loss Prevention in the Process Industries. 4th ed., Butterworth-Heinemann, 2012. The reference on quantitative risk analysis, fault trees and major accidents.
- Ulrich Beck. Risk Society: Towards a New Modernity. Sage, 1992 (German original, 1986). Spanish edition: “La sociedad del riesgo”, Paidós, 1998. Technological risk as a central feature of advanced industrial society.