Climate change

July 12, 2010

What is risk? A simple explanation

In chapter 7 of his book “The Nuclear Energy Option“, Prof Bernard Cohen wrote the following provocative statement:

It is very difficult to predict the future of scientific developments, and few would even dare to make predictions extending beyond the next 50 years. However, based on everything we know now, one can make a strong case for the thesis that nuclear fission reactors will be providing a large fraction of our energy needs for the next million years. If that should come to pass, a history of energy production written at that remote date may well record that the worst reactor accident of all time occurred at Chernobyl, USSR, in April of 1986.

How could he have the audacity to make such a prognostication? Simple — because he, like most scientists, engineers and actuaries, understands the meaning of  probability and risk (as well as the fundamental physics of modern reactor design). In chapter 8, called “Understanding Risk“, he goes on to say:

One of the worst stumbling blocks in gaining widespread public acceptance of nuclear power is that the great majority of people do not understand and quantify the risks we face. Most of us think and act as though life is largely free of risk. We view taking risks as foolhardy, irrational, and assiduously to be avoided. Training children to avoid risk is an all-important duty of parenthood. Risks imposed on us by others are generally considered to be entirely unacceptable.

Unfortunately, life is not like that. Everything we do involves risk. There are dangers in every type of travel, but there are dangers in staying home — 25% of all fatal accidents occur there. There are dangers in eating — food is one of the most important causes of cancer and of several other diseases — but most people eat more than necessary. There are dangers in breathing — air pollution probably kills 100,000 Americans each year, inhaling radon and its decay products is estimated to kill 14,000 a year, and many diseases like influenza, measles, and whooping cough are contracted by inhaling germs. These dangers can often be avoided by simply breathing through filters, but no one does that. There are dangers in working — 12,000 Americans are killed each year in job-related accidents, and probably 10 times that number die from job-related illness — but most alternatives to working are even more dangerous. There are dangers in exercising and dangers in not getting enough exercise. Risk is an unavoidable part of our everyday lives.

That doesn’t mean that we should not try to minimize our risks, but it is important to recognize that minimizing anything must be a quantitative procedure. We cannot minimize our risks by simply avoiding those we happen to think about. For example, if one thinks about the risk of driving to a destination, one might decide to walk, which in most cases would be much more dangerous. The problem with such an approach is that the risks we think about are those most publicized by the media, whose coverage is a very poor guide to actual dangers. The logical procedure for minimizing risks is to quantify all risks and then choose those that are smaller in preference to those that are larger. The main object here is to provide a framework for that process and to apply it to the risks in generating electric power.

The failure of the American public to understand and quantify risk must rate as one of the most serious and tragic problems for our nation. This chapter represents my attempt to contribute to its resolution.

In this post, Peter Lang provides a simple explanation of risk in relation to energy generation. In an Endnote, I quote a few passages from my recent book that also relate to this important — but often misunderstood — concept.


What is risk? A simple explanation

Guest Post by Peter Lang. Peter is a retired geologist and engineer with 40 years experience on a wide range of energy projects throughout the world, including managing energy R&D and providing policy advice for government and opposition. His experience includes: coal, oil, gas, hydro, geothermal, nuclear power plants, nuclear waste disposal, and a wide range of energy end use management projects.

A recent comment stated:

I for one am glad nukes are being forced to be orders of magnitude safer than coal because the risks are orders of magnitude greater

In fact, the risks from nuclear are orders of magnitude lower than coal, not greater. Let me explain.

Risk is Consequence of an event multiplied by the Probability of that event occurring.

We need to define what we mean by the Consequence.

For electricity generation the consequence could be (for example):

1. Fatalities

2. Work-days-lost

3. Total health effects

4. Total damage costs (including health, environmental, etc.)

Fatalities can be subdivided into ‘immediate fatalities’ and ‘latent fatalities’. Fatalities can be subdivided into ‘workers’ and ‘public’.

We must define which measure of ‘Consequence’ we are using. Let’s keep it simple and use ‘immediate fatalities’ as our measure of ‘Consequence’.

The consequence of an accident might be 30 immediate fatalities (as happened at Chernobyl). The probability of occurrence might be 1 in 14,000 GW-years (123,000 TWh). The risk of such an accident is 1 fatality per 4,000 TWh (equivalent to 1 fatality in 20 years from severe nuclear accidents if all of Australia’s electricity was generated by nuclear power).

Now refer to Figure 1. To understand what this chart is telling us, consider the pink dot labelled “Chernobyl”. This is plotted at 28 Fatalities on the x-axis. Reading off the y-axis we see the frequency of nuclear accidents causing 30 or more immediate fatalities is 1.1 x 10-4 GW-years (or 1 occurrence in 9,000 GW-years of electricity supplied). That is about 1 immediate fatality in 2,800 TWh (equivalent to about 1 immediate fatality in 14 years from severe nuclear accidents if all Australia’s electricity was generated by nuclear power).

Figure 1: Risks of severe accidents in the different energy chains in the EU. Original Source of this chart (link no longer available). Original Data is in Figures 7 and 8 is here.

Now look at the coal accidents (the brown line). For accidents with the same number of immediate fatalities as Chernobyl we see that the frequency is about 1.15 x 10-3 GW-years. So, the frequency of severe accidents that causes 30 or more early fatalities is 15 times greater for coal generation than for nuclear generation.

Also on Figure 1, notice the pink line in the lower left corner of the chart. This is the Probabilistic Safety Analysis (PSA) of nuclear generation. It indicates that nuclear is about 4 orders of magnitude (10,000 times) safer than coal generation.

This chart includes only the immediate fatalities caused by severe accidents. It does not include the latent fatalities. For coal generation most of the fatalities are latent fatalities and these occur in the general public, not in the workers. However, in nuclear and renewable energy generation most of the fatalities are amongst workers in the industry — workers anywhere in the chain from mining materials, processing, manufacturing, construction, transport decommissioning and disposal. The figures are from full life cycle assessment.

For nuclear and renewables the Fatalities per TWh of electricity supplied are roughly in proportion to the quantity of materials needed per TWh over the plant life. However, for fossil fuels, the fatalities are dominated by the fatalities to members of the public due to the toxic emissions. The fatalities to the workers are dominated by those involved in the fuel extraction.

Figure 2 compares the total health effects of the main types of electricity generation in the EU. It shows that, in the EU, nuclear is about 50 times safer than coal generated electricity. Nuclear is safer than all except hydro in the EU.

Figure 2: Mean values of health effects, presented as deaths/TWh for the respective forms of electricity generation throughout the EU (Source here).

Outside the OECD, fossil fuel and renewable energy generation is much more dangerous that in the EU so nuclear is even safer by comparison.


Endnote (Barry Brook): Here are some extracts on this topic from recent book, Why vs Why: Nuclear Power:

Nuclear safety and serious accidents

Safety is the most common fear about nuclear power, yet the nuclear power industry has an excellent operational safety record.

A study of 4,290 energy-related accidents by the European Commission’s ExternE research project examined the number of deaths per terawatt hour of energy for each of various technologies. It found:

  • oil kills 36 workers a terawatt hour
  • coal kills 25
  • gas kills 4
  • hydro, wind, solar and, yes, nuclear, all kill less than 0.2

(These figures ignore deaths from pollution and global warming.)

The fearsome reactor meltdown or terrorist act: what is the worst case scenario?

There is no limit to what the imagination can come up with regarding industrial accidents.

Imagine if a fire broke out in a natural gas refinery on the outskirts of a city. High winds then carried the hot embers aloft, setting ablaze nearby suburbs and the surrounding forest. What if this triggered explosions in adjacent chemical plants? This chain of events might ultimately lead to a city-wide conflagration that killed hundreds of thousands of people. Such a scenario is exceedingly unlikely, but not impossible. In the end, it is the probability that matters.

There is, for instance, some risk that a terrorist could hijack an aircraft, hit a reactor with pinpoint accuracy, breach containment, and cause the release of nuclear material. However, it is an incredibly low risk that all of these things will occur together. For instance, it has been estimated that only about one in every 1,000 direct aircraft strikes might crack a steel-reinforced concrete containment dome.

If we want to increase global security, then it is counterproductive to hope nuclear power will simply go away. We should instead discuss how to use this low-carbon energy source safely and cleanly, with minimum risk and maximum advantage. The risks of not employing nuclear power vastly outweigh the dangers of continuing to use fossil fuels or running out of energy.


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