Background on Risk

Risks for spent fuel and high-level waste transportation arise from conventional vehicular accidents and exposures to ionizing radiation under both normal and accident conditions. Radiation risks are primarily a concern for people who live near, or travel on, spent fuel shipment routes.

Risk considers both the likelihood of occurrence of a specific hazard and its consequences. Frequently, one considers several scenarios that involve different kinds of hazards, each with a different likelihood of occurrence and consequence. One way in which risk can be expressed is in terms of a triplet (Kaplan and Garrick, 1981):

Where, for spent fuel shipments,

  • Scenarios represents transport conditions that can lead to an exposure to ionizing radiation from either routine operations or severe accidents,

  • Probability expresses quantitatively the likelihood that a scenario will actually occur during one shipment; it is expressed as a dimensionless quantity that ranges in value from 0 (impossible) to 1 (certain)—for example, a probability of 0.5 indicates that a particular scenario has a 50 percent chance of occurring, and

  • Consequences describe the undesirable results if the scenario does occur: for example, undesirable health effects.

The risks from spent nuclear fuel transport can be characterized by several measures. For example, risk can be expressed in terms of the expected number of deaths per quantity of spent fuel transported, per number of packages shipped, or per number of package shipments. It also could be expressed in terms of the number of deaths expected for a specific subpopulation exposed to ionizing radiation, for example, the subpopulation of transportation workers. Although they are difficult to quantify, consequences may also include socioeconomic outcomes.

The choice of scenarios and consequences selected for a risk calculation can make a difference in how that risk is understood by potentially affected populations. A risk may be understood as low by one measure in comparison to another, even though the same risk is being considered (NRC, 1996). This has implications for informing decision makers, for communicating about risks with non-experts, and for the legitimacy of risk comparisons in the eyes of interested and affected people (NRC, 1989, 1996). Comparing risks arising from fundamentally different activities also requires care in the selection of appropriate scenarios and consequences. This point is discussed in more detail elsewhere in this chapter.

report is on transportation in the United States, much can be learned from international experiences. Spent fuel and high-level waste are being transported in many other countries, in some cases in much greater quantities than in the United States. The committee has drawn upon the experiences

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