Absolute Time Nuclear Waste Disposal

Absolute Time – Nuclear Waste Disposal

Knowledge of radioactive decay series is important for understanding how geologic ages of the Earth and its various events and features are determined. However, an understanding of radioactive decay also has other important practical applications.

In a nuclear power plant, 235U is split (nuclear fission) into two lighter isotopes during a nuclear chain reaction. During the process of fission, some mass is converted to energy and this energy is released as heat. The heat is used to convert water to steam, which in turn drives a turbine producing the electricity. The process of nuclear power generation produces two types of radioactive waste material. The fissile uranium isotope is introduced into the reactor core in fuel rods, which also contain some non-fissionable 238U. Once the 235U has been consumed in the reactor core, the fuel rod is no longer capable of sustaining a chain reaction. These “spent” fuel rods must then be disposed. Because they are highly radioactive, they must be isolated from human contact until they decay down to safe levels. The actual length of time required is determined by the characteristics of the uranium decay series.

Inside the reactor core, the water that is to be converted to stream is circulated through steel pipes. During normal operation of the reaction, neutrons continually bombard the elements in the steel alloy. This causes some of the elements to become radioactive. For example, the nickel used in some steel alloys is converted to 59Ni and 63Ni. As with all installations or machinery, a nuclear reactor has a finite useful lifetime. After this time, the reactor is shutdown or decommissioned. Because some of the internal core components of the reactor have become radioactive during the normal operation, they, just like the spent fuel rods, must be isolated from human contact until their radioactivity has decayed to levels that are no longer harmful.

Table 1 lists the half-lives of uranium (in the spent fuel rods) and nickel (in the parts of a decommissioned reactor). Although the period for which a radioactive element is hazardous depends upon its initial abundance and activity, for simplicity we will assume that these elements remain hazardous for five half-lives.

For each element in Table 1, calculate the hazardous lifetime and enter it in the blank column. Assume these elements remain hazardous for five half-lives.

Table 1: U and Ni half-lives
Decay series / Half-life (yr) / Hazardous lifetime
238U-206Pb / 4.5x109 / years
59Ni / 76,000 / years
63Ni / 100 / years

Which of the products of nuclear power generation, i.e. spent fuel rods or decommissioned reactors, will you have to isolate longer?

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What are the oldest human structures you know of, and how old are they?

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Considering how long the nuclear waste will be dangerous, how would you warn future civilizations and tell them when the waste repositories will become safe? (Really think about this and answer in a brief paragraph.)