User:David MacQuigg/Sandbox/Nuclear waste

A more realistic view of nuclear waste hazards

Fig.3 Dose Rate at Fuel Element Surface.

Figure 1 in the Nuclear_waste_management article does not tell us much about the long term danger of spent nuclear fuel. Long term radioactivity is mostly alpha particles and electrons (beta rays) harmlessly trapped in the spent fuel. The more dangerous high-energy photons (gamma rays) can pass through the fuel element metal casing, but they die out in a few hundred years. If we want to know how dangerous it is, we should look, not at the total radioactivity inside the source, but at the dose received by a person handling the spent fuel. Alpha particles can be stopped by a sheet of paper. Electrons are stopped by the outer layer of our skin. Instead of Becquerels per kilogram inside the source, we need dose rates at the fuel element surface. See Figure 3.

Dose is the amount of radiation energy absorbed by our tissues, measured in joules per kilogram of tissue. Gray is the scientific name for joules per kg. Figure 3 shows the dose rate in milligrays per day (mGy/d). The key feature of Figure 3 is that photon decay is relatively rapid. By year 600, almost all the photon emitters are gone. In fact, the photon dose rate is so low that, according to DOE rules, used fuel rods can be handled without any shielding at all. After year 600, the spent nuclear fuel must be swallowed in order to do any damage.