Biological Effects of Radiation. Because the many of the problems associated with nuclear power plants are associated with the biological effects of nuclear radiation, we will reverse the order and discuss biological effects first. It is known that the biological effects of exposure to radioactive materials can be serious, but in many cases the mechanisms of damage are not understood at the atomic level so we need to use information from biological experiments.

Units. Unfortunately many units are used to Describe radiation exposure: Energy exposure: 1 rad = 10-5 joules in 1 gram of tissue Quality factor: Takes account of the fact that Different kinds of radiation have different Biological effects 1 rem = (Quality factor)x 1 rad

Table 15-1, p. 505

Table 15-2, p. 506

Lethal Dose 50% Animals (including humans) exposed to intense radiation become ill and die of various diseases, mainly cancers. The deaths may occur quickly or over many decades and the responses are not the same for all individuals. The mechanisms of damage are not well understood and the available information is from data from experiments. The data can be arranged in a dose-response curve as shown on the next slide Note that what matters biologically is the total dose to the organism and not the rate of exposure.

The dose at which 50% die is called the lethal-dose 50 Figure 15.2: Dose-effect distribution curve for mice. The point at which 50% of the population dies is called the lethal dose-50, or LD-50. It is about 1000 rads in this example. Fig. 15-2, p. 507

Unfortunately we do have such data for humans as a result of the bombs dropped on Hiroshima and Nagasaki in 1945. By following the histories of 24,000 people exposed to intense radiation in those events, data including that displayed in the following graph was obtained.

Figure 15.3: Leukemia mortality dose-response curves for Hiroshima and Nagasaki. Fig. 15-3, p. 508

Note that the numbers are per million person years. This data implies roughly 2 cases per rem of radiation per million people per year for leukemia. There is debate concerning whether such high dose data can be extrapolated to determine risks at low doses, but most scientists believe that it can.

If there are 1200 leukemia deaths per million person years at a dose of 250 rads, what are your chances of getting leukemia from the background radiation of 360 millirads per year in a lifetime of 70 years? (assuming quality factor =1) A. 1 in 1200x106 B. 1 in 1200x(.360/(250x70))x106 C. 1 in 250 x 106/(1200x70x0.360) D. 1 in (1200x70 x.360)/(250 x 106) E. 1 in 1200x250x106/(70 x.360)

Answer C: Number of deaths per person year per Rad = 1200/(106 x250) Exposure of one person in 70 years = 1x70x.360 rads Number of deaths expected is the product = 1200/(250x106) deaths per person per rad per year x 1 person x 70 years x .360 rads Or one death for every ((250x106)/(1200x70 x.360) about 8200 people

Actual leukemia death rates are around 1 for every 10,000 people per lifetime. So the exercise gave an answer close to what is observed. However that is somewhat surprising. The curve is S-shaped, so the death rate would be expected to be much lower at low doses. This suggests that other origins of leukemia may be important.

Background radiation: In evaluating the risk of nuclear power plants it is necessary to take into account that humans are exposed to significant nuclear radiation in the absence of power plants or nuclear weapons. A summary appears on the next slide.

Table 15-5, p. 514

These radiation exposures do have health effects. Unless nuclear power significantly increases this exposure it would be rational to worry first about the other exposures. These include particularly radon. Measures you can take to protect yourself against excessive radon exposure are discussed in your book. Current national standards limit radiation exposure above background to 100mrem/yr

Table 15-3, p. 508

From these tables, what kinds of health effects can be expected from the average background exposure during a 70 year human life? (Assume QF=1.) A. None B. blood changes. C. injury and disability D. between B. and C. E. between A. and B. Table 15-3, p. 508

Answer: D. between B and C 360 mrem/yr x(1/1000 rem/mrem) x (1/QF (rad/rem)) X70 yr = 25.2 rad Some effects!

Decay of Radon, naturally occurring in common rocks Figure 15.4: Dose-response curves for low doses of radiation are based on the assumption of a linear response—that is, that the data from high doses (solid curve) can be extrapolated back to low doses (as in curve a). No threshold exists (as is assumed for curve b), so that any radiation is assumed to have a harmful effect. Fig. 15-5, p. 510

Medical Procedure exposures Table 15-4, p. 514

Figure 15.7: Key to major radon entry routes. Fig. 15-7, p. 512

Figure 15.6: Comparable risks from exposure to radon gas. Fig. 15-6, p. 511

Somatic and genetic damage: The biological effects discussed so far are to the individual irradiated (somatic) There are also effects on the descendants of the irradiated individuals through damage to the DNA molecules which carry genetic information. Such mutations are known to be a major source of the variation which, together with natural selection, drives biological evolution. They occur naturally, driven by the background radiation. However most mutations fail (do not produce improvements in survivability) and too many mutations will threaten the survivability of a species.

Exposures from various other hazards above ‘background’ Proposed standard for the Yucca Mountain high level waste storage site: 15mrem/yr for 10,000 years (may not be achievable) Normal nuclear reactor operation for electric power production: 170 mrem/yr (federal limit) Chernobyl (worst nuclear power accident until Fukushima) 200 mSv= .2 x100 rem/SV x 1000mrem/rem = 20,000mrem Nuclear bomb: Exposures and Hiroshima and Nagasaki were (for survivors) from 100-500 rads= 100,000-500,000 mrem ( if QF=1 bone marrow only)

Summary and conclusions: Radiation from radioactive (decaying) nuclei has mainly deleterious effects on biological organisms including humans. Humans are all exposed to substantial, naturally occuring radiation, most significantly from radon. Much the biggest biological threat to human health from nuclear radiation arising from human activity is from nuclear weapons, if they are ever used again. The other dangers are from nuclear power plant accidents and extremely long term storage of nuclear waste. The federal limit on exposure from normally operating nuclear power plants Is about 20% above normal background.