Will space travel force us to think dynamically about radiation harm?

This post assumes some familiarity with two competing radiation harm models, LNT and SNT. New subscribers might want to check out LNT is Nonsense and SNT for Dummies.

Figure 1. 1 and 2 mSv/d cancer mortality as a function of repair period and length of exposure. SNT with a 1 day repair period are the bottom lines. LNT and SNT with a 1 year repair time are the top lines.

In reporting on a radiation study, a nearly universal practice of the 'experts' is to show us only the subjects' total doses. They do this despite the fact that usually what is measured is the dose rate profile, often in the form of daily doses. The total dose is computed by adding up these daily doses, and then tossing aside everything but the total. Analyzing radiation harm by only looking at total dose is like an electrical engineer attempt to analyze a complicated circuit by only looking at the annual energy input.

The human body is an extremely complex circuit. It has to be analyzed dynamically. The essential element of SNT is not the shape of the acute dose response curve, it is chopping the dose rate profile into repair periods, and analyzing each period separately. I've repeated this literally ad nauseum, most recently in the Dumbest Graph of all time. But based on the comments I'm getting, it still hasn't stuck. So here's one more try.

SNT assumes a repair period of a day. This is a pessimistic approximation of the actual DNA repair period which runs from about 30 minutes to 20 hours. Suppose instead SNT assumed a repair period of a year. This would be biological nonsense; but it is the de facto assumption of most of our current radiation regulations which are based on annual dose.

Table 1 shows at low dose rates the difference in repair period results in roughly 1000 fold difference in predicted harm. The reason for this is pretty obvious. If the dose rate is constant, the repair period dose is inversely proportional to the repair period. The repair period dose for 1 day is 1/365th annual dose. At low dose rates, the SNT repair period harm goes as the 2.18 power of the repair period dose. So the repair period harm is (1/365)**2.18 = 2.60e-6. But we have 365 times as many repair periods, so the overall reduction in cancer incidence is 365 * 2.60e-6 = 0.000947 =~ 1/1000.

For SNT a very rough rule of thumb is: doubling the repair period, doubles the predicted harm. The repair period harm goes up a a bit more than four, but we have half as many repair periods. As we move up in dose rate and start suffering substantial mortality, we run into the upper mortality limit of 1.00, and the repair period difference starts to converge. The LNT result is the same for both repair periods, because LNT claims there is no repair.

Figure 1 shows the impact of the exposure time. The important takeaway from Figure 1 is that there is not all that much difference between LNT and SNT with a 365 day repair period. In fact, for all but the very long exposure periods, 365 day SNT predicts a higher mortality than LNT. LNT does do silly stuff like predicting mortality above 1.00 for high enough dose rates and long enough exposure; but the important point is that most of these people will die according to both LNT and SNT with a 365 day repair period. The key reason why standard SNT is so different from LNT is not the shape of the acute dose response curve. It's because SNT divides the dose rate profile into biologically reasonable repair periods.

Where would we encounter 1 and 2 mSv/d dose rate profiles for decades? That's an easy one. Space travel. The astronauts in Low Earth Orbit get between 0.5 and 1.0 mSv/d, with occasional spikes during solar flares. High Earth Orbit or a trip to Mars will about double that. If LNT were valid, the shielding requirements would be prohibitively expensive.

NASA can't afford LNT. That's why it ignores all the EPA and NRC limits. The EPA says more than 1 mSv per year is unsafe. NASA says 1 mSv per day is routine. That's the difference between the top and bottom of Figure 1.

NASA is not the only entity that cannot afford LNT. Space travel is a luxury that humanity may or may not be able to afford. The benefits of manned space travel are at best speculative. The benefits of cheap nuclear electricity are undeniable and cornucopic. If we can correctly trash LNT to go into space, surely we can junk this counterfactual hypothesis to get cheap nuclear.