Deep Geologic Hubris

Be warned, this is by far the Gordian Knot News' technically most demanding post. As usual, a properly referenced version can be be found at Deep Geological Hubris

Repository Studies

Despite the remarkably small amount of used fuel generated by nuclear power, despite the fact that the penetrating radiation in that fuel is effectively gone in 600 years, after which the fuel would have to be swallowed to be harmful, the nuclear establishment is adamant that long-lived radionuclides are an extremely difficult waste problem, requiring deca-billion dollar investments in deep geologic repositories. And even then they are a lurking, barely contained danger.

But there isn't much agreement about which isotopes we should worry about. Table 1 summarizes the results of five major repository studies

In the Finnish study, the top three exchange positions drastically depending on the scenario. These studies are based on a long chain of arguable and often arbitrary assumptions about what will happen over the next million years. The assumptions dictate the results, some of which are quite surprising.

1) C-14 and Cl-36 are not even fission products. They are activated scrap. Tiny amounts of scrap. Cl-36 arises from the activation of normal chlorine. But no part of a light water reactor is made of chlorine. Some reactor components can be contaminated by chlorine at ppm or lower levels. This contamination is where the Cl-36 comes from. Nor is there much carbon in a standard light water reactor.

Much is made of the fact that these elements are naturally in our bodies, so the uptake is high. But the biological half life of carbon in a human is 40 days. For chlorine, it's 10 days. Carbon-14 is so natural, it is something we ingest every day.

2) In terms of activity, Tc-99 represents over 90% of the long lived fission products; and, like carbon and chlorine, it can be highly mobile. High uptake of 50 to 80%, about the same as carbon and chlorine. But it shows up almost nowhere in the repository studies, presumably because of its very short (about 1 day) biological half-life,

3) Ra-226 is neither a fission product nor an activation product. It's a natural part of the U-238 decay chain. The amount of U-238 that is put back in the ground is slightly less than what was taken out. And the radium from this clean U-238 builds up very slowly. Yet according to the Swedes, this natural radiation is the most dangerous of all the isotopes. In their reference scenario, it represents close to 90% of the dose at the end of the study period.\cite{skb-2011}

4) Plutonium is highly immobile. The plutonium created by the natural Oklo reactor in Gabon has moved less than 3 meters in two billion years despite groundwater flowing through the formation.\cite{miller-2000}. Human uptake is near zero. If plutonium somehow were ingested, 99.997% would be excreted in a day or two.\cite{henriksen-2003} But somehow Pu-242 made it to the top of the USA list. Most of the other studies don't even mention plutonium. In the Finnish study, the worst case release of plutonium is one-trillionth of the worst case release of I-129. See Table 3. And that number does not account for the massive difference in uptake. The Yucca Mountain study is a weird outlier.

About the only thing, the studies agree on is that I-129 is important. I-129 has a half-life of 16.6 million years. It emits a modest (max 151 keV) beta and a weak 39 keV gamma. It concentrates in the thyroid. So let's take a look at I-129, and throw in Tc-99 for good measure.

I-129 and Tc-99

Table 2 compares the effect of ingesting 1 nanogram of I-129 and Tc-99 with ingesting 1 nanogram of I-131, the most dangerous isotope in the first week or two after a release.

The dose per gram from I-129 and Tc-99 is a billion times less than that from I-131. Long lived is synonymous with decays-very-slowly which results in correspondingly minuscule dose rates.

At Chernobyl, the thyroid doses to affected children were in the range of 560 mSv (average Belarus) and 770 mSv (average Ukraine).\cite{takamura-2016} Almost all of this dose was from I-131. The maximum measured thyroid dose was 39 Gy (Belarus) and 42 Gy (Ukraine). These two kids must have ingested about 55 nanograms of I-131.

Tc-99 is regularly injected into medical patients as a by-product of Tc-99m imaging. Tc-99m is by far the most popular form of internal gamma imaging. Reference \cite{doe-1996} says ``a total of approximately 38,000 diagnostic procedures involving radioactive isotopes are performed each day in the U.S. Most of these procedures use Tc-99m." Tc-99m has a decay half-life of 6 hours and a specific activity of 19.5e17 Bq/g, 300 million times higher than the Tc-99 to which it decays. It emits a 141 keV gamma. Yet it is approved by the FDA for all sorts of diagnostic purposes, including children. The approved dose varies with use; but in many cases it is in excess 1.0e9 Bq or about 52 nanograms of Tc-99m. This would be 65 mGy to the body and 3.45 Gy to the thyroid.

EPA correctly says this is safe because of the short decay half-life and the fact that the biological half-life is about 1 day.\cite{epa-2002} Every atom of Tc-99m that decays produces an atom of Tc-99. The EPA claims Tc-99 is hazardous because of its long decay half-life.\cite{epa-2002} Yet Tc-99 has the same 1 day biological half life as Tc-99m. The dose the patient receives from the supposedly hazardous Tc-99 is about 100 millionth of the dose the patient receives from the Tc-99m. The medical profession for once is correctly unconcerned.

Table 2 says a nanogram of I-129 results in eleven times the thyroid dose of a nanogram of Tc-99 or 10 millionth the Tc-99m dose. Put another way, if a child ingested 55 nanograms of I-129, the maximum at Chernobyl, she would receive a dose to the thyroid of 0.466 micro-Gy. To claim that Tc-99 or I-129 is a problem, you must come up with a delivery scenario that results in a rapid ingestion of a million times or more material than the largest ingestion after Chernobyl.

Repository Release Rates

Towards the end of their study of the Finnish Olkiluoto repository, the authors did something interesting. They simply listed the number of atoms of each isotope that was released into the geosphere from the repository in the worst case scenario for that isotope over the period, 2020 to 17020. Table 3 shows the results.

If you add up all the isotopes weighted by their activity, you come up with a total release of 1.3e10 Bq over the 15,000 year period. Chernobyl released 6.5e18 Bq of I-131 alone over 10 days. The Chernobyl release was 500 million times larger and took place 500,000 times faster. Of course, our ginned up Olkiluoto release will not be evenly spread over 15,000 years; but this error is in the noise compared with the error of combining mutually exclusive worst cases. For comparison, the Finns' reference case is shown in the right most column.

So how do they come up with max dose rates that are only a million or 10 million times less than the worst case at Chernobyl? The repository studies focus on a hypothetical most exposed person(MEP). You identify a dominant pathway in each scenario and you put your MEP at the end of that path. Our most exposed person drinks a couple of liters a day every day for a year out of the most contaminated well. The analysis invokes LNT multiple times --- usually silently -- including the assumption that dose rate is irrelevant.

But these studies make another unsupported assumption. They assume that our descendants are even stupider than we are. We know how to detect radiation down to a few counts per second. Technically it is not difficult. If a thousand years from now our descendants are still worried about dose rates that are orders of magnitude below background, do we think they won't have the capability to detect and respond to those dose rates? That's precisely what the repository studies assume.

The repository studies are exercises in monumental hubris. The idea that we can predict what will happen 100 years from now is preposterous. The idea that we can predict what will happen 1000 years from now --- well, there is just no word for it. And then we assume this omniscient species which can foretell the future for millenia, all of a sudden forgets how to measure radiation.

The humble, prudent, common sense approach is;

1) Shield and cool the used fuel adequately. We know how to do this. It is not difficult.

2) Keep the material where you can repair the shielding as necessary.

3) In no more than 600 years, effectively all the penetrating radiation will be gone. The valuable fissile and fertile isotopes can easily be extracted. The remainder will be low level waste that can be landfilled.

Forget about predicting the future for millenia and trying to come up with a system that will last that long.