Comment by neutronicus | Hacker News Reader

Comment by neutronicus | Hacker News Reader

The article is mostly correct, but a few corrections from a nuclear engineer:

1. What he refers to as "moderator rods" are actually control rods. The term "moderator" refers to a material that is unlikely to absorb neutrons, but likely to scatter them. This helps lower the average neutron speed inside the reactor, which actually increases the fission rate. Since water serves this purpose just fine, most reactors do not have specific "moderator rods", although BWRs actually run pipes of liquid water through the fuel assemblies for additional moderation.

2. BWRs are not run all-rods-out, like he claims. BWRs are generally run with significant control rod insertion, so that the water stays liquid for a greater portion of the height of the core.

3. He doesn't quite explain that the source of the hydrogen is a replacement reaction where zirconium and water react to create zirconium oxide and hydrogen gas.

4. Xenon-135 is a fission product, not produced by neutron activation, and neutron activation of coolant is nothing to sneer at (the half-life of tritium is 12.3 years).

Also, correct me if I'm wrong, but Cs-137 has a half life of 30 years or something. It certainly does not disappear quickly. Most of the Chernobyl contamination in Sweden was from Cesium, and that made berries and reindeer (which eat lichens) unsafe to eat for many years, if not decades.

Yeah, 30 years.

There was a lot of background radiation from Cs-137 from all the old nuclear testing that's disappearing now.

ummm half life means factor 2 in activity reduction. to obtain a factor 1000 in activity reduction you would have to wait 10 times as long. 300 years?

for 4. Tritium is a beta emitter. Wherever it ends up when it decays, the radiation won't get very far.

Still not good when it ends up in your body.

True, although in fairness it's a fairly weak beta emitter. The 100,000 picocuries of Potassium-40 your body has emits betas that are about 120 times more energetic, or sometimes emits gamma rays instead.

Probably better to not get exposed to tritium all other things being equal but you would receive more damage from radioactivity IMO inhaling someone's cigarette smoke. I suppose this depends on exactly how much tritium is present in the steam that was vented.

The original engineer also left out the many other radionuclides that are created that don't break down instantly, but his points about filtering most contaminants out before they are vented, and their destination of the vast Pacific Ocean is right on target.

Range of Betas in air (I believe at 1 MeV) is 12 feet. These aren't alphas here, they'll make it into living tissue.

Tritium betas are about 0.005 MeV, not 1.000, and can penetrate only 6mm of air. The only real threat from tritium is ingestion where you don't have air or dead skin layer to block them, but even then they would have to be present in vast, vast quantities to receive appreciable dose from them.

What is nicer is that your body naturally replaces most of its water every few days, so you arn't stuck like you would be with say sr-90 trying very hard to embed itself in your bones :(

Would you provide more detail on #4? I'm curious but not quite getting what the consequences of this are.

It means there are isotopes, including tritium, whose half-lives are long enough that they pose a risk to the health of anyone who happens to inhale or ingest them. The OP's dismissal of such risks is a bit facile.

But tritium pretty much can't be made in light water reactors, so there isn't going to be any in the released steam.

True, but the reactors aren't full of light water anymore, they are pumping sea water into them.

Where does the tritium come from? As I recall tritium is one proton and two neutrons. So, you are saying that one hydrogen, that is, one proton, from the water captures a neutron and becomes deuturium and then later the deuturium captures another neutron and becomes tritium? Is that correct? Net, to those two reactions have a signficant reaction rate in the context?

There's a significant amount of conversion from hydrogen to deuterium. The likelihood of neutron absorption by deuterium is much smaller (by a about a factor of ten) than the likelihood of neutron absorption by plain hydrogen, but it still happens since there's a lot of neutron flux.

Canadian reactors (the CANDU design) use heavy water as moderator because it doesn't absorb neutrons as readily, and the improvements in neutron economy allow them to avoid enriching their fuel.

Radiation from tritium is definitely something that we worry about as far as worker safety near piping containing irradiated water, but it's not something that concerns us too much as far as being vented into the environment (we try to minimize that, but there's only so much you can do, especially with a BWR).

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