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On 4/17/2019 at 7:01 AM, Just_A_Guy said:

Any links would be very much appreciated.  I’ve seen articles suggesting that the amount of extant waste, by volume, is much lower than commonly believed; but nothing suggesting the stuff reactors produce can be safe in under a couple of thousand years.

The vast majority of nuclear waste, the so-called "low-level waste", has a radioactive half-life measured in months or even weeks. It is currently stored on-site until it has decayed enough to be safe to throw away.

"High-level waste" is a small fraction of total nuclear waste. It consists of dangerous, highly radioactive material that must be sequestered. Most of this actually decays pretty fast, too, becoming safe in a time period on the order of years or maybe decades. But high-level waste also includes some "transuranics", very heavy (>uranium) artificially produced elements like neptunium, plutonium, einsteinium, etc. Some of these have a half-life of decades or even centuries; plutonium-239 has a half-life of over 24,000 years, and thus requires on the order of 240,000 years (ten half-lives) until it has decayed to a one-thousandth of its original radioactivity. It is these highly poisonous, extremely long-lived radioisotopes that present the greatest concern.

At this point, let me put in another plug for molten salt reactors. The basic design of any such reactor allows easy isolation of such transuranics, and also provides an easy way of destroying them: Keep them exposed to the nuclear processes in the reactor until the neutron flux breaks them down:

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Because a LFTR fissions 99%+ of the fuel (whether thorium, or plutonium from nuclear waste), it consumes all the uranium and transuranics leaving no long-term radioactive waste. 83% of the waste products are safely stabilized within 10 years. The remaining 17% need to be stored less than 350 years to become completely benign.

 

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35 minutes ago, Vort said:

Some of these have a half-life of decades or even centuries; plutonium-239 has a half-life of over 24,000 years, and thus requires on the order of 240,000 years (ten half-lives) until it has decayed to a one-thousandth of its original radioactivity. It is these highly poisonous, extremely long-lived radioisotopes that present the greatest concern.

Remember that it isn't the half-life alone that is the concern.  It is the quantity and rate of ionizing radiation.  The relationship between these is inversely, not directly proportional.  Vort, I realize that you are more familiar with radioactivity than most, but I've got to state this for the thread.

People hear about Pu 244 having a HL in the millions of years and think that's terrible.  No, quite the opposite.  The longer the half-life, the less radiation is being released per second.  The shorter the half-life, the less concern for long term storage.  So, there is a bell curve.  We're most concerned about materials near the top of the bell-curve.  The items that emit sufficiently threatening levels of radiation while still having a HL that will make it dangerous for at least a person's lifetime.

The more dense a product is, the longer the half-life, so the less radioactive.  But the more likely there are multiple stages of decay.  And among those stages of decay, some materials may be release that are near the top of the bell curve.

Then there is the question of states of matter.  Gasses are among the worst because they are difficult to contain or confine.  And if breathed in, even alpha particles can do damage to the lungs.  Whereas, the skin is a good barrier against alpha particles, somewhat weak against beta, and completely penetrated by gamma (the higher band).

Your link to the molten salt reactor was much appreciated.  And it does indeed work through a LOT of the issues.

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