Funny nuclear numbers

One of these two numbers is wrong.

The politicians of Cumbria County Council have cooled on the idea of burying hundreds of thousands of tonnes of nuclear waste but a kilometre beneath their feet.

Initially tempted by the huge economic benefits a £12bn nuclear research and disposal would bring, councillors now seem dismayed that the waste wouldn\’t be considered safe for another 100,000 years.

There are hundreds of thousands of tonnes of low level nuclear waste. This is entirely true.

But it\’s the high level (essentially, but not exclusively, the used fuel rods themselves) which \”are not safe for 100,000 years\”. Although it\’s a very much shorter period than that for them to be less radioactive than the rock they were originally made from.

But there ain\’t hundreds of thousands of tonnes of that high level waste. To get to that sort of weight you\’ve got to include the bed linen used by people having certain radiotherapies etc. It just ain\’t the same stuff at all.

7 comments on “Funny nuclear numbers

  1. “Although it’s a very much shorter period than that for them to be less radioactive than the rock they were originally made from.” Of order a thousand years. Since the Chinese keep boasting that theirs is the only civilisation to have lasted thousands of years, perhaps we should send all of it to them?

  2. But it’s the high level (essentially, but not exclusively, the used fuel rods themselves) which “are not safe for 100,000 years”. Although it’s a very much shorter period than that for them to be less radioactive than the rock they were originally made from.

    I am not sure that is true. It is certainly true of the daughter products of the fission process. They burn brightly but quickly as a general rule. So they produce most of the radiation in high level waste for the first few hundred years or so. But then the waste is dominated by the decay of the plutonium and minor actinides.

    I will ignore the minor actinides because they are a tiny percentage of the spent fuel. But plutonium is not – it is about 1%. About the same as U-235 in the natural ore actually (about 0.72%)

    U-235 has a half life of seven hundred million years. Pu-239 about 24,000 years. Pu-240 a bit over 6500 years. In other words, they are far quicker to decay – producing bad things on the way – than the U-235 they have replaced.

    Most of the rest of the natural ore and the spent fuel is U-238 in both cases so it is about the same – a bit less for the waste than the natural ore (the daughter products are about 3% of the total).

    I think the conclusion from that is that the high level spent fuel rods are going to be more radioactive than the ore for some time to come. Longer than a few hundred years.

  3. Pretty much all numbers about nuclear, in the press or elsewhere, are “funny”.

    Real numbers about nuclear are difficult to get hold of.

    Just as a note for SMFS – the whole fuel reprocessing saga is to reduce the amount of usable Pu (or other fuel isotopes) dumped in to waste storage. The presence of “neutron poisons” (neutron absorbers) in the daughter fission products means that fuel rods become ineffective when they still contain vast amounts of useable fuel.

  4. Although it’s a very much shorter period than that for them to be less radioactive than the rock they were originally made from.

    That’s unclear. High grade uranium ore might be 17% pitchblende. The nuclear fuel in fuel rods is a mixture of U-238 (mostly) and U-235 (perhaps 3%). Spent fuel will consist of U-238 (mostly), U-235 (perhaps 1%), various fission products, and various neutron-capture products including plutonium. Since there’s a higher concentration of both U-238 and U-235 in the spent fuel than in the original ore, it will always be more radioactive.

    But if you include the fuel rod cladding in the calculation, then it depends on what construction of fuel rod you’re comparing with what sort of ore.

  5. Surreptitious Evil – “Just as a note for SMFS – the whole fuel reprocessing saga is to reduce the amount of usable Pu (or other fuel isotopes) dumped in to waste storage.”

    But I assumed TW was talking about the fuel rod as a whole. Given that there is not a whole lot of reprocessing for fuel going on (weapons is another matter) it seemed reasonable.

    “The presence of “neutron poisons” (neutron absorbers) in the daughter fission products means that fuel rods become ineffective when they still contain vast amounts of useable fuel.”

    And when the percentage of U-235 falls too low. The present useage is incredibly wasteful and I doubt we will go on this way for long.

    4PaulB – “Since there’s a higher concentration of both U-238 and U-235 in the spent fuel than in the original ore, it will always be more radioactive.”

    I think I know what you mean but you are not phrasing it well. You seem to imply that if you concentrate gold into little gold bars, from thousands of tons of rock, you end up with more gold. There is less U-235 and U-238 in the waste stream than there was in the ore. Because a lot of it has burnt up and produced heat and fission products. And plutonium. Which means that as far as the Uranium is concerned, the waste U produces less radioactivity than the ore did. It is just more concentrated. You could put it back into the ground, mixed with the waste from the mine, and it would be as dilute – but still less radioactive.

    The problem is that some of the uranium has been turned into fission products, plutonium and the minor transuranic elements. These are a lot more radioactive than the uranium they came from.

    5johnny bonk – “i think they enrich to higher than that nowadays, 5 – 7 % – others may know better, would love to hear …”

    Depends on the reactor. Some use natural uranium like Britain’s Magnox and Canada’s Candu reactors. Some use slightly enriched. 3-5% is standard for light water reactors.

  6. SMFS: we’re not really disagreeing here. I’m comparing the spent fuel with the equivalent weight of ore; you’re comparing it with the ore it came from. Neither comparison is very meaningful.

    What I meant to emphasise was that spent nuclear fuel is hasn’t had its uranium used up to any significant extent. The U-238 is largely unchanged, and U-235 is present in a slightly higher proportion than it is in naturally occurring uranium. So it’s hard to see any measure by which the spent fuel should be thought of as less radioactive than the rock it came from.

    As you say, it has in addition P-239 and P-240, formed from U-238 by neutron capture. The 100,000 years Tim mentions is I think the time taken for the amount of P-239 to be much reduced (its half-life is 24,000 years).

    There’s a helpful discussion of fission and neutron-capture products here.

Leave a Reply

Name and email are required. Your email address will not be published.

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>