Re: Do CANDU reactors produce (net) nuclear waste?

On 29 Apr, 19:19, "daestrom" <daestrom@xxxxxxxxxxxxxxxxxxxxxxxx>
wrote:
"Alex Terrell" <alexterr...@xxxxxxxxx> wrote in message

news:1177830784.566453.312090@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



On 29 Apr, 01:10, "daestrom" <daestrom@xxxxxxxxxxxxxxxxxxxxxxxx>
wrote:
"Alex Terrell" <alexterr...@xxxxxxxxx> wrote in message

news:1177784784.933952.249470@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

On 28 Apr, 16:39, "Paul F. Dietz" <d...@xxxxxxx> wrote:
Alex Terrell wrote:
In short, it seems that "waste" from a normal reactor can be used as
fuel for a CANDU reactor. What waste though does a CANDU reactor
produce?

If a CANDU reactor's waste is no worse than waste from a normal
reactor, then such a reactor would generate power without adding to
the waste problem.

Is that correct?

No. Each will produce its share of fission products, which will end
up in the final spent fuel. You can't have fission that doesn't
produce
fission products.

But you can have a process which destroys fission products at the same
rate, or faster, than it produces them. Such a process will not add to
the waste problem.

Fission products are not destroyed by placing them in a neutron flux
(i.e.
an operating reactor). If they could be destroyed this way, then even
LWR's
would 'burn' them up. Both LWR and HWR designs create fission products
that
are much lighter atomic numbers than the original uranium.

Sorry - got the the terms wrong. Shouldn't a CANDU convert Actinides
into fission products.

Yes. But a LWR does this to some degree as well. The question is, "How
much?"

<snip>

The level of fission products should roughly equate to the amount of
energy extracted, give or take different paths. So using LWR waste as
fuel for a CANDU will increase the amount of fission products. IIRC
fission products, where radioactive, decay faster, resulting in
shorter waste hazard time (decades).

However, could the actinides be converted in to fission products at
the same rate as they are produced? In this case there would be no net
increase in actinide production. Given most of the fission products
would stop being radioactive in the core, the fuel rod coming out of
the CANDU may be no more radioactive than the fuel rod which went in.

What do you mean, "Given most of the fission products would stop being
radioactive in the core..."??

IIRC, fission products have half lives varying from days to years,
with a few lasting much longer. Given the fuel rod spends a few years
in the core, most of the fission products will decay to something
stable. Though I suppose they will be regenerated by actinide
splitting.

If you take LWR spent fuel and put it into a CANDU, and then extract more
energy from it by 'burning' some of the actinides and further burning the
U-235 (and perhaps some actinides), there will be more fission products than
when the fuel went into the CANDU. Has to be. More fissions, more fission
products. Yes, many short-lived ones will decay within the same time frame
as with the original LWR fuel cycle but the half-life of some fission
products far exceeds the length of time the fuel is loaded (e.g. Sr and Cs).

OK. I do recall Sr 90 being about the worst, with a half life of 28.9
years.

The overall radiation level may be similar to that of fuel immediately
discharged from an LWR (not necessarily the radiation level when ready for
insertion into CANDU). The heat load is similar to the LWR spent fuel, and
that means the short lived fission product levels are about the same, but
that says little about the long-lived fission product levels.

I think that was my original point. LWR + CANDU might give roughly the
same waste problem as LWR on its own, depending on the thermal
neutrons burning actinides. Mishagam believes fast neutrons are best
for actinide burn up, which would probably invalidate my hypothesis.

The crux of it is your statement that thermal neutrons don't do much
for actinide burnup. I'll need to look at this time permitting.
Certainly fission products are being produced, and given the low
concentration of U235 it seem likely that other actinides are involved
as well. Aren't thermal neutrons from thorium meant to be the means of
actinide burn up in proposed thorium reactors?

Clear pro nuclear, but worth a read:
http://www.nuclearfaq.ca/brat_fuel.htm
especially the bit on CANDU/LWR Synergism.

Nice read. But I wonder about the costs of cutting and re-sealing or
double-sheathing versus just manufacturing new fuel. In principle the
operation is 'easy', but with highly radioactive fuel pins, nothing is
'easy'. The amount of volatiles that would be released would generate a lot
of handling problems. And the fuel pellets in LWR fuel at discharge have
lost a lot of their physical integrity inside the pin (centerline melt,
radial cracking, 'bow-tieing'). Simple 'cutting of the spent LWR fuel rods'
would also require boring out some fuel pellet material to make a space for
fill gas, new compression spring and end sealing. Re-sheathing may require
cleaning of the fuel pin's outer surface to remove oxide buildup to get good
conductivity between old and new. Many a detail still involved, with highly
radioactive material, not like manufacturing new fuel.

That's probably why its proposed. This would need quite a few
expensive robots.

Although the 'dry' processing they discuss certainly has advantages over the
'wet'. 'Bake' out the volatils and re-form into new pellets is a lot fewer
steps than chemical reprocessing.

Don't know that much about CANDU fuel's thermal performance, but LWR fuel is
highly optimized for specific flow, LHGR and coolant conditions. If CANDU
fuel performs under much different conditions, these things would have to be
evaluated as well.

Getting more energy out of the fuel before final storage in general is a
good thing, but reducing the number of bundles that have to go to long-term
storage by a factor of two doesn't mean the total costs of long-term storage
are cut by the same factor.

I suspect there is a huge fixed cost to long term storage. For
starters, the length of the planning enquiry will be the same for
1000m3 as for 1,000,000m3.

This is a point in favour of new nuclear build. Building a fleet of
new reactors is not going to add much to the significant problems and
costs of getting rid of the existing waste.

And getting rid of excess weapons grade material is *always* a good thing
:-)
True, but by the time something new comes on stream, it should be
mostly gone.


.

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