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


<dave.walters@xxxxxxxxxxx> wrote in message news:1177799332.474637.200650@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Apr 28, 9:20 am, "Giuseppe" <g...@xxxxxxxxxx> wrote:
"Alex Terrell" <alexterr...@xxxxxxxxx> ha scritto nel messaggionews:1177765474.341114.73660@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

> I was looking up the CANDU reactor design.

> There's a description at:http://en.wikipedia.org/wiki/CANDU

> An interesting piece states:
> "Recyling of LWR fuel does not necessarily need to involve a
> reprocessing step. Fuel cycle tests have also included the DUPIC fuel
> cycle, or direct use of spent PWR fuel in CANDU, where used fuel from
> a pressurized water reactor is packaged into a CANDU fuel bundle with
> only physical reprocessing (cut into pieces) but no chemical
> reprocessing. Again, where light-water reactors require the reactivity

I don' understand how it's possible to recycle Lwr fuel rods with no
reprocessing,at least separation of uranium from neutron "poisons" fission
products; if criticality is still possible,it's however a great loss in
terms of energy "wasted"



> associated with enriched fuel, the DUPIC fuel cycle is possible in a
> CANDU reactor due to the neutron economy which allows for the low
> reactivity of natural uranium and used enriched fuel."

> 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?

Depending on what you mean by 'repreocessing'. Some CANDUs can be
'simply' engineered to take the LWR/PWR fuel rods by 'repackaging' the
pellets into the larger size rodes for the CANDUs.


Aside from the problems of opening up the rod and letting volatile products out, the fuel pellets in fuel discharged from LWR have lost a lot of their physical integrity. A lot of radial cracking happens early in the fuel cycle, causing pellets to expand against the interior of the cladding. If you were to open up such a rod and up-end it, none of the pellets would simply slide out of the cladding tube. You might get some 'fines' and dust, but that's probably about all.

So you either have to use the existing cladding tube over again, re-sheath it inside a larger diameter tube, or some how mechancially remove the cracked/ expanded pellets. (slice the tube lengthwise, bore it out from the end or ??)

> 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.

Correct. About 25-30 tonnes of spent fuel or three cubic metres per
year of vitrified waste for a typical large nuclear reactor (1000 MWe,
light water type). This can be effectively and economically isolated
*in any event*. The CANDUs in the Advanced models are having their
cores 'shrunk', literally, by 30 to 50% from normal CANDUS with the
SAME power output but with a similiar shrinkage of high level
"wastes". Other CANDUs modifed can eat THIS waste with almost nothing
left.


Come now, "almost nothing left"?? Just a fuel bundle with a lot of short and medium half-life fission products like Tc, Sr and Cs? No significant amounts weapons grade material, I'll grant you that. But the production of long-lived fission products is based on energy extraction. More energy extracted from burning the fuel in several different reactors means more long-lived fission products. Has to be, that's 'fission'.

Most of these fission products are already 'neutron-excessive' because they came from fission of heavy elements. A strong thermal neutron flux doesn't 'burn' them, they have very low affinity for neutron capture. There are a few exceptions of course (Xe comes to mind), but they are the exception, not the rule.

daestrom

.

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