Nuclear waste comparison between LWR and HWR

[oksuz_]

Hi,

If we compare LWR and HWR (CANDU), which reactor type produces more waste per used Uranium? And also, between these reactors, is there a difference at the time required for the spent fuel to be completely safe? For LWR, It is around 100 000 years. But what about HWR?

Thank you in advance.

 

[mathman]

Iam not a nuclear engineer. However the essential difference between light and heavy water is that a proton may absorb a neutron, while a deuteron does not, so the CANDU is more efficient. However the effect on fissionable material is the same in both cases. If there is a difference
in thw life of spent fuel, it would be as a result of other materials used.

 

[Astronuc]

CANDU fuel uses natural uranium or slightly enriched, and it achieves low burnup compared to LWR fuel. Typical discharge burnup for LWR fuel is on the order of 50 to 60 GWd/tU, while CANDU may get to 18 to 20+ GWd/tU, so has lower fission product inventory, but I've seen older numbers like 9-12 GWd/tU. At the lower burnup levels, CANDU spent fuel would reach relatively low activity levels after a few thousand years as opposed to 10s of thousands for LWR spent fuel.

I'm aware of some extended burnup fuel in CANDU reactors, but I don't know how common. I will check with some colleagues to see if statistics are readily available.
CANADIAN HIGH BURNUP FUEL EXPERIENCE - dated publication with a reference to data from 1983
https://inis.iaea.org/collection/NCLCollectionStore/_Public/22/060/22060819.pdf (dated paper)

Back in the 1970s, LWRs operated on annual cycles with discharge burnups on the order of 30 to 35 GWd/tU. Once recycling was not an option and when the DOE would not accept spent fuel, utilities then transitioned to longer 18 or 24 month cycles and higher discharge burnups of about 50 to 60 GWd/tU. Higher burnup means more fission products and transuranics.

 

[rpp]

I think you need to define your terms a little better. For example, what do you mean by "more waste per used Uranium"? Do you mean the total mass of the discharged fuel, the mass of the fission products, the mass of the actinides, or perhaps the heat load of the waste? As others have pointed out, lower discharge burnups will increase the total amount of mass, but will have decreased fission product inventories. If you know exactly what you are looking for, you can run a depletion calculation and find out the answer. The usual limits on a spent fuel repository are volume and heat load.

Second, how do you define "time required for the spent fuel to be completely safe"? There really isn't such a thing. You can always trip or choke on a piece of metal, so nothing is ever "completely safe". Maybe you mean the amount of time for the radioactivity of the spent fuel to match the radioactivity of natural uranium? Even so, does it matter if one is radioactive for 100,000 years and another one is radioactive for 120,000 years?

If you don't look too closely at the details, energy comes from fission, so the same amount of power generated by a CANDU or a LWR is going to require the same number of fissions, which will produce the same number of actinides and fission products. There may be some slight differences due to the neutron spectrums, but they will be similar numbers.