How spent are they...?  In an effort to turn swards into plow shares,
if the percentage is anything, reprocessing would make sense, on two
fronts, purifying the waist as well as recapturing U-238 for fuel rod
use.  Who knows, maybe a lower power reactor would make sense these
days, using the pure waist atoms...

Extracting U-238 from ore seems to me to be more expensive as
reprocessing.  Can't the spent pellets be added in the ore process
purification somewhere...?

It depends.   On the type of fuel for instance and the amount it is
used.   The terms "burnup", "rating", "dwell time", "depleted", and
"reactivity" may be useful in searches for a description of spent
fuel.

U238 is not really what you want.  That's the abundant U isotope.  You
might want to recover what U235 is still present (that's the kind
that's increased by enrichment in most reactor fuel).  In a thriving
fuel cycle market (but not today's conditions) you'd want to extract
the Pu (converted from U238) for use in fuel too.  Fast breeder
reactors are currently so out of fashion there's been work done on
"burner" reactors that _consume_ Pu and other stuff - it being rated
as having no value at present.   That's just one of several reasons
the nuclear power industry is not as profitable as was expected in the
1950s.

To get a reactor working you need a suitable combination of conditions
involving the fuel size, shape and composition.  You can't just settle
for a lower-power reactor using U238 any more than you can settle for
a low-power steam engine burning ash.  Your ash may have unburnt
material still in it but if it's not enough to keep up the temperature
and ignite its neighbour your fire will just fizzle out.  A reactor is
a bit like that - a "self-sustaining chain reaction" is needed.

Reprocessing is in any case compulsory for fuel types that are not
durable when spent.

Something to keep in mind:

The US Government, probably in response to the Three-Mile Island
incident (but don't quote me on that since I wasn't born yet),
reasoned under Jimmy Carter, that the US Government is the only entity
in the US that can own High-grade, and mid-grade special nuclear
material-- that is, materials greater than about 20 percent
enrichment.  I understand there are probably limited quatities that
organizations can own in this higly enriched state (probably less than
a few grams, but check www.nrc.gov for the requirements for liscensure
in these cases if you're interested).  So, the US government doesn't
let just anyone use this highly enriched fuel, or even own it.

A byproduct of a heavy water reactor, that is a reactor that burns
mixed-oxide fuels, is production of plutonium in the process of
producing these fuels.  Sometimes, plutonium is formed when a few
neutrons add to uranium without fission. According to Jimmy, only the
US government can own this, so really no one can make these reactors.

Thank your pal Jimmy for these laws-- France, the UK, et cetera-- have
been using mixed oxide reactors for years, with no problems.  Just my
comments about one bone head ex-Navy Nuke, from another bonehead
ex-Navy Nuke.

Just wanted to restate the obvious.  It would seem that reprocessing
the spent material, should be much more cost effective.  I guess I
still don't see the problem DOE...  Then we would have true waist...

I guess i'm a bode head too, never had the Navy pleasure though...