Re: Nuclear Power: Unsafe, Uneconomical, Unsustainable


"Bill Ghrist" <notmyname@xxxxxxxxxxxx> ha scritto nel messaggio
news:F_Kxh.9631$MH2.4655@xxxxxxxxxxx
dave.walters@xxxxxxxxxxx wrote:
...

Of course it's cheaper...it's smaller, eliminates a whole secondary
loop of heat-exchange, uses helium instead of steam, which can be
heated higher (more effienency) and is *totally inert* in terms of
reactivity, and it *never has to shut down* for refueling. There are
NO cooling systems at all as they are unnessary and is cooled
naturally by ambient air temperature. They can use almost any fuel,
including Thorium, which is non-weapons grade fuel, and are built
assembly-line style in modules. And, they are cheap per kwhr
standards. And, they are small, no bigger than 289 MWs (general
atomics version) and 160 or so (S. African/German version).


I think that PBMRs may be a promising technology, although I am not
convinced that the safety case has been made unequivocally, but what is
your basis for claiming that they can use thorium as a fuel? Thorium is
not fissile; it is usable in fission reactors not directly as fuel, but
as the basis for breeding of uranium-233, which then serves as the fuel
for the fission process. This generally is done using some sort of
"blanket/seed" arrangement where by a "seed" of enriched uranium is
surrounded by a "blanket" of thorium. The fissioning of the seed
uranium provides the neutrons that can be absorbed by the thorium to
produce breeding. It is difficult to envision how this could be
arranged in a PBMR "pebble," but perhaps you know of someone who has
worked this out. I have not seen any reference to such a thing.

Regards,
Bill Ghrist

In the '70-'80 both German pebble beds (Avr e Thtr) worked with high
enriched uranium/thorium cycle,achieving so called "near breeding",i.e.
conversion ratios slightly lower than unit
http://72.14.221.104/search?q=cache:yJPPmwk4rsQJ:www.oecdnea.org/html/scienc
e/docs/1976/neacrp-l-1976-169.pdf+pebble+OR+bed+OR+htr+OR+htgr+%22near+breed
ing%22&hl=it&ct=clnk&cd=1&gl=it

Clearly,thorium itself is not fissile,but it can be breeded to uranium 233
fissile by one or more fissile elements.Nowadays,instead high enriched
uranium it could be more interesting to use a
plutonium/minor actinides mixture discharged from current LWR fleet
Pebble beds (and heavy water reactors) could be good reactors to use thorium
because they have good
neutron economy (all ceramics,no metallic material,helium coolant in
transparent to neutrons...) and
can achieve very high burn-ups








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