Newsgroup: it.scienza.ambiente
Titolo: Reattori nucleari al torio (video da google tech)

Interessante video di presentazione sullo sfruttamento sicuro e pulito del
torio come combustibile nucleare in reattori a combustibile liquido (non
solido)
http://www.youtube.com/watch?v=VgKfS74hVvQ
Come ricorderete, ho scritto in passato diversi post sulla possibilit� di
produrre energia nucleare dal torio (non dall'uranio come oggi avviene) in
reattori a "sali fusi" (fluoruri), di cui sono un grosso sostenitore (a
differenza del nucleare "tradizionale"), senza tutti i problemi connessi
(sicurezza, scorie, costi/complessit�, tempi di costruzione/modularit�,
persino necessit� di acqua per raffreddamento/difficolt�  a reperire i siti,
ecc...) con la produzione di energia nucleare dalle tecnologie esisitente
Per chi non avesse seguito o ricordasse i miei post precendenti incollo un 
paio
di link :
1) 
http://www.theoildrum.com/node/5002
"....Our nuclear technology still has faults:
  a.. it uses only a fraction of the energy in the uranium we mine,
  b.. it leaves much more waste than is necessary, and
  c.. it presents proliferation hazards that could be avoided.
We should do better, and we can.
The USA has developed technologies to address all of these problems, and
then mothballed them.  The failure to develop our capabilities was not
technical, but political, and came mostly from within your own party.  This
is another luxury we can no longer afford.  These should go back on the
front burner as soon as humanly possible.
The neglected technologies are:
  a.. The molten-salt reactor (MSR)
  b.. The Integral Fast Reactor (IFR)
These two technologies have several very valuable properties in common:
  1.. They reprocess their fuel at the reactor site.
  2.. Because of the on-site reprocessing, there is no storage of spent
fuel.
  3.. Also because of this, the volume of waste is minuscule; the waste from
a reactor's entire lifetime can be stored on-site and not removed until
decommissioning.
  4.. They can use roughly 100 times as much of the raw fuel material as
today's reactors.
A ton of raw nuclear fuel (uranium or thorium) can make approximately one
gigawatt-year of electric power in an MSR or IFR.  The total electric power
needs of the USA could be satisfied by less than 500 tons per year of
either, and a great deal of this could come from material already mined or
even designated as "waste".  Because of these properties, the MSR and IFR
are potential solutions to both the USA's energy difficulties and the
nuclear waste problem.
The Molten-Salt Reactor (MSR)
The Molten-Salt Reactor was originally developed for nuclear aircraft, but
it was later tested as an alternative to water-cooled reactors.  An
experimental reactor at Oak Ridge National Laboratory was tested using three
different fuels:  enriched uranium-235, plutonium and uranium-233 (bred from
thorium).  It ran well on all of them.  The final run was intended to gather
data to evaluate the feasibility of a thorium-uranium fuel cycle, and was
apparently successful.
Molten-salt reactors have a number of advantages over today's water-cooled
technology:
  1.. They cannot suffer a meltdown, because the fuel is already molten.  If
the cooling systems are shut off, the reactors shut down through their
essential physics; they are inherently safe.
  2.. They cannot explode, because they run well below the boiling point of
the salts and require no pressure vessels.  This also makes their components
relatively lightweight and easy to manufacture.
  3.. They can run at relatively high temperatures, which increases their
efficiency and makes the heat usable for many industrial purposes.
  4.. They can remove fission wastes continuously, so there is never a
danger from "afterheat" when a reactor is shut down.
  5.. The extracted wastes are relatively pure rather than containing large
amounts of unused fuel, so their bulk is comparatively tiny.  The wastes can
be made ready for permanent disposal right at the reactor site.  Fuel cannot
be diverted for weapons because it never leaves the reactor building.
  6.. They can be started up with plutonium from spent nuclear fuel or
reclaimed weapons material, and can destroy this fuel while breeding new
fuel from thorium.
  7.. The physics of breeding thorium to uranium creates uranium-232 as well
as uranium-233, which is not a difficulty for power production but makes the
material unsuitable for use in weapons.  Even more so than light-water
reactors, molten-salt thorium breeders do not pose a risk of nuclear weapons
proliferation.
According to recent news, the USA has approximately 900,000 tons of
high-grade thorium reserves.  This is approximately 2000 years of supplies
at current rates of electric consumption, or hundreds of years if thorium
was substituted for all fossil fuel.  Lower-grade thorium resources include
coal ash.
In addition to reactors using molten fluoride salts, it appears to be
possible to make fast-breeder reactors using molten chloride salts.  This
has not yet been tested, but it probably should be.
.....
The Consequences of Breeders
Between the two technologies of the MSR and IFR, the USA's entire inventory
of spent nuclear fuel (43,000 tons of uranium as of 2002), depleted uranium
(roughly 6 times as much) and thorium (900,000 tons of reserves) become
available as domestic fuel reserves.  The entire electric demand of the USA
could be met with roughly 500 tons per year of this; the entire energy needs
of the USA would take perhaps 1500 tons.  We could export both clean,
no-carbon power generators and the fuel to run them.  If we are looking to
save the world from climate change, we have to grab these opportunities with
both hands... "
2)
http://thoriumenergy.blogspot.com/2009/04/kloosterman-on-tmsrlftr-technology.html
the MSR in combination with the thorium fuel cycle has many advantages:
- Fluoride inorganic salts are used as a carrier for the fuel and as a
coolant. They are among the most stable of chemical compounds and have
proven stable under reactor operating conditions. They have a high
solubility for actinides, very low vapor pressure, and good heat transfer
properties. Furthermore, they do not react with air or water, and are inert
to some commonly used structural materials.
- Soluble fission products can be removed on-line in a chemical processing
plant, while non-soluble fission products and the noble metals can be
extracted from the salt by helium bubbling. This enhances the neutron
economy. Together with the large number of neutrons liberated in U-233
fission events, new fissile material can be bred from abundantly available
thorium.
- There are no mechanical valves in the salt circuit. Flow is blocked by
plugs of frozen salt cooled by electrical fans. If the salt heats up to
levels above design values or if the power supply fails, the plugs will melt
and the salt will be drained into storage drums cooled by natural convection
(see the Figure).
- A fast excursion of the fuel temperature will lead to salt expansion
providing instantaneous negative reactivity feedback, which will slow down
or completely stop the fission process. Although heating of the graphite
moderator will generally introduce positive reactivity, this process is much
slower and can easily be controlled. Furthermore, a fuel salt temperature
too high will always lead to drainage of the fuel into passively cooled
storage tanks.
- The primary and secondary circuits are operated under ambient pressure,
which is considered a very important safety feature.
- The thorium fuel cycle produces much less long-lived nuclear waste.
Compared with the standard once-through fuel cycle in a Light Water Reactor
(LWR), a thorium fueled MSR produces 4,000 times less neptunium, plutonium,
americium and curium. Plutonium production is reduced even with a factor of
10,000.
- Among all nuclear reactors, the MSR is most suited to utilize the thorium
cycle. Neutron capture by Th-232 produces Pa-233, which decays with a half
life of 27 days to U-233. To avoid Pa-233 capturing an extra neutron, which
would produce the non-fissile U-234, part of it can easily be stored in a
hold-up tank to let it decay to U-233. This enhances the breeding process,
which makes the MSR, in combination with its excellent neutron economy, the
most attractive reactor for using thorium.
3) 
http://www.theoildrum.com/node/4971
" ...Famed Climate Scientist James Hanson, recently spoke of thorium's great
promise in material that he submitted to President Elect Obama:The
Liquid-Fluoride Thorium Reactor (LFTR) is a thorium reactor concept that
uses a chemically-stable fluoride salt for the medium in which nuclear
reactions take place. This fuel form yields flexibility of operation and
eliminates the need to fabricate fuel elements. This feature solves most
concerns that have prevented thorium from being used in solid-fueled
reactors. The fluid fuel in LFTR is also easy to process and to separate
useful fission products, both stable and radioactive. LFTR also has the
potential to destroy existing nuclear waste.(The) LFTR(s) operate at low
pressure and high temperatures, unlike today's LWRs. Operation at low
pressures alleviates much of the accident risk with LWR. Higher temperatures
enable more of the reactor heat to be converted to electricity (50% in LFTR
vs 35% in LWR). (The) LFTR (has) the potential to be air-cooled and to use
waste heat for desalinating water.LFTR(s) are 100-300 times more fuel
efficient than LWRs. In addition to solving the nuclear waste problem, they
can operate for several centuries using only uranium and thorium that has
already been mined. Thus they eliminate the criticism that mining for
nuclear fuel will use fossil fuels and add to the greenhouse effect.The
Obama campaign, properly in my opinion, opposed the Yucca Mountain nuclear
repository. Indeed, there is a far more effective way to use the $25 billion
collected from utilities over the past 40 years to deal with waste disposal.
This fund should be used to develop fast reactors that consume nuclear
waste, and thorium reactors to prevent the creation of new long-lived
nuclear waste. By law the federal government must take responsibility for
existing spent nuclear fuel, so inaction is not an option. Accelerated
development of fast and thorium reactors will allow the US to fulfill its
obligations to dispose of the nuclear waste, and open up a source of
carbon-free energy that can last centuries, even millennia... "
"...
Thorium is extremely abundant in the earth's crust, which appears to contain
somewhere around 120 trillion tons of it. In addition to 12% thorium
monazite sands, found on Indian beaches and in other places, economically
recoverable thorium is found virtually everywhere. For example, large-scale
recovery of thorium from granite rocks is economically feasible with a very
favorable EROEI. Significant recoverable amounts of thorium are present in
mine tailings. These include the tailings of ancient tin mines, rare earth
mine tailings, phosphate mine tailings and uranium mine tailings. In
addition to the thorium present in mine tailings and in surface monazite
sands, burning coal at the average 1000 MWe power plant produces about 13
tons of thorium per year. That thorium is recoverable from the power plant's
waste ash pile.
One ton of thorium will produce nearly 1 GW of electricity for a year in an
efficient thorium cycle reactor. Thus current coal energy technology throws
away over 10 times the energy it produces as electricity. This is not the
result of poor thermodynamic efficiency; it is the result of a failure to
recognize and use the energy value of thorium. The amount of thorium present
in surface mining coal waste is enormous and would provide all the power
human society needs for thousands of years, without resorting to any special
mining for thorium, or the use of any other form or energy recovery.
Little attention is paid to the presence of thorium in mine tailings. In
fact it would largely be passed over in silence except that radioactive
gases from thorium are a health hazard for miners and ore processing
workers.
Thorium is present in phosphate fertilizers because fertilizer manufactures
do not wish to pay the recovery price prior to distribution. Gypsum present
in phosphate tailings is unusable in construction because of the presence of
radioactive gasses associated with the thorium that is also present in the
gypsum. Finally organic farmers use phosphate tailings to enrich their soil.
This has the unfortunate side effect of releasing thorium into surface and
subsurface waters, as well as leading to the potential contamination of
organic crops with thorium and its various radioactive daughter products.
Thus the waste of thorium present in phosphate tailings has environmental
consequences.
The world's real thorium reserve is enormous, but also hugely
underestimated. For example the USGS reports that the United States has a
thorium reserve of 160,000 tons, with another 300,000 tons of possible
thorium reserve. But Alex Gabbard estimates a reserve of over 300,000 tons
of recoverable thorium in coal ash associated with power production in the
United States alone.
In 1969, WASH-1097 noted a report that had presented to President Johnson
that estimated the United States thorium reserve at 3 billion tons that
could be recovered for the price of $500 a pound - perhaps $3000 today. Lest
this sound like an enormous amount of money to pay for thorium, consider
that one pound of thorium contains the energy equivalent of 20 tons of coal,
which would sell on the spot market for in mid-January for around $1500. The
price of coal has been somewhat depressed by the economic down turn. Last
year coal sold on the spot market for as much as $300 a ton, yielding a
price for 20 tons of coal of $6000. How long would 3 billion tons last the
United States? If all of the energy used in the United States were derived
from thorium for the next two million years, there would be still several
hundred thousand years of thorium left that could be recovered for the
equivalent of $3000 a pound in January 2009 dollars..."

 (sicurezza, scorie,
Non mi ci si metta anche lei a definire "scorie" quello che finalmente
anche la legislazione italiana torna a definire "combustibile esausto" !!!
Roberto Deboni

 ha scritto nel messaggio
news:EtWdnaasMOUwcL3X...@giganews.com...

Non sto a sottilizzare sui termini, ma se davvero si vuole un sistema
nucleare senza praticamente scorie (e dico appunto "sistema" e non 
semplicemente singolo
reattore, perch� l' impianto a sali fusi include tutto il ciclo combustibile
nucleare - entra materiale naturale di miniera, o scorie gi� prodotte da
altri impianti ed escono solo prodotti di fissione a bassa vita/tossicit�),
allora la tecnologia a combustiubile liquido � di sicuro la migliore, mentre
quelle tradizionali sono pressocch� un vicolo cieco. Primo, perch� essendo
il combustibile liquido � nella forma pi� semplice/efficiente per essere
trattato e separato, non necessita (come avviene nelle barre di combustibile
odierne) che vengano tolta l' incamiciatura in lega metalica per poi essere
liquefatto; secondo, come si pu� leggere dalle slides di power point (n�45),
http://rethinkingnuclearpower.googlepages.com/aimhigh
il reattore � enormemente (centinaia di volte) pi� efficiente di un reattore
tradizionale ad acqua : per alimentare una centrale nucleare tradizionale da
1000 MWe occorrono 200-250 tonnellate di uranio naturale all' anno (di cui
350 kg/anno circa diventeranno elementi radioattivi a luga vita), mentre
occorre meno di una tonn di torio/anno per alimentare i 1000 MWe del
reattore a sali fusi e qui gli elementi radioattivi a lunga vita saranno al
max solo decine di grammi all' anno; per ultimo, il reattore si alimenta
esclusivamente con le scorie fin qui accumulatesi senza bisogno di estrarre
un solo grammo di nuovo uranio di miniera - a parte il torio stesso, le cui
necessit� sono appunto come si diceva davvero minime, in pratica una
tonnellata di torio produce fino a ~ 11 miliardi di kWh, per cui il torio 
potrebbe costare come l'
oro e non avere cmq alcun apprezzabile effetto sul costo dle kWh nucleare

 
http://www.youtube.com/watch?v=VgKfS74hVvQ
Ho notato solo adesso che l' autore ha anche un proprio sito, dove possono
essere visionate le slides del video ed altro materiale annesso
http://rethinkingnuclearpower.googlepages.com/aimhigh

 anche la legislazione italiana torna a definire "combustibile esausto" !!!
Esausto all'1-2%,beninteso.