"John Travolta Sardus" <pireddag_at_hotmail.com> ha scritto nel messaggio
news:i12va9$usd$1_at_tdi.cu.mi.it...
> Giulio Severini wrote:
>> Ho letto su it.scienza un thread interessante sul futuro del nucleare
>> in Italia. Non essendo quel ng moderato, quindi potenzialmente pieno
>> di troll chiedo qui, a voi, cosa ne pensate in merito.
>> Gradirei, se possibile, link a documenti che possano fornire numeri e
>> chiarire le idee a riguardo.
>> Grazie,
>>
>> Giulio.
>
>
> Io posso aggiungere qualche domanda mia. Una cosa che vorrei capire piu' a
> fondo e' la questione delle stime della quantita' di Uranio disponibile
> nella crosta terrestre.
...
> Qualcuno sa rispondere?
Ci provo io.
Se guardi bene i dati,
http://www.world-nuclear.org/info/reactors.html
http://www.uxc.com/
oggi da un kg di uranio naturale che ha un prezzo di mercato di < 100 $/kg
si producono da 40 a 55 mila kWh di elettricit�, per cui allo stato attuale
anche usando solo l' attuale e meno efficiente tecnologia nucleare che
sfrutta meno dell' 1% del reale contenuto energetico dell' uranio
(praticamente, semplificando grossolanamente, solo l' isotopo 235 che � il 7
per mille dell' U naturale, e non il 238, che � quasi tutto il rimanente),
il
prezzo dell' U incide praticamente zero nel costo finale del kWh nucleare;
si pu� tranquillamente decuplicare il costo di estrazione dell' U (lo stesso
ovviamente NON si pu� dire per carbone ed idrocarburi) senza impattare
apprezzabilmente nel costo del kWh - tieni conto che i soli costi di
combustibile + operazione e manutenzione del nucleare sono molto modesti, <
2
cent per kWh
http://www.eia.doe.gov/cneaf/electricity/epa/epat8p2.html
Secondo, con il nucleare a differenza dei combustibili fossili, � molto
semplice risparmiare il combustibile: se mai il prezzo dell' U dovesse
davvero aumentare esponenzialmente come sopra, cosa non improbabile visto la
pressione dei giganti asiatici (ma a quel punto se si accetta un costo di
estrazione x10, le risorse sarebbero enormemente pi� ingenti di quelli
attuali che si "fermano" a meno di 130 $/kg, cio� meno di un $ per barile di
petrolio equivalente), allora diventa economico il
riuso e riciclo dell' uranio depleto (il 238 che si diceva) e delle scorie
prodotte
come il plutonio e i transuranici, con il vantaggio indiretto di eliminare
gli elementi radiaottivi a lunga vita, abbiamo > 2000 anni di elettricit�
per l' INTERO pianeta solo riusando/riciclando le scorie GIA' prodotte (oggi
il costo dell' uranio � cmq troppo basso per rendere conveniente questi
cicli del combustibile che sono intrinsicamente pi� efficienti, ma anche pi�
complessi). E' quello che � stato realizzato alcuni decenni fa in Usa nell'
ambito del progetto chiamato "integral fast reactor",
http://en.wikipedia.org/wiki/Integral_Fast_Reactor
che � stato un reattore reale che si alimentava solo ed esclusivamente delle
scorie radioattive prodotte da altri impianti nucleari, producendo enormi
ritorni di energia
> Domanda di riserva. Pare che ci siano in vista (non so quanto nel futuro)
> reazioni basate sul Torio che darebbero delle scorie con vita media molto
> piu' breve che le scorie dell'uranio (decine di anni contro un paio di
> migliaia). Si tratta di fantascienza oppure nel giro diciamo di una
> ventina d'anni potrebbe essere una tecnologia accessibile?
Anche qui, di reattori al torio in passato ce ne sono stati diversi, non c'�
niente di nuovo nello sfruttare il torio come combustibile nucleare al posto
dell' uranio (anche negli attuali reattori ad acqua).
http://en.wikipedia.org/wiki/Thorium_fuel_cycle
http://www.atomicinsights.com/oct95/LWBR_oct95.html
Il vantaggio del torio
vs uranio � che anche senza la tecnologia ai neutroni veloci refrigerato al
sodio (come il Superfenix o l' integral fast reactor, IFR), e quindi anche
nei reattori attuali, � che con esso pu� essere impostato un ciclo
autofertilizzante e pu� essere sfruttato al 100% nel suo (solo) isotopo 232,
a differenza dell' uranio come visto sopra (< 1%, a meno di tecnologie come
l' IFR che ne brucino le scorie e l' uranio depleto).
Tuttavia oggi il prezzo di mercato dell' U, che � praticamente meno di un $
per barile di
petrolio equivalente, non rende conveniente un riuso/riciclo efficiente
delle scorie, operazione assolutamente necessaria se si vuole impostare un
ciclo autofertizzante che sfrutti il 100% della materia prima combustibile
(uranio o torio)
In particolare, � la tecnologia nucleare a combustibile liquido (anzicch� a
combustibile solido come oggi avviene) detti MSR (molten salt reactor o
LFTR, liquid fluoride thorium reactor), di cui fossero costruiti a suo tempo
due protopi di enorme successo
http://en.wikipedia.org/wiki/Molten_salt_reactor
a rendere l' uso del torio particolarmente interessante non solo per motivi
ecologici (nessuna produzione in pratica di scorie radioattive a lunga vita)
o di sostenibilit� (il torio cos� sfruttato sarebbe una fonte perfettamente
rinnovabile, illimitato su scala umana
http://www.energyfromthorium.com/images/thoriumVsUranium.jpg ), ma anche in
particolare 1) di sicurezza, perch� a
quel punto � fisicamente impossibile un incidente grave come Chernobyl o
Three mile islands in quanto la sicurezza non � pi� dovuta all' intervento
umano o l' attivazione di particolari macchinari, ma a semplici ed
inemendabili leggi fisiche (la gravit� o l' espansione dei sali in caso di
surriscaldamento, la rapida solidificazione del combustibile in caso di
perdita del refrigerante, ecc...) e 2) di economicit�, vista l' enorme
maggiore semplicit� tecnologica di un reattore di questo tipo rispetto ad
uno tradizionale ad acqua (per dirne una, il fluido � sempre a pressione
atmosferica contro le 70-140 atm di un reattore ad acqua)
Un articolo su "the oil drum" riassume pi� in dettaglio la tecnologia
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 "
E' abbastanza interessante il fatto che, come citato nel testo sopra, il
famoso climatologo Nasa James Hansen abbia proposto (tra le altre) queste
due tecnologie all' atto dell' insediamento alla casa bianca del pres. Obama
(ovvero, l' IFR per il bruciamento delle scorie e i MSR, ovvero la
tecnologia dei reattori a combustibile liquido, per la produzione di energia
dal torio)
http://www.columbia.edu/~jeh1/mailings/2008/20081121_Obama.pdf
"... Some discussion about nuclear power is needed. Fourth generation
nuclear power has the potential to provide safe base-load electric power
with negligible CO2
emissions.
There is about a million times more energy available in the nucleus,
compared with the
chemical energy of molecules exploited in fossil fuel burning. In today's
nuclear (fission)
reactors, neutrons cause a nucleus to fission, releasing energy as well as
additional neutrons
that sustain the reaction. The additional neutrons are 'born' with a great
deal of energy and
are called 'fast' neutrons. Further reactions are more likely if these
neutrons are slowed by
collisions with non-absorbing materials, thus becoming 'thermal' or slow
neutrons.
All nuclear plants in the United States today are Light Water Reactors
(LWRs), using
ordinary water (as opposed to 'heavy water') to slow the neutrons and cool
the reactor.
Uranium is the fuel in all of these power plants. One basic problem with
this approach is that
more than 99% of the uranium fuel ends up 'unburned' (not fissioned). In
addition to
'throwing away' most of the potential energy, the long-lived nuclear wastes
(plutonium,
americium, curium, etc.) require geologic isolation in repositories such as
Yucca Mountain.
There are two compelling alternatives to address these issues, both of which
will be
needed in the future. The first is to build reactors that keep the neutrons
'fast' during the
fission reactions. These fast reactors can completely 'burn' the uranium.
Moreover, they can
burn existing long-lived nuclear waste, producing a small volume of waste
with half-life of
only decades, thus largely solving the long-term nuclear waste problem.
The other compelling alternative is to use thorium as the fuel in thermal
reactors.
Thorium can be used in ways that practically eliminate buildup of long-lived
nuclear waste.
The United States chose the LWR development path in the 1950s for civilian
nuclear
power because research and development had already been done by the Navy,
and it thus
presented the shortest time-to-market of reactor concepts then under
consideration. Little
emphasis was given to the issues of nuclear waste. Today the situation is
very different. If
nuclear energy is to be used widely to replace coal, in the United States
and/or the
developing world, issues of waste, safety, and proliferation become
paramount.
Nuclear power plants being built today, or in advanced stages of planning,
in the United
States, Europe, China and other places, are just improved LWRs. They have
simplified
operations and added safety features, but they are still fundamentally the
same type, produce
copious nuclear waste, and continue to be costly. It seems likely that they
will only permit
nuclear power to continue to play a role comparable to that which it plays
now.
Both fast and thorium reactors were discussed at our 3 November workshop.
The
Integral Fast Reactor (IFR) concept was developed at Argonne National
Laboratory and it
has been built and tested at the Idaho National Laboratory. IFRs keep
neutrons "fast" by
using liquid sodium metal as a coolant instead of water. They also make fuel
processing
easier by using a metallic solid fuel form. IFRs can burn existing nuclear
waste and surplus
weapons-grade uranium and plutonium, making electrical power in the process.
All fuel
reprocessing is done within the reactor facility (hence the name "integral")
and many
enhanced safety features are included and have been tested, such as the
ability to shut down
safely under even severe accident scenarios.
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 solidfueled
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, albeit with less efficiency than in a fast reactor such as IFR.
Both IFR and LFTR operate at low pressure and high temperatures, unlike
today's
LWR's. 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
(40% in IFR, 50%
in LFTR vs 35% in LWR). Both IFR and LFTR have the potential to be
air-cooled and to
use waste heat for desalinating water.
Both IFR and LFTR 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.
It is commonly assumed that 4th generation nuclear power will not be ready
before 2030.
That is a safe assumption under 'business-as-usual". However, given high
priority it is likely
that it could be available sooner..."
Received on Thu Jul 08 2010 - 18:57:41 CEST
