Re: CANDU reactor questions


"Paul Ciszek" <nospam@xxxxxxxxxx> ha scritto nel messaggio
news:gv6lng$i4$1@xxxxxxxxxxxxxxxxxxxx

In article <qVzRl.23132$Ux.12232@xxxxxxxxxxxxxxxxxxxxx>,
Alessandro <miaposta_42@xxxxxxxx> wrote:

"Paul Ciszek" <nospam@xxxxxxxxxx> ha scritto nel messaggio
news:gv6bjt$b2h$1@xxxxxxxxxxxxxxxxxxxx

2) While CANDU reactors make better use of U-235 than the American fuel
cycle does, it sounds like, unless they are being fed recovered
plutonium from elsewhere (such as dismantled nuclear weapons) they
are essentially limited by the amount of U-235 available, which we
can't really afford to think of as inexhaustable.

Of course, there is no unlimited raw material in the earth, but today the
uranium market price is dirty cheap (~ one $ per oil barrel equivalent)
and
we are plenty of the fuel at a cost of x10 or x20 that, of course with
anyway an enormous return of the energy invested in mining. Today, the
need
for reprocessing is mainly ecological, not economic, because we get rid of
transuranics (which have very long hal life) waste

I thought I once saw a comparison of how long the world could meet its
energy needs using various fuel sources in which a U-235-only fuel cycle
didn't last that much longer than fossil fuels.


These aren't *all* uranium reserves, because they consider, as usual, only
low cost uranium resources, typically less than 130 $/kg of uranium mined
(corresponding, indeed, to only one $ per oil barrel equivalent), but with
nuclear electricity you can use uranium (even with current nuclear
technology, no thorium or breeders) at ten times that cost with negligible
escalation in nuclear kWh cost; of course, there is a market price for
uranium where breeders (and thorium breeders) become more economic than
current nuclear technologies

1) This business about "burning" "higher actinides"--do they mean
getting useful energy by *fissioning* assorted stray synthetic
elements that turn up in reactor waste? I thought that designing
a self-sustaining reaction around the "good stuff" like U-235,
Pu-239, or Th-233 was hard enough, and that most of those other
by-products like Am-241 just hurt a fission reaction. How does
one "burn" "higher actinides" at a net energy profit?


Because actinides/transuranics behave themself like fissile elements
(including uranium 235), i.e.
if we burn them we produce enormous quantity of energy, in the range of
more
than 3 tonn of coal equivalent per single gram of them

Again: I thought only a few isotopes could be made to fission on demand,
and the rest, like Americium-241, actually *interfered* with useful
fission.


Only with current nuclear technologies (using slow/thermal neutrons)
transuranics elements like americium tends to build-up and not fission, this
doesn't happen in reactors using fast (or simply "faster") neutron reactor
technology, like the integral fast reactors (IFR) or molten salts reactors
(MSR or LFTR). Pratically, they destroy *all* nuclear "waste" converting
them in useful energy (and fission products with half lifes mainly of only
years or tens of years) - pratically, we can produce all the world
electricity needs for several millenia, only consuming the uranium and
nuclear waste *still produced* and today stored somewhere in the planet,
with no need of single gram of new uranium to mine
http://en.wikipedia.org/wiki/Integral_Fast_Reactor
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 Integral Fast Reactor (IFR)
The IFR is another promising technology nixed by partisan politics. A
prototype reactor was killed by a Democratic congress in 1994, despite test
results showing great potential. Fifteen years have now passed, fifteen
lost years. It's time to go back to it.

The IFR is similar in some ways to the Molten Salt Reactor. It can convert
nearly 100% of the raw fuel (uranium in this case) to useful energy; it
reprocesses fuel at the reactor; it produces tiny amounts of waste
pre-packaged for disposal; the fuel processing does not separate
weapons-grade components; and the fuel from the reactor is always too
radioactive to be safe to divert.

Unlike Light Water Reactors which use fuel as ceramic (oxide) pellets and
the Molten Salt Reactor which uses salt mixtures, the IFR's fuel is
metallic. This fuel is cast into rods and cooled by liquid metal. Both
liquid sodium and lead-bismuth alloy have been suggested as coolants. Like
the MSR, the IFR operates at atmospheric pressure and requires no large
metal forgings. The last design tested was also proven to be passively
safe.

The IFR may seem redundant if we have MSRs, but it has proven capabilities
that MSRs do not, capabilities that we need:

1.. It is a fast-neutron reactor, so it can "burn" troublesome isotopes of
plutonium and americium rather than leaving them as a disposal problem.
2.. It can turn stocks of uranium to fuel, even the uranium in spent PWR
fuel.
3.. Because of this, it can ultimately eliminate the entire stock of
nuclear fuel piled up at present and past nuclear plants.
4.. It can also convert our entire stock of Depleted Uranium (DU) to fuel.
It may be possible to make a fast-neutron reactor using molten salts, but
the fuel chemistry and other details have not been tested; the IFR has. The
IFR needs to be taken to full-scale test ASAP, so that our big nuclear waste
problem can be turned into a small, short-lived one.

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) Supposedly you can load a CANDU-variant with Thorium-233 and
someone else's waste Plutonium, and get a self-sustaining
reaction. Once you start doing that, can you keep it going
with Thorium, or do you need a continuous supply of someone
else's Plutonium?

No, besides of course economics considerations, with a Candu reactor and a
thorium
cycle (that's physically impossible to do with natural or enriched uranium
cycle) you can produce at least slightly more fissile from the thorium
blanket (i.e. uranium 233) than the plutonium you consume. The following
feeds are enterily composed of uranium 233 and thorium, no new plutonium
need at all

So, once India gets their reactors going, they can get by on nothing but
fresh Thorium? Sweet.


That's right, moreover they are going to produce for a single tonn of
thorium about 100 times more the electricity than from one tonn of uranium
in current reactors, i.e. several TWh per tonn of natural thorium, instead
40-50 GWh per tonn of natural uranium mined

Actually, today the French don't *breed* any fuel, don't make confusion
between
simply reprocessing, allowing of course some fuel savings, and breeding
that
means you can produce more fisssile fuel than that you consumed, that's
possible only with 1) plutonium sodium (or metal) cooled fast breeders or
2)
thorium cycle with slow neutrons, that means even with current nuclear
technology
Again, the problem is mainly costs, if we want a self sustained (breeding)
fuel cycle we have to reprocess the spent fuel many times and, at least
with
a solid fuel approach, this is very costly and complex (for the
infrastructures we should need).

So, are you saying that a minority of France's nuclear power comes from
fissioning Plutonium?


Without fast breeders, we can achieve with plutonium/uranium recycle in LWR
only some savings, but not so much

Oh, one more question:

Is the Plutonium-238 that the US prefers for radiothermal generators
specifically a by-product of nuclear weapons production? That is,
if a nation were actively re-processing civilian reactor fuel and
making use of the Plutonium, would it be able to make the occasional
Pu-238 RTG?


I'm not an expert here, but I don't think a simple chemical separation is
not sufficient to extract Pu-238 from a typical LWR waste stream, a
different process of production/separation is needed, I guess ( honestly, I
don't
know how)






.

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