News update #2 for readers of "A serious but not ponderous book about Nuclear Energy"
In these updates, we try to bring you news of significant current scientific and
technological developments in the field of nuclear energy. New, still in development, not
yet completely tested, some will be tomorrow's news headlines, some may be obsolete
within months or years. Often we have to rely on information from the people who are
promoting them, who have a personal or financial interest in them, and who promise
results which may or may not materialize. Many numbers that we cite are estimates, and
differ from source to source; rarely are all the raw data they are based on available. We
do our best to sort out fact from hype and to be accurate and understandable. You'll be
the judge.
Walter Scheider Cavendish Press Ann Arbor, PO Box 2588, Ann Arbor, MI 48103
cavendish@worldnet.att.net
Thorium: Is It the Better Nuclear Fuel?
It may turn out to be a quantum leap in the search for
economy and safety.
Carlo Rubbia won a Nobel Prize in Physics in 1984 for the discovery of two
elusive high energy particles, called the W and the Z. The discovery was a feat not
only of physics, but of engineering. He is good at both, and now has another idea
which could revolutionize the methods we use to retrieve nuclear energy.
You may never have heard of thorium. It is a plentiful element; there is more of
it in the earth's crust than uranium. No, it is not fissionable. But it can be made into a
low weight isotope of uranium that is fissionable. Rubbia thinks it may be worth the
trouble to do that, even if it is a roundabout route to nuclear fission. countries.
A good introduction to Rubbia's idea is in "Megawatts and Megatons," (pp153163) by Richard Garwin and Georges Charpak, Knopf, NY 2001 (originally published
in 1997 in French). Another summary, just 3 pages long, is in the CERN Courier, a
publication of the European collider laboratory, of April 1995, available on the web at
http://einstein.unh.edu/FWHersman/energy_amplifier.html . The CERN report closes
with this sentence: "With the heavy ecological implications of present nuclear and
conventional energy sources, it is surprising how little R&D work is being invested
anywhere in this potentially rewarding alternative energy solution."
What is special about thorium?
(1) Weapons-grade fissionable material (uranium233) is harder to retrieve safely
and clandestinely from the thorium reactor than plutonium is from the uranium
breeder reactor.
(2) Thorium produces 10 to 10,000 times less long-lived radioactive waste than
uranium or plutonium reactors.
(3) Thorium comes out of the ground as a 100% pure, usable isotope, which does
not require enrichment, whereas natural uranium contains only 0.7% fissionable U235.
(4) Because thorium does not sustain chain reaction, fission stops by default if
we stop priming it, and a runaway chain reaction accident is improbable.
Besides, the priming process is extremely efficient: the nuclear process puts out
60 times the energy required to keep it primed. Because of this, the device is also
called, (quite inappropriately) an "Energy Amplifier."
Naturally occurring
232
thorium is in the form of the stable isotope, 90Th . Notice that thorium is just two
places removed on the periodic table from Uranium. In a sequence of nuclear
processes exactly like those by which the non-fissionable isotope, 92U238 is bumped up
through Neptunium to Plutonium, 94Pu239, Thorium can be bumped up to a light
weight isotope of Uranium, 92U233. (See p 135, Eq 15.01 and 15.02 of "A serious but
not ponderous book about Nuclear Energy".) In each case, a non-fissionable isotope
is converted to a fissionable one.
Plutonium, while highly radioactive, can be shielded and concealed for shipping
and storage, because the alpha rays that it emits do not penetrate lead. On the other
hand, uranium233, the weapons-grade material that could be recovered from the
thorium reactor, can not be as easily concealed. U233 is almost inextricably
accompanied by 0.1% of U232, which, after a series of dissociations (to thallium208)
emits gamma rays that penetrate everything.
Here is the thorium sequence in the Rubbia reactor: A neutron is captured by
232
Th
, which makes it 90Th233.
90
232
90Th
+
1
0n
233
90Th
->
[1]
Thorium-233 spontaneously emits a beta particle (an electron from the nucleus, see p
173), leaving behind one additional proton, and one fewer neutron. ("...Nuclear
Energy" p134) This is called "beta decay."
233
90Th
->
233
91Pa
+
ß
[2]
The element with 91 protons is Protactinium (Pa). The isotope 91PA233 also undergoes
beta decay,
233
91Pa
->
233
92U
+
ß
[3]
The U233 isotope that is produced in step [3] is fissionable, but has fewer neutrons
than its heavier cousin, Uranium-235, and its fission releases only 2 neutrons, not 3.
233
92U
+
1
0n
->
fission fragments
+
20n1
[4]
If this sequence [1 through 4] is to replicate itself, it would require one neutron
to generate the next U233 nucleus [1– 3] and another would be required to induce the
U233 nucleus to fission [4]. A chain reaction, then, could occur only with 100%
utilization of the 2 neutrons emitted in [4]. 100% utilization means none can be
allowed to get away, an ideal that can not occur in practice. With 98% utilization, the
generation ratio (p 87-93) would be 0.98, and the half-life of the decline of the
number of fissions per generation would be 50 generations. (1000 fissions in the
zeroth generation would decline to 1000/e, or 368, fissions in 50 generations.)
This means that by itself, the
fission process would die out very
quickly. With a steady supply of
"priming" neutrons, one can obtain, on
the average, 50 new fissions from each
priming neutron. There is, of course, a
cost in providing the priming neutrons.
But because the energy cost of the
priming neutron is about 30 to 60 times
less than the energy yield of the fissions
it triggers, there is a net gain of energy
of about 30 to 60. This is why it is called
an Energy Amplifier (EA).
The priming neutrons are emitted in
a process called "spallation," which is
the induced splitting of an otherwise
non-fissionable large nucleus. In the EA,
a proton beam impinges on lead, the
high energy protons splitting lead nuclei,
leading to release of neutrons. In
Rubbia's design, the molten lead doubles
also as primary coolant. The diagram at
the left shows the proposed arrangement,
most of it below ground level. High
energy protons emerge through a window in the tip of the proton beam tube inside the
core. Protons split lead nuclei, with neutrons emitted into the core. The molten lead
carries nuclear heat upward by convection.
Pumping is required only in the secondary coolant loop, which carries the heat to
where steam is made for the turbines. All other circulation is convection-driven, with
no moving machinery. The lead and air circulation is guided along partitions that are
not shown.
The lead vessel is nearly 30 meters long and 6 meters in diameter, and contains
10,000 tons of lead. Control rods are not needed, either to regulate energy production
or to stop fission in an emergency, because the fission rate is determined by the proton
accelerator. If the accelerator stops sending protons, fission stops almost instantly. In
an emergency, the proton accelerator can be switched off by a trigger signal, or it can
be shut off automatically if overheating causes the expanding lead to overflow into
the accelerator.
Once fission is stopped, there is still the heat released from radioactive fission
fragments that were produced before the shut-down. Although this rate of heat
generation is a small fraction of that during normal operation of the reactor, the aftershutdown heat can accumulate rapidly if it is not removed (p 208). In the conventional
uranium reactor this heat can be sufficient to melt the core and the bottom of the
containment.
There is no reason to believe that the prevalence of short-lived radioactive fission
fragments (after fission is stopped) will be much different in the Rubbia reactor from
that in uranium 235 reactors (p 208). But the EA is undoubtedly a safer reservoir for
the after-shutdown heat than the conventional reactor, because it is filled with heat
absorbing material (lead) that does not leak, does not require pumping to distribute the
heat evenly, and will not boil away or make bubbles, as water does. Simple
calculations suggest that the lead in this reactor has sufficient heat capacity to keep
the temperature in the reactor below 1300oC even in the worst case, if the cooling
system shuts down completely and no heat is removed from the reactor.
The radioactive waste from the thorium reactor contains vastly less long-lived
radioactive material than that from conventional reactors. In particular, plutonium is
completely absent absent from the thorium reactor's waste. While the radioactivity
during the first few days is likely to be similar to that in conventional reactors, there is
at least a ten-fold reduction of radioactivity in the waste products after 100 years, and
a 10,000 fold reduction after 500 years. From a waste storage point of view, this is a
significant advantage.
It is certainly premature to celebrate this technology yet. Much of the feasibility
data is from small scale tests and from simulations. There are technical challenges that
will have to be overcome. One of these is to find a containment material that does not
have the nasty tendency that steel has to dissolve in molten lead.
An encouraging fact is that so far, the simulations and tests have supported the
theoretical predictions, which is a testament to the engineering savvy of Carlo Rubbia.
In addition to the CERN group, several laboratories in the US, Japan, and Russia are
working on various aspects of the EA technology.
To NUCLEAR ENERGY UPDATE #1
Back to NUCLEAR ENERGY BOOK home page
Back to BOOKS home page
Back to Web site HOME page.
cavendishscience.org