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[Phys-l] Liquid Fluoride Thorium Reactor



Liquid Fluoride Thorium Reactor.


In this post I will be relating a new direction for the production of
energy from Nuclear power that may hold great promise. Nevertheless, it should
be made clear at the onset, that there remain many uncertainties concerning
the economic viability and practicality of this revolutionary new nuclear
energy paradigm. But certainly Humanity can't turn its back on a potential
source of virtually limitless energy without exploring the possibility.


The promise of cheap and limitless energy from Nuclear Power has not been
realized. The problem of long term wastes and the safety of nuclear power
plants continue to be divisive issues. While in my view the story of nuclear
power has not been quite as bleak as committed anti nuclear activists have
asserted, it is true that the technological choices made in utilization of
nuclear energy have been far from ideal. Part of the reason for these poor
choices relate to the birth of nuclear energy, born in the creation of
nuclear weapons. This has channeled nuclear energy into the utilization of
enriched Uranium and Plutonium rather than the far more abundant Thorium fuel
source. It might well be argued that the utilization of a Thorium fuel
cycle was hampered by its unsuitability for the production of fissionable
material for nuclear weapons. Today, however, this is clearly seen as an
advantage in the quest to expand energy production from Nuclear fission.
.


Basic Description of the Liquid Fluoride Thorium Reactor.




I will describe one particular design concept for this Thorium fuel
cycle, what is called the Liquid Fluoride Thorium Reactor. (LFTR) Other designs
may prove superior, this will be just one possible example of this new
technological approach to creating power from Nuclear Fission.

The idea for a molten liquid fueled reactor was born and implemented with
considerable success in one of the craziest cold war ideas of all times, the
creation of a nuclear powered bomber. The idea of flying nuclear power
plants over our towns and cities is hardly an auspicious start for this new
technology but the very insanity of this idea prompted the creation of a very
unique Nuclear power plant design.

In conventional Nuclear Power plants, the fuel consists of solid rods of
slightly enriched Uranium in combination with various metal allows. The
coolant used to transfer heat must be brought to the fuel. In the event of a
failure of the primary coolant system, some form of emergency coolant must be
provided to prevent a core meltdown, even after the reactor is shutdown.

In contrast in LFTR, the fuel is integral with the coolant and is
maintained in a liquid state during reactor operation. LFTR fuels consists of
liquid Uranium and Thorium tetrafluoride in a solution of Liquid Lithium and
Beryllium Fluoride. The Lithium used is depleted in Lithium 6 content to
minimize the production of tritium, a significant radiological hazard. These
salts being ionic compounds have excellent chemical stability. The use of the
structural alloy Hastelloy -N had proven corrosion resistant in the
environment of these high temperature salts and fairly resilient against neutron
embrittlement.


The Reactor where fission takes place utilizes a special low porous
graphite structure which contain both the fuel flow channels and the control and
shutdown poison rods. The presence of Lithium and Beryllium in the coolant
fuel matrix causes some of the moderation to be effected in the coolant.
This provides an important safety feature, a rapidly acting negative
reactivity temperature coefficient that makes power control simple and provides for
automatic power reductions should the fuel heat up beyond established
limits. In effect, the reactor power will automatically follow demand and the
control rods will normally only set the operating temperature of the
reactor. In addition, having the fuel in this form allows continuous on line
refueling and reprocessing. The most troublesome fission poison, Xenon 135, will
be removed by a fuel spray device at the primary coolant pump making for
far greater reactor stability during power operation and during start up
shut down cycles. In addition, because of the on line fuel refueling and
reprocessing, reactor availability should be far higher than conventional light
water reactors.

Since the fuel-coolant mixture will be intensely radioactive, the primary
circuit will be contained behind heavy shielding. The heat from the primary
circuit will therefore transfer heat to a secondary coolant loop
containing liquid Lithium and Beryllium Fluoride. This coolant will transfer its
heat energy into a gaseous working fluid, possibly Helium, to power a Brayton
cycle power plant.


The high temperature gas will flow into a turbine turning an electric
generator. The gas exhausted from the turbine will flow into a Recuperator
transferring heat to gas reentering the secondary coolant loop-gas heat
exchanger, than to a heat sink heat exchanger into a compressor driven by the
turbine. The compressed gas is cooled again in a second heat sink heat
exchanger and then pumped to the Recuperator and back to the secondary loop-gas
heat exchanger.


Because of the high temperature of the fuel coolant mixture (663 degrees
centigrade) and the more efficient Brayton cycle, overall efficient of the
LFTR plant should approach 50% or almost 60% higher than conventional
nuclear power plants This means far less heat pollution, simpler ultimate heat
sink requirements, and based on the amounts of available thorium, thousands
of additional years of energy supply. Continuous on line refueling and
reprocessing is accomplished by an addition flow circuit for the coolant fuel
mixture from the flow input of the reactor and back to the reactor output
flow line.

At the reactor input, at the discharge of the primary coolant pump, there
will be a freeze plug of solid fuel-coolant. Upon loss of power, the freeze
plug melts, draining the fuel coolant into emergency dump tanks which are
passively cooled. I will expand on the safety characteristics of LFTR later
in this post.


The Thorium Fuel Cycle

Thorium is believe to be between three and five times more abundant than
Uranium. Given that the LFTR utilized a breeder cycle, we can expect the
fuel supply to last thousands of years. Of course Thorium cannot sustain a
chain reaction and is therefore not directly fissionable. In the initial fuel
loads enriched Uranium would be needed. However once in operation all
additional fuel will come from Thorium breeding, primarily U233 but also other
fissionable fuels in small amounts.

Fuel in the LFTR comes primarily from the interaction

0n1+ 90th232 = 90th233=[ beta(-) 22.2m] 91Pa233 = [beta(-) 27 d]= 92U233


U233 is directly fissionable by low energy neutrons and has an excellent
neutron yield over a wide neutron energy level. This broad spectrum of
neutrons per fission event allows a breeding ratio of 109% in thermal nuclear
reactors. Breeding of plutonium requires neutrons using fast neutrons making
plutonium breeding more problematic. Fast Neutron reactors require highly
volatile coolants and are considered to be more difficult to control. As we
shall see in the section on safety, the LFTR has inherently safe power
stability and superior traits during start up and shut down cycles.


In addition to the interaction above, the following interaction will occur.


0n1+ 90th232= 90Th231 = 2*(0n1)

90Th231 = [Beta(-) 25.5h]= 91Pa231


0n1+ 91Pa231= 91Pa232 = [beta(-) 1.32 d] = 92U232



This means that if the Uranium is processed from the spent fuel, it
will be contaminated by significant amounts of U232. U232 decays includes
a hard gamma photon. This makes the utilization of fissionable Uranium for
nuclear weapons highly problematic. The high radiation level created by
U232 makes the storage of nuclear weapons using U233 very difficult, but even
more important, the hard gamma flux will destroy plastic bonded explosive
lens and electronic components. This makes the thorium fuel cycle
essentially useless for nuclear weapon production, which perhaps explains its lack of
popularity during the early days of nuclear energy development.

Also nuclear waste produced from the Thorium cycle contains much lower
percentage of transuranic isotopes which are the primary cause of the long
lived hazard of nuclear waste. This is because U233 has much smaller non
fission absorption cross section minimizing the buildup of these isotopes by
neutron capture and even when neutron non fission capture occurs it will, by
subsequent neutron captures, transformed to U235 which is available for
fission. In addition, because of the continuous on line fuel reprocessing, any
transuranic isotopes produced can be mixed with the new fuel burring this
source of long lived radioactivity into shorter lived fission products or
stable isotopes. However, Pa231 produced in the interaction related above is
a source of long lived radioactivity with a mean lifetime of 3.27E4 years.
In addition, the production of Pa233, also related above, is a significant
neutron absorber and does affect the neutron economy. Its mean lifetime of
27 days also allows it to provide some challenge to reactivity control in
LFTR.



LFTR SAFETY


The safety of nuclear power system depends on the successful containment
of the radioactive materials produced by the reactor systems throughout the
entire fuel cycle. This includes the release of small amounts of
radioactive materials in the day to day operation of the plant, but especially the
catastrophic release of fission products and transuranic elements in an
accident. The release of small amounts of radioactive material to the
environment may be somewhat more challenging for LFTR, due to the need for on site
reprocessing. However, this is offset by eliminating the need for fuel
fabrication and Uranium enrichment. However, this requires a careful analysis and
is beyond the scope of this post. Therefore I will restrict this
discussion to possible catastrophic release of large amounts of radioactive
material.

In Nuclear reactors this catastrophic release can occur in three ways. A
breach of the containment system by external forces, airplane crash,
violent weather etc. With regard to this hazards LFTR ranks equal with other
reactor systems, assuming the same high quality construction and that a well
engineered containment structures house the nuclear reactor system. The two
issues I will expand on are a loss of coolant causing melted fuel to breach
the containment and an out of control power excursion causing a reactor
explosion. We have had several examples of these kind of events in the Nuclear
Industry, out of control power excursions at SL1 and Chernobyl, and loss
of coolant at TMI as examples.


In a conventional nuclear reactor a loss of coolant and a failure of the
emergency cooling systems can result in a core meltdown and a breach of the
containment structures. (though that a breach will in fact occur is not
certain even under a full core meltdown) Given the high redundancy of the
active emergency core cooling systems of a conventional reactor, it is highly
unlikely that a meltdown can occur. At TMI however, the operators confused
about what their instrumentation was telling them, turned off the redundant
cooling systems causing the core to partially melt. In today's Nuclear Power
plants, operators are better trained in understanding what their
indicators are telling them and special instrumentation designed to flag dangerous
thermodynamic conditions and a modification of the control of safety
systems addressing this problem have been added to all nuclear power plants.


However, in LFTR, fuel meltdowns are not a worry because the fuel under
normal operation is already melted. And since the fuel and coolant are
integral, a loss of coolant in the sense of a loss of coolant in conventional
reactor is impossible. However even a total loss of heat transfer from the
fuel coolant poses no danger. Should this happen the fuel would heat up
shutting down the fission process, and fuel temperature would stay well within
safe limits. Also since the primary system operates under atmospheric
pressure, a violent breach of the primary coolant circuit is not possible. Should
any leak develop or all electrical power be lost, the fuel coolant is
dropped into emergency dump tanks where the fuel-coolant will be kept well
within limits by passive cooling.


Even in the other class of reactor accidents that can cause catastrophic
release of radioactive material i.e. uncontrolled power excursions, LFTR has
superior characteristics. Uncontrolled power excursions are caused when the
reactor core has so much reactivity than it no longer needs delayed
neutrons to maintain criticality. Neutrons produced by fission consist of two
main types. Prompt neutrons which are released directly from the fission event
and delayed neutrons released from certain daughter products of fission.
The reactivity of the reactor must be kept low enough to need these delayed
neutrons to sustain the fission chain reaction. In other words K_eff must
be kept low enough to prevent a chain reaction from prompt neutrons alone.
This condition where this isn't so is called prompt criticality and is fatal
to any nuclear power reactor.

This can be seen from the following equations. For neutron cycles with
delayed neutrons you have;




tau= (betabar - rho)/(lamda*rho)+ L/rho


Where betabar is the precursor atom effective fraction, a value
proportional to the fraction of delayed neutrons in the neutron cycle, lambda is the
weighted average decay factor of the precursor atoms, L is the mean prompt
neutron life cycle time in the fission process and rho is the reactivity
which equals;


Rho= K_eff-1/K-eff

Where K_eff is the effective neutron multiplication factor per cycle.

Betabar varies with various fissionable fuel ratios but can roughly be
given by 70E-4, Lambda is approximately 0.1, and L = 1E-4 seconds. ( Fast
neutron cycles are far shorter.)


However, for prompt neutron cycles the rate equation reduces to


tau=L/rho

Let's assume a reactivity of 80E-4, a bit above the delayed fraction. We
then have;


Tau= 1E-4/80E-4= 1.25E-2


Now given


P=P(0)*exp[t/tau]= P(0*)exp[80*t]


Given P(0)= 1E-3 % power we have in one-second.

1E-3*exp[80] = approx. 1E32 % reactor power.

This makes clear the danger of achieving prompt criticality in a nuclear
reactor.


Can this happen in the LFTR? Like all commercially licensable nuclear
reactors in the United States and Western Europe, this is all but impossible
when the reactor power level is above the point of adding heat. That is when
the reactor is at a power level where increasing reactor power increased
the temperature of the reactor system i. e the fuel, but especially the
moderator. In LFTR, since the fuel is mixed directly with part of the moderator,
the Lithium and Beryllium Fluorides, there is a faster temperature
response as power increases making power level very stable. ( Unlike Chernobyl
type reactors)



This leaves two possible situations where prompt criticality may occur,
during startup when reactor power is below the point of adding heat, and
during refueling. However, a refueling accident cannot happen for LFTR because
all fueling is done above the point of adding heat during reactor operation
and for a normally configured shutdown condition there will be no fuel at
all in graphite core structure which is needed to create a critical mass.

During start up, the LFTR, just like conventional reactors, depends on its
nuclear instrumentation and redundant shut down safety systems, as well a
careful calculation of expected control and shutdown rod alignment needed
to sustain a critical mass in the reactor.


The Reprocessing Issue.

One very big unknown is the issue of on site fuel processing. This is
turning over a responsibility to utilities that they have never dealt with
before. This would require capital expenditure on costly and complex equipment
and the need for many additional highly trained and reliable personal. This
would be a daunting challenge for most, if not all utilities.

However, on the flip side, LFTR eliminates the need for very expensive fuel
fabrication and Uranium enrichment. Would the savings here balance out the
added costs of on site fuel processing? Also even fast breeders require on
site fuel processing and fabrication. If nuclear energy is to be a
significant source of energy, it seems that on site processing is a must.


I think from this post, its clear that the expansion of nuclear power will
require a better technological approach and that such an approach may be
possible, though many questions remain. However, if renewable energy sources
are shown to be unable to address Humanity's need for a non green house
gas generating source of energy, this new Nuclear Technology might be just
what we need.



Bob Zannelli