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Figure 1. The concentration of CO2 in the atmosphere in

ppm as a function of time during the past ten thousand years

(up to 2005). Data source: IPCC

-

EARTH ENERGY: GIFTS FROM NATURE

(The English Poster Version)

Mitigating Climate Change

Human burning of fossil fuels

has increased the atmospheric

concentration of carbon

dioxide from 280 ppm before

the industrial revolution to

395 ppm at the time of the

writing of this article (Fig. 1).

Overwhelming scientific

consensus holds that this

increase is the main cause of

modern climate change. To

avoid climate catastrophe, we

need to transition away from

burning fossil fuels. In this

article, we introduce two

alternative paths that are being

developed at ASIAA.

Nuclear Breeder Reactors

Conventional nuclear energy based on fissioning U-235 (enriched relative to natural

uranium) in light water reactors (LWRs) is not a sustainable replacement for fossil

fuels because there is only six years of energy use at the levels needed globally in

2050 if all that energy were to come from U-235 in high-grade uranium ore. In

addition, LWRs are (unfairly) perceived to possess a nuclear waste problem, safety

issues against the massive release of radioactivity into the environment, and security

issues against weapons proliferation.

U-238 is more than 100 times as abundant as U-235, and adding a neutron to U-238

makes U-239, which becomes Pu-239 after two beta decays to turn two neutrons into

two protons. Pu-239 is fissile. Such a program of breeding to turn a fertile (U-238) into a fissile (Pu-239) raises the high-grade uranium ore use (if all power came from

fission reactors) to 600 years. Uranium-bearing minerals are soluble in seawater,

leading to Japanese proposals to use polymer filters to trawl for uranium from

seawater. The supply of uranium in the oceans suffices to power a plutonium economy for hundreds of thousands of years.

The potential for thorium breeder reactors is even better. Thorium has only one stable

isotope, Th-232, which eliminates the need for expensive isotope separation.

Moreover, while Th-232, an even-even nuclide with 90 protons and 142 neutrons, is

only fertile, it can be made fissile by absorbing a neutron. This turns Th-232 into

2/10

Th-233, which, after two beta decays that convert two neutrons into two protons,

produces U-233. An even-odd nuclide with 92 protons and 141 neutrons, U-233 is

fissile. When U-233 has a slow neutron added to it (one with a spin opposite to the

unpaired neutron that must be in U-233 because it has an odd number of neutrons),

the increase in the energy of the large nucleus is enough to cause the resulting nucleus

to vibrate violently into two uneven pieces, called fission products. Fission products

from the breakup of a neutron-rich parent are too neutron rich to remain in such states

without spitting out an additional 2 or 3 neutrons. When a U-233 nucleus absorbs a

slow neutron and fissions, an average of 2.49 (fast) fission neutrons will be produced

in the aftermath.

Because this average output of neutrons per fission is greater than 2, apart from the 1

neutron needed to sustain the chain reaction, another is available to turn a neighboring

Th-232 nucleus into Th-233, that then decays into a new fissile U-233. If the neutron

economy is managed properly by building the reactor core out of materials that do not

absorb fission neutrons parasitically while slowing them down to low speeds, the

extra 0.49 neutrons on average per fission reaction can make more U-233 from

Th-232 than we started out with. In principle, then, thorium breeder reactors could

exponentially expand their numbers until we have enough to supply the total energy

needs of the world.

Thorium is 3 to 4 times as abundant as uranium in the crust of the Earth. What is a

600 year depletion time for high-grade uranium ore becomes something more like

2000 years for the depletion of high-grade thorium ores. As a chemical element,

thorium behaves oppositely to uranium in one important respect: thorium minerals are

not soluble in seawater. Thus, they are not found in the oceans of the Earth, but are

ample in beach sand of a variety black in color called monazite. Lots of monazite exits

on Taiwan beaches. If you think it is not enough, just go out in the ocean and get some

more from the ocean bottom. Because thorium has no other commercial applications,

no one has surveyed how much thorium might exist in the world as potential nuclear

fuel. The reserves are likely to last millions of years, if not billions, if one were to go

to lower grades of ore. Thus, thorium MSBRs are sustainable.

Molten Salt Breeder Reactor

Our more detailed discussion of MSBRs begins with the observation that it offers a

solution to the nuclear waste problem that has accumulated from half a century of

operating LWRs.

Figure 2 schematically provides the solution. The high-level nuclear waste from the

spent fuel rods of LWRs consists of three main components:

Unreacted U-235, mixed with U-238,

Pu-239 and higher actinides from collateral neutron irradiation of U-238,

Fission products from the splitting of fissile nuclei.

3/10

Frank H. Shu

Yucca

Mtn

MSRs Can Rid LWR Waste &

Safely Breed for U-233

LWR spent fuel Th-232 Blanket

U-238, U-235

Pu/actinides

Fission prods

Th-232

Ground

300 yr

IFR or

TWR

Core

Chain reaction, breeding, and processing in liquid salt

Enough in Lehmi Pass for

1,000 yr of USA energy use

Pu in core

turns

Th-232

into

U-233

U-233

in core

gives

breeder

2/15/13

Blanket processing: UF4 (liquid) + F2 (gas)

! UF6 (gas)

both U-233 & U-232

9

Figure 2. Schematic diagram of how solving the nuclear waste

problem of LWRs provides a method to start up MSBRs.

ASIAA

Unreacted uranium can be safely separated from the Pu-239 and minor actinides by

the standard process of

fluorination to produce a

gas UF6 that rises out of a

molten salt system. Once

separated, the large

amounts of U-238 mixed in

with the U-235 (converted

from the UF6 form to more

stable oxide forms) makes

this material unsuitable for

bomb making, and it can

either go to a geological

repository (like Yucca

mountain or its

replacement), or be given as fuel for proponents of reactor technology like the integral

fast reactor (IFR) or traveling wave reactor (TWR). A process called pyroprocessing developed at the Idaho National Laboratory then safely separates the Pu-239 and

minor actinides from the fission products.

With a few unimportant exceptions, the fission products contain radioactive elements

that have half-lives of order 30 years or shorter. Such material can be packed in dry

casks and stored underground for 300 years, after which their radioactivity has

dropped below background levels. The casks can be opened to retrieve rare substances

that have great economic and medical value.

The Pu-239 and minor actinides are chemically made into fluoride compounds, such

as PuF3, and dissolved in eutectic NaF/BeF2 molten salt (our preferred choice of the

carrier solvent salt). We pump enough of PuF3/NaF/BeF2 fuel salt into the core of a

molten salt converter reactor (MSCR) to achieve a critical mass and to sustain a chain

fission reaction. The excess neutrons above what is needed to sustain the chain

reaction (against parasitic neutron captures by non-fissiles in the system) random

walk their way out of the core to irradiate a blanket salt in a pool surrounding the

reactor core that consists of ThF4 dissolved in molten eutectic NaF/BeF2. The thorium

is entirely in the form Th-232, and neutron captures by Th-232 result, after two beta

decays, in U-233. When the Pu-239 and minor actinides are consumed, we have

solved the nuclear waste problem of LWRs.

The solution for LWR waste has two side benefits:

It has eliminated the dirty bomb risk from the existence of LWR plutonium.

4/10

Figure 3. One design possibility for a two-fluid MSBR (patent

pending). Four molten-salt pumps in the foreground, fuel salt

circulates into the vertical channels in the black-colored core.

Reaching a compact configuration with moderator graphite all

around it, the fuel salt sustains a chain reaction. Pumps in the

background pull blanket salt through the core in horizontal

channels that alternate with the vertical channels, but separated

from them by walls of graphitic material. Heat from fission

reactions in the vertical channels conducts across the graphite

into the blanket salt in the horizontal channels. The blanket salt

then flows into a secondary heat exchanger in the background

outside the pool. The secondary heat exchanger transfers the

heat from the radioactive blanket salt to a non-radioactive

working salt (e.g, the NaAc/KAc used for supertorrefaction of

biomass). After the secondary heat transfer, the cooled blanket

salt flows to rejoin the pool at the top. The cooler blanket salt

lying above the hotter blanket salt induces a convection patter

that keeps the blanket salt well mixed. In the interim the cooled

fuel salt flows out of the core into the foreground pumps,

where any fission gases in the salt are flushed out of the system

by helium gas flowing through the white pipes. The fuel salt

then circulates back into the core via the red pipes to begin the

process anew. ASIAA

It offers a way to start up MSBRs when

U-233 does not exist

in nature.

The manufactured U-233

in the blanket salt exists

chemically as UF4 in the

pool. To extract it, we

continuously pump small

amounts of the pool salt to

a chamber where gaseous

F2 bubbled through the

molten salt combines with

UF4 in solution to form a

gas UF6 that bubbles out of

the liquid. The UF6 then

flows to another chamber

where it attacks metallic

Be to produce UF4 and

BeF2. When we dissolve

the 233

UF4 in eutectic

NaF/BeF2 molten salt and

pump this fuel salt into the

core of the reactor, the

replacement fissile has

turned a MSCR (converter

reactor) into a MSBR

(breeder reactor).

Electrolysis of the BeF2

can recover the Be and F2

needed to process the next

batch of 233

UF4. The

chemical processing is

straightforward and can be

carried out remotely

without endangering the

operators. The energy

needed for the chemical

processing is minuscule (~

10-5

) compared to the

nuclear energy benefit.

Because the fuel salt in MSBRs circulates indefinitely until all fissiles are consumed,

there are only fission products to deal with by underground storage for 300 years.

Thus, MSBRs have no waste problem of their own without a good solution.

What about security? Cannot U-233 be used to make bombs? No, when one has fast

fission neutrons flying around, one cannot avoid reactions with one fast incoming

5/10

neutron and two outgoing neutrons. Such reactions create U-232 that accompanies the

U-233. In its decay chain, U-232 is a powerful gamma emitter, and U-232 is almost

impossible to separate from U-233. Even if martyrs were willing to make a bomb

using unseparated U-233/U-232, the presence of the U-232 would make the bomb

easily detectable by Geiger counters if one tried to smuggle it into a city, say, in a port

container. The gamma rays would also interfere with the sensitive electronic control

mechanisms that must be part of any weapons assemblage. No nation or terrorist

organization would attempt to make a bomb this way, when much simpler alternatives

are possible. Thus, MSBRS are secure.

Figure 3 shows a possible design for a two-fluid molten-salt breeder-reactor of a type

described schematically in Figure 2. To slow the fission neutrons from the fast speeds

at which they emerge from the fission reactions without absorbing them, we build the

reactor core entirely out of carbon-based materials (except for metallic nuts and bolts).

Graphite is impervious to chemical attack by hot NaF/BeF2 as long as there is no

water in the salt. Doubled for safety of containment, the walls of the pool are made of

metal (Hastelloy N resistant to attack by the salt). The random walking neutrons in the

pool will be mostly absorbed by Th-232 (in the form of ThF4 dissolved in molten

NaF/BeF2 in the pool) before they can strike the walls of the pool and activate the

metal to become nuisance low-level waste.

Nuclear Accidents

All nuclear reactors are designed to shut themselves off automatically in the case of

an emergency. The MSBR is no different, it just has larger safety margins. No reactor

accident has ever occurred because of a runaway chain reaction (with the exception of

the Chernobyl reactor, which had a horrible flaw in its design that could never pass

the nuclear regulatory review outside of the former Soviet Union). Most nuclear

accidents occur after the reactor has shut down safely. They arise because of problems

in dissipating the decay heat from the fission products.

For reactors with fixed solid fuel elements, the possible problems are exemplified by

Fukushima. An emergency arises (a tsunami of historical proportions strikes the

station). The reactors shut down safely, but the fuel rods continue to put out decay

heat that is a few percent of reactor full power. Something knocks out the cooling

systems normally used to cool the fuel rods (the whole electrical grid goes down

because of the earthquake and tsunami). Emergency equipment has to cool the fuel

rods while they remain in the same cramped space of the operational configuration.

The auxiliary power goes out (fuel for diesel generators swept away, batteries run

down), and there is a loss of coolant fluid (because the water boils away). Now, the

plants are in big trouble. Without active cooling of the fuel rods, the rods melt down.

Steam interacts with the superhot fuel rods, generating hydrogen. The hydrogen

escapes into the containment buildings and explodes. Not designed to be strong, the

buildings blast apart. Containment is breached, and massive amounts of radioactivity

escape into the environment.

None of these events would have occurred in two-fluid molten-salt breeder-reactors of

the design in Figure 3 because of the following safety features:

6/10

MSBRs do not use water, so they do not need to be located near large bodies of

water, like rivers or ocean sides, where people like to live. They can survive

earthquakes and cannot be overwhelmed by tsunamis.

Molten salt reactors run themselves, without operator intervention needed;

Neutron absorber elements buoyant in the blanket salt automatically descend into

the core if the pool loses coolant (the blanket salt of the pool).

If the fuel salt overheats for any reason, a drain plug melts that dumps the fuel salt

into an air-cooled tank absent of moderators and of a geometry where reaching

accidental criticality is impossible.

In MSBRs, if reactions run too fast, the fuel salt heats up. The molten fuel salt will

then expand partially out of the core, and the reactions will slow. Conversely, if we

need extra power, we pump on the blanket salt harder. This cools the fuel salt, causing

it to contract into the core more, thereby making the reactions run faster. These

principles are exactly how the Sun, having a gaseous core that expands when heated

and contracts when cooled, regulates its thermonuclear fusion reactions in the core to

balance what is lost in radiation from the surface. We no more have to worry about a

molten salt reactor overheating or overcooling than we have to worry that the Sun

tomorrow wont be the same as it is today.

The idea of a drain plug originated at Oak Ridge National Laboratory, who invented

the concept of reactors with liquid fuel elements. With solid fuel elements, as we have

seen in the example of Fukushima, if something goes wrong with the primary cooling

system, the problem needs fixing with the equipment in the same place where

something broke. With liquid fuel, we can move it to another place (the dump tank)

where we have prepared a separate emergency cooling system. We choose the coolant

to be air, because although we can lose water, and we can lose molten salt, it is almost

impossible to lose air.

To be able to use air to cool nuclear power equipment, however, the decay heat cannot

be overwhelming. This is where online cleaning of the fission products (needed to

maintain the breeding ratio above unity) makes its contribution to reactor safety it allows even reactors with fairly large full-power operations to have relatively little

decay heat when one has reactor shutdown in an emergency. To be supersafe, we

should avoid building reactors that are too big (because the amount of decay heat

scales with operational full power).

Nevertheless, it is conceivable that with complete station blackout (as happened with

Fukushima), the power needed even to run fans wont be available. Suppose the fuel salt then melts through the air-cooled dump tank. For this contingency, weve added a steel salt catcher into which the molten salt will spread into a thin sheet, conducting

its heat to inside the steel as it flows. The design is such that the salt freezes in less

than 10 seconds to immobilize any fission products that the fuel salt might contain.

Because solid salt has a very low vapor pressure, no radioactive gases will escape.

One extra precaution must be taken: a containment dome that can prevent intrusion by

jet airplanes that try to crash into the reactor. We have to design the dome so that in

case the unthinkable happens, and the operators have to abandon the site, the reactor

is walk-away safe. This means that decay heat cannot be trapped inside the dome, but

7/10

Figure 4. Torrefaction of woody plant material. Data source: Bergman et

al. 2005).

Figure 5. The Crankberry machine for tabletop supertorrefaction.

ASIAA

needs to be able to work its way out. A good design is exemplified by the

Westinghouse AP 1000, which has a thin steel cap that traps gases inside but allows

conduction of heat to the upper surface, which is cooled by convection in a protective

concrete dome partially open to circulating outside air. Finally, MSBRs can be located

in remote places where any accident would have a minimal impact on surrounding

human populations. Thus, MSBRs are walk-away safe.

Supertorrefaction of Biomass into Biofuel

Torrefaction is

generally recognized

as the most efficient

way of harnessing

biomass energy (Fig.

4). The traditional

method involves

burning a fuel and

letting the flue gas

heat biomass in a

partially enclosed

environment that has

a limited intake of

oxygen in air. The

process drives out

volatile organic

compounds (VOCs),

including water

vapor, leaving

behind a blackened

solid residue,

charcoal. The VOCs

are usually burned to

supplement the fuel,

which can be natural

gas or a portion of

the biomass or its

torrefaction

products.

Supertorrefaction

(patent pending) is

an improved process

conceived as part of

a general program

using molten salts to

generate alternative

energies by the first

author and brought

to maturity at

8/10

Figure 6. Examples of charcoal making by supertorrefaction with molten acetate

salt (NaAc/KAc) from different biomass feedstocks. ASIAA

Academia

Sinica.

Supertorref

action uses

molten salt

as a

medium to

transfer

heat to the

biomass

with which

the salt is in

direct

contact.

Immersion

beneath the

surface of

the salt

excludes

oxygen and

air. In

contrast with traditional torrefaction, where many hours are required for the

completion of the charring process, supertorrefaction requires typically only ten

minutes because the heat capacity of molten salt per unit volume is about 2000 times

larger than that of flue gas if both heat-transfer fluids are at atmospheric pressure and

a given temperature.

The second author of this article designed a tabletop machine (crankberry, Fig. 5) which automates the process of supertorrefaction on a laboratory scale. Using the

crankberry, the third author and his group have supertorrefied a wide variety of

biomass feedstocks, with uniformly good results (Fig. 6).

If the temperature of the salt is 300 oC, a product ecocoal results that is a

clean-burning, carbon-neutral, replacement for natural coal; whereas if the

temperature is 500 oC, the product biochar is a fine carbon-negative soil amendment

(Fig. 7). We note that burying bichar is a carbon-negative activity, beneficial not only

to the host country, but to the whole world.

Because the VOCs driven from the biomass are recovered rather than burned, the

economic return per unit weight of the biomass is higher than in traditional

torrefaction. In particular, apart from water (which we recover and recycle for

washing and recovering the salt in the finished biochar), acetic acid is the most

abundant component of the VOC yield. We are able to generate acetone and

Na2CO3/K2CO3 if we take NaAc/KAc above 460 oC. By reacting the Na2CO3/K2CO3

with acetic acid, which is a fast acid-base reaction, we are able to recover the

NaAc/KAc that we decomposed (plus CO2 and H2O).

Acetone is a high-value chemical, useful as an industrial solvent as well as a

feedstock for general aviation fuel, so the technique not only creates a

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Figure 7. Scanning electron microscope (SEM) images of (left) ecocoal made

from leucaena supertorrefied at 300 oC for ten minutes, and (right) biochar made

from leucaena supertorrefied at 500 oC for eleven minutes. The bar at the bottom

left of the left image is 10 microns; of the right image, 20 microns.

Supertorrefaction at 300 oC drives out VOCs from ecocoal, but leaves many

microstructures within cell walls, whereas supertorrefaction at 500 oC decomposes

some acetate salt into carbonate salt and leaves behind only cell walls. Below the

image we give the Brunauer-Emmett-Teller (BET) measure of porosity (area per

unit mass) in m2/g. ASIAA

high-throughput solid biofuel to compete with natural coal, but also a liquid feedstock

to lessen the dependence on petroleum for one segment of the transportation industry.

We also get uncondensed gases combustible as a replacement for natural gas.

Supertorref

action

allows a

greatly

reduced

size of the

equipment

needed to

produce a

given

throughput

(tonne per

day) for the

biomass

processing,

even when

the slight

loss of the

salt encased in the pores of the charcoal is taken into account. This reduction lowers

considerably the initial investment of capital equipment. Indeed, it is possible to have

supertorrefaction throughputs that generate attractive economic returns with

batch-process equipment compact enough to be transportable by truck to remote batch

supertorrefaction sites where the biomass is harvested. These capabilities make

commercialization of supertorrefaction possible in startup environments that hold

many barriers for traditional torrefaction technologies.

For example, a bad situation exists in Western North America, where winters that are

too mild, combined with drought-like conditions in the summers, are blamed for an

outbreak of pine bark beetle disease in mountain forests stretching from Southern

California to British Columbia. Hundreds of thousands of pine trees fall per day. We

propose that the felled trees should be supertorrefied before they become ground

tinder for wildfires, or rot and release greenhouse gases into the atmosphere, or have

falling limbs that bring down power lines and cause expensive and dangerous outages.

We would bury the resulting biochar in the same forests, not only sequestering for

thousands of years the resulting carbon, but also encouraging new growth that would

lock up more carbon.

The forest crisis affects more than just North America. A survey published in Nature

magazine in 2012 found that 70% of 226 forest species in 81 forests of the world are

on the verge of dying from the stress placed on root systems when there is too little

water in the soil. This existential threat deserves an adequate response.

The Grand Challenge

Climate change is the grand challenge of the twenty-first century. The fate of human

10/10

civilization may well depend on whether we rise in a rational and scientific manner to

meet this challenge. The ultimate goal of our group is to marry the technologies of

molten salt reactors and supertorrefaction. We can transfer the heat carried in the

radioactive blanket salt (ThF4/NaF/BeF2) to a non-radioactive working salt

(NaAc/KAc) via a secondary heat exchanger (an easy coupling depicted in the

background of Fig. 3). We can then use nuclear heat to produce from biomass, at very

high throughputs, biochar, acetone, and syngas cheaper and cleaner than the highly

invasive processes of extracting coal by strip mining and mountain-top removal,

petroleum by drilling in the ocean deeps, and natural gas from the hydraulic fracturing

of shale rock. Baseload electric power can be generated from syngas; liquid

transportation fuels can be made from acetone; and carbon-negative sequestration can

be achieved with biochar.

Coal, oil, and natural gas are valuable Earth resources, and they would not contribute

to climate change if they were used to make durable goods, rather than burned. We do

not need fossil-fuel companies to go out of business; we need them to go into a

different business. Other researchers may have even better ideas for effecting a

realistic transition from an economy based on fossil fuels. If so, they should get to

work. Through nearly fourteen billion years of the evolution of the physical universe,

nature has given us a bounty of Earth energy that can, in principle, replace fossil fuels.

It is time for us to do our part.

Authors/Frank Shu, Michael Cai, Fen-Tair Luo; Translator/Chun-Hui Yang; Reviewers/ Michael Cai

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