1/16

【徐遐生院士談地球新能源】

原文網路全文版

EARTH ENERGY: GIFTS FROM NATURE

(Online English Full Version)

Frank H. Shu1, Michael J. Cai

2, Fen-Tair Luo

3

1Institute of Astronomy and Astrophysics, Academia SInica

Research Corporation, University of Hawaii2

Institute of Chemistry, Academia Sinica3

Introduction

The astronomical heritage of the Earth makes it rich with energy. In its oceans are

water molecules that contain two isotopes of hydrogen that date back to the big bang.

The light form is what powers thermonuclear fusion in the Sun; the heavy form

underlies the hope behind thermonuclear fusion on Earth. Helium is the second most

abundant element in the Universe after hydrogen, but none of the helium remaining

on Earth came from the big bang; they all come from the alpha particles (helium

nuclei) spit out from unstable isotopes of heavy elements that make up the rocks of

Earth. These radioactive elements are relics from supernovae that made neutron stars

and provide the heat that keeps the interior of the Earth hot. The most neutron-rich of

these heavy elements, uranium, forms the basis of fission reactions that power most of

today’s terrestrial nuclear reactors. The moderator that slows down fission neutrons is

the same as the coolant that carries away the heat from the core of these reactors,

water with the light form of hydrogen. This use gives these machines their name: light

water reactors (LWRs). LWRs have no emissions of carbon dioxide, but they play a

controversial role in Earth energy because of misconceptions that they lack 4 S’s:

Solutions (for the nuclear waste problem)

Safety (with respect to massive release of radioactivity to the environment)

Security (with respect to weapons proliferation)

Sustainability (of high-grade uranium ore)

Radiation from the thermonuclear powered Sun is the natural energy source that

sustains all life on Earth. Sunlight passes through an optically transparent atmosphere

to warm the surface of the earth. If sunlight falls on the oceans, heating the water

causes some of it to evaporate. The salt of the seawater is left behind, so when the

water vapor precipitates, the rain is a source of fresh water. If it is cold, and snow

instead falls on high mountain passes, when the snows melt, the streams of fresh

water collect into mighty rivers. If the rivers are dammed, high reservoirs of water

build up behind the dams. Released from these great heights, the falling water can

rush past water turbines that turn powerful magnets inside coils of wire that hum with

alternating current.

On inhomogeneous terrain, and because of night and day variations, the heating by

sunlight is uneven and gives differences in temperature and pressure that create wind,

which can power turbines also generating electricity (at about 50% efficiency versus

90% for hydroelectricity). Because air is 800 times less dense than water,

wind-electricity is considerably more expensive than hydroelectricity.

2/16

If sunlight strikes solar panels, the photovoltaic effect generates solar electricity (at

efficiency up to 20%). Solar electricity ceases at night and is highly variable during

cloudy days, so it requires backup from other sources of “base-load” power.

If the sunlight falls on green plants, photosynthesis is able to take the energy in the

photons to convert the carbon dioxide in the air and water in the ground into the

organic compounds necessary for plant growth and reproduction (at about 1%

efficiency). These organic compounds contain proportionally fewer O compared to C

and H than present in CO2 and H2O, so free molecular oxygen O2 is released to the

atmosphere as a byproduct of photosynthesis. Conversely, when plants die, the

incompletely oxidized C and H in organic matter can combine with the O2 in air,

releasing heat in the process, and reform CO2 and H2O, both of which are greenhouse

gases (GHGs). If the reactions occur in a flame, we call the process “burning,” with

the heat of combustion used perhaps to boil water that causes the steam to expand past

a steam turbine that can again generate electricity (at about 20% efficiency if the

biomass is burned directly). If the reactions occur more slowly in animals, we call the

process digestion, with the animals (unicellular or multicellular) making use of the

food energy (at a low efficiency dependent on the species) and exhaling or excreting

the waste products, CO2 and H2O.

Biomass that got buried in past eons deep into the Earth, where there is no oxygen but

ample heat and pressure, produced the fossil fuels, coal, petroleum, and natural gas

that powers the modern technological society. Coal burning is used mostly for

electricity generation (at about 35 to 40% efficiency), with noxious emissions of

volatile heavy metals (like mercury) because coal is dug out of the ground with small

bits of stone in it that contain such heavy metals.

Petroleum holds an almost unassailable position as the feedstock of choice for

transportation fuel because it of its advantage in ETUDES:

Extraction, with historical energy return on investment (EROI) ratios > 10

Transportation, worth doing because petroleum is an energy dense liquid

Upgrading, refining to separate low and high molecular weight hydrocarbons and

processing to produce a variety of chemical products (e.g., plastics)

Distribution, extensive network of suppliers and outlets for products

Establishment, with market penetration into all segments of society

Storage, e.g., in gasoline tanks, available for usage when one wants

In the public mind, natural gas is a clean burning cooking fuel with almost no noxious

emissions and yields CO2 about a half that of coal with the same energy content. But

natural gas can also be burned so that the expanding flue gas turns turbines to

generate electricity at an efficiency that can reach 60% in so-called “combined cycle”

power plants where the waste heat in the flue gas is used to help boil water in the

steam boiler of a coal-fired power plant. Natural gas in the United States produced by

the method of hydraulic fracturing of shale has unbelievably low production costs.

Other nations, notably China, are joining the “rush to gas.”

For these various reasons, natural gas is often touted as the “bridge fuel” to a

carbon-free future, where human energy needs are entirely supplied by renewables

like solar and wind. One can question how natural gas can serve this temporary role

given that it is needed to take up the slack when the wind is not blowing or when the

3/16

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

Sun is not shining in the sky. Building more wind and solar makes humans more

dependent of natural gas, not less.

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. Because of space limitations,

we do not discuss the evidence that supports this conclusion. We hope that a future

issue of the ASIAA Quarterly can focus on this important subject.

From the perspective of

mitigating the effects of

climate change, we can divide

the major terrestrial energy

sources mentioned above into

four categories:

Category I, sources that

produce copious emissions of

carbon dioxide:

coal

oil

natural gas

Although always lumped

together, the three fossil fuels

are not equal. Coal powered

the Industrial Revolution; for

the Age of Innovation, we

need something better. But if we are to stop using coal, thought has to be given to

how we salvage the investment made on all the new coal-fired power plants that are

springing up in China, India, and Germany (which shut down its nuclear power plants

because a tsunami disabled three nuclear reactors in Japan).

For sound technical reasons, civilization uses oil as the transportation fuel of choice.

Easy to extract, transport, upgrade, distribute, and store, it is priced per unit energy at

ten times the value of coal and shale gas for its convenience of use.

Natural gas is cheap in some parts of the United States because of the practice of

hydraulic fracturing. In its low-density state as a gas; transporting it in pipelines is

very expensive compared to doing the same for oil, because to carry the mass

mass-flow the natural gas pipes have much larger diameters. Shipping natural gas

overseas is economically feasible only if it is liquefied into a denser state. Liquefied

natural gas (LNG) requires cryogenically low temperatures and high pressures, so by

the time LNG reaches Taiwan from the United States the cost of natural gas has

increased by a factor of six. As a result of these difficulties, shale gas is not today

transported from where it is produced, with the result that local supply greatly exceeds

4/16

the local demand, which explains why current prices for shale gas are so low.

Moreover, if there are leaks during extraction, then methane, which makes up 90% of

what is in natural gas, is, as a GHG, 72 times worse than the equivalent amount of

CO2 for 20 years, and 25 times worse for 100 years. Methane is gradually destroyed

by oxidization in the atmosphere, but its potential for harm in the environment if not

used wisely does not bode well for it being a panacea for humanity’s problems with

climate change.

Group II. Sources that are renewable and reliable, that produce essentially zero

emissions of carbon dioxide:

hydroelectric power

biofuels

geothermal

solar thermal

Hydroelectricity is a wonderful twentieth century technology. It has little room for

expansion in the twenty-first because almost all the large rivers of the world have

already been dammed.

Biofuel technologies are often judged on their ratio of energy return on energy

invested (EROI). Economists argue whether corn ethanol is being produced in the

United States with EROI > 1 or < 1. Brazil claims that its EROI for producing

sugar-cane ethanol averages about 8.3; however, their calculation does not count as

input the bagasse (the material left after the sugar has been pressed out of the cane)

burnt in the fields to help power the plant. If this input is included, the Brazilian EROI

is probably closer to 2.

Corn ethanol is notorious for driving up worldwide food prices. Researchers hope to

proceed to second-generation biofuels where the feedstock does not compete with

food. To accomplish this aim requires using (a) non-food feedstocks, e.g., waste wood,

wild grasses, etc; (b) marginal lands not suitable for the planting of food crops. The

second requirement is at odds with having biomass yields per hectare high enough to

sustain economic biofuel production. Almost by definition, marginal lands either lack

water or lack the soil nutrients necessary for productive vegetative growth, or both. To

supply this water and/or the chemical fertilizer (which is today produced by the

petrochemical industry) requires large fossil-fuel inputs that may be self-defeating if

the goal is to reduce our dependency on fossil fuels. This realization has spurred some

to look at oil produced by algae, where the effort is in a state of relative infancy.

When the source of Earth heat is close to the surface, as in Iceland, geothermal is a

reliable, established technology, especially when used for space heating. In warm

climes, like Taiwan, it makes more sense to look at using cold seawater at depth as a

source for air chilling in the summertime. To drill ten km deep to tap geothermal heat

where it is not available from the surface, as some have proposed, seems an

unnecessary invasion of the environment, given the bad accidents that have occurred

with deep drilling for oil.

In solar thermal, the heat of the Sun is captured by parabolic east-west troughs and

stored in molten salt for energy conversion at night. Solar thermal suffers from the

dilute nature of sunlight and the inefficient use of its energy compared to

5/16

photovoltaics, which directly converts sunlight into electricity.

Group III, sources that are renewable but unreliable, and produce essentially zero

emissions of carbon dioxide:

wind

solar PV

With hydroelectricity, we can control the release of water behind dams to satisfy the

timing of human demands. The wind changes speed and direction according to the

vagaries of a turbulent lower atmosphere of the Earth. During hot or cold spells, when

one needs electricity the most, the wind can stop blowing for weeks on end. Wind is

strongest at night, when the cold air is descending and everybody is sleeping with

little need for electricity. Thus, wind behaves like a car with a mind of its own,

starting when the traffic light is red, and stopping when it is green.

Solar photovoltaics (PV) is intermittent because it ceases when the Sun sets, which is

when we need to turn the lights on. It is not completely dependable even during the

daytime because passing clouds can interfere with the efficient operations of solar

panels. Nevertheless, because electricity demand is highest around noon, solar PV is

well matched to “peak-load” power.

Solar PV is the only energy generation technology that offers personalized action, i.e.,

each family and business can own and control their own system to reduce the

electricity demand on the power grid. The main failing of solar PV is its heavily

subsidized costs, including installation. As long as solar PV needs government

subsidies, which can change in democracies with each election cycle, making the

market for solar panels highly volatile, it cannot have an impact much greater than its

current contribution of about 0.01% of total world energy usage. (Beware that articles

about solar PV usually quote nameplate power. Nameplate power refers to electricity

generation on a clear day at noon when the Sun is highest in the sky. The average

contribution is typically only 20% of nameplate power.)

Group IV, sources that are reliable, sustainable, and have essentially zero emissions of

carbon dioxide:

advanced fission nuclear reactors

thermonuclear fusion

Nuclear power based either on fission or fusion are not renewable because the fuel –

uranium or thorium in the case of fission, deuterium in the case of terrestrial fusion –

are irreversibly transformed into non-fissionable and non-fusionable substances.

Nevertheless, the stock of deuterium in the oceans is so large that fusion could supply

all the world energy needs until the Sun turns itself into a red giant. In that sense,

fusion energy is not renewable, but it is sustainable. Unfortunately, fusion power is

unlikely to become a commercial reality in time to help with climate change. Thus, it

remains a terrestrial energy source for the future.

In contrast, if U-235 continues to be the world’s sole source of fissile material, then

the stocks of high-grade uranium ore are sufficient only to supply about six years of

total projected world energy needs in 2050. We cannot even make it to 2050 at that

6/16

rate. Fissioning U-235 for terrestrial power is neither renewable nor sustainable.

Nuclear Breeder Reactors

Molted salt breeder reactors (MSBRs) offer solutions for the nuclear waste problem,

safety against the massive release of radioactivity into the environment, security

against weapons proliferation, and sustainability of the nuclear fission option. Before

we discuss MSBRs, however, we briefly review the subject of breeder reactors more

generally.

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. Experiments have been carried out showing that the technology is

economically viable. The supply of uranium in the oceans suffices to power a

“plutonium economy” for hundreds of thousands of years. Thus, U-238 breeder

reactors are a sustainable energy resource for the Earth. Bill Gates has invested money

in this technology.

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

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

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

7/16

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 prod’s

• 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

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 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

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.

8/16

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

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 moleten

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

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

9/16

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

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.

10/16

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:

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 won’t 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 won’t be available. Suppose the fuel

salt then melts through the air-cooled dump tank. For this contingency, we’ve added a

11/16

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

Bergman et al. 2005).

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.

There is no water in the system, so hydrogen will not be generated to cause an

explosion. The salt is composed of elements on opposite sides of the periodic table,

one being very electropositive and the other being very electronegative. No other

element can get between them, so there are no chemical reactions that can threaten the

system. In other words, salt cannot catch on fire.

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

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

With oil’s advantages

in ETUDES (which

have made them rich

and powerful), oil

companies are tough to

displace with

technologies that

depend on primitive

micro-organisms

performing

fermentation reactions

at room temperature,

where all chemical

reactions are slow. (If

they were not slow, the

organisms would char.)

The strategy of our

research group is to

fight fire with fire, or

more accurately, with

supertorrefaction. 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.

12/16

Figure 5. The Crankberry machine for tabletop supertorrefaction.

©ASIAA

Figure 6. Examples of charcoal making by supertorrefaction with

molten acetate salt (NaAc/KAc) from different biomass feedstocks.

©ASIAA

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 Academia Sinica. Supertorrefaction 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). From data that

we have accumulated

from such experiments

and using the same rules

of calculation as Brazilian sugar cane ethanol, we estimate that the EROI ratio for a

demonstration-scale supertorrefaction project is of order 40:1. If we include internal

inputs of energy from renewable sources in the denominator, but not the crude

glycerol that should be charged to biodiesel making, the EROI drops to about 9.6:1,

still very good by Brazilian standards, and comparable to the record of established oil

companies. With “peak oil,” our EROI will improve relative to that of the oil industry.

Moreover, burning our products is a carbon-neutral activity.

13/16

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

The molten salt we use for supertorrefaction is a eutectic mixture of sodium acetate,

NaAc, and potassium acetate, KAc. (The same combination is used to flavor

“salt-and-vinegar” potato chips). This salt mixture melts at 235 oC and decomposes to

sodium carbonate, Na2CO3, and potassium carbonate, K2CO3, plus acetone if the

temperature exceeds 460 oC. 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. Thus, in principle,

biochar production and burial can become the basis of true carbon trading, where, for

example, oil companies that extract a tonne of petroleum from anywhere in the world

are required to pay someone else to bury a tonne of biochar on land in need of

improvement in soil quality. The resulting flow of money from the rich to the poor in

rural communities facing desertification is a win-win proposition, with everyone

receiving the benefits of a cleaner environment.

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. As mentioned earlier, 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

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.

Supertorrefaction 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

14/16

Figure 8. Leucaena fields occupy 70% of the land area of Penghu

and threaten to invade the remaining 30%. In autumn and winter the

plants are very dry, in optimum condition for harvest and

supertorrefaction. (Photo taken Oct 19, 2012).

©ASIAA

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.

The next step may be

to conduct a

demonstration project

in Penghu County to

prove the economic

feasibility of scaled-up,

mobile, batch-process

supertorrefaction. Our

target biomass is a

bush called leucaena

that has over-grown

70% of Penghu County

(Fig. 8). Introduced to

Taiwan under the

Japanese occupation,

leucaena was originally

cultivated for firewood.

Leucaena is

nitrogen-fixing and

requires no chemical

fertilizer to grow in

poor soil. Now that everyone uses natural gas or propane for cooking, the leucaena,

with its adaptive advantages, has become an invasive species that threatens the

habitats of the native vegetation of Taiwan (and, indeed, much of Southeast Asia).

Using this biowaste as a bioresource is consistent with the sustainable development

goals of Penghu County.

Taiwan’s Council of Agriculture (CoA) prefers to try to eradicate this invasive species.

Eradication of established leucaena is impossible without digging up its deep roots,

and killing all viable seeds dispersed on and in the soil. To harvest the leucaena, we

would therefore clear-cut the branches, allowing the CoA to experiment with

eradication schemes. If eradication efforts fail, as is likely from experience in other

parts of the world, each topped bush will regenerate new growth in ensuing seasons,

recovering fully in about three years.

Another 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 (Fig. 9). 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

15/16

Figure 9. Pine trees in Colorado dying or dead from bark beetle

infestation (AP/Colorado Forest Service/Jen Chase).

Figure 10. Left (picture taken in July 2010): how an abandoned silver mine in Hope, Colorado looked

for a century before the addition of biochar soil amendment made by torrefying diseased pine trees.

Right (picture taken in August 2011): how the same mine tailings site looked a year later after the

application of biochar soil amendment at a rate of about 100 tonne per hectare. (Photo credit: Troy

Hooper).

growth that would lock

up more carbon.

The forest crisis affects

more than just North

America. A survey that

appeared 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.

Biochar is also useful for land reclamation. Experiments carried out at an abandoned

silver mine in Hope, Colorado show that each hectare treated with 100 tonne of

biochar will permanently require 17% less water to rejuvenate vegetative growth (Fig.

10). We propose to use charcoal fines, generated by supertorrefaction whenever one

has bark mixed in with the woody stems, in experimental trials to see whether the use

of charcoal fines as a soil amendment stimulates a similar dramatic improvement in

soil productivity of Penghu’s infertile soil while decreasing the share of water that

needs to be devoted to agricultural irrigation. With the data in hand, Penghu County

can make better informed decisions whether it should (a) undertake a systematic effort

to eradicate leucaena over the next decade, (b) passively harvest leucaena as a

bioresource while controlling its spread, or (c) actively cultivate leucaena, but without

the application of ammonium fertilizers that are based on petroleum feedstocks.

16/16

The Grand Challenge

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

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. There are physical and economical reasons

why it is hard to beat natural gas for turbine electricity generation, or to beat natural

gas as the input heat for supertorrefaction. But we do not have to use the nuclear heat

from a MSBR for turbine electricity generation (a difficult coupling). Instead, 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). Then we have a combination that can

beat natural gas at both tasks. Although nuclear electricity is expensive, nuclear heat

is cheap – much cheaper than natural gas. We can therefore use nuclear heat to

produce from biomass, at very high throughputs, biochar, acetone, and syngas cheaper

and cleaner than mined coal, drilled petroleum, and fracked shale gas. 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;

Reviewer/ Michael Cai）

天聞季報海報版與網路版由中央研究院天文及天文物理研究所製作，

以創用 CC 姓名標示-非商業性-禁止改作 3.0 台灣 授權條款釋出。

天聞季報網路版衍生自天聞季報海報版。超出此條款範圍外的授權，請與我們聯繫。

創用 CC授權可於以下網站查閱諮詢 https://isp.moe.edu.tw/ccedu/service.php。