TAR SANDS
TAR SANDS
tertiary extraction is reduced by the energy consumed by the processes.
In some regions, large sand deposits are permeated with Because an oil well occupies only a few hundred
heavy oil and an oil-like substance called bitumen, which square meters of land, most cause relatively little envi-
are too thick to be pumped. The richest tar sands exist in ronmental damage. However, oil companies have begun
Alberta (Canada), Utah, and Venezuela. to extract petroleum from fragile environments such as
In Alberta alone, tar sands contain an estimated 1 the ocean floor and the Arctic tundra. To obtain oil from
trillion barrels of petroleum. About 10 percent of this the sea floor, engineers build platforms on pilings driven
fuel is shallow enough to be surface mined (Fig. 19–21). into the ocean floor and mount drill rigs on these steel
Tar sands are dug up and heated with steam to make the
Petroleum and Natural Gas 349
Figure 19–20 An offshore oil drilling platform. (Sun Oil)
bitumen fluid enough to separate from the sand. The bi-
OIL SHALE
tumen is then treated chemically and heated to convert it to crude oil. At present, several companies mine tar sands
Some shales and other sedimentary rocks contain a waxy, profitably, producing 11 percent of Canada’s petroleum.
solid organic substance called kerogen. Kerogen is or- Deeper deposits, composing the remaining 90 percent of
ganic material that has not yet converted to oil. Kerogen- the reserve, can be extracted using subsurface techniques
bearing rock is called oil shale. If oil shale is mined and similar to those discussed for secondary and tertiary
heated in the presence of water, the kerogen converts to recovery.
petroleum. In the United States, oil shales contain the en- ergy equivalent of 2 to 5 trillion barrels of petroleum, enough to fuel the nation for 315 to 775 years at the 1994 consumption rate (Fig. 19–22). However, many oil shales are of such low grade that they require more energy to mine and convert the kerogen to petroleum than is generated by burning the oil, so they will proba- bly never be used for fuel. Oil from higher-grade oil shales in the United States would supply this country for nearly 75 years if consumption rates remained at 1994 levels. Oil shale deposits in most other nations are not as rich, so oil shale is less promising as a global energy source.
Water consumption is a serious problem in oil shale development. Approximately two barrels of water are needed to produce each barrel of oil from shale. Oil shale occurs most abundantly in the semiarid western United States. In this region, the scarce water is also needed for agriculture, domestic use, and industry.
When oil prices rose to $45 per barrel in 1981, major oil companies built experimental oil shale recov- ery plants. However, when prices plummeted a few years later, most of this activity came to a halt. Today, no
Figure 19–21 Mining tar sands in Alberta, Canada. large-scale oil shale mining is taking place in the United
(Syncrude Canada, Limited)
States.
350 CHAPTER 19 GEOLOGICRESOURCES
Billions of barrels of petroleum
Known and Estimated
Petroleum
Petroleum
estimated petroleum
in Canadian in oil shale
petroleum in the U.S.
tar sands
in the United
reserves available
States
in the with secondary United
recovery Figure 19–22 Secondary recovery, tar sands, and oil States
methods shale increase our petroleum reserves significantly.
䊳 19.9 NUCLEAR FUELS
systems fail, the control rods fall into the reactor core
AND REACTORS
and quench the fission.
The reactor core produces tremendous amounts of
A modern nuclear power plant uses nuclear fission to heat. A fluid, usually water, is pumped through the reac- produce heat and generate electricity (Fig. 19–23). One
tor core to cool it. The cooling water (which is now ra- isotope of uranium, U-235, is the major fuel. When a U-
dioactive from exposure to the core) is then passed 235 nucleus is bombarded with a neutron, it breaks apart
through a radiator, where it heats another source of wa- (the word fission means “splitting”). The initial reaction
ter to produce steam. The steam drives a turbine, which releases two or three neutrons. Each of these neutrons
in turn generates electricity (Fig. 19–25). can trigger the fission of additional nuclei; hence, this
type of nuclear reaction is called a branching chain THE NUCLEAR POWER INDUSTRY reaction . Because this fission is initiated by neutron
bombardment, it is not a spontaneous process and is dif- Every step in the mining, processing, and use of nuclear ferent from natural radioactivity.
fuel produces radioactive wastes. The mine waste dis- To fuel a nuclear reactor, concentrated uranium is
carded during mining is radioactive. Enrichment of the compressed into small pellets. Each pellet could easily
ore produces additional radioactive waste. When a U- fit into your hand but contains the energy equivalent
235 nucleus undergoes fission in a reactor, it splits into of 1 ton of coal. A column of pellets is encased in a 2-
two useless radioactive nuclei that must be discarded. meter-long pipe called a fuel rod (Fig. 19–24). A typical
Finally, after several months in a reactor, the U-235 con- nuclear power plant contains about 50,000 fuel rods bun-
centration in the fuel rods drops until the fuel pellets are dled into assemblies of 200 rods each. Control rods
no longer useful. In some countries, these pellets are re- made of neutron-absorbing alloys are spaced among the
processed to recover U-235, but in the United States this fuel rods. The control rods fine-tune the reactor. If the re-
process is not economical and the pellets are discarded. action speeds up because too many neutrons are striking
In Chapter 15 we discussed proposals for storing ra- other uranium atoms, then the power plant operator low-
dioactive wastes. To date, no satisfactory solution has ers the control rods to absorb more neutrons and slow
been found.
down the reaction. If fission slows down because too In recent years, construction of new reactors has be- many neutrons are absorbed, the operator raises the con-
come so costly that electricity generated by nuclear power
U-235 U-235
Decay product
U-235
Neutron
Decay product
Twelve Fifteen
decay
neutrons
additional additional
products
decay
neutrons products
Figure 19–23 When a neutron strikes a uranium-235 nucleus, the nucleus splits into two roughly equal fragments and emits two or three neutrons. These neutrons can then initiate additional reactions, which produce more neutrons. A branching chain reaction accelerates rapidly through a sample of concentrated uranium-235.
(a)
(b)
Figure 19–24 (a) Fuel pellets containing enriched uranium-235. Each pellet contains the energy equivalent of 1 ton of coal. (b) Fuel pellets are encased into narrow rods that are bundled together and lowered into the reactor core. (Courtesy
352 CHAPTER 19 GEOLOGICRESOURCES
Containment building
Steam
Electric generator
Control Primary rods
water circuit
Figure 19–25 In a nuclear
circuit
power plant, fission energy cre- ates heat, which is used to pro-
Secondary Condenser
Cooling tower
duce steam. The steam drives
a turbine, which generates Reactor core
water
Steam generator
circuit
electricity.
plants. Public concern about accidents and radioactive of the total electricity consumed that year. Those num- waste disposal has become acute. The demand for elec-
bers will decline in the coming decade because no new tricity has risen less than expected during the past two
plants have been started and old plants must be decom- decades. As a result, growth of the nuclear power indus-
missioned. Forbes business magazine called the United try has halted. After 1974, many planned nuclear power
States nuclear power program “the largest managerial plants were canceled, and after 1981, no new orders were
disaster in U.S. business history, involving $1 trillion in placed for nuclear power plants in the United States.
wasted investment and $10 billion in direct losses to In 1994, 109 commercial reactors were operating in
stock holders.”
the United States. These generators produced 22 percent