Energy and Society
Week 11 Section Handout
Section Outline:
1. Rough sketch of nuclear power (15 minutes)
2. Radioactive decay (10 minutes)
3. Nuclear practice problems or a discussion of the appropriate role of nuclear power in our energy future
(25 minutes)
I. Nuclear Power Basics (15 minutes)
If our ultimate goal is evaluating any technology is to decide whether it should be a part of our energy
future, we should have a sense of the upsides, downsides, and unknowns of that technology.
Question to the class: What are the pros, cons, and uncertainties of using nuclear power?
You were irradiated with information last week, and many of you are likely wondering what we’d like
you to internalize from the nuclear lectures and readings. We don’t expect you to return to your textbooks
and learn or re-learn atomic physics. What we would like is to make sure that you:
1) Have a basic, qualitative understanding of the different pieces that make up the lifecycle of nuclear
power generation, including:
a. The nuclear fuel cycle — what are the major steps and what happens at each step?
b. Nuclear reactions — what happens during the reaction and why do you get energy out?
c. Different reactor types — what are the basic differences between pressurized water reactors
and boiling water reactors? Light water reactors and heavy water reactors?
d. Nuclear waste and storage — what are the main risks posed by nuclear waste?
2) Be able to work quantitatively with basic policy-relevant questions that involve nuclear power,
including:
a. Fuel characteristics and requirements
b. Waste and exponential decay
Nuclear Fission Basics
Fission splits an atom into 2 (or more) daughter products. The mass of an atom is less than the sum of its
constituent parts (e.g., protons, neutrons); the extra mass of the parts becomes the “binding energy” of an
atom, the energy required to hold the atom together. E = mc 2 describes the equivalence of mass and
energy. Fissioning 1 U-235 atom liberates about 200 MeV of energy.
We can purposefully promote fission reactions by creating a system in which neutrons are free to slam
into readily fissionable atoms (like U-235). This fission reaction liberates energy, and this energy can be
used to boil water that is used to turn a turbine and create electricity. The fission of one U-235 atom with
a neutron releases, on average, 2.5 neutrons. These neutrons can, in turn, fission other U-235 atoms in a
chain reaction. Since more neutrons are liberated by a fission reaction than are required to start it, a
nuclear reactor might see an exponential increase in the number of neutrons in the system and a
corresponding exponential increase in fission events. Various mechanisms (e.g., control rods) are in place
to control the population of neutrons to prevent “supercriticality,” or increasing power levels.
II. Radioactivity (10 minutes)
Isotopes = different “versions” of the same element – same # of protons, different neutrons also nuclides.
U-235 had 92 protons and 143 neutrons. Many (most) isotopes are stable, some undergo radioactive
decay – these are radioisotopes or radionuclide.
Radionuclides naturally undergo nuclear fission. In such an event, a part of the atom is spontaneously
emitted. This emission changes the atom, creating an isotope of a new element (e.g., C-14 into N-14). The
rate at which they decay is an intrinsic property; some decay rapidly, while isotopes can take billions of
years to undergo decay. A common way to characterize the rate at which a given isotope will decay is by
its half-life, the amount of time it takes for half of a given stock of a radionuclide (say, 1 g or 100 atoms)
to undergo decay. Half-lives can range from the very short (0.00016 for Po-214) to the very long (4.5
billion years for U-238).
Radioactive decay in an exponential process, meaning that the number of decays at any given moment is a
function of the size of the stock of radionuclides. (More radionuclides, more decays/second.) To describe
the change in the size of the stock of radioactive isotopes, we call on the exponential growth equation:
I = I0e-t
Where:  is the decay constant and is equal to ln(2)/half-life.
Example Problem 1
Plutonium (Pu) is created by the decay of 239U into 239Pu and ultimately to other Pu isotopes. 239Pu has
a half life of 24,000 years.
A large nuclear power plant produces 0.50 tons of reactor grade Pu per year, about 60% of which is 239P.
How much of the 239Pu produced in 2009 will still be around in the year 10,000 CE?
III. Practice Problems or Discussion (25 minutes)
Practice Problem 2
What mass of uranium ore (in kg) enriched to 3% 235U is required to produce 6132 GWh of electricity
(equivalent to a 1 GW power plant running at 70% capacity factor)? Assume that each fission of 235U
produces 200 MeV (3.2 x 10-11 J), that all neutrons absorbed by 235U cause fission, and that the nuclear
power plant has a thermal efficiency of 33%.
IV. Supplemental Information

Sources of nuclear fuel
o Uranium Supplies. Canada and Australia account for 40 percent of global uranium production;
the United States accounts for 3 percent. According to a 2010 Organization for Economic
Cooperation and Development (OECD) and International Atomic Energy Agency (IAEA) report,
uranium resources are adequate to meet nuclear energy needs for at least the next 100 years at
present consumption levels. More efficient fast reactors could extend that period to more than
2,500 years. [Beware of forecasts in both directions!]
o Mining and Milling. Uranium miners use several techniques to obtain uranium: surface (open
pit), underground mining and in-situ recovery. Uranium also is a byproduct of other mineral
processing operations. Solvents remove the uranium from mined ore or in-situ leaching, and the
resulting uranium oxide—called yellowcake—undergoes filtering and drying.
(Source: Nuclear Energy Institute)

Processing
o Uranium Conversion. The yellowcake then goes to a conversion plant, where chemical
processes convert it to uranium hexafluoride. The uranium hexafluoride is heated to become a gas
and loaded into cylinders. When it cools, it condenses into a solid.
o Uranium Enrichment. Uranium hexafluoride contains two types of uranium, U-238 and U-235.
The percentage of U-235, which is the type of uranium that fissions easily, is less than 1 percent.
To make the uranium usable as a fuel, its U-235 content is increased to between 3 percent and 5
percent. This process is called enrichment. The concentration of U-235 is so low in enriched
uranium that an explosion is impossible.
o Fuel Fabrication. After the uranium hexafluoride is enriched, a fuel fabricator converts it into
uranium dioxide powder and presses the powder into fuel pellets. The fabricator loads the
ceramic pellets into long tubes made of a noncorrosive material, usually a zirconium alloy. Once
grouped together into a bundle, these tubes form a fuel assembly.
(Source: Nuclear Energy Institute)

Fuel consumption
o Fission. In a nuclear reactor, fission occurs when the nuclei of atoms are split through the capture
of neutrons. This fissile process can release new neutrons (on average more than one), some of
which get absorbed by non-fissile material, some of which leak through the reactor core, and
some of which get absorbed by fissile material. Re-absorption by fissile material is what
continues the chain reaction.
o Nuclear Reaction. The goal for a reactor operator is to maintain a balance between the rate of
neutron production and neutron absorption/loss. When the rates are equal, we have a “critical”
reaction. When there is a net productivity of neutrons, the reaction is “super-critical”—the
nuclear operator must increase the rate of neutron absorption to bring the reaction back into
balance (e.g., by inserting a control rod; or, to shut down a reactor, by inserting safety rods).
When there is a net loss in neutrons, the reaction is sub-critical. In comparison, a nuclear weapon
is designed to achieve a super-critical reaction.

Used fuel processing and disposal
o Interim Storage. Following use in the reactor, the fuel assembly becomes highly radioactive and
must be removed and stored under water in a spent fuel pool at the reactor for several years. Even
though the fission reaction has stopped, the spent fuel continues to give off heat from the decay of
radioactive elements that were created when the uranium atoms were split apart. The water in the
pool serves to both cool the fuel and shield the operators from any radiation. As of 2002, there
were over 165,000 spent fuel assemblies stored in about 70 interim storage pools throughout the
United States. After cooling a few years in the pool, the spent fuel element may be moved to a
dry cask storage container for further on-site storage. An increasing number of reactor operators
now store their older spent fuel in these special outdoor concrete or steel containers with air
cooling.
o Reprocessing. Less than 4% of the uranium loaded into the reactor is consumed in nuclear
reactions. The rest of the uranium remains unchanged. Chemical processing of the spent fuel
material to recover the remaining portion of fissionable products for use in fresh fuel assemblies
is technically feasible. Some countries, such as France, reprocess spent nuclear fuel, but it is not
permitted in the United States.
o Final Disposal. The final step in the nuclear fuel cycle is the collection of spent fuel assemblies
from the interim storage sites or future reprocessing facilities, and the disposal of any remaining
high-level nuclear waste in a permanent underground repository. The United States currently has
no such repository.1
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1
Nuclear Waste. The main environmental concerns for nuclear power are radioactive wastes such
as uranium mill tailings, spent (used) reactor fuel, and other radioactive wastes. These materials
can remain radioactive and dangerous to human health for thousands of years. Radioactive wastes
are classified as low-level and high-level. The radioactivity in these wastes can range from just
above natural background levels, as in mill tailings, to much higher levels, such as in spent
reactor fuel or the parts inside a nuclear reactor.
Half-life. The radioactivity of nuclear waste decreases with the passage of time through a process
called radioactive decay. The amount of time necessary to decrease the radioactivity of
radioactive material to one-half the original level is called the radioactive half-life of the material.
Radioactive waste with a short half-life is often stored temporarily before disposal in order to
reduce potential radiation doses to workers who handle and transport the waste, as well as to
reduce the radiation levels at disposal sites.
Brief document about Yucca Mountain: http://www.nei.org/keyissues/nuclearwastedisposal/yuccamountain/
o
o
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Low-level Waste. By volume, most of the waste related to the nuclear power industry has a
relatively low-level of radioactivity. Uranium mill tailings contain the radioactive element
radium, which decays to produce radon, a radioactive gas. The other types of low level
radioactive waste are the tools, protective clothing, wiping cloths, and other disposable items that
get contaminated with small amounts of radioactive dust or particles at nuclear fuel processing
facilities and power plants.
High-level Waste. High-level radioactive waste consists of “irradiated” or used nuclear reactor
fuel (i.e., fuel that has been used in a reactor to produce electricity). The used reactor fuel is in a
solid form consisting of small fuel pellets in long metal tubes. (Source: US Energy Information
Administration)
Note: A typical 1000 MWe light water reactor will generate (directly and indirectly) 200-350 m3
low- and intermediate-level waste per year. It will also discharge about 20 m3 (27 tonnes) of used
fuel per year, which corresponds to a 75 m3 disposal volume following encapsulation if it is
treated as waste. Where that used fuel is reprocessed, only 3 m3 of vitrified waste (glass) is
produced, which is equivalent to a 28 m3 disposal volume following placement in a disposal
canister. (World Nuclear Association)
Types of Reactors
For economic reasons, a few reactor types have dominated the commercial market. Most commercial
reactors in the world use low-enriched uranium, e.g., 90% are light-water reactors (LWRs) where the
fresh fuel is UO2 (the U is enriched to about 4-5% U-235), and heavy-water reactors (mostly in or
supplied by Canada) where the U is natural uranium (0.711% U-235); and a few reactors use MOX fuel (a
blend of uranium and plutonium), e.g, some fast reactors and some LWRs.
There are two dominant subtypes of light-water reactors: boiling water reactors (BWR) and pressurized
water reactors (PWR). The primary difference is that in BWRs, the water that contacts the core is
converted directly to steam, which then passes through the turbine. The PWRs use a primary closed loop
of cooling water that flows through the reactor core and passes through a heat exchanger to generate
steam in a secondary loop of water flow.
In response to the difficulties in achieving sustainability, a sufficiently high degree of safety and a
competitive economic basis for nuclear power, the U.S. Department of Energy initiated the Generation IV
program in 1999. Generation IV refers to the broad division of nuclear designs into four categories: early
prototype reactors (Generation I), the large central station nuclear power plants of today (Generation II),
the advanced light-water reactors and other systems with inherent safety features that have been designed
in recent years (Generation III), and the next-generation systems to be designed and built two decades
from now (Generation IV). These next-generation systems are based on three general classes of reactors:
gas-cooled, water-cooled and fast-spectrum.
(Source: Lake 2002)
Types of Fuel Processing
The current “once-through,” or open, nuclear fuel cycle uses freshly mined uranium, burns it a single time
in a reactor and then discharges it as waste. This approach results in only about 1 percent of the energy
content of the uranium being converted to electricity. It also produces large volumes of spent nuclear fuel
that must be disposed of in a safe fashion. Both these drawbacks can be avoided by recycling the spent
fuel—that is, recovering the useful materials from it.
Most other countries with large nuclear power programs—including France, Japan and the U.K.—employ
what is called a closed nuclear fuel cycle. In these countries, used fuel is recycled to recover uranium and
plutonium (produced during irradiation in reactors) and reprocess it into new fuel. This effort doubles the
amount of energy recovered from the fuel and removes most of the long-lived radioactive elements from
the waste that must be permanently stored. It should be noted, though, that recycled fuel is today more
expensive than newly mined fuel. Current recycling technology also leads to the separation of plutonium,
which could potentially be diverted into weapons.
(Source: Lake 2002)
http://what-if.xkcd.com/29/