Transcript Ambiente & Energia

Environment & Energy
Nuclear Energy
Valentim M B Nunes
Unidade Departamental de Engenharias
Polytechnic Institute of Tomar, march, 2015
Introduction
In 1990, nuclear plants provide more than 20% of the electricity consumed in
the United States and, in 2001, were in operation more than 100 nuclear
power plants in the USA. In France, more than 50 power plants are
operational, providing more than 75% of electricity consumed in the country.
The Japan has about 40 plants, producing more than 13% of their needs. As a
whole, there are more than 400 nuclear power plants around the world, giving
account of about 17% of the global needs of electricity consumption.
However, in the last two decades of the 20th Century, nuclear power plants
have faced mistrust of public opinion. The accident at the nuclear power plant
of The Three Mile Island in the USA in 1979 and that of Chernobyl in Ukraine
in 1986, and the latest in nuclear fuel processing plant in Tokaimura in Japan
in 1999 raised significant doubts about the production of electricity by nuclear
means. Also the problem of storing high-level radioactive wastes as a result of
the processing is a problem still not solved.
The nuclear accident of Fukushima
The Fukushima I nuclear accident resulted from a series of equipment failures
and release of radioactive materials in Nuclear power plant of Fukushima, in
Japan, as a result of the damage caused by the Tōhoku earthquake and tsunami
that happened at 14:46 JST on March 11, 2011.
Currently, the cost of investment in
a new nuclear power plant is two
or more times greater than that of
a thermal power station with
steam cycle or combined cycle,
including the costs of investment in
emission control systems required
in these systems.
Resources
Nuclear resources are much more abundant than fossil fuel resources. It is
estimated that the ores with high uranium purity can supply existing reactors
for about 50 years, but the use of smaller ore purity (although with the
consequent increase in the price of refining) can last for centuries. The use of
thorium and fast breeder reactors can extend the nuclear resources for
thousands of years. Thus, it is possible that nuclear energy will gain support
from the public opinion as they become economically competitive compared
to other energy resources.
Nuclear energy
Nuclear energy results from the bonding forces (the "strong“ forces) that
maintains the nucleons (protons and neutrons) in the atomic nucleus. The
bond strength by nucleon is greater for the elements in the middle of the
periodic table and is smaller for the lighter and heavier elements. When light
nuclei fuse together, energy is released. When heavier nuclei undergo fission
also releases energy. When an 235U nucleus (an isotope of uranium) is
bombarded with a neutron, it separates into several products with the release
of 2 or 3 times more neutrons than those who were absorbed. For example
one of the reactions is the splitting of 235U in 144Ba and 89Kr, with the release of
3 neutrons and 177 MeV of energy:
235U
+ n → 144Ba + 89Kr + 3n + 177MeV
1 electron volt (eV) is the energy acquired by an electron when it is accelerated
through a potential difference of 1 volt. 1 eV = 1.60210−19 J;
1 MeV = 1.60210−13 J.
Fission
Einstein equation
The energy released in a nuclear fission reaction can be calculated using
equation of Einstein:
E  mc
2
The mass of 235U = 235.04394 amu (1 amu. = 1.66 10−27 kg); the mass of the
neutron is n = 1.00867 amu; the mass of 144Ba = 143.92 amu; the mass of 89Kr
= 88.9166 amu. If we subtract the masses of the products by the mass of the
reactants is there a mass deficit Δm = -0.19 amu. This difference in mass is
converted to energy :
E = mc2 = 0.19 amu × 1.66 10−27 kg × (3 108)2 m2 s−2 = 2.84 10−11 J
E =177 MeV.
The fission of 235U produces than 2.8410−11J×6.0221023 mole−1÷0.235 kg
mol−1 = 7.31013 J kg−1. Comparing with combustion, the combustion of carbon,
for example, produces 3.3 107 J kg−1, what represents about 2 million times
less energy than that of the fission reaction!!
Chain Reactions
The majority of the fission products are radioactive. Since more than one
neutron is produced in the fission reaction, it develops a chain reaction, with an
increase of energy release. Most (about 80%) of the energy released is
contained in the kinetic energy of the fission products, and manifests itself as
sensitive heat. A portion of the remaining power is immediately released in the
form of ϒ and β rays and also neutrons. The rest of the energy is contained in
later radioactivity of products. At the same time as the 235U separates in fission
products with the release of 2 – 3 neutrons, some of the neutrons can be
absorbed by the more abundant 238U of fuel, converting a series of reactions in
the isotope of plutonium, 239Pu
238U
+ n → 239U +  → 239Np + β→ 239Pu + β
Radioactivity
Radioactivity is the spontaneous process of decay of a nucleus, usually the less
stable isotope of a given element, which can be natural or artificially obtained,
and which is accompanied by the release of very energetic radiation. After the
emission of radiation an isotope of that element, or a new element, is formed,
usually more stable than the element or isotope. The radiation comes from
the atomic nucleus, not from the atom as a whole. This is important because
the X-rays, although equally dangerous, comes from internal electronic layers
and not atom the nucleus.
There are three types of radiation: α, β, and ϒ. Only the latter is a form of
electromagnetic radiation; the first two are particle emissions with very high
energy. The three are called ionizing radiation because they create ions as the
energy is absorbed by crossing the matter. The degree of penetration in the
matter depends on the type of radiation and energy.
Radiation 
In α radiation, a helium nucleus containing two protons and two neutrons, He2+,
is emitted. Once it is loosed two protons of the original nucleus (and
consequently two electrons from electron cloud) the isotope son corresponds
to the second element that precedes the original on the periodic table. For
example, the isotope 239Pu disintegrates in the isotope 235U with the issuance of
an α particle:
239Pu
→ 235U + α(4He)
Another example is the disintegration of a radon isotope, 222Rn (which is a gas
under normal conditions) in polonium, 218Po. The latter emits another α
particle with the formation of a stable isotope of lead, 214Pb:
222Rn
218Po
→ 218Po + α(4He)
→ 214Pb + α(4He)
Since α particles are relatively heavy, their penetration in matter is very small,
on the order of a millimeter. A sheet of paper or a layer of skin on a person can
stop the α radiation.
Radiation 
In β radiation it is emitted an electron. This electron does not come from the
electronic cloud but from the nucleus: a neutron becomes a proton with the
emission of an electron. In this conversion, an isotope becomes another
isotope that corresponds to the next element in the periodic table. As an
example we have the decay of strontium 90Sr in Yttrium, 90Y :
90Sr
→ 90Y + β(0e)
Once the 90Sr is one of the products of uranium fission, this isotope is one of
the main sources of radiation wastes from spent fuel in a nuclear reactor. The
emitted electron is relatively light, and can penetrate into matter on the order
of centimeters. To protect against this type of radiation is required a metal
shield, as for example lead.
Radiation 
Generally, β radiation is followed by ϒ radiation emission. Once the number
of protons and neutrons does not change with this type of radiation, the
isotope does not change position on the periodic table. Gamma radiation is
the emission of electromagnetic radiation of very short wavelength, then
extremely energetic. As an example we have the emission of radiation by the
cobalt isotope 60Co:
60Co
→ 60Co + 
The radiation ϒ from 60Co has medicinal use, in the destruction of cancer cells,
and therefore, has therapeutic effects for certain types of cancer. Once the ϒ
rays don't carry mass, the penetration in matter is very deep, of the order of
several meters, and very large shields are necessary for protection against
radiation.
Radioactive decay
The rate of radioactive decay of a set of radioactive nuclei, to which the
radioactive activity is proportional, corresponds to a first order kinetics:
dN
 kN
dt
where N is the number of nuclei that decay, or its mass, at the moment of
time t, and k is a kinetic constant (with units of t−1). The integration results in:
N  N 0 exp( kt)
where N0 is the number of nuclei, or the mass, at a given moment of initial
time.
Half-live time
The instant of time after which half of the radioactive nuclei of a given
sample decayed is designated half life time, t1/2:
t1/ 2
ln 2

k
Dosage and effects of radioactivity
The levels of radioactivity of a sample of substance is measured by the
number of disintegrations per second. The SI unit is the becquerel (Bq), which
represents one disintegration per second. A more practical unit is the curie
(Ci), which is defined as 3.71010 Bq, or 3.71010 disintegrations per second.
The radioactivity of 1 gram of Radium-233 is 1 Ci, and 1 gram of cobalt-60 is 1
kCi. For mixtures of isotopes, as for example in nuclear waste, the level of
radioactivity cannot indicate the composition of the waste but only the total
amount of disintegrations.
The exposure of humans to radiation α, β, or ϒ can be dangerous, and
practical units are required for radiation exposure. The SI unit for absorbed
radiation dose is the gray (Gy), which is equal to 1 J of energy absorbed per
kilogram of matter penetrated by radiation. Another common unit is the rad,
which equals 1 10-2Gy.
The absorbed energy is not fully adequate to measure the level of danger of
ionizing radiation in humans, since the type of radiation is also important. For
take into account this we use the sievert (Sv), to measure what is known as
equivalent dose. As the gray, has dimensions of J/kg. The sievert takes into
account the type of radiation absorbed. An equivalent dose of 1 Sv is received
when the dose measured in grays multiplied by dimensionless factors Q
(quality factor) and N (other multiplicative factors), is 1 J/kg. The factor Q
depends on the nature of radiation and is 1 for x-rays, radiation ϒ and β
particles; 10 for neutrons and 20 for α particles. N is a factor which takes into
account the energy distribution along the dose. An alternative is the rem, set
equal to 1 10-2 Sv.
On average, a person on Earth receives about 2.2 mSv.y−1. A dose of 1 Sv causes
temporary disruptions. A dose of 10 Sv is fatal. After the Chernobyl accident,
the average dose absorbed by the resident populations in the affected areas
within 10 years, between 1986 and 1995, was of 6 to 60 mSv. The 28 deaths
from effect of radiation at Chernobyl received over 5 Sv in a few days.
Biological effects of radiation
The greatest risk associated with the operation of a nuclear power plant is
associated with radioactivity. This affects humans and animals, causing
somatic and genetic effects. Somatic effects may be acute, when the body is
subjected to high doses of radiation or when there is chronic exposure to low
levels of radiation but for very prolonged periods.
The acute effects include vomiting, bleeding, increased susceptibility to
infections, burns, hair loss, blood changes and, in extreme cases, death.
Chronic effects, which manifest themselves for many years, include cataracts
in the eyes, and several types of cancer, such as leukemia, thyroid cancer, skin
cancer or cancer of the lungs. May also manifest itself genetic effects in
future generations.
Nuclear reactors
A nuclear reactor of a thermonuclear power plant is a vessel under pressure
containing the nuclear fuel that will undergo a chain reaction, generating heat
that is transferred to a fluid, usually water, which is pumped from the reaction
vessel. The heated fluid may be water vapor, which flows through a turbine to
generate electricity; or it can be hot water, gas or a liquid metal, which
generates steam in a heat exchanger.
The first nuclear reactor was built by Enrico Fermi in 1942. It was built under
the stadium at the University of Chicago. The reactor had 9 m wide, 9.5 m long
and 6 m high. It contained about 52 tons of natural uranium, about 1350 tons
of graphite as a moderator and cadmium bars as controllers. The reactor
produced only 200 W and for a few minutes!!
The first commercial-scale nuclear power station, with an installed power of
180-MW, entered into operation in 1956 at Calder Hall, England.
The fuel rods contain the isotopes that undergo fission of 235U and/or 239Pu.
Natural uranium contains approximately 99.3% of 238U and 0.7% of 235U. The
concentration of 235U in natural ore is not sufficient to sustain a chain reaction
in most nuclear reactors. Thus this isotope has to be enriched to 3 to 4%. The
bars contain uranium metal, solid uranium dioxide (UO2), or a mix of uranium
oxide and plutonium oxide called MOX, and constructed in ceramic pellets.
These pellets are inserted into zircalloy or stainless steel tubes with about 1
cm in diameter and up to 4 m in length.
The moderators are used to slow down the energetic neutrons that are
originated by the fission chain reaction, giving rise to slow neutrons, also
called thermal neutrons. This increases the probability of these neutrons
being absorbed by another nucleus, propagating the chain reaction. The
moderators contains atoms or molecules whose nucleus are able to diffract
the neutrons and low tendency to absorb neutrons. The typical moderators
are the common water (H2O), heavy water (D2O), graphite (C) and beryllium
(Be).
The control rods contain elements whose nucleus has a high probability to
absorb thermal neutrons, and that are not available for further fission
reactions. In the presence of control rods, the chain reaction is controlled or
even stop. The control rods are typically made in boron (B) or cadmium (Cd).
The chain reaction inside the reactor is governed by the neutron economy
coefficient, k. In steady state the number of thermal neutrons is invariant
through time, i.e. dn/dt = 0, and k 1. The reactor is then in critical condition.
When k < 1, the reactor is in subcritical condition; When k > 1 is in
supercritical condition. A reactor enters critical condition when the control
rods are high, and more than one neutron released by the fission survives
without being absorbed by the control rods. The position of the control rods
determines the reactor power. Plants usually operate at full power due to
economic reasons
The heat generated by the chain reaction has to be constantly removed from
the reactor. The heat is generated not only by the chain reaction but also by
the radioactive decay of fission products. This heat is removed by a cooling
fluid which can be boiling water, pressurized water, a liquid metal (sodium
liquid), or a gas (CO2 or helium).
BWR
The water serves simultaneously as the cooling fluid (refrigerator) and
moderator. Once the control rods are removed the chain reaction starts and
the water boils. The saturated steam at a temperature of about 300 ºC and a
pressure of 7 MPa is separated from the condensate and forwarded to a
turbine. After expansion in the turbine steam condenses in the condenser
and is pumped back into the reactor. This cycle has the advantage of
simplicity and a relatively high thermal efficiency, since the steam generated
in the reactor is forwarded directly to the turbine. The thermal efficiency of a
BWR reactor is about 33%.
PWR
Because of the heat exchanger the heat efficiency of a PWR reactor is slightly
lower than the BWR, about 30%.
Breeder reactor
In a reactor of this type, the fissile nuclei are produced from fertile nuclei.
The main mechanism is the conversion of 238U to 239Pu. The intermediary 239U
has a half-life time of 23 minutes, turns into 239Np with a half-life of 2.4 days,
which in turn decays in 239Pu, with a t1/2 of 24000 years. The 239Pu formed,
although suffers fission, does not participate in chain reaction, but
accumulates in the spent fuel, being subsequently extracted and reused.
Unlike the 235U fission, who suffers fission with slow neutrons with energy in
the range of tens of eV, the 238U effectively captures fast neutrons in the range
of MeV. To obtain this spectrum of energy is necessary a different moderator.
The most widely used is sodium liquid.
These reactors can also use thorium as fuel. The 232Th is a fertile core that can
be converted to 233U through reactions :
232Th
+ n +  → 233Th + β → 233Pa + β → 233U
The nuclear fusion
Like fission, a huge amount of energy is involved when light nuclei undergo
fusion. Examples:
The advantages of fusion over the thermonuclear fission-based power stations
are:
(a) the fuel available for fusion reactors is virtually unlimited!
(b) The fusion reactions produce a minimal amount of radiation; some
radioactive isotopes can be created due to the absorption of neutrons in the
materials surrounding the fusion reactor. The tritium is slightly radioactive
emitting β radiation with low-energy and t1/2 of about 12 years.
c) no waste from which we can extract ingredients to manufacture atomic
weapons.
The difficulty in achieving controlled Fusion is to overcome the enormous
forces of repulsion between positively charged nuclei. To win these repulsive
forces, the colliding nuclei must have kinetic energies that correspond to
temperatures of millions of °C. At these temperatures the atoms are
completely dissociated in positively charged nuclei and free electrons. We call
to this plasma state. To obtain a significant release of energy many nuclei must
collide and therefore they should be confined in a small volume at high
pressure
The most optimistic estimates predict that there will be nuclear fusion
operating in the next 40 years. The most pessimistic say the nuclear fusion
will never be practiced because it will be too expensive.
Tokamak
The plasma confinement in a given volume is based on the confinement in a
magnetic field. The magnetic field is created inside of cylindrical coils through
current circulation. These cylindrical coils form a circle, and the magnetic field
is toroidal (doughnut). Plasma particles move through revolutions through the
helical field lines. The first toroidal magnetic was built in the former USSR, and
hence the use of the acronym Tokamak which, in Russian, means toroidal
magnetic chamber.
Fusion Tokamak-type machines have already worked in Russia, Europe, Japan
and USA. In 1993, a laboratory reactor of the Physics of Plasmas lab in
Princeton using deuterium-tritium fusion, reached a temperature of 100
million °C and a 5 MW power for approximately 4 seconds.
ITER will cost 5 billion euros during the construction that will last 10 years
and another 5 billion euros over 20 years of operation.
A commercial reactor is not expected before 2045 or 2050, however there are
no guarantees of success of ITER.
Problem 1
(a) Calculate the mass deficit in Atomic mass units (amu) of the following
fission reaction:
235U + n → 139Xe + 95Sr + 2n
(b) Calculate the energy (MeV) released in one fission.
(c) Calculate the energy released by 1 kg of 235U.
Problem 2
(a) Calculate the mass deficit in amu for the following fusion reaction:
2D
+ 3T → 4He + n
(b) Calculate the energy released (MeV) per fusion.
(c) Calculate the energy released by 1 kg of deuterium.
Problem 3
A nuclear accident occurs with a release of 90Sr that emits ϒ radiation with a
half-life of 28.1 years. Assuming 1 μg is absorbed by a newborn child, how
much of that remains in the body after 18 years and after 70 years, assuming
no losses due to metabolism.
Problem 4
The 129I isotope has a half-life of 15.7 years. In an accident of a thermonuclear
plant, 1 kg of this isotope is dispersed around the grounds of the facility. How
much remains after 1, 10 and 100 years?