what is physics

What Is Physics?

Physics is considered to be the most basic of the natural sciences. It deals with the fundamental constituents of matter and their interactions as well as the nature of atoms and the build-up of molecules and condensed matter. It tries to give unified descriptions of the behavior of matter as well as of radiation, covering as many types of phenomena as possible. In some of its applications, it comes close to the classical areas of chemistry, and in others there is a clear connection to the phenomena traditionally studied by astronomers. Present trends are even pointing toward a closer approach of some areas of physics and microbiology.
Although chemistry and astronomy are clearly independent scientific disciplines, both use physics as a basis in the treatment of their respective problem areas, concepts and tools. To distinguish what is physics and chemistry in certain overlapping areas is often difficult. This has been illustrated several times in the history of the Nobel Prizes. Therefore, a few awards for chemistry will also be mentioned in the text that follows, particularly when they are closely connected to the works of the Physics Laureates themselves. As for astronomy, the situation is different since it has no Nobel Prizes of its own; it has therefore been natural from the start, to consider discoveries in astrophysics as possible candidates for Prizes in Physics.

Big Bang Theory - The Premise

The Big Bang theory is an effort to explain what happened at the very beginning of our universe. Discoveries in astronomy and physics have shown beyond a reasonable doubt that our universe did in fact have a beginning. Prior to that moment there was nothing; during and after that moment there was something: our universe. The big bang theory is an effort to explain what happened during and after that moment.

According to the standard theory, our universe sprang into existence as "singularity" around 13.7 billion years ago. What is a "singularity" and where does it come from? Well, to be honest, we don't know for sure. Singularities are zones which defy our current understanding of physics. They are thought to exist at the core of "black holes." Black holes are areas of intense gravitational pressure. The pressure is thought to be so intense that finite matter is actually squished into infinite density (a mathematical concept which truly boggles the mind). These zones of infinite density are called "singularities." Our universe is thought to have begun as an infinitesimally small, infinitely hot, infinitely dense, something - a singularity. Where did it come from? We don't know. Why did it appear? We don't know.

After its initial appearance, it apparently inflated (the "Big Bang"), expanded and cooled, going from very, very small and very, very hot, to the size and temperature of our current universe. It continues to expand and cool to this day and we are inside of it: incredible creatures living on a unique planet, circling a beautiful star clustered together with several hundred billion other stars in a galaxy soaring through the cosmos, all of which is inside of an expanding universe that began as an infinitesimal singularity which appeared out of nowhere for reasons unknown. This is the Big Bang theory.
Big Bang Theory - Common Misconceptions
There are many misconceptions surrounding the Big Bang theory. For example, we tend to imagine a giant explosion. Experts however say that there was no explosion; there was (and continues to be) an expansion. Rather than imagining a balloon popping and releasing its contents, imagine a balloon expanding: an infinitesimally small balloon expanding to the size of our current universe.

Another misconception is that we tend to image the singularity as a little fireball appearing somewhere in space. According to the many experts however, space didn't exist prior to the Big Bang. Back in the late '60s and early '70s, when men first walked upon the moon, "three British astrophysicists, Steven Hawking, George Ellis, and Roger Penrose turned their attention to the Theory of Relativity and its implications regarding our notions of time. In 1968 and 1970, they published papers in which they extended Einstein's Theory of General Relativity to include measurements of time and space.^{1, 2} According to their calculations, time and space had a finite beginning that corresponded to the origin of matter and energy."³ The singularity didn't appear in space; rather, space began inside of the singularity. Prior to the singularity, nothing existed, not space, time, matter, or energy - nothing. So where and in what did the singularity appear if not in space? We don't know. We don't know where it came from, why it's here, or even where it is. All we really know is that we are inside of it and at one time it didn't exist and neither did we.
Big Bang Theory - Evidence for the Theory
What are the major evidences which support the Big Bang theory?

• First of all, we are reasonably certain that the universe had a beginning.
• Second, galaxies appear to be moving away from us at speeds proportional to their distance. This is called "Hubble's Law," named after Edwin Hubble (1889-1953) who discovered this phenomenon in 1929. This observation supports the expansion of the universe and suggests that the universe was once compacted.
• Third, if the universe was initially very, very hot as the Big Bang suggests, we should be able to find some remnant of this heat. In 1965, Radioastronomers Arno Penzias and Robert Wilson discovered a 2.725 degree Kelvin (-454.765 degree Fahrenheit, -270.425 degree Celsius) Cosmic Microwave Background radiation (CMB) which pervades the observable universe. This is thought to be the remnant which scientists were looking for. Penzias and Wilson shared in the 1978 Nobel Prize for Physics for their discovery.
• Finally, the abundance of the "light elements" Hydrogen and Helium found in the observable universe are thought to support the Big Bang model of origins.

Big Bang Theory - The Only Plausible Theory?
Is the standard Big Bang theory the only model consistent with these evidences? No, it's just the most popular one. Internationally renown Astrophysicist George F. R. Ellis explains: "People need to be aware that there is a range of models that could explain the observations….For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations….You can only exclude it on philosophical grounds. In my view there is absolutely nothing wrong in that. What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that."⁴

In 2003, Physicist Robert Gentry proposed an attractive alternative to the standard theory, an alternative which also accounts for the evidences listed above.⁵ Dr. Gentry claims that the standard Big Bang model is founded upon a faulty paradigm (the Friedmann-lemaitre expanding-spacetime paradigm) which he claims is inconsistent with the empirical data. He chooses instead to base his model on Einstein's static-spacetime paradigm which he claims is the "genuine cosmic Rosetta." Gentry has published several papers outlining what he considers to be serious flaws in the standard Big Bang model.⁶ Other high-profile dissenters include Nobel laureate Dr. Hannes Alfvén, Professor Geoffrey Burbidge, Dr. Halton Arp, and the renowned British astronomer Sir Fred Hoyle, who is accredited with first coining the term "the Big Bang" during a BBC radio broadcast in 1950.

Atomic Nomenclature

Introduction:

Throughout this web site we will use different terminology to describe atomic processes. It is important that you know this terminology, or "nomenclature", so that you can understand what we are talking about! In this page we are going to highlight some of the vocabulary you must understand.

Atomic Number:

The atomic number of an atom is essentially the number of protons in its nucleus. For example, carbon has 6 protons. Thus, its atomic number is 6. Earlier, we talked about isotopes. Remember that each isotope has the same number of protons? Well, every isotope of an element has the same atomic number.

Atomic Mass:

The atomic mass of an element is the sum of the number of protons AND neutrons. For example, carbon has six protons. One isotope of carbon also has 6 neutrons. This means that its atomic mass is 12, or 6 protons + 6 neutrons. Another isotope of carbon has 8 neutrons. This means that its atomic mass would be 14, or 6 protons + 8 neutrons.

What exactly is an Isotope?

An isotope is a type of an atom. One element can have many different isotopes. This is because an atom is defined by its number of protons, not it's number of neutrons. Isotopes are simply atoms of an element with different numbers of neutrons, but the same number of protons.

Each isotope has one proton but a different number of neutrons. Note: Picture Not To Scale

Referring to Isotopes:

Elements are often referred to by their symbol, and not their name. For instance, we might call Beryllium Be. (Scientists are lazy people. Why write the full name when they can abbreviate it?) So, how do we distinguish between different isotopes? They are the same element, after all. Well, it turns out that there is a notation that says what type of element we're talking about AND tells the isotope. This is done by giving the element symbol and the isotope mass in smaller text to the left of the symbol. This is written like this: 244Pu .
Pu is the symbol for plutonium, an element used in nuclear fission. The number 244 is the atomic mass. This tells the isotope because we know that plutonium has 94 protons. This means that we are referring to the isotope of plutonium with 150 neutrons. 244Pu is pronounced "plutonium 244" or "Pu 244".

Periodic Table of Elements:

The Periodic Table of Elements is a listing of all the known elements (types of atoms) that exist. They are listed in order of atomic number (number of protons). In our table, we have each element's atomic number at the top of each box, its atomic symbol in the center, and its atomic mass at the bottom. The atomic mass of each element is not a whole number. At first, this appears to make no sense. How could the sum of an atom's protons and neutrons be 1.01 (the atomic mass of hydrogen)? It is impossible, after all, to have .1 neutrons or .1 protons. To understand this, first recall that the same type of atoms can have different numbers of neutrons, and thus different atomic masses. The reason that the atomic masses are not whole numbers is because the atomic masses of the isotopes are averaged to get the overall atomic mass for the element. We will explain more about this later. For now, remember that the mass of the isotope of an element that occurs most often in nature is the atomic mass rounded to the nearest whole number. For example, hydrogen's (H) mass is 1.01. So, th e mass of its most common isotope is 1. Click here to view our Periodic Table of Elements.

Nuclear Disasters and Accidents

Introduction:

One of the scariest things about nuclear power is when something goes wrong and an accident occurs. Radiation is released into the environment and people get hurt. Two of the most famous nuclear accidents occurred at the Three Mile Island reactor 2 in the United States and the Chernobyl reactor 4 in the former Soviet Union. In this text we will discuss these two disasters, along with correcting a few common misconceptions about nuclear accidents.

The Myth of a Reactor Explosion:

It is impossible for any PWR or LWR nuclear reactor to explode like an atomic bomb. This is because in order for an uncontrolled chain reaction to occur that is similar to an atom bomb, the uranium fuel must be extremely enriched, much more than the 4% 235U that is present in regular, commercial nuclear reactor fuel. So, if it can't explode, what does happen in a nuclear reactor? The answer is what is called a meltdown. When a meltdown occurs in a reactor, the reactor "melts". That is, the temperature rises in the core so much that the fuel rods actually turn to liquid, like ice turns into water when heated. If the core continued to heat, the reactor would get so hot that the steel walls of the core would also melt. In a complete reactor meltdown, the extremely hot (about 2700º Celsius) molten uranium fuel rods would melt through the bottom of the reactor and actually sink about 50 feet into the earth beneath the power plant. The molten uranium would react with groundwater, producing large explosions of radioactive steam and debris that would affect nearby towns and population centers. In general a nuclear meltdown would occurr if the reactor loses its coolant. This is what occured in the two disasters that we will discuss. Without coolant, the core's temperature would rise, resulting in the meltdown scenario we explained above.
You may be wondering, "Why can't they just drop the control rods in the reactor if it starts to get out of control?". The answer is that they can. The problem is that, even if the control rods are completely dropped in and the nuclear chain reaction stops, the reactor is still extremely hot and will not cool down unless coolant is put back in. The residual heat and the heat produced from the decay of the fission products are enough to drive the core's temperature up even if the nuclear chain reaction stops.

Three Mile Island:

Outside View of the Three Mile Island Nuclear Power Plant Photo Courtesy Nuclear Regulatory Commission

On an island 10 miles from Harrisburg Pennsylvania resides the Three Mile Island Nuclear Power Station. There are two reactors at the plant, dubbed Unit 1 and Unit 2. One of them is inoperable. Unit 2 experienced a partial reactor meltdown on March 28, 1979. A partial nuclear meltdown is when the uranium fuel rods start to liquefy, but they do not fall through the reactor floor and breach the containment systems. The accident which occurred at Unit 2 is considered to be the worst nuclear disaster in US history. Why did it happen? There are many reasons for the accident, but the two main ones are simple human error and the failure of a rather minor valve in the reactor. In the following paragraphs, we will explain how it was possible for the accident to happen and both its psychological and physical effects on the American people. The accident at TMI (Three Mile Island) began at about four in the morning with the failure of one of the valves that controlled coolant flow into the reactor. Because of this, the amount of cool water entering the reactor decreased, and the core temperature rose. When this happened, automatic computerized systems engaged, and the reactor was automatically SCRAMmed. The nuclear chain reaction then stopped. This only slowed the rate at which the core temperature was increasing, however. The temperature was still rising because of residual heat in the reactor and energy released from the decaying fission products in the fuel rods.
Because the pumps removing water from the core were still active, and a valve that controlled the cool water entering the core failed, water was leaving the core, but not coming in. This reduced the amount of coolant in the core. There wasn't enough coolant in the core, so the Emergency Core Cooling System automatically turned on. This should have provided enough extra coolant to make up for the stuck valve, except that the reactor operator, thinking that enough coolant was already in the core, shut it off too early.
There still wasn't enough coolant, so the core's temperature kept increasing. A valve at the top of the core automatically opened to vent some of the steam in the core. This should have helped matters by removing the hot steam, but the valve didn't close properly. Because it didn't close, steam continued to vent from the reactor, further reducing the coolant level. The reactor operators should have known the valve didn't close, but the indicator in the control room was covered by a maintenance tag attached to a nearby switch. Because the operators didn't know that the valve had failed to close, they assumed that the situation was under control, as the core temperature had stopped rising with the first venting of steam from the core. They also thought that the coolant had been replaced in the core, because they didn't know that the pump outlets were closed. A few minutes later the core temperature began to rise again, and the Emergency Core Cooling System automatically switched on. Once again, an operator de-activated it, thinking the situation was under control. In reality, it was not.
Soon, because of the coolant lost through the open valve at the top of the reactor, the core temperature began to rise again. At this point the fuel rods started to collapse from the intense heat inside the core. The operators knew something was wrong, but didn't understand what it was. This was about 5 minutes after the initial valve failure. It took almost 2 hours for someone to figure out that the valve releasing steam at the top of reactor hadn't closed properly. During those 2 hours, precious coolant continued to be released from the reactor a meltdown was underway. At approximately 6AM, an operator discovered the valve at the top of the core was open and closed it.
During the day hydrogen gas began to accumulate inside the reactor and caused an explosion later in the afternoon. This explosion did not damage the containment systems, however. Two days later, the core was still not under operator control. A group of nuclear experts were asked to help evaluate the situation. They figured out that a lot of hydrogen gas had accumulated at the top of the core. This gas could have exploded, like the explosion on the first day of the accident, or it could have displaced the remaining coolant in the reactor, causing a complete nuclear reactor meltdown. No one really knew what to do about the hydrogen build-up. A hydrogen recombiner was used to remove some of the hydrogen, but it was not very effective. However, hydrogen also dissolves in water, which is what the coolant was composed of. Thus, over time the hydrogen that had collected at the top of the core completely dissolved in the coolant. Two weeks later the reactor was brought to a cold shutdown and the accident was over.
No one was directly injured as a result of the accident. However, some radioactive gas and water were vented to the environment around the reactor. At one point, radioactive water was released into the Susquehanna river, which is a source of drinking water for nearby communities. No one is really sure what effects these radioactive releases might have had on people living near the power plant.

Chernobyl:

About 80 miles (130 km) north of Kiev, in what is now the Ukraine, is located the Chernobyl nuclear power plant. At this plant the worst reactor disaster to ever occur took place on April 26, 1986. It happened largely because normal reactor operations were suspended; an experiment was to take place in the reactor. As a result, normal safety guidelines were disregarded, and the accident occurred. However, as with most accidents of this type, it was a result of many small mistakes adding up to create a catastrophe. In the following paragraphs, we will outline just how the event transpired: Early in the day, before the test, the power output of the reactor was dropped in preparation for the upcoming test. Unexpectedly, the reactor's power output dropped way too much, almost to zero. Because of this drop, some control rods were removed to bring the power back up. (As you recall from the fission power text, the more control rods there are in a reactor, the more free neutrons are absorbed and the less fissioning that goes on. So, more control rods means less energy and power output.) The reactor's power output raised up, and all appeared to be normal.
More preparation for the test began later when two pumps were switched on in the cooling system. They increased water flow out of the reactor, and thus removed heat more quickly. They also caused the water level to lower in a component of the reactor called the steam separator. Because of the low level of water in the steam separator, the operator increased the amount of feed water coming into it, in the hopes that the water level would rise. Also, more control rods were taken out of the reactor to raise internal reactor temperature and pressure, also in the hopes that it would cause the water level in the steam separator to rise. The water level in the steam separator began to rise, so the operator adjusted again the flow of feed water by lowering it. This decreased the amount of heat being removed from the reactor core.
Because many control rods had been removed and the amount of heat being taken from the core by the coolant had been reduced, it began to get very hot. Also, there was relatively low pressure in the core because the amount of incoming water had been decreased. Because of the heat and the low pressure, coolant inside the core began to boil to form steam.
The actual test began with the closing of the turbine feed valves. This should have caused an increase in pressure in the cooling system, which in turn would have caused a decrease in steam in the core. This should have lowered the reactivity in the core. Thus, the normal next step when closing the turbine feed valves was to retract more control rods, increasing reactivity in the core. This is what the operator at Chernobyl did. The only problem was that in this case there was no increase in pressure in the cooling system because of the earlier feed water reduction. This meant that there was already a normal amount of steam in the core, even with the turbine feed valves closed. Thus, by retracting more control rods to make up for a reduction in steam that didn't happen, the operator caused too much steam to be produced in the core.
With the surplus of steam, the reactor's power output increased. Soon, even more steam was being produced. The operator realized there was a problem and SCRAMmed the reactor, completely disabling all fission reactions. However, it was too late. The temperature and pressure inside the reactor had already risen dramatically, and the fuel rods had begun to shatter.
After the fuel rods shattered, two explosions occurred as a result of liquid uranium reacting with steam and from fuel vapor expansion (caused by the intense heat). The reactor containment was broken, and the top of the reactor lifted off. With the containment broken, outside air began to enter the reactor. In this particular Soviet reactor, graphite was used as a moderator instead of water. (water was the coolant) As air entered the core, it reacted with the graphite. Graphite is essentially just carbon, so oxygen from the air chemically combined with the carbon to form CO (carbon monoxide). Carbon monoxide is flammable and soon caught fire. The fire emitted extremely radioactive smoke into the area surrounding the reactor. Additionally, the explosion ejected a portion of the reactor fuel into the surrounding atmosphere and countryside. This fuel contained both fission products and transuranic wastes.
During the days following the accident, hundreds of people worked to quell the reactor fire and the escape of radioactive materials. Liquid nitrogen was pumped into the reactor core to cool it down. Helicopters dumped neutron-absorbing materials into the exposed core to prevent it from going critical. Sand and other fire-fighting materials were also dropped into the core to help stop the graphite fire. All in all, over 5000(metric) tons of material were dropped into the core. After the fires were brought under control, construction of what is called "the sarcophagus" began. The word "sarcophagus" is usually used to describe the elaborate coffins the ancient Egyptians used to entomb their dead. In this case, the sarcophagus is a structure erected from about 300,000 metric tons of concrete that surrounds the reactor. It was designed to contain the radioactive waste inside. It has served its purpose well, but, now, ten years after the accident, several flaws have been found in it. Holes have begun to appear in the roof, allowing rainwater to accumulate inside. This water can corrode the structure, further weakening it. Also, birds and other animals have been seen making homes in the sarcophagus. If they should ingest radioactive material, they could spread it around the countryside. Additionally, with time the sarcophagus has become worn down. It is conceivable that an intense event like an earthquake, tornado, or plane crash directly on the sarcophagus could lead to its collapse. This would be catastrophic, as radioactive dust would once again rain down on the surrounding areas. Scientists and engineers are working on ways to repair or replace the structure.
One of the great tragedies of the accident was that the Soviet government tried to cover it up. Clouds of fallout were traveling towards major population centers such as Minsk, and no one was warned. No one outside the Soviet Union knew about the accident until two days later, when scientists in Sweden detected massive amount of radiation being blown from the east.
The effects of the disaster at Chernobyl were very widespread. The World Health Organization (WHO) found that the radiation release from the Chernobyl accident was 200 times that of the Hiroshima and Nagasaki nuclear bombs combined. The fallout was also far-reaching. For a time, radiation levels in a Scotland were 10,000 times the norm. 30 lives were directly lost during the accident or within a few months after it. Many of these lives were those of the workers trying to put out the graphite fire and were lost from radiation poisoning. The radiation released has also had long-term effects on the cancer incidence rate of the surrounding population. According to the Ukrainian Radiological Institute over 2500 deaths resulted from the Chernobyl incident. The WHO has found a significant increase in cancer in the surrounding area. For example, in 1986 (the year of the accident), 2 cases of childhood thyroid cancer occurred in the Gomel administrative district of the Ukraine (this is the region around the plant). In 1993 there were 42 cases, which is 21 times the rate in 1986. The rate of thyroid cancer is particularly high after the Chernobyl accident because much of the radiation was emitted in the form iodine-131, which collects in the thyroid gland, especially in young children. Other cancer incidence rates didn't seem to be affected. For example, leukemia was no more prevalent after the accident than before.
What caused the accident? This is a very hard question to answer. The obvious one is operator error. The operator was not very familiar with the reactor and hadn't been trained enough. Additionally, when the accident occurred, normal safety rules were not being followed because they were running a test. For example, regulations required that at least 15 control rods always remain in the reactor. When the explosion occurred, less than 10 were present. This happened because many of the rods were removed to raise power output. This was one of the direct causes of the accident. Also, the reactor itself was not designed well and was prone to abrupt and massive power surges.

Nuclear Weapons

Introduction:

Since 1945, when the first nuclear bomb was exploded by the Manhattan Project team in the US, nuclear weapons have proliferated across the globe. Currently, the US has about 7,000 warheads and the nations of the former Soviet Union have approximately 6,000. There are enough nuclear weapons in the world to destroy all civilization as we know it. They are perhaps the most powerful forces that man has ever wielded. Other countries that possess known nuclear capabilities are the United Kingdom, France, the People's Republic of China, Pakistan, and India. When first developed, nuclear weapons were completely strategic weapons. That is, they were not designed to destroy enemy weaponry; they were designed to destroy entire cities. However, there are now small, tactical nuclear weapons in addition to the others. Besides how powerful a nuclear weapon is, there are other differences between them. They can be either a fusion or a fission device, and they can be dropped from an airplane, fired from an artillery gun, or attached to various types of rockets.

The Fission Bomb:

A fission bomb uses an uncontrolled nuclear fission chain reaction to release an enormous amount of energy in a small amount of time. Previously, you read about different ways that a fission chain reaction is controlled in a nuclear reactor. In a bomb all those safeguards are removed. There are no control rods and when uranium is used as the fuel, it is much more enriched than the 4% 235U in nuclear fuel rods. This means that there is less 238U and thus less neutrons are captured. The fissile material (plutonium or uranium) in the bomb is usually surrounded by conventional explosives (non-nuclear). When the bomb is detonated, the conventional explosives are ignited. These explosives are designed to blow inwards, crushing the fissile material they surround. This compaction of the uranium or plutonium increases the chance that a stray neutron will strike a nucleus, inducing fission and the result runaway chain reaction. Once the nuclear material is compressed to the point of criticality (able to undergo a runaway nuclear reaction), a neutron "gun" fires. This "gun" shoots extra neutrons into the critical mass of fuel. This adds a lot of extra neutrons, which increase the chain reaction. As the chain reaction begins to grow exponentially, the inside of the bomb gets hotter and hotter. At a certain point, the pressure and heat are too intense for the casing, and it is ripped apart. Then the bomb explodes into the surrounding area and atmosphere, releasing fall-out and radioactivity into the environment. Additionally, the shock wave from the blast can level immense areas around the bomb. However, this area depends on the strength of the bomb. When the bomb explodes, the particles that made up the bomb are vaporized and enter the atmosphere as a fall-out cloud. The radioactive fall-out that is released consists of the fission products and the transuranic neutron-capture products, just like those in spent fuel rods. The difference is that the reaction in a bomb occurs at a much, much faster rate than that in a power plant.

Thermonuclear Fusion Bombs:

Fusion bombs have two main stages. These are called the "primary" and the "secondary". The primary reaction is a regular fission chain reaction. The radiation from this reaction is used to heat the interior of the bomb to temperatures where a fusion reaction can be sustained. Also, the neutrons produced from the fission reactions are used in the secondary (fusion) part of the reaction. The secondary is composed of lithium-deteuride (deuteride is basically deuterium, which is 2H). The lithium deteuride, under intense heat, splits apart into lithium (6Li) and deuterium ions. The neutrons produced from the primary (fission) reaction react with the 6Li to produce 4He and 3H. This reaction can be expressed in the following equation: 1 neutron + 6Li 4He + 3H
3H + 2H 4He + 1 neutron
A thermonuclear fusion bomb is generally a lot more powerful than a fission bomb. Interestingly, the harmful fall-out from a fusion bomb is generated mostly from the products of the primary (fission) reaction. These are the fission products and the transuranic products, just like in the fall-out of a standard fission bomb.

Aftermath:

If all the nuclear weapons in the world were used, then all of humanity would most like be destroyed. This is for several reasons. Firstly, most major cities would be destroyed by incoming warheads. However, this would leave some areas untouched. Thes e areas would most likely be reached by radioactive fall-out blown by the wind. These would be the immediate repurcussions. Later, the world would go into what is called "Nuclear Winter". Global temperatures would drop significantly, as well as the amount of sunlight received by the earth. This is very similar to what is believed happened to the dinosaurs. It is believed th at a large asteroid collided with the earth, and stirred up a lot of dust into the atmosphere. This blotted out the sun, and plants died. With very few plants to eat, the dinosaurs (and many other animals) went extinct. Nuclear winter would be a lot like this. The only difference is that there the dust would be raised up by impacting nuclear warheads and their explosions. Additionally, the dust would be radioactive. The combination of radioactivity, lack of food, and lowering temperatures cause a Nuclear Holocaust, with the chances of humans surviving it very low.

New Nuclear Power Technologies

Introduction:

There are many new waste disposal technologies which could prove to be somewhat of a solution to the problem of nuclear waste.

Reprocessing, The Missing Step:

Although not a new technology, reprocessing can be part of the solution to nuclear waste. When nuclear power was first developed, it was assumed that spent nuclear fuel would go through a process called reprocessing. In reprocessing, one of the major transuranic wastes, 239Pu, is extracted from the spent fuel rods. This 239Pu (plutonium-239) is fissile and can be reused in power plants. The advantages of this process are somewhat obvious: The volume of waste is lessened and more fuel is created for nuclear reactors. However, as with all things, politics can get in the way. In the US plutonium reprocessing was banned because the recovered 239Pu is weapons grade material. If, after reprocessing, the fuel is stolen, it could be used by anyone to construct a nuclear weapon. As of a few years ago, the ban against reprocessing in the US was lifted, but there are still no operating reprocessing plants in the US because of the heavy regulations and the anti-nuclear sentiment of the general public. There are a few countries which do reprocessing, however. France, for instance, regularly reprocesses its spent fuel.

High Temperature Breeder Reactors:

Many of us are familiar from television (and hopefully not from real life experience) of the bar-room game in which a very large man holds a mug of beer on top of his head and challenges people to punch him. If his opponent punches him hard enough, the beer falls off and spills all over the man holding it. The harder the punch, the better chance that the beer will fall off and the puncher will win. Also, the bigger the man is who is getting punched, the harder the punch must be to knock the beer down. You might be wondering why we are talking about a bar-room game. Think of the guy holding the beer as an atom and the guy punching as a neutron. The transuranic elements are bigger than uranium and generally don't fission (get their beer knocked off) in a regular reactor. The neutrons aren't excited enough (don't punch hard enough) to induce fission in them. However, if they are placed in a high-temperature reactor in which the neutrons are much more excited (and carry more punch), there is a much better chance that they will fission. In a reactor being developed by Argonne National Laboratory in the US, almost 100% of the transuranic nuclear wastes produced through neutron capture can be caused to fission. Generally, the fission products created have shorter half-lives and are not as dangerous. This reactor, dubbed EBR-II, uses liquid sodium as a coolant, which means that the internal reactor temperature is much, much hotter than that of a normal PWR reactor, which uses water as a coolant.
Another advantage of EBR-II is that its fuel is not weapons grade quality. When the transuranic wastes are separated from the other wastes in the spent fuel rods, the resultant mix of isotopes can not be used in a bomb. Thus, the mix can be used as fuel for EBR-II without a chance of it getting stolen by a terrorist group for use in an explosive device.
Breeder reactors "breed" fuel. That is, they are designed to create 239Pu from 238U through neutron capture. This "waste" can then be used as fuel.

Types of Nuclear Waste

Introduction:

One of the greatest problems with nuclear energy is the waste produced. The waste is generally radioactive, and thus toxic. There are also a few different kinds of waste, depending on how it was produced. Nuclear waste is produced in many different ways. There are wastes produced in the reactor core, wastes created as a result of radioactive contamination, and wastes produced as a bi-product of uranium mining, refining, and enrichment. The vast majority (99%) of radiation in nuclear waste is given off from spent fuel rods. However, fuel rods make up a relatively small percentage of the volume of waste. The largest volume of nuclear waste is composed of the leftovers from the mining process. This waste, however, doesn't give off much radiation. Some of the nuclear waste is extremely long-lived, meaning that it lasts a long time without its toxicity decreasing all that much, and some of it is very short-lived. Some types of nuclear waste are considered high-level and some are considered low level. The difference is in the amount of radioactive nuclei in relation to the mass of the waste. If there are a large amount of radioactive nuclei relative to the amount of waste, it is considered high level nuclear waste.

Fission Bi-Products:

When a 235U atom splits, it can produce a number of different products. Many of these are radioactive elements. For example, the following reaction produces 90Sr, which has a half-life of about 29 years. 1 neutron + 235U 2 neutrons + 90Sr + 144Xe
Although its half-life is 29 years, a quantity of 90Sr is not considered safe for 290 years. After 290 years, 10 half-lives would have passed. So, if we started out with half a ton (1000 lbs.) of 90Sr, after 290 years there would be 1000 x (1/2)¹⁰ left. This is about a pound. The rest of the 90Sr would have undergone ß^- decay, producing 90Y. 90Y is also radioactive, but is has a very short half-life of about 2.67 days. The 90Y undergoes ß^- decay, forming 90Zr, which is a stable, non-radioactive isotope. 90Sr is particularly dangerous because it shares many of the same chemical properties as calcium (Ca), and, if ingested, can take calcium's place in your bones. Then, when 90Sr decays, the radiation released in your body can cause cancer.
This is just one example of a radioactive isotope that is produced from fission. There are hundreds of other fission products, many of which are radioactive. Their half-lives, however, vary greatly from less than a second to many, many years.
The fission products, or fragments, usually remain within the fuel rods of the reactor. When most of the 235U in a fuel rod is spent, the rod must be removed. The radioactive fragments are what make the spent rods toxic. The fission products can be long-lived or short-lived.

Transuranics:

In previous texts we talked about how the 238U in a fuel rod is not fissile and is a neutron absorber. We made the point that because it absorbs neutrons, it stops the chain reaction in a nuclear power plant from running away (and producing a nuclear bomb effect). This is a good thing. However, think about what it means when we say "238U is a neutron absorber". The following reaction expresses that statement: 1 neutron + 238U 239U
When 238U "captures" a neutron, it is added to the original uranium nucleus, producing the radioactive isotope of uranium, 239U. This isotope has a half-life of 23.45 months. It decays, through ß^-, into 239Np. 239Np is also radioactive and decays into 239Pu. 239Np has a short half-life of about 2 days. This sequence of decays can be expressed like this:
1 neutron + 238U 239U
239U (ß^- decay) 239Np
239Np (ß^- decay) 239Pu
239Pu is also radioactive, and has a half-life of approximately 24,000 years. That's a long time!! A lot of 238U is turned into 239Pu through this sequence of decays. 239Pu is called a transuranic element. Any element with a higher atomic number (and thus more protons) than uranium is considered to be transuranic. This applies to all of the elements to the right of uranium in the Periodic Table. In the equations above we showed how a neutron can be captured by a nucleus and, through a series of ß^- decays, can produce an isotope with a higher atomic number than the original atom. More than one neutron can be captured. So, for instance, a neutron can be captured again by 239U. This produces 240U, which decays into 240Np. If 240Np captures another neutron, it becomes 241Np, which then decays into 241Pu and then into 241Am, which has a half-life of about 400 years. This sequence of decays and neutron additions can be expressed in the following reactions:
1 neutron + 238U 239U
1 neutron + 239U 240U
240U (ß^- decay) 240Np
1 neutron + 240Np 241Np
241Np (ß^- decay) 241Pu
241Pu (ß^- decay) 241Am
This is only one example of how higher-atomic number transuranic elements can be produced. There are many other pathways involving ß^- decay and neutron capture/addition that can produce transuranic elements besides neptunium (Np), plutonium (Pu), and americium (Am).
The transuranic neutron addition products usually remain in the fuel rods, where the original 238U from which they were produced was located. This adds to the rods' toxicity, and makes it harder for them to be disposed. In general, transuranic wastes are long-lived. However, this depends on the isotope produced. The biggest transuranic waste produced is 239Pu. This is an extremely toxic and extremely long-lived compound. 239Pu is fissile. In fact, when a nuclear reactor's fuel rods are almost spent, as much as 30% of the reactor output can come from the fissioning of 239Pu. Thus, the plutonium transuranic "waste" produced in a nuclear reactor can actually be used as fuel. We will discuss more on this later.

Waste from Uranium Mining and Enrichment:

When uranium is mined, it has to be separated from rock. This produces pure uranium ore and "tailings", essentially leftover rock that has had the uranium stripped from it. This rock often still contains radioactive nuclides and is somewhat dangerous. The tailings are generally long-lived, but are considered to be low-level waste. That is, the concentration of radioactive nuclei in them is small, and thus they are not extremely radioactive. As we explained previously, uranium ore is only about .7% 235U. It must be enriched to bring the percentage of 235U up to about 4%. The enrichment process produces a lot of waste. This is because for every gram of enriched uranium fuel produced, there are about 4 grams of 238U waste. 238U is radioactive and has a half-life of 4,468,000,000 years. This means that it is long-lived, but not extremely dangerous. However, some of its "daughter products" are radioactive. Thus, wastes produced as a result of enrichment must be kept in storage. By the way, a "daughter product" is an isotope that results from a decay of another, "parent", isotope. For example, when 238U decays, it produces 234Th, which is very radioactive and has a half-life of about 24 days. The decay can be expressed in the following equation:
238U ( decay) 234Th + particle

Contaminated Stuff:

A major portion of nuclear waste is comprised of spent fuel rods. These contain the fission products and transuranic wastes we mentioned above. However, a lot of other waste is produced in the reactor besides the fuel rods. This occurs as a result of radioactive contamination. A nuclear reactor is extremely hot. This means that the particles inside the reactor are very energetic and are flying around at incredible speeds. Occasionally, an atom that is in a fuel rod can get knocked out. These atoms that get knocked out can be many different types, ranging from fission products to uranium to transuranic elements. Most are radioactive. Atoms that escape the fuel rod careen all over the inside of the reactor core. Eventually these atoms can strike something solid. This is a lot like a bullet hitting a wall. If the wall is small, it might pass through. However, if the wall is big enough, the bullet will smash into the wall and "stick" there. So it is with a nuclear reactor. Occasionally an atom can smash into a structural component of the reactor, implanting itself into it. Because many of the nuclides (fancy term for atomic nucleus) careening about the core of a fission reactor are radioactive, when they smash into a structure and "stick", they make that structure appear to be radioactive. This is because there are many radioactive nuclides embedded in it, which give off radiation. Thus, many of the structural components of a reactor become radioactive over time, as they absorb radioactive nuclei into themselves. Also, many of the pipes and other components of a reactor become radioactive. These must be replaced eventually because over time the extreme radiation inside the reactor weakens them. The biggest problem, however, arises when a nuclear reactor is turned off for good, or "decommissioned". Disposal of the reactor core is a huge problem because it is extremely radioactive.