The Chemistry of Fusion Bombs

In the previous article, we learned about how nuclear fusion occurs naturally, specifically in stars throughout the universe. Now that we understand the very extreme set of conditions that must be present for fusion to occur, we can shift to the oppositeend of the spectrum: man-made fusion. As we have already learned, the fusion reaction gives off a great amount of energy which can be used for many practical purposes that would benefit our daily lives. Throughout the ages, it seems that it has been humanity’s goal to harness ever increasing power for its own benefit which could also destroy humanity altogether. Whether it is a bomb, or an alternative energy source in the form of a nuclear fusion reactor, those inventors who are able to achieve both are definitely making a great contribution to all mankind as long as humans are seeking peaceful cohabitation at all times.

Let’s start off with the largest nuclear fusion bomb ever detonated. The device officially designated RDS-220, known to its designers as Big Ivan, and nicknamed in the west Tsar Bomba was the largest nuclear weapon ever constructed or detonated. The nickname Tsar Bomba is areference to a famous Russian tradition for making gigantic artifacts for show. The bomb was detonatedin 1961 on Novaya Zemlya islands in Arctic Russia. This three-stage weapon was actually a 100 megaton bomb design, but the uranium fusion stage tamper of the tertiary (and possibly the secondary) stage(s) was replaced by one(s) made of lead. This reduced the yield by 50% by eliminating the fast fissioning of the uranium tamper by the fusion neutrons, and eliminated 97% of the fallout (1.5 megatons of fission, instead of about 51.5 Mt), yet still proved the full yield design. (http://www.tsarbomba.org/)

The result was the “cleanest” weapon ever tested with 97% of the energy coming from fusion reactions. The effect of this bomb at full yield on global fallout would have been tremendous. It would have increased the world’s total fission fallout since the invention of the atomic bomb by 25%. Despite the very substantial burst height of 4,000 m (13,000 ft) the vast fireball reached down to the Earth, and swelled upward to nearly the height of the release plane. The blast pressure below the burst point was 300 PSI, six times the peak pressure experienced at Hiroshima in 1945. The flash of light was so bright that it was visible at a distance of 1,000 kilometers, despite cloudy skies.

As we discussed in the first blog post, there are two types of nuclear fusion reactors that have been developed. They include the magnetic confinement reactor and the inertial confinement reactor. In the late 1940’s it seemed an irresolvable problem to scientists as how to enclose the plasma, since any contact to the reaction container wall would let the surface layer of the wall evaporate and cool the plasma rapidly, causing the fusion cease. In 1951, Lyman Spitzer had the idea to enclose the plasma in a magnetic cage. As the plasma is ionized, it consists of charged particles (positive ions and electrons) that can be influenced by a magnetic field. Their trajectory has two components: a circular motion at right angles to the magnetic field and a linear motion across the magnetic field. The Tokamak was invented by Soviet physicist Igor Tamm and Andrei Sakharov in 1952. Toroidal magnetic fields were used to avoid the particles that escaped at the poles of the magnetic field. However, a toroidal magnetic field was not able to hold the plasma in an equilibrium force balance because the field strength decreased from the inside to the outside of the toroidal field with the effect that the particles drift towards the wall. Therefore, the field lines may not take a circular course about the axis of the torus, but need to be helically looped. The scientists were enthusiastic and predicted in 1955 that in 20 years time, nuclear fusion would provide us with limitless energy. However, the magnetic confinement turned out to be much trickier than assumed. To this day, experiments and tests are being run in nuclear fusion facilities in France to prevent particles from leaving the magnetic field. (http://www.world-nuclear.org/info/Current-and-Future-Generation/Nuclear-Fusion-Power/

Now that we know that fusion is the future of clean and virtually unlimited power for all of our electrical needs, we can move forward into our next topic: Fusion in the Future, and specifically, cold fusion. Get excited to really focus on the chemistry concepts involved with this unique type of energy creation. Until then, keep reading to find out!

Advertisements

Pons, Fleishmann, and their Cold Fusion Success

After the completion of their cold fusion success, Pons and Fleishmann went to the United States department of Energy to request more funding for further research. However, they were denied because others who had attempted to replicate their results had not been successful. Cold fusion, they concluded, was impossible and the experiment carried out by the two electrochemists was flawed. In the last blog post we discussed the details of the experiment, and today we will explain some theories as to why these false results were obtained.

A research team at the California Institute of Technology extensively attempted to reproduce the results of Pons and Fleishmanns experiment to no avail. They then attempted to implement different experimental errors to see if they could reproduce the same results. When the Caltech team did not put a stirrer in the tester cell, temperature differences within the cell led to false temperature recordings, which led to data that was similar to that of Pons and Fleishmanns data. These researchers at Caltech also suggested that the helium neutrons which suggested to Pons and Fleishmann that there was nuclear fusion transpiring could have simply been atmospheric traces of helium which are natural.

Some researchers also believe that Dr. Pons’ son, who helped by watching over the lab some days, might have turned off the electrical current briefly. This, Dr. Lewis of Caltech says, will cause the chemical reaction on the surface of the palladium to cease, and the excess deuterium to “bubble out” causing a fire hazard and thus, exhibited a pseudo-raise in the temperature of the cell.

Many even suggested that Pons and Fleishmann outright lied about their data. There was a great deal of mystery surrounding the experimental set-up, and the two electrochemists purposely shielded their experiment from testing and criticism by the scientific community. If you want to see for yourself how suspicious (or innocent) Dr. Pons and Dr. Fleishmann were, you can watch this video of the electrochemists at a press conference, addressing their critics. Notice how well they avoid the questions — it’s more like politics than science.

Chemistry of Cold Fusion

In our group’s previous post, we talked about nuclear fusion in the past but now its back to the future, with cold fusion, (and specifically the Pons-Fleishmann experiement)!

Cold fusion was a concept thought up by Stanley Pons and Martin Fleishmann in 1989. These electrochemists claimed that they had been able to achieve nuclear fusion at room temperature, and dubbed this miracle cold fusion. Had these claims been true, nuclear fusion would have been significantly more feasible for more widespread use. However, when other scientists attempted to replicate this experiment, they were not able to obtain the same of level success, and cold fusion came to be regarded as a myth. Why were other scientists not able to get the same result, you ask? Well lets look at the experiment in detail.

Stanley Pons (University of Utah) and Martin Fleishmann (University of Southampton) hypothesized that nuclear fusion may occur if electrolysis was carried out with deuterium in palladium metal. Electrolysis is simply the process by which a chemical reaction is instigated using an electrical current. Pons and Fleishmann believed that electrolyzed deuterium would have a very high compression ratio and level of mobility, which would allow it to undergo nuclear fusion, and thus produce exorbitant amounts of energy. Palladium naturally experiences a chemical reaction on its surface which causes it to absorb large amounts of hydrogen into its metal. Fleishmann and Pons believed that if enough deuterium (an isotope of hydrogen) atoms were absorbed at once, they may undergo nuclear fusion. To test this theory, they put a palladium cathode in a calorimeter with heavy water, or deuterium oxide (contains more of the deuterium isotope than regular water). Through electrolysis, the deuterium oxide broke down into its elemental components, and the deuterium was absorbed into the palladium. This electrical current was applied over several weeks, and the heat change was measured. For most of the experiment the temperature remained stable at around 30 degrees celsius, but then the temperature would suddenly rise to 50 degrees celsius for two or more days at a time, although the input energy would not change. Thus, during these phases the calculated output energy was significantly higher than the input energy

It is widely believed that the Pons-Fleishmann experiment was flawed, especially in its sources of experimental error. Based on how the experiment was just outlined, what do you think are the sources of experimental error? Check to see if you guessed correctly when we reveal the answer in our group’s next post!

Nuclear Fusion in Stars

In the previous article, we discussed what nuclear fusion is and learned that this complicated chemical process can only occur under a very extreme set of conditions. To the average person, it may seem that nuclear fusion can only occur in a laboratory setting, where ideal conditions are created and maintained by scientists. However, that is not the case. Believe it or not, nuclear fusion is one of the more common chemical reactions, as it occurs naturally in stars throughout the universe.

 

Our sun, like all stars, fuses elements together to create denser elements, in doing so creating enough energy to maintain the star’s high temperature. If a certain temperature (100 million Kelvins) is not maintained, the sun will collapse in on itself. Fusion is possible in the sun (and any star for that matter) only because of the star’s immense mass, leading to an astronomically large gravitational field. In our solar system, the gravitational field is responsible for the aligned orbits of the planets. The size of the star causes extremely high pressure and temperature within its core, where large amounts of hydrogen are present (http://www.sciencealert.com.au/features/20130711-24990.html).  In these conditions, hydrogen atoms can overcome the electromagnetic repulsion between themselves, and undergo nuclear fusion. The magnitude of the repelling force increases with the electrical charges on the two nuclei. To keep this force small therefore, the interacting nuclei should have the lowest possible charge(or atomic number).

Fusion primarily takes place during the main sequence of a star. The main sequence is the time in which the star turns all of its available hydrogen into helium, which in turn produces light and heat energy. The exact length of the main-sequence stage depends on the star’s mass: the lower the mass of the star, the longer it takes to burn up all its hydrogen, chiefly due to the relationship between mass and gravity. The main sequence length depends on the mass of the star in question. For example, our sun is about halfway through its 10 billion year main sequence. (http://www.astronomynotes.com/evolutn/s4.htm). During this stage, the star is in hydrostatic equilibrium. This means that the gravity of the star and the opposing thermal pressure created by fusion are pushing against each other equally, creating stability within the star. Hydrostatic equilibrium is the pressure gradient force that also prevents gravity from collapsing theEarth’s atmosphere into a thin, dense shell, while gravity prevents the pressure gradient force from diffusing the atmosphere into space. 

Now that we have talked about what fusion is along with where and why it occurs naturally in the universe, in our next post, we will discuss how this complicated chemical process has already been used (on Earth), so keep on reading!

(pictures from http://www.buzzle.com/articles/nuclear-fusion-in-the-sun.html)

An Introduction into Nuclear Fusion

Have you always wanted to learn more about nuclear fusion, but can’t understand the complex scientific writing? Well then you’re in the right place! In our posts, you’ll find plenty of information about nuclear fusion told in a way anyone can understand. But before we talk about all the cool things nuclear fusion can do, we have to understand how it works and what it does.

Nuclear fusion is a reaction which produces large amounts of energy by combining two hydrogen or helium protons (positively charged atoms), hence the term “fusion.” Fusion is not to be confused with fission, which is the opposite process. While fusion forms larger atoms out of smaller ones, fission splits larger atoms into smaller atoms. The energy released is primarily in the form of heat. Fission creates a large amount of energy, but also releases potentially dangerous radioactive byproducts. This “nuclear waste”  has to be collected and stored in safe facilities to ensure no leaks occur, which costs money and requires resources.  However, fusion releases even more energy, and does not release radiation. Also, hydrogen, which is used in fusion reactions, is the most abundant gas in the universe (Check it out here). Because it is such a sustainable source of energy, many people hope that Fusion will be the energy source for the future.

Fusion reactions combine different isotopes (same element but with a variation in the number of neutrons) of helium and hydrogen, including the Hydrogen isotopes Protium, Deuterium, and Tritium and the Helium isotopes Helium-3 and Helium-4. If the nucleus of the newly created nuclei needs less energy to hold it together than the old ones, energy will be released and mass will be “lost.” Different combinations of the hydrogen isotopes result in an isotope of helium and the release of high-energy particles, as shown in this fusion diagram.  However, since fusion brings two protons together, their positive charges cause them to repel each other. The only way to get protons to overcome this repulsion is to put them at extremely high temperatures and pressures.  In order for the protons to have enough energy to bond with each other, they need to be in an environment that is about 100 million Kelvins (six times hotter than the sun’s core). They also need to be under a lot of pressure in order to push them close enough together to fuse (1×10-15 meters away from each other). At first glance, these temperatures seem unreachable, as no known substance can withstand them.

To compensate for this fact and achieve these conditions, scientists use magnetic or inertial confinement reactors. A magnetic confinement fusion reactor contains plasma (superheated gas) which is spun with a magnetic field around the inside of the reactor so that the plasma does not touch the walls of the reactor. At the same time, the necessary pressure and heat comes from magnetic and electric fields themselves. An inertial confinement reactor (ICR) works differently. To compress and heat the fuel, energy is delivered to the outer layer of the target ( a pellet containing a mixture of Deuterium and Tritium ) using high-energy beams of laser light, electrons, or ions. The most common practice of ICR has used lasers. The heated outer layer explodes outward, producing a reaction force against the remainder of the target, accelerating the energy inwards, compressing the target. This process is designed to create shockwaves that travel inward through the target. A sufficiently powerful set of shock waves can compress and heat the fuel at the center so much that fusion reactions occur. However, due to current technological limitations, fusion reactions have to take place on a very large scale in order to achieve an efficiency high enough that the reaction will be productive.Fusion Diagram

With all these conditions required for nuclear fusion, do you think it could occur naturally? Where in the universe might these extreme conditions be present? Check out our next post to find out! Also, watch this video for a brief recap of the process discussed in this blog post.