Question

Question: Write at least two paragraphs discussing things that you learned from the background portion of this lab. Be sure to include your interpretation of why nuclear fusion does not occur naturally on the Earth as well as why we do not yet have the technology to replicate the process.

lab/Background Information: Nuclear fusion is a very important physical process that governs how the Universe evolves. It occurs naturally in stars, but has not been observed to occur naturally anywhere else in the Universe, especially not here on Earth. The link (Links to an external site.)Links to an external site. here while being a bit technical in places, is an excellent reference source. There are many difficulties in achieving nuclear fusion, although some scientists believe they are getting close. The process is understood, but presently we do not have the technology to sustain nuclear fusion reactions that will provide positive net energy output. Some of the issues are explained in the article linked here (Links to an external site.)Links to an external site. that cites an ongoing project between MIT and private business. Stars form from molecular clouds. These clouds can have masses from thousands to one million times the mas of our Sun. If a large enough density spike occurs, that region can collapse under gravity to form a star. This is referred to as the protostar phase. More information about this process can be found on this website. (Links to an external site.)Links to an external site. (Make sure to click the links within the previous website to view key images). While this star is forming, the core becomes a plasma. This means that the molecular bonds in the molecules are broken and the electrons orbiting the nuclei are stripped away. This leaves a very high temperature fluid consisting of protons (hydrogen), electrons, and heavier atomic nuclei. These molecular clouds are typically 70 to 75 percent hydrogen. This means that regardless of the specific composition, once the plasma forms there will be a multitude of protons available for fusion. The first fusion process is referred to as the proton-proton chain and converts hydrogen into helium. The following link starts with three diagrams showing the three steps of the proton-proton chain. These steps are summarized here and presented in a more detailed discussion within the link.

Step One: Two protons fuse together to form deuterium.

Step Two: A proton combines with deuterium to form 3-helium.

Step Three: Two 3-helium nuclei fuse to form 4-helium and release two protons back into the plasma.

Click the Link here (Links to an external site.)Links to an external site. for more information. Our Sun is still running on this process. We believe that the Sun is roughly 4.5 billion years old and will continue the proton-proton chain for another 6 to 7 billion years.

Answer 1

When two light nuclei come close to one another, within the range of attractive nuclear force, they may combine to form a bigger nucleus. It is possible because the binding energy per nucleon is small for light nuclei and increases with A. To bring the light nuclei within a separation of about a femtometre (10^-15 m) is a difficult task. When atoms are pushed closer, their electrons cause them to repel each other. Even the single nucleus are positively charged and strongly repel each other. One needs a temperature of order of the 10^9 K if deuterons are to be fused. The temperature inside the sun is estimated to be 1.5*10^7 K . Yet fusion is the main source of energy in the sun which it ultimately radiates to the universe including the earth. As these reactions take place at high temperatures, they are also called thermonuclear fusion or thermonuclear reactions. The requirement of high temperature is the reason for Nuclear reaction not occurring naturally on earth.

FUSION IN SUN:-

Among the celestial bodies in which energy is produced, the sun is relatively cooler. In sun and other stars, where the temperature is less than or around 10^7 K the fusion takes place dominantly by proton proton cycle as follows:

¹H +¹H = ²H + ⁺e + v

²H + ¹H = ³He + gamma ray

³He + ³He = ⁴He + 2(¹H)

Totally, 4(¹H) = ⁴He + 2⁺e + 2v + 2 gamma photons

About 26.7 MeV energy is released in this cycle.

FUSION IN LABORATORY:-

In stellar objects, the material remains confined at high temperature to high gravitational pull . If we wish to make fusion as energy producing device in laboratory, the major problem is to confine the hot plasma in a small volume for extended time intervals. Producing a high temperature is a major task but confinement at such high temperature is more challenging. Solid walls cannot be used as containers as no solid can sustain the high temperatures needed for fusion. The easiest thermonuclear reaction held on earth is the fusion of two deuterons or the fusion of a deuteron with a triton.

²H₁ + ²H₁ = ³He₂ + n + 3.3MeV

²H₁ + ²H₁ = ³H₁ + ¹H₁ + 4.0MeV

²H₁ + ³H₁ = ⁴He₂ + n + 17.6MeV