In natural radioactive decay, an unstable isotope of an element decays into a different atom and an emission (alpha/beta particle, or energy in the case of gamma/? radiation). This is a spontaneous natural process with a random rate. Nuclear fission is also the splitting of a nucleus into two smaller parts, but each of these is an element in itself. Fission does not happen spontaneously; it requires a trigger. An unstable nucleus bombarded with a neutron will “elongate and divide itself like a liquid drop”  as its surface tension is decreased due to its charge .
Reaction 1 – a possible fission reaction of uranium Source  100 words Explain the role of hydrogen nuclei and helium nuclei in the synthesis of elements in stars. Give a detailed explanation of the nuclear changes that happen when lithium forms in stars. Chemical elements were originally created in stars such as the Sun, through processes collectively called nucleogenesis . Different elements are produced under the different conditions in each star; in the Sun, the primary “building blocks” are hydrogen nuclei.
These hydrogen nuclei fuse to form a helium nucleus, as shown in reaction 1. 4 1H –> 4He + subatomic particles Reaction 1 – a fusion reaction of hydrogen-1 to form helium-4  The energy released by this reaction allows more fusion reactions. Once most of the hydrogen is used up, the helium nuclei start a new series of reactions. In the first route and the first reaction of the second route, the nuclei fuse, simply adding together the protons and the neutrons together to form a larger nucleus. However, the second reaction of the second route is more complicated.
The negative electron joins with one of the positive protons in beryllium-7, forming a neutron,  in a way that can be thought of as the reverse of beta-decay. This decreases the atomic number by 1, so lithium is formed, but the atomic mass remains the same. Image 1 – route 2 in the synthesis of lithium Source  190 words Describe, with the use of examples, the main characteristics of fission and fusion reactions. Explain how each type of reaction produces energy and describe how these reactions are controlled.
Outline the main advantages and disadvantages of using fission and fusion processes for generating electricity. Fission Image 2 – fission Source  Both of the new nuclei are approximately half the size of the original nucleus. The neutrons given off go on to trigger more fission reactions, in a chain reaction. Fission reactions give off energy through conversion of some nuclear mass into energy. There is a difference in mass between the reactants and the products, as seen in reaction 13. 1n + 235U –> 91Kr + 142Ba + 3 1n Reaction 13 – a possible fission reaction of uranium-235
Source  The mass of the uranium + the neutron is 0. 1866971 amu  (atomic mass unit) greater than the mass of the products combined. The rule of conservation of mass-energy, according to Einstein’s E=mc2 (where E is energy in Joules, m is mass in kilograms, and c is the speed of light in a vacuum, 2. 99792 x 108m/s), explains that the lost mass has in fact been converted into energy – approximately 1. 68 x 1010 kJ/mole . If uncontrolled, fission will accelerate out of control due to more and more neutrons being given off.
Nuclear reactors generate electricity using a mixture of 238U and 235U. 238U does not undergo fission; it absorbs neutrons, interrupting the chain reaction so helping to control it. Image 3 – reactor and heat exchanger Source  The controlling features in the reactor: o The graphite moderator slows the neutrons from their high speeds (at which they are less likely to cause a fission reaction on collision with 235U). The high energy of neutrons produced in fission led to the development of the first nuclear bombs.
The control rods absorb neutrons, taking them out of the chain reaction. The rods can be moved to different depths to absorb different amounts of neutrons, controlling the rate of reaction. If they are fully inserted, all of the neutrons are absorbed and the reactor shuts down. The reactor is cooled by passing fluids (often molten sodium metal or carbon dioxide) through pipes around the reactor. This then boils water to form steam, which is used to turn turbines, generating electricity.