Applications of Nuclear Physics

Explain the basic principles of a fission reactor

A fission reactor uses a nuclear reaction to generate electricity. As with all generators, this involves producing rotation to turn a generator. In a nuclear reactor, heat from the nuclear reaction is used to produce steam which turns a turbine, in the same way that burning coal generates steam in a coal power plant. As outlined before, there are several requirements for a controlled reaction. These must be met in a fission reactor to ensure that firstly a reaction takes place, and secondly that the reaction doesn’t go out of control and produce an explosion. In addition to this, there are several key components to a fission reactor. Fuel rods consisting of enriched uranium are placed inside the reactor to provide the critical mass required. Control rods consisting of cadmium or boron are also placed in the reactor, such that they can be moved in and out to control the reaction. The control rods absorb excess neutrons to prevent the reaction from taking place too quickly. When they are lowered, more neutrons are absorbed and the reaction slows, and when pulled out the reaction rate increases. The entire reactor is immersed or surrounded by a moderator to slow down neutrons and thus increase the rate of reaction. The moderator consists of either heavy water, graphite, or various other organic compounds. A coolant is required to extract heat from the reaction and to prevent the reactor from melting. The coolant flows through the reactor then out into a heat exchanger that takes heat extracted from the coolant and uses it to boil water. Spent fuel rods that have been depleted in the reactor are extracted and processed or stored. They are extremely radioactive, making them very difficult to dispose of. Finally, the reactor is surrounded by multiple layers of shielding. There is a graphite shield that reflects neutrons back into the core, followed by a thermal shield to prevent unwanted heat loss from the core, a pressure vessel surrounding the core to isolate and contain everything inside the core, and lastly a biological shield of about 3 metres of concrete mixed with lead pellets, to absorb gamma rays and neutrons.

Figure 1

Remember- A nuclear reactor has a reactor core with fuel rods, control rods and a moderator. Coolant is heated inside the core and pumped out where it boils water. The steam produced turns a turbine, and is then condensed back into water. Shielding is used inside the reactor to prevent radiation and heat from escaping.

Gather, process and analyse information to assess the significance of the Manhattan Project to society

The Manhattan Project was one of the most significant scientific undertakings of the 20th century because of the dramatic impacts it had on society. It consisted of American efforts to produce nuclear weaponry, which were eventually successful and resulted in the deployment of nuclear weapons over Japan in 1945. In terms of impact on society, there were direct scientific impacts, namely the development of nuclear power offering a possible solution to the depletion of fossil fuels and a way of reducing greenhouse gas emissions from power generation. Much more significant however, were the social impacts that atomic weaponry had on global politics. To begin with, nuclear power had terrified the world with its incredible destructive power as witnessed in Japan. As a result, countries with nuclear weapons, primarily the USA and Russia in the period following WW2, became very reluctant to use them, firstly because of their long term destructive power, but secondly because of fear that retaliation would take the form of nuclear reprisal. As a result, although significant political tension built between Russia and USA, it never broke out into conflict, as either side was concerned that aggressive action would result in nuclear warfare, resulting in mutually assured destruction. Where a conventional war would have broken out previously, peace was maintained due to the development of nuclear weaponry. In modern times however, nuclear power is proving to be a dangerous bargaining chip for rogue states such as North Korea and Iran which are using nuclear weapons as leverage in negotiations with the Western world. It has led to a situation where small nations with comparatively weak conventional forces can use the threat of nuclear warfare to negotiate equally with large nations such as the USA. This has led to significant problems in terms of global politics and the power balance between nations necessary to maintain peace. Arguably however, even in these situations the threat of mutually assured destruction is preventing warfare. As a result of the nuclear threat, the UN and the USA are focussing on a diplomatic, sanctions-based approach to resolving conflict rather than an aggressive military approach. Overall, although the Manhattan Project led to the deaths of many Japanese people in Hiroshima and Nagasaki, and although it resulted in a build-up of nuclear arsenals across many nations providing a constant threat to global security, in the end the resulting nuclear stalemate has prevented several wars and therefore averted many possible deaths.

Remember- Although the Manhattan project led to many deaths at the end of WW2, the threat of nuclear war has prevented conflict in the decades after.

Describe some medical and industrial applications of radioisotopes

There are many applications of radioisotopes in medical and industrial fields. In the medical field, radioisotopes are mainly used for imaging/diagnosis and for treatment. In imaging, the transmission of radiation through the body and the degree to which radiation is absorbed can be used to remotely examine the body. They are often used to examine brain activity (using Positron Emission Tomog- raphy). By injecting radioisotopes into the body and examining where they end up (made possible because the radioisotopes are emitting radiation), the circulatory system can be investigated. Fi- nally, radioisotopes are frequently used to kill cancer cells, the radiation destroying them. In industry, they are used to examine stress fractures in metals such as in aircraft wings (because although the fractures may not be visible, radiation can pass through them), detecting leaks in pipes that may be otherwise difficult to find (since radiation will escape from a leaking pipe), and to irradiate medical supplies and food to kill bacteria.

Remember- Radioisotopes are in medicine used for imaging and cancer treatment, while in industry they are used to examine stress fractures and to sterilise objects and food.

Identify data sources and gather, process and analyse information to describe the use of a named isotope in medicine, agriculture and engineering

Iridium-192 is used in medicine to kill cancerous tumours. Iridium-192 pellets are implanted into the tumour where gamma emissions kill the cancer cells. Because the cancer cells are directly exposed to the radiation since they surround the pellet, damage to healthy cells is minimised (as opposed to external irradiation). Because it has a half-life of around 80 days the iridium must be surgically removed after treatment is complete to prevent over-exposure to radiation. The iridium undergoes beta decay, and transmutates to turns to inert platinum that poses no health risk. This makes iridium implants an extremely effective way to treat cancer.

In engineering, cobalt-60 is used to detect stress fractures in metals, particularly in aircraft. Stress fractures occur when metals are repeatedly exposed to strong forces, such as those experienced by the wings of an aircraft. Small fractures can form in the metal, which can eventually result in a catastrophic failure (e.g. there were several cases where early jet aircraft had fuselage explosions because the metals used to construct the aircraft eventually broke apart due to stress). These fractures are the precursors to actual breaks in the metal, but they are extremely hard to detect. By placing cobalt-60 on one side of the metal, and a gamma detector on the other side (often photographic film), the cracks can be identified easily and non-destructively because the gamma radiation only penetrates in areas where stress fractures have formed.

In agriculture, the elements that plants require can be substituted with radioisotopes of the same element. This allows the path of the material to be tracked through the plant’s structure. For example, replacing the phosphorus in soil with a mix containing radioactive phosphorus-32 will allow the path of phosphorus to be tracked into plants. By measuring how radioactive the plants are, how much phosphorus was used by the plant can be determined, as well as the areas in the plant where the phosphorus is concentrated. This has benefits in terms of better understanding the conditions favourable for plant growth, thereby maximising yield and increasing efficiency of the farming process.

Remember- Iridium-192 is implanted to kill cancer cells, cobalt-60 is used is used to detect fractures in metal, and phosphorus-32 is used to trace element flow in plants.

Describe how neutron scattering is used as a probe by referring to the prop- erties of neutrons

In the same way that an electron microscope uses electrons to probe materials, neutrons too can be used in microscopes. However, unlike electrons, neutrons do not carry charge and are therefore not affected by the nuclei of atoms which would deflect electrons. They are therefore extremely useful for imaging crystal structures, as well as substances containing light atoms such as hydrogen. While electrons do pass through crystals and diffract, they are deflected by charges in the crystal, resulting in errors. Neutrons penetrate crystals very effectively, allowing for a clearer and more accurate interference pattern to be produced. According to the de Broglie equation, neutrons have a wavelength shorter that light, similar to the wavelength of an electron.

Remember- Neutrons can be used as effective probes because they have wavelengths similar to electrons. However, because they are not charged they can image objects where charge interferes with electrons.

Identify ways in which physicists continue to develop their understanding of matter, using accelerators as a probe to investigate the structure of matter

Physicists now develop their understanding of matter by examining the component of atoms to better understand them. This necessitates separating the atom into its components, requiring large inputs of energy and sophisticated equipment that was previously unavailable. However, modern particle accelerators are used to break atoms into their components, which are then examined, developing our understanding of matter.

All particle accelerators use magnetic fields to accelerate charged atoms or particles to very high velocities. The three most common types are the linear accelerator, the cyclotron and the synchrotron. A linear accelerator is simply a very long track down which an atom is propelled. It is simple, but its main constraint is size, and energy input is dependant on the length of the accelerator. A cyclotron uses high-frequency AC current to generate a magnetic field that causes the electron to accelerate in a spiral. This reduces the size of the reactor, while at the same time using relatively simple equipment. A synchrotron is a complete circle in which a particle travels. While the particle can be accelerated indefinitely, powerful computers are required to manipulate the magnetic field in the synchrotron in order to propel the particle. Particle accelerators also usually have the ability to generate collisions between high-energy accelerated particles, allowing scientists to examine the properties of matter from the collision. So through using particle accelerators, scientists are able to develop their understanding of matter.

Remember- Scientists are now trying to understand the components of atoms. Accelerators provide the high energies required to break atoms into their components.

Discuss the key features and components of the standard model of matter, including quarks and leptons

The standard model of matter is a theory that states all matter is composed of small elementary particles that exist by themselves or group together to form subatomic particles and to transmit force (because under quantum theory, forces that result due to a field are caused by particles travelling between the objects). There are broadly 3 types of particles. Bosons are force-carrying particles, examples of which include photons that carry electric and magnetic force, gluons that carry the strong nuclear force, and gravitons that cause gravity. Leptons are single elementary particles that exist by themselves and are not affected by the strong nuclear force. They include electrons, muons and taus as well as their neutrino subsidiaries (the electron, muon and tau neutrinos respectively). Quarks are the building blocks of hadrons, which are groups of quarks.

There are 6 types of quarks- up, down, top, bottom, strange and charm, each with different properties. Baryons are groups with 3 quarks, such as protons and neutrons, while mesons are pairings of a quark and an antiquark. Because quarks each have a half-integer spin value (spin being one of Pauli’s quantum numbers), combining two gives a whole integer, while combining three gives a half-integer. Therefore, baryons have half-integer spin values while mesons have whole integer spin values. Thus baryons are also fermions, as fermions are particles that have half-integer spins and therefore obey Pauli’s exclusion principle. Bosons are not fermions.

Figure 2