AQA A2 Physics A Nuclear Physics Section 15 Fission

→ + Fission is where a large nucleus splits into two approximately equally sized nuclei whose total mass is less than the parent nuclei. The reduction in mass is accounted for by the emission of a finite amount of energy determined by Einstein’s equation. MAMA MBMB MCMC M A > M B + M C  E fission = (M A -(M B + M C )) c 2

Fission in terms of Binding Energy Consider a nucleus decaying under fission to two identically sized nuclei. The binding energy per nucleon of the parent nucleus is E 1 and the binding energy per nucleon of the daughter nuclei is E 2.  E = Energy released from the construction of B&C –Work required to dismantle A  E = (BE B +BE C ) –BE A → + A B C Number of nucleons = A Number of nucleons = A/2  E = (E 2 A/2+E 2 A/2) –E 1 A  E = E 2 A –E 1 A  E fission = (E 2 –E 1 )A There is a positive emission of energy if E 2 > E 1.

The Binding Energy per Nucleon Curve Only large nuclei can undergo fission because of the nature of the gradient to the right of the binding energy per nucleon curve. BE/A A Fission This is because the daughter nuclei has a higher binding energy per nucleon and hence a positive amount energy is released with the fission.

Fission Nuclei that are susceptible to fission are known as fissile. Uranium-235 is a common fissile nucleus. Fission is induced in uranium-235 if it absorbs a neutron. Uranium-236 undergoes spontaneous fission – the nucleus can split one of several ways (just two are shown below). Each fission reaction is followed by the subsequent release of several neutrons.

Fission The mass of a proton = u The mass of a neutron = u The mass of an electron = u u = x kg The following mechanism describes the fission of a uranium nucleus u u 91.91u Calculate the mass defect involved in this fission. Calculate the energy released as a result of this fission.

The released energy from fission mostly appears as the kinetic energy of the fission products and the neutrons. This kinetic energy is absorbed by the surroundings (such as a core in a nuclear reactor) and is converted into heat. The released energy for the previous reactions is shared out as follows: 83% - The kinetic energy of the fragments. 3% - The kinetic energy of the neutrons. 4% -  rays emitted when the fission takes place. 10% - the subsequent radioactive decay of the fragments after fission. Fission

Fission Chain Reaction In a sample of uranium-235, the neutrons released by fission can induce further fission by colliding and being absorbed by other urnaium-235 nuclei. This releases even more neutrons resulting in a chain reaction. Hyperlink

Fission Chain Reaction The fate of a neutron resulted from fission might be any of the following: 1. The neutron could collide with a uranium nucleus and cause fission. 2. The neutron could be captured by a stray nucleus, for example Xe. 3. The neutron could fly out of the mass of uranium without colliding with anything. The number of neutrons that go on to cause another fission is known a k.

Fission Chain Reaction Sub-critical (k < 1) If the production of neutrons from fission is decaying (there are less neutrons being produced by fission than are leaving the sample and being absorbed without inducing fission -  N/  t < 0) then eventually the chain reaction will cease and fission will end. Such a situation is known as sub-critical. Critical (k = 1) If the production of neutrons from fission is steady (there are as many neutrons being produced by fission as are leaving the sample and being absorbed without inducing fission -  N/  t = 0) then fission and energy production will continue at a steady rate. Such a situation is known as critical. Super-Critical (k>1) If the production of neutrons from fission is increasing (there are more neutrons being produced by fission than are leaving the sample and being absorbed without inducing fission -  N/  t > 0) then fission and energy production will increase. Such a situation is known as super-critical.

Generating Energy Fission is used to generate energy in nuclear reactors. Concrete Shielding Water Steam Fuel Rods: Fuel rods contain uranium oxide. They are enriched to have a greater percentage of fissile uranium-235 (0.7% → 5% ) than would naturally occur (most naturally abundant uranium is uranium-238).

Generating Energy Fission is used to generate energy in nuclear reactors. Concrete Shielding Water Steam Coolant: Coolant is a fluid (carbon dioxide/liquid sodium) pumped around the reactor core to transport the heat of the reactor to the water pipes to generate steam. A coolant must have a: large specific heat capacity, low viscosity, poor ability to absorb neutrons and low susceptibility to become radioactive.

Generating Energy Fission is used to generate energy in nuclear reactors. Concrete Shielding Water Steam Moderator: Neutrons released by fission have a high speed (2MeV). The probability of them inducing fission at this speed is small (0.1MeV). The moderator reduces the energy of the neutrons by collision so that they slow down to a speed where they have a lot greater probability (x450) of inducing fission.

Generating Energy Fission is used to generate energy in nuclear reactors. Concrete Shielding Water Steam Boron Control rods: Boron control rods maintain a constant rate of energy production. They are lowered further into the reactor to absorb neutrons when the chain reaction goes beyond critical and raised from the reactor to stop the chain reaction going sub-critical.

Shielding a Nuclear Reactor The core of a reactor is in a thick steel vessel designed to withstand high temperature and pressure. The thick steel absorbs beta radiation, some gamma radiation, and neutrons from the core. The core is in a building with very thick walls of concrete which absorb the neutrons and gamma radiation that escape from the steel vessel.

Shutting Down a Reactor in an Emergency In an emergency the reactor can be shut down by fully inserting the boron control rods into the reactor to stop fission.

Waste Products After the fuel rods are used they are much more radioactive than they were before because of the neutron rich fission products that have been formed. This radioactive waste is categorised as high-level waste. Radioactive materials with a lower activity are known as intermediate-level waste and laboratory equipment and protective clothing is categorised as low-level waste.

Storing and Handling Waste Products High-level Radioactive Waste High level waste is removed by remote control and stored in cooling ponds for a year as they continue to release heat due to radioactive decay. It is then stored in sealed steel containers and buried underground. Intermediate-level Radioactive Waste Intermediate level waste is stored in sealed drums that are encased in concrete. Low-level Radioactive Waste Low level waste is stored in sealed drums and buried underground.