Unit 1 Physics Detailed Study 3.3 Chapter 12.3: Nuclear Fissions Reactors

Thermal nuclear reactors ∗ The production of electrical power from fission reactions has been undertaken since the 1950’s. ∗ To produce power from a fission reaction, there are a few barriers that need to be overcome. ∗ The first of which is that fission of Uranium-235 requires the absorption of a slow moving or thermal neutron. However, in a fission reaction neutrons are released with very high speeds, so the neutrons need to be slowed. ∗ Next, each reaction releases ~2.5 neutrons, if uncontrolled this can lead to a chain reaction and cause an explosion. ∗ Finally, we need to capture the heat generated somehow and convert that into electric energy. Section 12.3 Nuclear Fission Reactors

Thermal nuclear reactors ∗ Thermal nuclear reactors come in a variety of designs, however, they all have three main features. ∗ Fuel rods: These are aluminium tube around 3-5 meters long that contain pellets of enriched Uranium-235 (to ~3%) inside. ∗ A fuel rod will generally last for around 4 years before the fuel inside is depleted. ∗ The Moderator: this is a material that is placed in the nuclear reactor to slow the speed of the neutrons from fast moving to slower, thermal neutrons. ∗ Common moderators are water, heavy water (contains Deuterium 1 2 H), carbon dioxide and graphite. Moderators should not absorb a significant number of neutrons, as this will slow the reaction. ∗ Control Rods: as we learnt, when a chain reaction is uncontrolled an explosion will result. The job of control rods is to absorb neutrons so that the reaction is sustained but does not get our of control. These are commonly made of Cadmium and Boron steel. Section 12.3 Nuclear Fission Reactors

Fast breeder reactors ∗ In a thermal reactor, only around 40% of the thermal neutrons are involved in a fission reaction, around ~35% of these remaining neutrons are absorbed by the Uranium-238 present in the fuel, while the rest are lost in the process. ∗ The absorption of neutrons by Uranium-238 creates the unstable Uranium-239 which decays into Plutonium-239 within a couple of days. ∗ So a highly fissile material is a by-product of these reactions. This Plutonium is extracted from spent fuel rods and used in another type reactor, known as the fast breeder reactor. ∗ A fast breeder reactor, is similar to a thermal reactor, except it does not require a moderator, as the reaction requires fast moving neutrons. ∗ The fuel rods contain Plutonium-239 in the core, blanketed by Uranium-238. The reaction produces neutrons, some of which continue the reaction, while others are absorbed by the Uranium-238 which will decay into Plutonium-239, thus breeding plutonium. Section 12.3 Nuclear Fission Reactors

Disposing of nuclear waste- the nuclear fuel cycle ∗ Nuclear reactors are capable of creating loads of energy, although they are also create a lot of other, unstable, radioactive by-products, the disposal of which is of big concern. ∗ Nuclear waste is categorised as either low, intermediate or high-level waste. ∗ Low-level, generally waste from hospitals and labs. This waste is generally incinerated and buried if solid, or just released into the environment if liquid. ∗ Intermediate-level, such as chemical sludge’s and contaminated materials from decommissioned reactors, these are solidified in bitumen or concrete and buried underground. Section 12.3 Nuclear Fission Reactors

Disposing of nuclear waste- the nuclear fuel cycle ∗ High-level, generally spent fuel rods, are treated differently depending on the country of origin. Spent fuel rods from the USA and Canada, are used once and stored permanently in cooling ponds. However, France, Japan and the UK have adopted a method in which they reprocess the spent fuel rods. ∗ In doing so, Uranium and Plutonium are filtered out to be enriched/reused, while the other products are placed in stainless steel drums in reinforced concrete chambers. Section 12.3 Nuclear Fission Reactors