Upcoming:

1. Upcoming: • Quiz Friday, Nov 3 on EFP chapter 11&12 • Course evaluations: • October 29 through November 20. • Go to wku.evaluationkit.com and use your WKU NetID and password. • Early access to grades and prizes for completing all course evaluations

2. Nuclear reactor • In a nuclear power plant, the energy to heat the water to create steam to drive the turbine is provided by the fission of uranium, rather than the burning of coal. • Fuel is 3% 235U and 97% 238U. 235U is an isotope of 238U. The chain reaction will only occur in the 235U, but naturally occurring uranium has both present in it. • The neutrons coming from a fission reaction have an energy of 2Mev. They are too energetic to sustain a nuclear reaction in 235U. • Need to slow them down to energies on the order of 10-2 so they can sustain fission in the 235U

3. Slowing the neutrons down • A moderator is used to slow down the neutrons and cause them to lose energy • The moderator could be water or graphite • The lower energy neutrons are called thermal neutrons • Some of the neutrons will be absorbed by 235U instead of causing a fission reaction or by 238U and resulting in the emission of a gamma ray in both cases. • Absorption of a neutron by 238U can result in the creation of 239Pu which is also fissionable

4. Creating Plutonium • So: 238U captures a neutron creating 239U • 239U undergoes a beta decay (a neutron is converted to a proton and an electron) with a half life of 24 minutes and becomes 239Np (Neptunium) • 239Np then beta decays with a half life of 2.3 days into 239Pu. • 239Pu has a half life of 24,000 years • 239Pu can also undergo fission by the slow neutrons in the core, with an even higher probability • So as it builds up in the core, is contributes to the fission reaction

5. Breeder reactor • A reactor designed to produce more fuel (usually 239Pu ) than it consumes. • 239Pu does not occur naturally, and it is more fissile than 235U. • Leads to the possibility of reactors that can create their own fuel, and only need limited mounts of naturally occurring uranium to operate. • Also leads to the danger of countries creating additional nuclear fuels for weapons development • Caution-reactor must be designed to produce weapons grade plutonium, jut because someone has a nuclear reactor does not mean they create weapons grade plutonium

6. Reactor design • PWR – pressurized water reactor • Core – where the action is. Fuel assembly is kept in here (fuel is usually in the form of fuel rods) • Fuel rods are surrounded by the water which acts as the moderator. This water is kept under high pressure so it never boils-it heats a seconds water source which turns into steam • Control rods are slid in and out from the top to control the fission rate-in an emergency they can be dropped completely into the reactor core, quenching the fission • Once the steam is generated, this works just like a fossil fuel power plant • Can run without refueling for up to 15 years if the initial fuel is highly enriched • Used in submarines and commercial power systems

7. Reactor design • BWR –Boiling water reactor • Core – where the action is. Fuel assembly is kept in here (fuel is usually in the form of fuel rods) • Fuel rods are surrounded by the water which acts as the moderator and the source of steam • Control rods are slid in and out from the bottom to control the fission rate-in an emergency they can be dropped completely into the reactor core, quenching the fission. Also, boron can be added to the water which also efficiently absorbs neutron • Once the steam is generated, this works just like a fossil fuel power plant

8. Fuel Cycle • Fuel rods typically stay in a reactor about 3 years • When they are removed, they are thermally and radioactively hot • To thermally cool them they are put in a cooling pond. • Initial idea was that they would stay in the cooling pond for 150 days, then be transferred to a facility which would reprocess the uranium and plutonium for future use.

9. Nuclear waste disposal • This idea ran into problems. • Fear that the plutonium would be easily available for weapons use halted reprocessing efforts in 1977 • Note that it is very difficult to extract weapons grade plutonium from spent fuel rods • Plan is now to bury the waste deep underground, in a place called Yucca Mountain, Nevada

10. Nuclear waste • The spent fuel rods are radioactive • Radioactivity is measured in curies • A curie is 3.7x1010 decays per second • A 1000 MW reactor would have 70 megacuries(MCI) of radioactive waste once it was shut down • After 10 years, this has decayed to 14 MCi • After 100 years, it is 1.4MCi • After 100,000 years it is 2000 Ci

11. Yucca Mountain

13. Transportation scenarios

14. Transportation scenarios

15. What can go wrong? • Nuclear power plants cannot explode like a nuclear bomb. • A bomb needs a critical mass in a confiuration which is not present in the reactor core. • Even a deliberate act of sabotage or terrorism could not cause such an explosion. • The worst that can happen is a core melt down. • 2 classes of accidents-Criticality and Loss of Coolant (LOCA) accidents

16. Criticality accident • If the control rods were removed and/or the control systems failed, a runaway reaction would occur. • The tremendous heat produced would melt the containment system and the reactor core would sink into the Earth • Radioactive material would enter the ground and be released as steam (a radioactive cloud) into the air • The area around the reactor would be highly contaminated with radioactivity • The cloud could travel for hundreds or even thousands of miles, and could spread dangerous levels of radioactivity around the world.

17. Loss of coolant accident • After a reactor is shut down, it is still hot enough to experience a core melt down if cooling system fails. • Emergency coolant systems are in place to prevent this • Big part of reactor design is the prevention of such accidents

18. Probability • To determine the likelyhood that such an accident would occur something called an event tree is constructed. • This determines the consequences of a particular event occurring • Each component (pump, valves etc) has a failure probability assigned to it • Bottom line-most recent studies indicate that for all 104 reactors operating the US, over their 30 year operating lifetime, there is a 1% probability of a large release of radioactivity