19.6 Nuclear energy

• Fission=splitting a heavy nucleus into 2 with smaller mass numbers. Causing an unstable nucleus.

• Fusion=combining 2 light nuclei to form a heavier nucleus. (more stable nucleus)

n4MoBaCf 10

10642

14256

25298

nHeHH 10

42

31

21

19.7 Fission

– In some cases a nucleus can be induced to undergo fission by bombardment with neutrons.

Write an equation for the following:The Uranium isotope with 143 neutrons is

bombarded with a neutron. It produces the Kr isotope with 56 neutrons, the Ba isotope with 85 neutrons and some additional isolated neutrons.

nn 10

14156

9236

10

23592 3 Ba Kr U

Fission of U-235 causes an unstable nucleus.

Fission is a chain reaction. It is sustained through the neutrons that

are produced.

• A chain reaction is a self-sustaining series of nuclear fissions caused by the absorption of neutrons released from previous nuclear fissions.

• Critical Mass – the mass of fissionable material needed to produce a self sustaining chain reaction.

19.8 Nuclear reactors

• Nuclear plants generate electricity by producing near to boiling water using the heat of nuclear fission reactions. The resulting steam spins the turbines of giant generators, producing electrical energy.

Schematic of the reactor core.

1. Fuel rods – uranium; fission reaction takes place

2. Moderator – core; contains substance to slow down neutrons so they react with rods

3. Control rods – absorb neutrons to control reaction

4. Coolant – extracts the heat; after steam cools, it returns

Diagram of a nuclear power plant.

Fuel rods and Nuclear Power

• U-238 – radioactive, but not fissionable• U-235 – fissionable, but only 0.7% of

natural uranium.• 3% U-235 needed to for fuel rods.• 90% U-235 in bombs

Breed fissionable material. Used in Europe but not in the US.

• Breeder reactors – Take nonfissionable material (U-238), bombard with n’s U-239 Np-239 Pu-239which is fissionable.

• Controversial – Pu is more easily made into bombs but it is also very toxic/poisonous and flames in the air.