Fission

Physics 12 Adv

Comprehension Check

1. Two deuterium nuclei fuse to form a tritium nuclei and a proton. How much energy is liberated?

2. A deuterium nucleus fuses with a nitrogen-14 nucleus to produce boron-10 and lithium-6. If the nuclear masses are 2.014102u, 14.00307u, 10.01294u and 6.015122u respectively; would this release energy?

Comprehension Check

1. 4.03MeV

2. No; the final mass is larger than the initial mass so it requires energy be converted into mass

Fission

Nuclear fission is a process in which an unstable nucleus splits into two fragments of comparable size

Fission was discovered in 1938 through experiements of Otto Hahn and Fritz Strassman in Germany based upon earlier work by Fermi

They bombarded uranium with neutrons and found an isotope of barium and krypton

Fission Example

When uranium-235 absorbs a neutron, it becomes uranium-236 (excited state)

One of the possible fission decay processes is uranium-236 decaying into barium-141, krypton-92 and three neutrons.

Determine the energy released in this process if the masses are 235.043930u, 140.914411u and 91.926156u respectively

Fission Example

MeVE

JxE

um

nKrBanU

172

1077653.2

186033.0

3

11

10

8936

14456

10

23592

Fission Example 2

However, another possible fission decay process for uranium involves uranium-235 absorbing a neutron but in this case it decays into xenon-140, strontium-94 and two neutrons.

Determine the energy released from this fission process if the mass of the products are 139.921641u and 93.915361u respectively

Fission Example 2

MeVE

JxE

um

nSrXenU

184

10959061.2

198263.0

2

11

10

9438

14054

10

23592

Nuclear Chain Reactions

Nuclear reactors depend on nuclear chain reactions in order to produce energy

Once started, these reactions will continue until the fuel has been spent or the reaction is stopped

A common reaction is the fission of uranium-235 which releases 3 neutrons; these 3 neutrons can then release 9 neutrons; the 9 can release 27; and so on

Fuel for Fission

The two nuclei that are most likely to undergo fission are: Plutonium-239 Uranium-235

Plutonium is a manmade element while uranium occurs in nature

However, U-235 exits is very small percentages in nature (~.7%)

Moderators

In order for a nucleus to undergo fission, it needs to absorb a slow neutron

Unfortunately, the neutrons released from fission are fast neutrons

Before the chain reaction can continue, these fast neutrons must become slow neutrons

Moderators A moderator is a substance that will slow

a fast neutron A good moderator will slow the neutrons

and offer a very low chance of absorption Some moderators included

(collions/absorption) Water (18/560) Heavy Water (25/1) Beryllium (90/16) Graphite (114/6)

Control Rods

A nuclear chain reaction can be maintained by either enriching uranium or by using heavy water as a moderator

However, the rate of the chain reaction must be controllable

Control rods (made of cadmium or boron) are used to absorb neutrons to slow the reaction as required

Coolants

There is a primary coolant that transfers heat from the reactor to a boiler

This coolant must have a high boiling point, not be corrosive, not absorb neutrons, be chemically stable, have god heat transfer properties and pump easily

Some common coolants include water, heavy water, some organic compounds and even some gases

Coolants

The secondary coolant never comes into contact with the primary coolant as they are both in contained systems

The heat from the primary coolant is transferred to the secondary coolant which is used to drive a turbine

Nuclear Waste

Reactor fuels (uranium and plutonium) are radioactive but emit alpha particles

Fission products are highly unstable and as a result release large amounts of radiation

A nuclear reactor will mange all nuclear waste on site; normally waste is stored under water for a year and then is transferred to concrete containment units

CANDU

CANadian Deuterium Uranium (CANDU) is one of the most respected and efficient nuclear reactors in the world

Unlike many reactors, it does not require enriched uranium and does not produce weapons grade plutonium

It uses heavy water as a moderator and primary coolant

Fuel Bundles The CANDU reactor uses uranium oxide

pellets that are inserted into tubes and made into fuel bundles

Each bundle contains about 22kg of UO2 and will remain in the reactor for about 17 months

The reactor can be refuelled while in operation

Each fuel bundle provides as much thermal energy as 400t of coal

Fusion Reactors

Fusion reactors would be able to release more energy than a fission reactor and do not result in radioactive water produts

However, at the present time we cannot sustain a fusion reaction for long enough to produce more energy than is required to start the reaction

Work is ongoing at Tokamak reactors in order to solve this issue

Energy in one gram of uranium

If the natural abundance of uranium-235 is 0.75% in a sample of uranium, assuming that fission is 100% complete, how much energy can be released from the fission of one gram of uranium (the remaining mass, uranium-239 does not undergo fission)

Problems

Page 925 1-3

Page 933 1-7

Page 934 1-21