Chapter 16.1 Announcements

The rest of the semester:

- Thursday, 4-22, Nuclear Weapons & Reactors

- Tuesday, 4-27, catch-up, review, some fun, last day of class

Final:

Tuesday, May 4, 2:00 pm – 5:00 pm

Practice test will be put on web, Final is CUMULATIVE!!

- Reading Assignment: Get ready for final. Bring questions.

- Go through old test and practice test.

- Go through homework and lecture notes.

- All grades will continue to be posted at: http://www.wfu.edu/~gutholdm/Physics110/phy110.htm

- Listed by last four digits of student ID

Chapter 16.1 Nuclear reactions and nuclear bombs

- chain reaction - atoms- atoms consist of a nucleus and electrons- chemical reaction - nuclear reaction- fission - fusion

Demos and Objects Concepts

Atomic force microscope image of gold surface

The building blocks of matter

• All matter consists of atoms (greek: atomos = not sliceable)

• All atoms consist of a nucleus surrounded by electrons

• Nuclei consist of protons and neutrons. The sum of neutrons and protons in the nucleus of a particular element is called the atomic number of the element.

Solids and molecules:

An arrangement of atoms

An atom: Nucleus surrounded by electrons

Table salt (75x)

10,000,000x

10,000x

Table salt

1m : (meter) child

10-1m (decimeter, dm): hand

10-2m (centimeter, cm): penny

10-3m (millimeter, mm): needle, width

10-4m (100 m): hair, width

10-5m (10 m): human cell

10-6m (1micrometer, m): bacterial cell

10-7m (100 nm): chromosome width (optics limit)

Size scales: Powers of ten

10-8m (10 nm): simple viruses

10-9m (1 nanometer, nm): DNA (radius)

10-10m (1 Ångstrom 1Å): atom

Limit of electron and other ultrahigh resolution microscopes

10-11m (10 picometer, pm): inner electrons in atom

10-12m (1 picometer, 1 pm): nothing

10-13m (100 femtometer): nothing

10-14m (10 femtometer): nucleus

10-15m (1 femtometer 1 fm): proton and neutron

Size scales: Powers of ten, continued

Atoms of a gold surface

THE PERIODIC TABLE OF ELEMENTS

Atomic number = # of protons in nucleus

# of nucleons = # of protons + # of neutrons

The number of neutrons can vary slightly for a given element (isotopes)

Atomic weight is equal to average number of nucleons in nucleus

Chemical reactions

One molecule or element reacts with another one.

Get a rearrangement (different combination) of elements.

No new elements are created (C, H, O before and C, H, O after)

CH4 + 2O2 CO2 + 2H2O + some energy

Nuclear reactions

• Very different from chemical reactions.

• In these reactions (fusion and fission) the chemical element changes to a different element.

• Fusion: The nuclei of lighter elements (like hydrogen or helium) combine to form a heavier element (e.g.: H + H He). That’s what’s going on in the sun.

• Fission: The nuclei of heavy elements split to form new, lighter elements.

• These reactions emit an enormous amount of energy (~ 10 million times more than chemical reaction, per kilogram)

• These reactions can be very difficult to start!!

Natural (radioactive) decay (fission)

Neutron-induced fission

• Many heavy elements (eg. Uranium) decay (slowly) into lighter elements (natural decay)

• However, this fission can also be induced by an incoming neutron.

• Fission reaction release a lot of energy.

• Fission often creates new neutrons!!

Fission and chain reaction

Fission releases neutrons …

… these neutrons cause new fission reactions in surrounding Uranium …

… creating more neutrons …

… chain reaction …. huge explosion

http://lectureonline.cl.msu.edu/~mmp/applist/chain/chain.htm

Conditions for a chain reaction to occur (nuclear bomb):

1. Need a source of neutrons to trigger chain reaction.

2. Bomb material needs to be fissionable (splits when hit by a neutron.

3. Each fission has to produce more neutrons.

4. Each fission needs to induce more than one subsequent fission ((super-)critical mass = 60 kg; 7 inch sphere for 235U).

235U (Uranium isotope with 92 protons and 143 neutrons) works!! Releases 2.5 neutrons when fissioning.

238U (Uranium isotope with 92 protons and 146 neutrons), which naturally occurs more abundantly (99.28%) does not work.

For bomb: Need to enrich 235U (very difficult, fortunately).

Assembly of supercritical mass

(to initiate 235U bomb)

Hiroshima, Aug. 6, 1945“Little boy”

Two-thirds of Hiroshima was destroyed. Within three miles of the explosion, 60,000 of the 90,000 buildings were demolished. Clay roof tiles had melted together. Shadows had imprinted on buildings and other hard surfaces. Metal and stone had melted. Hiroshima's population has been estimated at 350,000; approximately 70,000 died immediately from the explosion and another 70,000 died from radiation within five years.

At the time this photo was made, smoke billowed 20,000 feet above Hiroshima while smoke from the burst of the first atomic bomb had spread over 10,000 feet on the target at the base of the rising column.

239Pu (Plutonium) bomb

Supercritical mass (density) of Plutonium core is achieved when explosion crushes 239Pu core.

Nagasaki, Aug. 9, 1945“Fat man”

A dense column of smoke rises more than 60,000 feet into the air over the Japanese port of Nagasaki, the result of an atomic bomb, the second ever used in warfare, dropped on the industrial center August 8, 1945, from a U.S. B-29 Superfortress.

Approximately 40 percent of Nagasaki was destroyed. Though this atomic bomb was considered much stronger than the one exploded over Hiroshima, the terrain of Nagasaki prevented the bomb to do as much damage. Yet the decimation was still enormous. With a population of 270,000, approximately 70,000 people died by the end of the year.

The fusion or Hydrogen bomb

• A fusion (e.g. H + H He (two hydrogen atoms make a Helium atom)) is extremely difficult to start,because it requires extremely high temperatures (like inside sun).

• Fusion bombs release even more energy than fission bombs

• Use a fission bomb to create 100 million degrees

• This will trigger the fusion bomb

• Controlled nuclear reactions (add material that slows down reaction) are used in nuclear reactors to create energy.

• All reactors thus far are fission reactors. (problem: radioactive waste)

• Fusion reactions can not be controlled yet (just bomb).

• Active research is underway to control fusion reactions (How to initiate reaction, how to hold material once reaction is going?)

Civil use of nuclear reactions