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Note: This is text from a New Zealand Anti-Nuke Web Page, that doesn't seem to exist any longer. It was not copyrighted, and is reproduced here because it is excellent writing and informative.
p.83, Uranium Enrichment & Nuclear Weapon Proliferation, SIPRI, A. Krass et al.
The collapsed core is then irradiated with external neutrons to start fission. The number of nuclei fissioning during later generations is proportional to how many nuclei you have fissioning initially. This depends on the initial neutron flux.
At the centre of the core you can have solid Li6D. Some time before detonation you could also inject the core with T gas, from some exterior device. The idea is to achieve a fusion reaction in the central pit.
During implosion, the core gains a high enough temperature and pressure for fusion reactions to occur. The synergy between fusion and fission reactions greatly improves chain reaction efficiency. Neutrons released from fusion have much higher energies than those released via fission processes. When these ultra fast neutrons hit U-235 nuclei, the splitting releases more neutrons than when U-235 are hit with fission neutrons. The chain reaction multiplies rapidly, leading to higher temperatures, and more fusion. The ultimate result is more U-235 splitting than you would have without the boost.
The shell may be made of U-235 or Pu-239. The first Chinese test employed a U-235 shell.
Another technique is to introduce a tamper of some sort, which is another shell surrounding the fissionable material. This shell is usually made of Beryllium or U-238. It's purpose is to reflect escaping neutrons back into the reaction. This has the effect of reducing the amount of fissionable material needed for a reaction.
The Fat Man bomb used a hollow core, but present day weapons might employ a solid (subcritical) core of fissile material. The density increase in a solid core reduces the amount of critical mass required for a chain reaction. This is because the reaction cross section is substantially increased.
To increase the impact of the tamper on the fissionable shell, you can levitate the shell. You leave an air gap between the two materials. This has the effect of increasing momentum transfer, and hence facilitating implosion. According to Ted Taylor, in John McPhee's book (see Refs):
[1] Uranium Detonator
-----------------
Comprised of 2 parts. Larger mass is spherical and concave.
Smaller mass is precisely the size and shape of the `missing'
section of the larger mass. Upon detonation of conventional
explosive, the smaller mass is violently injected and welded
to the larger mass. Supercritical mass is reached, chain
reaction follows in one millionth of a second.
[2] Plutonium Detonator
-------------------
Comprised of 32 individual 45-degree pie-shaped sections of
Plutonium surrounding a Beryllium/Polonium mixture. These 32
sections together form a sphere. All of these sections must
have the precisely equal mass (and shape) of the others. The
shape of the detonator resembles a soccerball. Upon detonation
of conventional explosives, all 32 sections must merge with the
B/P mixture within 1 ten-millionths of a second.
____________________________________________________________________________
- Diagram -
-------------
____________________________________________________________________________
|
[Uranium Detonator] | [Plutonium Detonator]
______________________________________|_____________________________________
_____ |
| :| | . [2] .
| :| | . ~ \_/ ~ .
| [2]:| | .. . ..
| :| | [2]| . |[2]
| .:| | . ~~~ . . . ~~~ .
`...::' | . . . . .
_ ~~~ _ | . . ~ . .
. `| |':.. | [2]\. . . . [1] . . . ./[2]
. | | `:::. | ./ . ~~~ . \.
| | `::: | . . : . .
. | | :::: | . . . . .
| [1] | ::|:: | . ___ . ___ .
. `. .' ,::||: | [2]| . |[2]
~~~ ::|||: | .' _ `.
.. [2] .::|||:' | . / \ .
::... ..::||||:' | ~ -[2]- ~
:::::::::::::||||::' |
``::::||||||||:'' |
``:::::'' |
|
|
|
|
[1] = Collision Point | [1] = Collision Point
[2] - Uranium Section(s) | [2] = Plutonium Section(s)
|
|
______________________________________|_____________________________________
- Diagram for Plutonium Bomb -
--------------------------------
[Gravity Bomb - Implosion Model]
--------------------------------
-> Cutaway Sections Visible <-
/\
/ \ <---------------------------[1]
/ \
_________________/______\_________________
| : ||: ~ ~ : |
[2]-------> | : ||: : |
| : ||: : |
| : ||: : |
| : ||: : |
| : ||: : |
| : ||: : |
| : ||: : |
| : ||: : |
| : ||: : |
| : ||: : |
| : ||: : |
| :______||:_____________________________: |
|/_______||/______________________________\|
\ ~\ | : |:| /
\ |\ | : |:| /
\ | \ | :__________|:| /
\ |:_\ | :__________\:| /
\ |___\ |______________| /
\ | \ |~ \ /
\|_______\|_________________\_/
|_____________________________|
/ \
/ \
/ \
/ _______________ \
/ ___/ \___ \
/____ __/ \__ ____\
[3]_______________________________ \ ___|
/ __/ \ \__ \
/ / \/ \ \
/ / ___________ \ \
/ / __/___________\__ \ \
./ /__ ___ /=================\ ___ __\ \.
[4]-------> ___||___|====|[[[[[|||||||]]]]]|====|___||___ <------[4]
/ / |=o=o=o=o=o=o=o=o=| <-------------------[5]
.' / \_______ _______/ \ `.
: |___ |*| ___| :
.' | \_________________ |*| _________________/ | `.
: | ___________ ___ \ |*| / ___ ___________ | :
: |__/ \ / \_\\*//_/ \ / \__| :
: |______________:|:____:: **::****:|:********\ <---------[6]
.' /:|||||||||||||'`|;..:::::::::::..;|'`|||||||*|||||:\ `.
[7]----------> ||||||' .:::;~|~~~___~~~|~;:::. `|||||*|| <-------[7]
: |:|||||||||' .::'\ ..:::::::::::.. /`::. `|||*|||||:| :
: |:|||||||' .::' .:::''~~ ~~``:::. `::. `|\***\|:| :
: |:|||||' .::\ .::''\ | [9] | /``::: /::. `|||*|:| :
[8]------------>::' .::' \|_________|/ `::: `::. `|* <-----[6]
`. \:||' .::' ::'\ [9] . . . [9] /::: `::. *|:/ .'
: \:' :::'.::' \ . . / `::.`::: *:/ :
: | .::'.::'____\ [10] . [10] /____`::.`::.*| :
: | :::~::: | . . . | :::~:::*| :
: | ::: :: [9] | . . ..:.. . . | [9] :: :::*| :
: \ ::: :: | . :\_____________________________[11]
`. \`:: ::: ____| . . . |____ ::: ::'/ .'
: \:;~`::. / . [10] [10] . \ .::'~::/ :
`. \:. `::. / . . . \ .::' .:/ .'
: \:. `:::/ [9] _________ [9] \:::' .:/ :
`. \::. `:::. /| |\ .:::' .::/ .'
: ~~\:/ `:::./ | [9] | \.:::' \:/~~ :
`:=========\::. `::::... ...::::' .::/=========:'
`: ~\::./ ```:::::::::''' \.::/~ :'
`. ~~~~~~\| ~~~ |/~~~~~~ .'
`. \:::...:::/ .'
`. ~~~~~~~~~ .'
`. .'
`:. .:'
`::. .::'
`::.. ..::'
`:::.. ..:::'
`::::::... ..::::::'
[12]------------------> `:____:::::::::::____:' <-----------------[12]
```::::_____::::'''
~~~~~
- Diagram Outline -
---------------------
[1] - Tail Cone
[2] - Stabilizing Tail Fins
[3] - Air Pressure Detonator
[4] - Air Inlet Tube(s)
[5] - Altimeter/Pressure Sensors
[6] - Electronic Conduits & Fusing Circuits
[7] - Lead Shield Container
[8] - Neutron Deflector (U-238)
[9] - Conventional Explosive Charge(s)
[10] - Plutonium (Pu-239)
[11] - Receptacle for Beryllium/Polonium mixture
to facilitate atomic detonation reaction.
[12] - Fuses (inserted to arm bomb)