How do external neutron initiators work in boosted fission bombs? Where are they placed to initiate fission reactions? Are they sent through tube and how?

You get an alpha emitter (such as a suitable isotope of Polonium) and a material that emits neutrons (such as Be-9) when struck by an alpha. The products are C-12 and a neutron. A high-activity source would have a short half-life and wouldn’t last long

You’d place them such that they came together when you’d set the bomb off; details would depend on the bomb design.

IIRC Frederick Forsyth describes this in one of his books (The Fourth Protocol), leaving out some details, of course. 

 

Can you mention how a boosted fission bomb can be designed to use external neutron initiator.

14 hours ago, Brainee said:

Can you mention how a boosted fission bomb can be designed to use external neutron initiator.

It's injected just before implosion. Tritium has a half-life of only 12 years, so it needs to be accessible for servicing.

How do you inject the nuetrons, the deuterium and tritium are in way? I mean in boosted fission bomb.

4 hours ago, Brainee said:

How do you inject the nuetrons, the deuterium and tritium are in way? I mean in boosted fission bomb.

This sounds like an oddly specific question. Are you trying to design and build a boosted fission bomb yourself by any chance ?

If so, a word of warning - You will find  it very hard indeed to acquire quantities of Tritium in particular - without attracting the sort of attention from the authorities that trying to buy large quantities of green ink and certain types of paper will usually attract in the USA.

Some years ago I heard a story about a physics student in Britain who embarked on a similar 'proof of concept' project and got arrested at the point where he  tried to obtain the Tritium he required. His design was sent off for expert assessment which indicated that it would have been a  'fizzle' - but a fizzle of around 1 kiloton yield ! The guy allegedly wound up working at the top secret Aldermaston atomic weapons R&D station.