can bootcamp be used with Windows install disk on an external drive (internal is broken) ?

No. Windows will neither install on nor boot from an external drive.

With Boot Camp, you must install Windows onto in internal HDD (or SSD). If you are referring that your internal Optical drive is broken, you might be able to install Windows from an external drive, but you might not. I have heard varying levels of success with that. I'm not sure what the official stance is, but I do know that if youe machine shipped with an internal optical drive, then you must use that to install Windows using Boot Camp and not swap it out for a "data doubler" and try to install from an external optical drive.

Generally it is not possible to install Windows from an external DVD drive if the machine has an internal (whether working or not)

The answer is to fix the internal.