one tun decoction?

[BrewWhat]

I'm doing up an Altbeir with a Munich malt  base. Is there a way to decoct in a cooler type tun? Is there a way to do the decoctions and then do a Mash out in a plastic tun or do I have to be able to heat the tun?

[bonjour]

Yes,

You remove a thick portion of the mash and bring the removed portion to a boil, stopping at sacrification temp.  You get different impacts depending on how long you boil the grains.

Add the decoction back into the mashtun to to raise temp.

Pull the liquid only to bring to a boil for a mashout.

Fred

[UselessBrewing]

Agreed
An alternative would be to pull enough liquid half way through and boil this down, add it back as a mash out. Randy Mosher's "Radical Brewing" talks about this in more detail.
Cheers
Preston