What should be the atmosphere inside any mill to grind explosive material?

In an atmosphere which is inert for that explosive.

All other answers are some what correct, But not always and entirely. I am writing this answer with3 years experience of working with explosives during my PhD project.

For starters, It all depends on What kind of Explosive Material you have. If it is oxygen positive, Then inert/nitrogen atmosphere will not do anything.

If it is hypergolic, or conductive as solution, then wet environment might not be suitable. It can even cause production loses. (or worst, agglomerations etc which could lead to voids in later processing).

if if it is extremely sensitive, then none of the above are suitable.

Let's say you are working with Ammonium nitrate, Then, Humid atmosphere (below%RH) would be good but above that, AN will start to absorb moisture leading to other issues. If you are working with CL- or Fox- or GuDN, then humid atmosphere (up to%RH) is fine as these are water insensitive materials.

In short, it is like comparing apples with oranges. So, Unless you tell about what kind of explosive material it is. It will be very hard to properly identify the safe route.

And other questions to ponder upon:

1. does it gives fumes or decompose during milling process ? what kind of gases it evolves? are they reactive?

2. What type of milling is being used ? dry or Liquid (water based or otherwise) conditions?

3. Is the system remotely controlled ? as it should be .

4. What is the pressure conditions ? How pressure release system works ?

5. What is the quantity ? if it is few grams or few kilograms ? explosives are always processed in batch conditions.

It depends on the type of explosive but normally it takes in normal environment but wet and with non-sparking material likely to be contact with the explosives.

In an inert atmosphere such as Nitrogen atmosphere

in meduim without any oxegine