Fixing the leak in the central heating boiler

[capinss]

I have rented an old building which has an old central heating boiler which leaks.
I have located the approximate area which leaks, but its not reachable by any means. The boiler is welded one-piece box.

So I am guessing are there any substances available in the market to pour in the circulating water system to stop the leak?
I have heard that people use mustard powder, but i'm afraid i'll need something more serious.
The leaking in the boiler is like 1 pill every second... is that even possible to fix it with adding some substances to the water?

 

[BingoBongo]

when you say boiler do you mean copper cylinder?

 

[capinss]

No, I mean the boiler where you put the wood in, which heats the water in the radiators.