Hi,
What I don't understand is : where are these BCH really !???
Exactly where you sent them.
If you try a transfert a transfer from a BCH adress to a BTC one, why do u have a "success" notice transfer from Kraken !???
Because they have successfully sent them to the address you asked them to send to.
I really need more information to understand blockchains...If the transfer occured, then these BCH must be somewhere, no ??
Yes. They are exactly where they were sent to.
These BCH are not lost for everyone...
That depends on what you mean by "lost".
If the recipient refuses to help you, then you're going to have a difficult time accessing that money.
It's too easy for the receving exchange to say I can't do anything...
Yep. You can try and use courts and law to force the recipient to do something. That's likely to be expensive and might not be successful. Are you willing to risk that cost?