Actually there are two questions.
The first one is just as the title. Does CKB adopt any method to avoid bribery 51% attack? And the second one is how CKB thinks of the problem that when network starts at the beginning, there is not so much hashpower in the network as it in the bitcoin network now, so someone can attack CKB network much more easier.
IMO, this is problem to all the PoW projects, not just for Nervos CKB. For bribery 51% attack, from this view it’s the same as someone buy a lot of hash power to take a 51% attack.
I have to admit that in the begining the hash power is small, what we could do is to do our best to have more hash power, to ensure the security of the Common Knowledge Base.
Yes, it seems that all PoW projects have the same problem. But is it inherited or just not solved yet? Nervos CKB is different with others so I think “maybe CKB has something special because it is Nervos after all!” 
If you want to know about this, Plz wait for our economic model white paper (as well as consensus paper all together). We designed to reward miners to make the whole network stable and secure in the long term.
For 51% attack, more hash power in the network means the cost of an attack will be higher, as well as the cost for bribery. (the reward might be great but it will hurt the whole system in the long term).
And they will know Nervos CKB is worth HODL 