When we want to tight the reactor bolts , is it necessary to have hot bolting?
However for bolt tighting , how much torque we need in the cold condition ot hot condition?
bolt tighting torque value will depends on size of bolt & nuts your using regardless of hot or cold condition...
manufacturer has its value on data sheet or else you can refer to client specification..
This topic has been covered multiple times - please use the search.
My recommendation is that you should hire an expert in bolted flange joints. This is a specialty that takes decades to learn and hone - things that cannot be had for free anonymously over the internet. This recommendation doesn't necessarily mean that you should hire a bolting contractor, though.
What kind of reactor?
What is the reactor fabricator and manufactor requirements? HINT: They know how they designed the bolts and pressure vessel head!