Interest can be determined by the function of the risk and the compensation of the investor.
The risk-free rate is the rate that assumes zero default in the market which is more or less equivalent to the rates offered by the sovereign.
The PDS Group is an organized market that was formed out of the financial distress in 1997. On a normal basis, they release the Treasury Bills on which the risk-free rate is based on.
Bond valuation is a technique for determining the theoretical fair value of a particular bond.