A public procurement notice sets the tender participation fee at 70,000 levs ($50,197/35,785 euro), or up to 3.0% of the contract value, business daily Dnevnik (www.dnevnik.bg) reported.
The deadline for submitting bids is May 9.
The nuclear plant seeks to extend the two units' life by 15 to 20 years, Dnevnik added.
The operational licences of the 1,000-megawatt (MW) Units 5 and 6 expire in 2017 and 2019, respectively. The plant launched Unit 5 in 1988 and Unit 6 five years later.