MOSCOW, Aug 11 (Reuters) - It may sound like science fiction but the first nuclear-powered underwater oil tanker could soon be a reality, cruising from Russia under the Arctic ice cap. 

A Russian naval architect who designed nuclear submarines for the Soviet navy said on Tuesday his firm had already won interest in its blueprints from companies wanting to transport oil from northern Russia to Japan under the frozen polar sea. Boris Dronov, a chief designer at the Malakhit design bureau in St Petersburg, said the first 30,000-tonne submarine -- about the capacity of a medium-sized surface oil tanker -- could be launched within four years if a sale can be clinched this year. 

``Our bureau has finished all the blueprints and our first clients are familiarising themselves with them,'' he told Reuters by telephone from the Baltic port. ``The first client to show an interest in our tanker was Russia's Arctic Transport Company.'' 

He said the two sides had agreed to sign a deal this year to build an underwater oil transporter for $150 million at Severodvinsk, near Arkhangelsk on the White Sea. 

Malakhit designed the first Soviet nuclear submarines in the 1950s and worked on subsequent generations of the vessels seen by the Kremlin as the backbone of its strategic nuclear forces. 

But the post-Communist cash crunch and a planned navy cuts have forced Malakhit to find a new niche. 

Dronov said state-owned oil company Rosneft (PFGS.RTS) had financed the design work. Gas monopoly Gazprom (GAZPq.L) and the Arctic Transport Company had funded more recent researches. 

Malakhit unveiled a mock-up of its design in Brussels last year, sparking talks with British, South Korean and Japanese oil companies. 

``The era of ice-breakers is coming to an end, underwater tankers are an all-weather transport and can carry oil and gas all the year round,'' he said. 

Russia's Arctic north coast, close to many of its biggest oil and gas fields, is ice-bound for most of the year. 

Dronov said the company has several designs capable of carrying 10,000, 20,000 or 30,000 tonnes at speeds of 19 to 21.5 knots up to 400 km (250 miles) offshore. 

He said it was envisaged that the new tanker could deliver oil and gas from Russia's northern Yamal peninsula to western Europe and Japan, South Korea and other big Asian consumers. 

``Our tanker can call on ice-bound ports, simply breaking the ice from below. We have a great deal of experience of doing this with military nuclear submarines,'' Dronov said. 

But he added that the new underwater tanker would not be simply a modification of a military submarine. ``We abandoned an idea of using decommissioned nuclear submarines for carrying oil and other fuel, and our newly-designed tanker is completely different,'' Dronov said. 

``Only superficially does it look like a submarine.'' 

 ^REUTERS@