The major coal project in this Siberian town is a child of detente, conceived in 1974 by the Russians in an area of southern Yakutia where only forest tundra once spread to the endless horizon.

A few years earlier, after the discovery of huge natural gas reserves a few hundred miles northwest of here, Soviet prospectors had moved southward into this region that was the domain of bears and reindeer to test drill for oil and gas. Each probe came up with evidence of rich coal deposits to suggest a dazzling energy potential--not only to the Soviets but also to energy-hungry Japanese and some American firms.

The climate of detente at the time seems to have brushed away Siberia's inclement weather and the absence of basic facilities to make the prospect of cooperative development of the Soviet Wild East attractive. Moreover, the Russians committed huge sums of money to build a northern trans-Siberian railway known as Bam for exploitation of energy and raw material resources that Japanese and American investments and machinery would help develop.

But worsening relations between Washington and Moscow subsequently blocked plans for building a 1,200-mile pipeline to carry natural gas from Yakutia to the Pacific coast. The Japanese made clear that they were not prepared to finance cooperative ventures without U.S. participation.

Before the deterioration of relations, however, the Japanese decided to extend $450 million in credits for purchase of Japanese equipment to develop a huge coal mine here, while the Russians committed the equivalent of $1.2 billion for technology and manpower to open up the vast reserves locked in the frozen wastelands outside Neryungri.

This sole Japanese-Soviet cooperative venture got off the ground in 1975 and is now in the final stages of development. When it reaches the projected capacity, the Neryungri coal complex will produce 13 million tons annually, of which 5.5 million tons of coking coal will be shipped to Japan each year to pay off the credits.

It is difficult to avoid being impressed by the sheer scale of the effort and the scope of the complex. Neryungri had been selected for it because it has the most accessible coal deposits, which lie in a compact, bowl-shaped formation six miles wide. According to project director Viktor Zhdamirov, there are 400 million tons of coal nearby to be mined from open pits.

To strip away a layer of rock and dirt overburden, which at its deepest point goes down 350 yards, the Russians bought huge Sumitomo-Marion hydraulic bucket shovels, 120-ton dump trucks from Momatsu, Unit-Rig and Kato cranes and drilling equipment. This was augmented last year by the first Soviet-made mechanical power shovels and dump trucks of even greater capacity.

As with most Soviet projects, the Neryungri complex is two years behind schedule. Zhdamirov and other officials say this was mainly due to problems with the Japanese and U.S. equipment, which had not been geared for the harsh climate of Siberia. The hydraulic systems of the imported excavators tend to freeze and burst at temperatures under 35 degrees below zero Fahrenheit, which are common here.

Other vehicles have suffered from a combination of permafrost surface and low temperatures that make rubber, plastics and steel, unless specially toughened, as brittle as glass.

Yet the Russians have pressed on with the project, which is now scheduled for completion in 1985. But it is clear to any visitor that the original Soviet allocations of $1.2 billion have not been sufficient to meet the objectives and that cost overruns may have brought the total investment in the project well above $3 billion.

According to Soviet plans, Neryungri is intended to become not only a focal point of new energy but also the first industrial city of Yakutia. The mine itself is already producing 5 million tons of high-grade coal annually.

Prefabricated housing for 40,000 people, factories and other support facilities have sprung up around the new settlement. A spur of the new Bam (for Baikal-to-Amur) railway already reaches Neryungri. New and rich deposits of iron and high-grade copper at nearby Udokan back officials' claims that Neryungri will become the biggest steel and industrial center east of the Urals.

At present, only the metal frames of a coal enrichment plant and associated washing and loading facilities are being built. But the chief engineer, Viktor Vodopianov, insisted that they will be operational by 1985, when deliveries of enriched coal to Japan are due to start.

The hope here is that Neryungri would be turned into a metallurgical base for the production of special steels for extreme cold conditions--something the Russians badly need--and eventually for nonferrous smelting as well.

Over the long term, Moscow's investments here and in the second trans-Siberian railway are bound to become profitable, and the linking of eastern Siberian resources to the industries of the Pacific rim is likely to play a major role in the country's overall development.

But there are about 400 miles of Bam to be built before the line from Ustkut, near Lake Baikal, to Komsomolsk on Amur in the Far East is to be completed. The line is scheduled to begin operating by the end of 1984.

Meanwhile, however, the projects are likely to be a significant drain on the Soviet economy. Because of the permafrost terrain and inclement weather, construction costs here are four times higher than in western Russia. Labor costs are even higher because of high wages, bonuses, longer holidays and other perks needed to attract workers.