ALBUQUERQUE, N.M. (AP) _ The Iron Curtain may no longer prevent U.S. and Russian scientists from sharing nuclear discoveries, but red tape can still get in the way.

A model of a Russian nuclear rocket engine was to have been part of a symposium here last month. But the U.S. government wouldn't let it into the country.

It was the third time in a year the government has interfered with attempts by a California company to acquire or borrow examples of Russian nuclear systems for U.S. scientists' use.

Ned Britt, vice president of International Scientific Products of San Jose, Calif., said his company is disappointed.

''In this way, they're sort of saying, no license for it will be given, and it will go home and it should go home,'' Britt said.

Bob Mantel, senior adviser in the State Department's bureau of political and military affairs, said the company didn't provide enough information in its application and never gave an adequate reason for the model to enter the United States.

''It doesn't have a valid license. ... The only choice is to send it back,'' Mantel said last week.

Britt said the model would remain with Customs officials in Los Angeles awaiting a Russian freighter, which could take several weeks.

Last year, the company displayed a model of a Soviet Topaz 2 nuclear power system at Albuquerque's annual Symposium on Space Nuclear Power Systems.

Bringing that model to the United States was no problem. Getting it back to the Soviet Union was.

The Nuclear Regulatory Commission refused to let it go, saying it was illegal to export nuclear technology to the Soviet Union - even though it was designed, built and owned by the Soviets. It took more than five months to work out a deal that released the model.

Another problem arose after a major announcement at last year's conference that U.S. researchers would buy a Topaz 2 with an eye toward developing the technology for American use.

A Topaz destined for U.S. scientists has been in a warehouse in St. Petersburg, Russia, for six months, awaiting U.S. approval to bring it in, according to Joseph R. Wetch, president of International Scientific Products.

The State Department said last week the company hadn't submitted paperwork for it.

International Scientific Products said it used the same procedure to bring in the rocket engine model as it used to bring in the Topaz 2 model for last year's symposium.

But Britt said Customs officials this time asked the State Department to determine whether the rocket engine was a military article.

In its memo denying the engine's entry, Britt said, the Office of Defense Trade Control acknowledged it is a model but said it is not in the country's foreign policy interest to keep it.

The Russians may be wearying of the hassles.

Nikolai N. Ponomarev-Stepnoi, a top Russian scientist, intended to use the engine model at the Albuquerque conference to push for joint ventures with U.S. space scientists.

Without mentioning any countries by name, Ponomarev-Stepnoi said that if such programs can't be arranged, Russian researchers may have to work instead on nuclear projects for ''ambitious Third World countries.''