Documents released yesterday by the Nuclear Regulatory Commission show that it conducted an informal safety rating of 51 nuclear power plants in the United States that placed 10 of the plants in a "below average" category.

The 10 plants received a "C" rating grade from the NRC for their safety performance in 1975 the only year the NRC made up the equivalent of report cards on nuclear power plants. A spokesman said the NRC conducted three different ratings of the 51 plants to see if it could develop a system to improve plant performance and the allocation of inspectors.

The documents containing the ratings of the 51 plants in operation during 1975 were released by the NRC as a result of a suit brought by the Union of Concerned Scientists under the Freedom of Information Act.

Two-thirds of the 51 plants rated by the NRC were termed "average" in their approach to safety and received grades of "B" One-sixth were called "above average" and won "A's." The remaining 10 got "C's" for what the NRC said was "below average" performance.

The 10 plants with a "C" included the Surry nuclear plant near Newport News. operated by Virginia Electric Power Co. They also included plants run by Commonwealth Edison Co., one Containing three nuclear reactors just outside Chicago and the other containing two reactors near Moline, III. Also on the "C" list was the reactor at the Indian Point complex of Conslidated Edison Co. 30 miles from New York City.

"It is deeply disturbing," said Robert D. Pollard, spokesman for the Union of Concerned Scientists, a public interest group in Cambridge. Mass, oposed to nuclear power, "that several of the reactors the NRC rates as the poorest are located near areas such as New York and Chicago, where millions of citizens reside."

In rating the safety performance of the 51 plants, the NRC graded them against each other and not against some standard put forth by the NRC. Sair the NRC: "Th is practically assures a mix of A's, B's and C's."

Its system of grading the 51 plants did not take into account "violation" of NRC safety regulations, the most serious charge against a nuclear power plant. It only included "infractions," "deficiencies" and "licensee event reports" where nuclear plants had experienced an operating event like a temporary shutdown that had to be reported to the NRC.

In the documents released the NRC pointed out that if it included "violations" of NRC regulations like an accidental exposure of plant workers to radiation, two of the 51 plants getting a "B" rating would have been down graded to "C." This would bring to 12 the number of plants listed as "below average" in safety performance.

The NRC identified these two plants as the Zion plant of Commonwealth Edison Co. in Illinois and the Millstone 1 plant of the Northeast Nuciear Energy Co. outside New London, Conn.

Violations also were not counted against the Indian Point plant of Con Ed. which had already received a "C" for its safety performance. Said the NRC: "It would significantly reinforce the "C" categorization" of the Indian Point facility.

In its won documents accompanying the NRC documents, the Union of Concerned Scientists criticized the NRC for not having enough safety inspectors, for not conducting enough inspections and for not levying stiffer fines against electric power companies violating its regulations.

Asked for comment, the NRC said it now has 235 inspectors who will conduct an estimated 3,000 reactor safety inspections this year. The NRC said that since 1973 it has levied 28 civil penalties for safety infractions against companies operating nuclear power plants. Fines imposed totaled $434,750, the NRC said.