Investors apparently had expected a prolonged costly shut-down of Babcock & Wilcox-built reactors for utilities. When they learned otherwise today, some utility issues scored gains.

The reactor designs are similar to that of Three Mile Island, where the nation's worst nuclear accident took place a month ago. When the Nuclear Regulatory Commission announced today that the utilities which operate Babcock & Wilcox plants had agreed to close them voluntarily to modify the reactors, stock prices of the two companies most deeply involved-Babcock parent J. Ray McDermott and Duke Power-rose sharply.

Duke Power was the third most active stock on the New York exchange today, closing up 1 3/8 at 17 7/8. McDermott was eighth most active, up 1 5/8 at 18 1/2.

Overall, however, the market was down in moderate trading. The Dow Jones average of 30 industrial stocks ended the day at 856.64, down 4.33.

Duke Power stood to suffer the most frmo a long shutdown of its nuclear facilities. Analysts had worried that the company could have been forced into brown-outs or blackouts if all four of its nuclear plants were down during the heat of the summer.

The utility from which it would be most likely to borrow power-Virginia Electric & Power Co.-has its three nuclear plants shut down for modifications.

But Duke said that it can make the modifications desired by the Nuclear Regulatory Commission and still be able to avoid an extended closing of its plants several of which are shut down now for scheduled maintenance. The company also said that the cost of the modifications will be minor and have little impact on its earnings.

Even General Public Utilities Corp., which owns the severely damaged Ghree Mile Island facility, closed up 3/8 at 10 7/8.

Investors are gambling, apparently, that despite the severe costs General Public will face as a result of the damage to its $1.1 billion reactor, the company will stave off bankruptcy and get the rate increases it needs from public utility commissions.

General Public announced Thursday day that it would cut the first quarter dividend to 25 cents per share upon 45 cents and would trim salaries of officers and directors by an unspecified amount.

The move has designed to conserve needed cash and to convince regulatory commissions that the company and its shareholders were doing their part in shouldering some of the costs of the accident at Three Mile Island.

General Public will have to go to utility commissions in Pennsylvania and New Jersey to request rate increases in the wake of the accident.

Big Board volume totaled 29.61 million shares, down from 32.40 million Thursday. Nationwide turnover in NYSE-listed issues, including trades in those stocks on regional exchanges and in the over-the-counter market came to 34.03 million shares.

The NYSE's composite index lost .12 to 57.40 Standard & Poor's index of 500 stocks was down .21 at 101.80.

On the American Stock Exchange the market value index rose .22 to a new high of 183.65. The NASDAQ composite index for the over-the-counter market closed at 134.37, off 27. CAPTION: Graph, no caption, The Washington Post