MOSCOW (AP) _ Nikolai I. Ryzhkov, the new premier of the Soviet Union, is a technocrat who personifies the emergence of a younger generation of Soviet leaders under party chief Mikhail S. Gorbachev.

Ryzhkov, who turned 56 today, replaces 80-year-old Nikolai A. Tikhonov, who said he is retiring on his doctors' advice, according to the official Soviet news agency, Tass.

As premier, Ryzhkov will head the Council of Ministers and take charge of the massive apparatus responsible for day-to-day government operations.

He is the youngest member of the 13-man ruling Politburo after Gorbachev, who is 54. The Friday announcement of Tikhonov's retirement did not say whether he was leaving the Politburo, but it is assumed he is.

Ryzhkov was named a full member of the Politburo, the Communist Party's policy-making body, on April 23, barely six weeks after Gorbachev became party leader following the death of 74-year-old Konstantin U. Chernenko.

Since his appointment, Ryzhkov has been considered one of Gorbachev's chief executives in the campaign to boost the sagging economy. He has appeared at the Soviet leader's side during virtually every major party meeting on that subject, and was with Gorbachev on a recent trip to western Siberia and Kazakhstan.

Ryzhkov first came to prominence in November 1982 when, under then-Soviet leader Yuri Andropov, he was named a Central Committee secretary in charge of economic administration.

A Communist Party member since 1956, his background is in heavy industry, which is important to Soviet defense manufacture. He is considered a technocrat and a backer of economic reforms initiated by Andropov and continued by Gorbachev.

Ryzhkov graduated from the Ural Polytechnical Institute in 1959 with an engineering degree. He worked in various capacities in mining, smelting and machine-building enterprises.

In 1975, he was named the Soviet Union's first deputy minister of heavy and transport machine building, and in 1979 he became a first deputy chairman of the State Planning Committee.

According to his official biography, Ryzhkov is Russian by nationality. His birthplace has not been disclosed.

His official writings in party and state newspapers have concentrated on economic management, plan fulfillment and industrial production.

A draft of the next five-year plan, scheduled for adoption at the 27th Communist Party Congress in February, has yet to be approved by the Politburo. An earlier draft was rejected as inadequate, and the plan to be adopted in February - along with a new party program - likely will bear Ryzhkov's stamp.