Interior Secretary Cecil D. Andrus yesterday reaffirmed his intention to invoke executive branch authority to protect almost 100 million acres to Alaska land from development.

He said he would act because Congress had failed to pass the controversial Alaska lands bill.

Andrus released a lengthy analysis of the environmental effects his actions by Dec. 18, based on some combination of three laws. That is when current safeguards for the lands expire.

He called for public comment by Nov. 20.

The Interior Department undertook the survey in August when hopes for passage of the Alaska lands bill began to wane. It catalogs Alaska's wealth of unspoiled wilderness, waterways, wildlife and native people's lifestyles, among other features, that need protection from development and industrial misuses.

Although Congress "struggled with this issue for seven years" and failed, it documented the need for "ambitious conservation" in Alaska, Andrus said in a statement.

The interior secretary has three options at his disposal to shut off public lands from use. The 1906 Antiquities Act enables him to recommend areas for presidential designation as national monuments, blocking hunting, mining, mineral leasing and other developments.

The Antiquities Act, the strictest of Andrus' options, provided for such national landmarks as the Grand Canyon and the Statue of Liberty. Its use can be revoked only by Congress.

Andrus might also use a section of the Bureau of Land Management Organic Act, to "segregate" from use public lands for two years, simply by officially announcing his intention to make them preserved areas.

A third authority is the requirement that the secretary study all U.S. roadless areas over 5,000 acres for their possible addition to the nation's wilderness system.Such studies could last for years, and the land must remain unused in the interim.

Andrus emphasized that he might exercise any of the possible alternatives, except the choice of doing nothing.

Although there is no danger that building or mining projects will begin immediately if Dec. 18 passes without executive branch action, Andrus purpose is "to give additional layers of protection" to the land until a bill is passed, his assistant Cynthia Wilson said.

Suporters of this year's failed legislation, Sen. Ted Stevens (D-Alaska) and Reps. Morris K. Udall (D-Ariz.) and John Seiberling (D-Ohio), among others, have said they will get a bill through Congress in 1979.