A $644 million loan by the Export-Import Bank to finance construction of a nuclear power plant in the Philippines is being challenged by Congress because the plant will be built by a relative of Philippine President Ferdinand Marcos on a site surrounded by five volcanoes.

The largest single loan transaction ever guaranteed by the Ex-Im Bank will be the subject of a hearing today by Rep. Clarence D. Long (D-Md.), chairman of the House Appropriations subcommittee on foreign operations. Long is challenging the loan on the twin questions of safety and conflict-of-interests, which he charges are more than enough to disapprove the loan.

The safety issue has been raised in questions to the Ex-Im Bank from outside of Congress. The White House Council on Environmental Quality and the Nuclear Regulatory Commission have both criticized privately the wisdom of constructing a 620,000 kilowatt nuclear plant on a site less than 100 miles from five volcanoes, four of them described as "active."

One of the active volcanoes is Mt. Natib, located 10 miles from the nuclear power plant's site at Nepot Point at the edge of the South China Sea. The outer edge of a huge mudflow that resulted from the last eruption of this volcano is fewer than two miles from the power plant site where ground has already been broken and where the foundation has begun to rise.

One volcano of the five is considered inactive. Named Mt. Mariveles, it is 12 miles away. The other three volcanoes are between 60 and 90 miles from the site and are all active. Their names are Tall, Mt. Banakoa and Mt. San Cristobal.

In an internal 1977 memorandum, the NRC said it made a study of the site at the request of the Philippine Atomic Energy Commission and concluded that "all volcanic hazards should be considered possible at the site." The NRC identified the volcanic hazards at the site as ranging from "ash fall and lava flow to volcanic earthquake."

At the hearing today, Long will question Export-Import Bank President John L. Moore Jr. about the $644 million loan guarantee for the $1.2 billion plant, which is to be built and insured by companies owned by Herminio Disini. One of the wealthiest men in the Philippines, Disini is married to a cousin of Imelda Marcos, wife of president Ferdinand Marcos. Disini's wife is the private physician to Imelda Marcos.

Less than a month ago, Marcos ordered a government takeover of three of the 35 companies controlled by Disini after U.S. news accounts of Disini's accumulation of wealth based on his relationship to Marcos. The Philippine embassy said yesterday that the scope of Disini's involvement in the nuclear power project "is under very intensive review by the government in Manila."

Last December, Washington Post correspondent Jay Mathews reported that Disini's Asia Industries was the Philippine agent for Westinghouse Electric Corp., which is to supply the reactor and turbine for the nuclear power plant. Asia Industries is also to be the construction company heading the nuclear plant project.

At the same time, a company named Summa Insurance, which is also controlled by Disini, has won a contract to write a $668 million insurance policy on the nuclear project. The policy would require premium payments of $10 million a year, so big that the Philippine government's Government Service Insurance System agreed to cover any liability Summa might incur.

The $644 million Ex-Im Bank loan guarantee includes an outright loan of $277.2 million and a guarantee that the Ex-Im Bank will cover defaults on $367.2 million in private bank loans arranged for the nuclear project by the Philippine government.

The loan guarantee has already been approved, but Long believes his hearing may convince NRC to deny an export license for the reactor on safety and conflict-of-interest grounds. The NRC must also approve export of enriched uranium fuel to the Philippines for use in the power plant when it is ready.

Besides being near five volcanoes, the Philippine nuclear plant will also be located less than six miles from the entrance to the Subic Bay Naval Base, where the U.S. Navy's Seventh Fleet stores 110 million gallons of diesel oil, 1.7 million gallons of jet fuel and ammunition for 20 surface warships, two aircraft carriers and the 200 warplanes assigned to the two carriers.

Long points out that the NRC rejected a request by Baltimore Gas & Electric Co. a year ago to build a nuclear power plant at Perryman, Md. at least in part because it was too close to the Aberdeen Proving Grounds, an Army installation where ordinance and new weapons are tested.

The NRC admits Perryman's proxim-Baltimore G & E's request mostly because the Perryman site exceeded its population criterion of 500 people per square mile for a nuclear power plant. The NRC admist Perryman's proximity to Aberdeen was a factor but not the major one.

The volcanic hazard is clearly the most serious one, according to internal NRC documents. Hazards from the closest volcanoes include lava and mud slides that could cover the power plant. Ash falls from all five nearby volcanoes could cover the site like heavy snowfalls, clogging filters and cooling ponds and raising havoc with the nuclear plant's complex machinery. CAPTION: Picture, PRESIDENT FERDINAND MARCOS . . . in-law to build $1.2 billion plant