The construction of the Calvert Cliffs nuclear power plant 45 miles from Washington sparked massive protests a decade ago over the possible effects of thermal pollution on oysters, fish, and the delicate balance of other marine life in the Chesapeake Bay.

Few if any of the environmentalists protesting the Maryland plant raised the specter of a "China Syndrome" problem: a loss of coolant, an uncovered nuclear reactor core, a meltdown of the superheated core and a release of dangerous radioactivity.

Further, in their contingency planning, neither Baltimore Gas & Electric Co. - Which owns and operates the plant - nor their government overseers even considered the possibility of such an occurence.

Now, with the well-publicized series of problems at Pennsylvania's Three Mile Island plant fresh in their minds, the utility owners and government overseers of the Calvert Cliffs plant still estimate that no accident of similar magnitude could occur there.

As far as the nuclear industry and its critics are concerned, the Calvert Cliffs plant is, in a word, average.

A 1976 Nuclear Regulatory Commission survey gave the plant a "B" grade on a scale of "A" to "C" - the same grade Three Mile Island got for overall operations and safety. Last year, Three Mile Island scored slightly higher than Calvert Cliffs.

Other nuclear reactor watchers regard Calvert Cliffs as a relatively "clean" plant. Of the 25 "abnormal occurrences" reported to Congress since 1975, none has involved the Marland facility.

As a study of four years' worth of utility and government-authored reports on the facility shows, even the most average of nuclear plants is not without risks.

The Types of problems that have presented themselves since the plant began operations in 1975 include:

Leaky valves, bad welds and other mechanical problems that might, uncorrected, result in a serious loss of cooling water, which keeps the reactor from overheating. The problems usually have been quickly corrected. For instance, on April 11, 1978, a utility employe discovered a valve leaking 10 gallons per minute because the packing around it was worn. The leak was reduced by interim measures until the packing could be replaced eight days later.

Control rods - equipment designed to shut down the fission process in an emergency - dropping unexpectedly into the core. This could create overheating problems if the rods are not removed. On Oct. 17, 1978, at 9:41 p.m. a rod unexpectedly dropped into the core but was removed by shortly after 11 p.m.

Severe wear at the end of the "guide tubes" containing the control rods - wear which, if not corrected, could cause the rods to jam in an emergency. This defect could prompt the NRC to shut the plant down. It is "a generic safety concern" for plants, like Calvert Cliffs, built by Combustion Engineering Co. of Connecticut.

Occasional environmental problems including fish kills or acid spills. On Feb. 28, 1979, a pipe weld near a sulfuric acid storage tank failed, spilling less than five gallons of sulfuric acid into the bay. "No environmental impact is considered due to the small amount of acid released," said the report on the incident.

At the same time as they are coping with such problems as these, plant officials and government regulators are analyzing and reanalyzing and setting up contingency plans to deal with an array of other problems that could affect the plant's two reactors.

Among these are the possible long-term consequences of simple mechanical failures of pumps and valves to the more exotic and less probable threat that a cloud of gas from a liquified natural gas spill nearby could envelope the plant and explode.

In the weeks since the problems developed at Three Mile Island, BG&E officials have received numerous questions on the probability of a similar incident at Calvert.

"If you put the same exact set of circumstances here that happened [at Three Mile Island], it could happen here, absolutely," said a Baltimore Gas & Electric Co. spokesman at a recent press conference called to dispel doubts about Calvert Cliffs in the wake of those events.

"The probability is less at Calvert Cliffs," emphasized Robert M. Douglass, former chief engineer at the plant who now is BG&E's quality assurance manager.

If Calvert Cliffs presents an "acceptable risk" by present standards, it also highlights the fact that risk exists. The constant question before the NRC and utility companies is whether the probability of risk has been reduced to an acceptably low level.

Utility officials portray the nuclear power plant as no more than a minor intrusion on the Chesapeaks Bay's western shore. The visual impression is fortified by the presence of utility-owned tobacco fields plowed within site of the plant's visitor center.

"BC&E people take a lot more interest in plant operation and safety than others I've run into," said Ronald Fluegge, a former BG&E employe and NRC inspector turned nuclear critic. Fluegge, now a consultant on nuclear medicine safety and licensing, had special praise for Douglass.

However, after an April 1978 visit to the plant one NRC inspector described Douglass as "anti-NRC." The plant's philosophy "is 2.5 and survive," the report alleged without elaboration, using Navy jargon for the lowest possible passing grade.

"They don't do anything above that which is required toward plant safety," said the report. "Its attitude toward safety appears to be that meeting literal NRC requirements is sufficient."

"Not so," said Douglass. "I think that was just one person's feeling."

While some of the problems confronting Calvert Cliffs are common-place to such power plants, unique to Calvert Cliffs is the proximity of a liquefied natural gas terminal 3.5 miles away at Cove Pint. The terminal's very presence poses the risk of a bizarre, doomsday-like accident - the odds against which NRC officials contend are more than a million to one.

The scenario begins with a gas spill from a tanker bound for the terminal or anchored nearby. The LNG vaporizes on the water into a highly-flammable, low-oxygen cloud of methane gas.

"It is remotely possible," BG&E acknowledged last year, "with the appropriate atmosphere and bay conditions, that the LNG vapor cloud emanating from the spill could engulf the plant and then ignite. The resultant energy [heat] released would be hazardous to personnel and could damage equipment. Additionally, should a flammable mixture of LNG vapor fill a confined space or room, an explosion could occur when it ignited."

The NRC concluded risk of this occurrence was "acceptably low."

An NRC tehnical adviser raised general questions in 1975 about the wisdom of locating an LNG facility near a nuclear power plant, listing it with subdivisions and ammunition plants as neighbors "that would have made the site unacceptable [for the nuclear power plant] if known before licensing."

However, the plant's preparations for an on-site emergency have been of continuing concern to the NRC staff.

In July, for instance, NRC inspectors found emergency equipment in the control room, guardhouse and a nearby Calvert County hospital "either missing, or defective and not maintained in a state of readiness." These problems since have been corrected.

The NRC also found that, contrary to its own emergency plan, Calvert Cliffs had failed to conduct on-site fire drills with the county fire department. BG&E officials since have agreed to hold such drills at least once every two years.

As recently as March, the NRC wrote a 15-page critique of the plant's fire emergency plans. Federal inspectors, the report said, had found "many safety related areas . . . not provided with automatic fire protection." BG&E's own safety analysis, the NRC said, lacked enough information "to conclude that the capability to safely shut down the plant will be observed in the event of a major fire."

In the "maximum hypothetical accident" envisioned at Calvert Cliffs by BG&E, cooling water that keeps the fuel from overheating streams from both ends of a ruptured 42-inch diamteter pipe. Some radiation from the water eventually enters the atmosphere, but, even with only half the backup safety systems working, as assumed in the analysis, radiation emissions remain acceptably low.

Short of such an accident, records reflect occasional small losses of cooling water at Calvert Cliffs, usually replenished before damage occurs.With 60,000 gallons of water circulating through the core, some leakage is expected. "We have drips here and there," said BG&E's Douglass.

When monitors show excessive leakage, operations are slowed or shut down to allow for repairs.

Environmental concerns originally voiced during the construction of Calvert Cliffs led to a landmark court ruling requiring the utility to file environmental impact statements to win U.S. approval. The plant met these criteria and a November 1978 state report said that the plant's impact has been cinsignificant."

Yet, the NRC files on Calvert Cliffs are punctuated with occasional "environmental deviations," usually kills of thousands of small fish, which the utility owners term "fish" impingements.

What to do with the used nuclear fuel-not radioactive enough to sustain heat-producing fission but radioactive enough to be dangerous for a quarter of a million years or more - is a problem facing Calvert Cliffs and all other reactors.

When Calvert Cliffs first was proposed, the solution was said to be recycling. "Spent" fuel was to be trucked to a reprocessing plant where the remaining uranium and plutonium would be recaptured and reused in new fuel assemblies.

However, Because of technical problems, economics and worker exposure to radiation, no reprocessing facilitics have operated in the United States since 1976. In 1977, President Carter halted the development of nuclear fuel reprocessing to discourage the spread of the technology that could lead to proliferation of nuclear weapons.

For the time being, spent fuel - the equivalent of about 30 metric tons each year for each reactor is stored instead at the plant on racks in concrete-lined stainless steel "swimming pools."

Meanwhile, the presence of the radioactive used fuel at Calvert Cliffs continues to concern the state of Maryland, which passed a law in 1970 barring permanent storage and limiting temporary storage to two years. BG&E argues it is bound only to follow NRC dictates and not the state's more stringent requirements.

To avoid a legal battle that state lawyers have acknowledged they could lose, the Maryland legislature twice has pushed back the deadline, most recently to 1980. CAPTION: Picture, Overall view of the Calvert Cliffs nuclear power plant. The first unit went into operation in May 1975, the second in operation April 1 this year. Total plant capacity is over 1,600,000 kilowatts. The Washington Post; Map, Map locates Calvert Cliffs plant. Cove Point is liquified gas dock terminal. By Richard Furno - The Washington Post