Washington — The issue of dumping radioactive wastes at sea is heating up, with major developments in Europe and the United States. Great Britain is scheduled today to begin dumping some 4,000 tons of atomic waste 400 miles northwest of the coast of Spain. Environmentalists have organized an ''international day of protest'' with demonstrations in front of several British embassies. British trade unions have told their members not to handle the waste, and protestors have been arrested for chaining themselves to British government office buildings.
The Swiss government recently announced it would stop such dumping beginning next year, the Netherlands has decided to do the same, and a majority of the 30 countries who are parties to the so-called London Dumping Convention have voted to suspend at-sea disposal of radioactive waste.
In the US, the period for public comment on a US Navy plan to dispose of 100 old nuclear submarines - perhaps by scuttling them, nuclear reactors and all - has just ended. A coalition of environmental and church groups, as well as several coastal states, have come out strongly against the Navy proposal.
It remains to be seen whether the actions by opponents to radioactive ocean dumping - particularly the seamen, transport, and train operators unions - will affect this week's British operation. The British vessel ''Atlantic Fisher'' is scheduled to carry out a joint Belgian/Swiss disposal next September as well.
All of this reflects growing international concern over the environmental impact of dumping radioactive garbage in the world's oceans. And it puts the United States at odds with the growing trend in political and scientific thought regarding the issue.
The US was one of the minority of states that voted last February against a Spanish resolution to suspend such dumping, at a meeting of the International Convention on the Prevention of Marine Pollution by Dumping Waste and Other Matter (the London Dumping Convention), formed in 1972.
The US has not disposed of radioactive wastes in the oceans since 1970. But the Environmental Protection Agency is developing new rules for the possible resumption of such dumping, and the Energy Department is studying the possibility of disposing of waste from atomic weapons tests at sea.
Meanwhile, the US Navy has to find a final resting place for the nuclear subs that will be taken out of service over the next 30 years, and the sea remains a prime candidate for these old vessels.
Under one option, the subs' radioactive reactor compartments would be removed and buried at government disposal sites at Hanford, Wash., and Savannah River, S.C. Then the rest of the subs would be cut up for scrap, or they would be flooded and sunk in the Atlantic off Cape Hatteras, N.C., and in the Pacific off Cape Mendocino, Calif.
The other option is to remove the radioactive fuel, seal off the reactor compartment (which would still be radioactive) and scuttle the whole sub in the Atlantic and Pacific. This is the option that most concerns environmentalists.
The Navy, in its draft environmental impact statement, says there would be ''very low radiation-related impacts'' and that ''either option would be acceptable.'' Navy scientists calculate that the sea floor in the potential dump sites ''is very sparse,'' and that ''none of the animals is used by man or form part of a food chain known to lead to man.''
Potential radiation exposure from scuttling 100 decommissioned nuclear submarines, the Navy says, would be less than already occurring naturally in ocean water, and also less than people receive from watching television or flying cross-country at high altitude.
One advantage to ocean dumping over land disposal, the Navy reports, is lower cost: $8.1 million per submarine less than salvaging the remains, and $2 million per sub less than burying the reactor and sinking the rest.
In a recent study, however, the coalition of environmental and church groups found that the Navy has failed to address important scientific and technical matters, including monitoring, the impact of radiation accumulating over time, potential for accidents, and indirect effects. It noted that the total amount of radioactivity associated with the subs would be 60 times the level of radiation dumped at sea by the US between 1946 and 1970.
There is also concern that ocean disposal of radioactive wastes would not end with submarines.
''We believe the Navy's sub scuttling plan is the cutting edge of an effort to resume sea disposal of America's radioactive waste,'' Oceanic Society President Christopher Roosevelt warned recently.
Congress has temporarily banned ocean sub dumping through 1984. But the Navy already has decommissioned five radioactive submarines. And the temporary storage sites at Navy shipyards will continue to grow until a solution is found.