The cost of insuring nuclear reactors against property damage will rise as much as 63 percent this year, and the increase is "wholly attributable to the cost of Three Mile Island," according to the industry's primary provider of coverage.

The industry insurance pool, American Nuclear Insurers, already has paid $88 million to General Public Utilities, the holding company for the Three Mile Island plant, to cover the cost of the damaged fuel core and of decontamination efforts undertaken so far. The insurers could be liable for as much as $300 million of the $400 million General Public Utilities estimates is needed to get the crippled plant back in operation.

This amount would effectively wipe out American Nuclear Insurers' reserve for compensating nuclear facilities againist future property loss, accroding to the pool's president, Burt Proom.

The rates for liability insurance are scheduled to rise 10 percent this year, and Proom anticipates further increases as his company evaluates individual reactors on the basis of a toughened set of standards. Proximity to population centers and health precautions for plant workers will be taken into account during these evaluations, he said.

Proom said that American Nuclear Insurers had paid out a little more than $1.2 million so far to individuals who have claimed damages from the accident at Three Mile Island. Several class actions ranging from charges of physical damage to "diminution of enjoyment of life" still are pending against General Public Utilities.

Each suit could result in judgments against the utility up to the $560 million limit now set by the Price-Anderson Act. But Proom asserted that the chance of any of the suits succeeding is "not good" and that his company hasn't figured the possibility of huge damage settlements into the current liability rates.

Utility companies with nuclear plants sill aren't certain how they can pass on their increased insurance costs to their customers.

"It will be considered a normal business expense when we determine the amount of our rate request," said Doug Cochran, spokesman for Virginia Electric and Power Co.

"We will bring these increases up with the rate-setting commission," said Ken McKay, a spokesman for General Public Utilities. "But compared with the $233 million we've had to pay to purchase the power we lost when Three Mile Island went down, a couple of million more in insurance premiums doesn't matter much."