Officials say the damage could instead have happened in other equipment, including piping or the spent fuel pool.
Operators have been struggling to keep cool water around radioactive fuel rods in the reactor's core after the March 11 earthquake and tsunami cut off power supply to the plant and its cooling system.
Damage could have been done to the core when a March 14 hydrogen explosion blew apart Unit 3's outer containment building.
This reactor, perhaps the most troubled at the six-unit site, holds 170 tons of radioactive fuel in its core. Previous radioactive emissions have come from intentional efforts to vent small amounts of steam through valves to prevent the core from bursting. However, releases from a breach could allow uncontrolled quantities of radioactive contaminants to escape into the surrounding ground or air.
Operators stopped work Friday at units 1 through 3 to check on radiation levels.
The official death toll from Japan's devastating earthquake and tsunami passed the 10,000 mark Friday and was still climbing two weeks after the magnitude-9 quake struck off the northeastern coast and unleashed a cascade of disasters.
Earlier Thursday, two workers in Japan suffered radiation burns trying to lay electrical cable at the crippled nuclear plant.
They apparently stepped in radioactive water. Another worker was also exposed.
About two dozen people have been injured trying to contain the radiation at the plant.
A remotely-operated water cannon has been rushed from Australia to help cool reactors.
In the meantime, demand for bottled water has spiked in Tokyo a day after the government reported that radioactive iodine in the tap water measured more than twice the level considered safe for babies.
The Japanese government has pleaded for calm in Tokyo. Officials urged residents to avoid panicked stockpiling, sending workers to distribute 240,000 bottles - enough for three small bottles of water for each of the 80,000 babies under age 1 registered with the city.
Many shops are now rationing water, milk and other products.
The U.S. and Australia were halting imports of Japanese dairy and produce from the region, Hong Kong said it would require that Japan perform safety checks on meat, eggs and seafood, and Canada said it would upgrade controls on imports of Japanese food products.
Radioactive iodine is short-lived, with a half-life of eight days - the length of time it takes for half of it to break down harmlessly. However, experts say infants are particularly vulnerable to radioactive iodine, which can cause thyroid cancer.
The death toll from the March 11 earthquake and tsunami disasters has reached 9,700. An estimated 16,500 people are missing.
The Associated Press contributed to this report.
