A thousand gallons of radioactive water accidentally flowed out of a pipe on to a basement floor at the Virginia Electric and Power Co.'s North Anna Nuclear Power Station early this week, slightly contaminating 13 employes and-hour plant shut-down.

Both Vepco and the Nuclear Regulatory Commission say the incident was not serious, that no one was exposed to radiation beyond legal levels set by the NRC, and that no radiation escaped from the plant into the atmosphere beyond legal levels.

However, an NRC spokesman said yesterday that the incident is being studied to see if "enforcement action" will be taken against Vepco to prevent a recurrence.

"We want to see that if there's a weakness in the program something is done to take care of it," said Michael S. Kidd, NRC's resident inspector at the plant, which is located near Mineral, Va., north of Richmond. "It appears that a chain of events allowed it to happen and it was probably no one individual's fault.

The North Anna plant has been in operation since April 5, and Vepco says there have been no serious accidents there.

According to Kidd, other NRC officials and a Vepco spokesman, here is what happened this week.

On Tuesday evening in the plants master control room, and "annunciator" light suddenly flashed and a horn blasted. The men in the room saw that the water level was sinking rapidly in a storage tank-holding coolant water that had been used in the reactor cooling system.

They worked quickly trying to find out what was wrong, and within two or three minutes were able to guess that the problem was related to repair work being done on a pipe that led from the storage tank back to the reactor itself.

The storage tank, along with many other supplementary systems, is located in an "auxilliary building" next to the main reactor building. Water is taken out of the reactor building periodically, treated chemically, and then stored in the tank to be eventually returned to the reactor cooling system when needed.

Because the water in the storage tank has been in contact with the reactor itself, it is somewhat radioactive.

The men in the control room, which is located in a third area outside both the main and auxiliary building, used the public address system to try to reach the man who was doing the pipe repair work - but he couldn't hear them because of the loud machinery around him.

Failing to reach him, two of the control room men rushed toward the auxiliary buildind, where they donned protective clothing and raced down two flights of stairs to the basement.

There they saw the water pouring out of two valves in the pipe - valves that had inadvertently been left open in the course of the repair work on the pipe. They shut the valves and stopped the flow.

These two men and 11 workmen on the floor above were mildly contaminated by the water: the two by direct contact with it and the other by breathing its vapors.

Those same vapors went through the ventillation system and into the atmosphere, but since the water's radioactivity was so low, legal limits of exposure were not reached. The water itself had poured from the open valves on to the basement floor and thence into a sump area designed to collect water in case of an accident.

The basement area itself is not a normal working area and is crowded with pipes and valves. In spilling out on the floor, the water got into a switching system and caused the shut-off of a secondary cooling system serving one of three pumps that run the main reactor cooling system.

The temperature of this pump began to rise. When the man in charge in the control room saw this, he decided to shut down the reactor. He pushed the "reactor trip" or "scram button," which dropped the control rods into the reactor and shut it down.