What is 400 tons of seawater doing in the cooling system at the Hamaoka nuclear plant?

Couldn't help but notice the waves. Might actually be surfable there

From NHK:

At the Hamaoka nuclear power plant in central Japan, seawater has been found in coolant at one reactor.

Five nuclear reactors at the Hamaoka plant in Omaezaki City, Shizuoka Prefecture, were all shut down on Saturday due to concern that a massive earthquake might hit the area. The move was in line with a request by Prime Minister Naoto Kan.

In the course of shutdown, plant operator Chubu Electric Power Company found impure substances in coolant water at the No.5 reactor.

The company reports damage to a pipe connected to a condenser, a system that turns the steam generated by a nuclear reactor to water through the use of seawater.  Chubu Electric Power Company says 400 tons of seawater may be mixed into the cooling water that goes through the reactor.

It says 400 tons would not severely affect the reactor, and that no radioactive substances were detected outside the building.

But in order to prevent the reactor being eroded by seawater, the operator will take measures to remove salt from the cooling water.

Edit:  Here's slightly more info about what happened:

"Chubu Electric Power Co. said Sunday that trouble with the cooling systems at the Hamaoka power plant prevented its No. 5 reactor from achieving a cold shutdown for about two hours but did not result in the release of radiation.

The problem at the coastal plant in Shizuoka Prefecture was caused by seawater that leaked into a steam condenser at the reactor. The condenser is a device used to cool the steam as it comes out of the turbines and turn it back into water.

The problem prompted the utility to switch to another system to cool and stabilize the reactor, which achieved cold shutdown status shortly past noon, it said."

How on earth did they allow seawater to leak in, especially since they have had months after the quake to safety check all systems?  Did they botch the shutdown procedure? After what they've learned from Fukushima, this should not be happening.