TOKYO (REUTERS) - Tepco halted the cooling system for a spent fuel pool at its Fukushima Daiichi nuclear plant on Monday, the third time a cooling system has been offline there in the past five weeks, underlining the challenges the utility faces in trying to shut down the facility.

The company, formally known as Tokyo Electric Power Co , said in a statement that it halted the system for the No.2 unit's spent fuel pool for inspection after it found dead rats near a transformer.

The inspection was expected to take 3 to 4 hours from 11:36 a.m. (10.36am, Singapore time).

Tepco estimated that temperatures in the pool would rise less than one degree during the inspection from around 14 degrees Celsius before it halted the system, it said.

Two years after an earthquake and tsunami crippled the plant, Tepco faces a raft of hurdles, including groundwater flooding into damaged reactors, as it works to decommission the complex. The clean-up effort is expected to take decades.

Last month, a 29-hour power supply halt affecting nine facilities, including four spent fuel pool cooling systems, was caused by a rat touching exposed wires in a temporary switchboard, triggering a circuit breaker.

In early April, the No.3 unit's spent fuel pool cooling system stopped, after workers appeared to have had inadvertently caused a power outage when they were trying to install a net to keep small animals from crawling into the reactor building.