Tokyo Electric Power Co has warned the tsunami-hit Fukushima nuclear plant may not achieve "cold shutdown" by January.
Efforts to decontaminate highly radioactive water at the facility have been delayed by repeated breakdowns of caesium absorption instruments, which have caused water leakage and malfunctioning of pumps.
This has threatened to delay the process of stabilising the stricken plant at the heart of the world's worst nuclear crisis in 25 years.
"It's possible that decontamination may not be completed as planned by the year-end, although we have not yet decided to change the target," a Tepco spokesman said.
"That could affect the cold shutdown process."
His comments follow a similar statement earlier this week from Junichi Matsumoto, a Tepco spokesperson on nuclear issues.
Cold shutdown is usually defined as meaning the water in a reactor is below 100 degrees C at normal atmospheric pressure, making its radioactive fuel safe from heating up again.
Tepco also confirmed it is testing a new radioactive water processing instrument as it struggles to cope with the damage wreaked by the magnitude 9.0 earthquake and tsunami that devastated Japan's northeast coast in March.
With only an estimated 42,000 tonnes of highly contaminated water having been processed by last week, roughly 120,000 tonnes are still left in the basement in turbine buildings and elsewhere.
Tepco had set a goal of processing 200,000 tonnes of radioactive water by year-end since more water will be contaminated in the process of bringing the damaged reactors under control.
Tepco officials and nuclear crisis minister Goshi Hosono plan to hold a joint news conference to explain progress on work to bring the damaged reactors under control.
The test run of Toshiba Corp's caesium adsorption instrument, known as Sarry, is being carried out by running water with low-level radiation through equipment from French nuclear firm Areva SA and U.S. nuclear waste management company Kurion Inc.
It is scheduled to start operating fully later this week, Tepco officials said.
Sarry is relatively easy to maintain because it has fewer pumps, the breakdown of which have been a major reason for delays.
Even if the system's overall operating rate holds steady at 90 per cent, roughly 195,000 tonnes will be processed this year.