Temperatures rose unexpectedly in one of the reactors at the embattled Fukushima nuclear power plant this week. The Japanese utility Tepco injected the reactor with boric acid in an effort to prevent a nasty chain reaction.

Starting last Thursday, the temperature at a Fukushima facility reactor began rising steadily even though the whole nuclear power plant has been in a state of cold shutdown since December. A magnitude 9.0 earthquake and tsunami last March caused three of the plant’s reactors to melt down, triggering large evacuations in the area.

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On Monday, the temperature in Reactor Number 2 reached 158 degrees Fahrenheit, well up from where it was only a few days ago. The temperature should have been stable in the reactor, and the utility says it doesn’t know why there’s been a spike.

Operators at the plant responded by injecting boric acid into the reactor, according to Bloomberg’s Tsuyoshi Inajima. Boric acid is used to capture radiation and help to prevent the radiation from leaking, a spokesman for Japan’s nuclear safety agency told the Associated Press after the disaster in March.

The plant also increased the rate of cooling water being injected to try and lower the temperature. Bringing that temperature down is crucial to prevent a state known by physicists as “re-criticality.” This is one of the things nuclear workers fear the most.

Here’s TIME.com blogger Eben Harrell description of criticality: “he fissile material in a reactor core — be it enriched uranium or plutonium — undergoes a spontaneous chain reaction, releasing a flash of aurora-blue light and a surge of neutron radiation.” Criticality happens within fractions of a second and is so unpredictable that it can kill workers without warning, he added.

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But as the head of the Atomic Energy Research Institute at Kinki University told Inajima, there are also enormous risks to the higher water injection rate. Inevitably more radioactive water will accumulate in the basements at the plant.

As of Tuesday, the reactor’s temperature had dropped only slightly to 157.28 degrees Fahrenheit. Tepco’s latest plant status simply says, “We will monitor the progress continuously.”

Photo: A screenshot on from Tepco’s live feed of the Fukushima nuclear power plant reactors showing how they looked on Monday. Credit: Tokyo Electric Power Company, Inc.