The temperature is either 88 degrees Celsius, or 20 degrees Celsius, depending on which thermocouple you believe.
Or none of the above, and both thermocouples are broken. Given the previous history (from 2012 when one thermocouples after another was failing inside the Containment Vessel of Reactor 2), it is possible that both are broken.
Mainichi Shinbun (4/7/2015) reports:
Thermocouple inside the Containment Vessel of Reactor 2 at Fukushima I Nuclear Power Plant shows abnormal rise in temperature
On April 7, TEPCO disclosed that a thermocouple inside the Containment Vessel of Reactor 2 at Fukushima I Nuclear Power Plant showed abnormal temperature.
The temperature was 20.9 degrees Celsius at 5AM on April 3 but it rapidly rose to 70 degrees Celsius at 11AM. On 5PM on April 5 it further rose to 88.5 degrees Celsius. Other thermocouples show normal temperatures. TEPCO says "It does not affect the control of the reactor," and is currently investigating.
Here's the latest temperature parameters for Reactor 2 Containment Vessel from TEPCO's plant parameters page, which omits the particular thermocouple that spiked:
So which thermocouple spiked? It is the thermocouple "supply air D/W cooler(TE-16-114K#1)". There is a site that continues to plot charts from the plant parameter data disclosed by TEPCO, and here's the chart plotting all "supply air D/W cooler" thermocouples:
Looking at the chart, it is more likely that this particular thermocouple broke, and the temperature inside the Containment Vessel remains between 20 and 30 degrees Celsius.
Quickly checking the measuring devices installed inside Reactor 2 (information from TEPCO's site), broken and malfunctioning devices are numerous.
As of July 2014, only 8 thermocouples out of 24 inside Reactor 2 Containment Vessel are monitored in accordance with the safety procedures set by regulatory authorities (meaning only 8 are properly functioning), according to TEPCO's information.
In contrast, 16 thermocouples out of 21 are functioning properly in Reactor 1 Containment Vessel, and all 19 thermocouples are functioning properly in Reactor 3 Containment Vessel.