The pressure readings of the Reactor 1 RPV had been used by TEPCO to defend their position that the RPV was not damaged, or was damaged but not by much. It had been also used by TEPCO to claim that there was water inside the RPV. Say goodbye to both fantasies for good.
From METI's press releases on the Fukushima Plant Parameters on June 3 and June 5:
Reactor 1 RPV Pressure Measurement in megapascals (MPa):
June 3:
Gauge A: 0.578 MPa g
Gauge B: 1.573 MPa g
June 5:
Gauge A: 0.025 MPa g
Gauge B: discontinued
MPa g = MPa abs - atmospheric pressure (0.1013 MPa)
("g" stands for "gauge, and "abs" stands for "absolute". )
So, if you see the RPV number in MPa g equal to zero, it means the RPV pressure is equal to the atmospheric pressure. If the number is negative, like the Reactors 2 and 3 RPVs, the RPV pressure is lower than the atmospheric pressure.
The next fantasy to go will be the pressure of the Reactor 1 Containment Vessel. It is again being used as the reason to believe the melted core is still within the Containment Vessel. (Never mind that the contaminated water is leaking inside the reactor building...) TEPCO says they will install the pressure gauge for the Containment Vessel soon.
For now, the pressure numbers for Containment Vessel show that it is above the atmospheric pressure in the Drywell, and almost at the atmospheric pressure in the Suppression Chamber. That's not surprising about the SC, as we've seen the gushing of "hot" steam (4 sieverts/hour) from the compartment that houses the SC.
Reactor 1 Containment Vessel Pressure Measurement in megapascals (absolute), from METI's press release on June 5:
Drywell: 0.1301 MPa abs [or 0.029 MPa g]
Suppression Chamber: 0.110 Mpa abs [or 0.0087 Mpa g]
(Why can't they just measure everything in either MPa g or MPa abs?)