Tepco has released the results of isotope testing in the water of the spent fuel pool of unit 4, which holds 1331 nuclear fuel assemblies:

• Caesium 137: 93,000 Bq/l (half life: 30 years)
• Caesium 134: 88,000 Bq/l (half life: 2 years)
• Iodine 131: 220,000 Bq/l (half life: 8 days)

The figures suggests the fuel assemblies are damaged, but Tepco believes the damage is minor and think some of it may have been caused by fragments of the building collapsing into the pool after a hydrogen explosion and fire.

The pressure reading of one of the sensors in the reactor pressure vessel (RPV) of unit 1 has been steadily rising since March 22 even though the temperature reading has been fluctuating (rising and falling) over the same period. This suggests a buildup of hydrogen from a reaction between steam and zirconium or a buildup of hydrogen and oxygen from radiolysis (breakdown of water into its elements by gamma radiation). Unit one is supposed to have suffered the most severe core damage of the three units in operation when the quake struck.

The RPV is designed for a maximum pressure of 8.7 MPa. The pressure reading on April 15 was about 0.953 MPa. If too much hydrogen accumulates, the operators may be forced to vent the reactor pressure vessel and/or the reactor containment. To minimize the risks of a renewed hydrogen explosion, nitrogen has been pumped into the RPV since April 6, but venting from the containment could also release considerable amounts of radioactivity into the atmosphere, as during he first days of the disaster.

Mobile power generators and fire fighting equipment is being relocated to a higher location to be able to cope better with the possibility of a tsunami triggered by another strong aftershock.