The document provides an assessment of the status of fundamental safety functions for Unit 1 of the Fukushima Daiichi Nuclear Power Plant as of May 18, 2011. It finds that stable cooling is not achieved and long-term closed-loop heat removal capability has not been established. Containment integrity is achieved but the reactor pressure vessel is assumed to be leaking. Radiation monitoring is available but additional releases are anticipated and radioactive releases should be limited and monitored.

1. TABLE 1. UNIT 1 REACTOR: FUKUSHIMA DAIICHI NUCLEAR POWER PLANT: 18 MAY 2011
ASSESSMENT OF STATUS IN TERMS OF FUNDAMENTAL SAFETY FUNCTIONS FOR ACHIEVING A SAFE STATE
Necessary safety function
Observation Evaluation of safe state TEPCO Roadmap countermeasures
and conditions
Reactor is subcritical and - No power spike or power increase ACHIEVED No additional countermeasures
sub-criticality is - No significant neutron flux measured and reported Caution: reported
Control of
demonstrated and - Short lived fission products (e.g. La-140) are not reported at - Continue detection of The need for boric acid injection is
reactivity
maintained present neutrons and short- being investigated by TEPCO
lived isotopes
Stable cooling shall be - The rate of water injection into the reactor pressure vessel of Unit NOT ACHIEVED Nos 1, 3, 4, 7, 8, 9, 13, 14, 16 and 17
assured: 1 was changed from 8 m3/h to 10 m3/h at 20:28 UTC 15 May and Establishment of a long
- Keep the coolant was still 10m3/h at 20:00 UTC 16 May. term closed-loop heat Actions aimed to establish closed-
temperature sufficiently - Based on available measurements (at 21:00 UTC on 16 May): removal circuit is loop cooling are in progress:
below the boiling point • Reactor vessel temperatures are: 92.8 °C at feedwater nozzle advised 1. Installation of primary and
at atmospheric pressure and 80.7 °C at the lower head secondary closed-loop cooling
- Cover the damaged core • Reactor pressure: (A) 5.96 atm / (B) 14.59 atm) based on systems (planned for 31 May)
2. Flooding of the containment
Residual heal removal
adequately with water available measurements
- Off-site and back-up - In accordance with TEPCO information, after calibration of the to provide water supply for the
power supply shall be reactor water level gauges, the actual water level in Unit 1 primary system
available reactor pressure vessel has been lower than was indicated. The
- Achievement of long results of provisional analysis show that fuel pellets melted and
term closed-loop heat fell to the bottom of the reactor pressure vessel. TEPCO also
removal capability reported that leakage of cooling water from the reactor pressure
vessel is likely to have occurred. TEPCO considers that the
actual damage to the reactor pressure vessel is limited, on the
basis of the temperatures at present measured around the reactor
pressure vessel.
- Off-site power supply and backup power from portable diesel
generators are available
- Fresh water injection is provided; however, closed-loop heat
removal is not yet established
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2. Necessary safety function
Observation Evaluation of safe state TEPCO Roadmap countermeasures
and conditions
- Containment pressure - Pressure in the primary containment vessel is maintained at about ACHIEVED Nos 2, 6, 11 and 15
maintained below design 1.0 atm which is well below the design operating pressure of 4.85 Caution:
Containment
integrity
limits atm - Pressure and H2
- Hydrogen explosion to - Nitrogen injection is maintained concentration of the
be prevented containment to be
further monitored
- Reactor pressure vessel - Reactor pressure vessel is assumed to be leaking most probably PARTIALLY Nos 5, 10, 29–46, 50, 54 and 55
Confining radioactive
(including connected through connected recirculation system (pump seal loss of coolant ACHIEVED TEPCO has on 13 May commenced
systems) should not leak; accident) Caution: the preparatory work for the
material
or if so the leakage shall - Exiting gap in pressure containment vessel is assumed (ongoing - radioactive releases installation of a cover for the reactor
be confined injection of nitrogen has not led to increase of pressure in including venting building of Unit 1.
- Leakages from the pressure containment vessel) operations should be
containment should be limited, controlled and
prevented or controlled, monitored
or shall be confined
- No additional releases - Intermittent releases have been observed PARTIALLY Nos 12, 47–49, 51–53, 55–63
Limiting effects of
shall be anticipated - Radiation monitors are available ACHIEVED
- Radiation monitoring - Reactor pressure vessel and pressure containment vessel are Caution: In order to reduce air radiation levels
releases
measurements shall be assumed to be leaking. - radioactive releases inside the reactor building, a filtered
available - Opening of the airlock in the reactor building did not lead to including venting air circulation system and system
measurable increase in the releases to the environment operations should be circulating outside air through the
limited, controlled and building have been installed
monitored