- The spent fuel pool at Fukushima Daiichi Nuclear Power Plant Unit 4 has partially achieved necessary safety functions, but further actions are needed. - TEPCO assumes the spent fuel assemblies are intact based on water sampling, but more assessment is required. Fresh water is being sprayed into the pool to cool it. - The pool temperature on May 9th was reported as 81°C after spraying, though no recent data was available. Continued water spraying and makeup is needed to fully cool the pool.

1. TABLE 4. UNIT 4 REACTOR, SPENT FUEL POOL: 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 and TEPCO Roadmap
Observation Evaluation of state
conditions countermeasures
Spent fuel pool shall be subcritical - According to TEPCO statement and to its PARTIALLY ACHIEVED: Maintaining the geometry of spent
Keff < 0.95 (design value), and sub- assessment from the water sampling of the spent CAUTION fuel pool (including aftershocks)
criticality shall be demonstrated fuel pool, TEPCO assumes that the fuel - Risk of criticality a priori excluded reinforce by countermeasure 20,
Control of reactivity
and maintained assemblies are ‘intact’ (sampling 26 April 2011) by TEPCO (provided that 21, 26
- New sampling performed (280 mL on 7 May). geometrically safe configuration is Make-up/cooling of spent fuel pool
The activity concentrations (Cs-134/Cs-137) are maintained, etc.) by external water injection
increasing compared with previous - Further assessment needed (countermeasures 18, 22, 28)
measurements. Analysis by TEPCO is - In order to avoid any potential No borated water injected
continuing problem due to further aftershocks, No countermeasure explicitly
- Only fresh (non-borated) water is being sprayed make-up with borated water to avoid mentioned
into the spent fuel pool (120 tonnes on 11 May) any risk of criticality is advised
- Hydrazine is being continuously injected - Monitoring of neutrons and short
- No neutron flux measured reported lived isotopes to be considered
Stable cooling shall be assured Latest temperature reported: 81 °C after spraying PARTIALLY ACHIEVED: Cooling of spent fuel pool by
- Spent fuel pool temperature on 9 May (no updated data available) CAUTION external water injection
should be similar to the spent (countermeasures 18, 22, 28)
fuel pond design values Sampling of steam/pool water and
Residual heal removal
Make-up (with borated water) to be
(approx. 30 °C) continued measurement of radioactive
- To recover the spent fuel pool Spent fuel pool temperature is not materials (countermeasure 19)
water level adequately with reported on May 16 Circulation cooling of spent fuel
(borated) water to about 7 m pool
above the top of active fuel. (countermeasures 23, 24, 25, 27)
- To restore spent fuel cooling
[Type text]

2. Necessary safety function and TEPCO Roadmap
Observation Evaluation of state
conditions countermeasures
- To recover the spent fuel pool - Fuel assemblies are covered by water ACHIEVED: - Cooling of spent fuel pool by
Confining radioactive material
water level adequately to CAUTION external water injection
ensure some form of shielding - to continue spraying and/or make (countermeasures 18, 22, 28)
above the top of active fuel. up - Circulation cooling of spent
- to continue monitoring of major fuel pool (countermeasures 23,
spent fuel pool parameters 24, 25, 27)
(temperature, water level) - Sampling of steam/pool water
- to improve radiation protection and measurement of radioactive
materials (countermeasure 19)
- Reactor building cover
(countermeasures 5, 50, 54,
55)
- Releases shall be only within - Uncontrolled release are not expected so far PARTIALLY ACHIEVED: - Nos 10, 12, 29–46 cooling of
the agreed allowed legal limits - Radiation monitoring positions are available CAUTION spent fuel pool by external
- Radiation monitoring - According to a TEPCO statement, fuel - Radioactive releases to be prevented water injection
Limiting effects of releases
measurements shall be assemblies are assumed to be ‘intact’. by sufficient water make-up (countermeasures 18, 22, 28)
available However, further assessment by TEPCO of - Reactor building cover to be pursued - Circulation cooling of spent
- Leakages from the spent fuel the latest sampling should confirm this fuel pool (countermeasures 23,
pool should prevented or statement 24, 25, 27)
controlled, or if not shall be - Sampling of steam/pool water
confined and measurement of radioactive
material (countermeasure 19)
- Reactor building cover
(countermeasures 5, 50, 54, 55)