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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

conditions Observation Evaluation of state TEPCO Roadmap countermeasures

Con

trol o

f rea

ctiv

ity

Spent fuel pool shall be subcritical Keff < 0.95 (design value), and sub-criticality shall be demonstrated and maintained

- According to TEPCO statement and to its assessment from the water sampling of the spent fuel pool, TEPCO assumes that the fuel assemblies are ‘intact’ (sampling 26 April 2011)

- New sampling performed (280 mL on 7 May). The activity concentrations (Cs-134/Cs-137) are increasing compared with previous measurements. Analysis by TEPCO is continuing

- Only fresh (non-borated) water is being sprayed into the spent fuel pool (120 tonnes on 11 May)

- Hydrazine is being continuously injected - No neutron flux measured reported

PARTIALLY ACHIEVED: CAUTION - Risk of criticality a priori excluded

by TEPCO (provided that geometrically safe configuration is maintained, etc.)

- Further assessment needed - In order to avoid any potential

problem due to further aftershocks, make-up with borated water to avoid any risk of criticality is advised

- Monitoring of neutrons and short lived isotopes to be considered

Maintaining the geometry of spent fuel pool (including aftershocks) reinforce by countermeasure 20, 21, 26 Make-up/cooling of spent fuel pool by external water injection (countermeasures 18, 22, 28) No borated water injected No countermeasure explicitly mentioned

Res

idua

l hea

l rem

oval

Stable cooling shall be assured - Spent fuel pool temperature

should be similar to the spent fuel pond design values (approx. 30 °C)

- To recover the spent fuel pool water level adequately with (borated) water to about 7 m above the top of active fuel.

- To restore spent fuel cooling

Latest temperature reported: 81 °C after spraying on 9 May (no updated data available)

PARTIALLY ACHIEVED: CAUTION Make-up (with borated water) to be continued Spent fuel pool temperature is not reported on May 16

Cooling of spent fuel pool by external water injection (countermeasures 18, 22, 28) Sampling of steam/pool water and measurement of radioactive materials (countermeasure 19) Circulation cooling of spent fuel pool (countermeasures 23, 24, 25, 27)

Necessary safety function and conditions Observation Evaluation of state TEPCO Roadmap

countermeasures

Con

finin

g ra

dioa

ctiv

e m

ater

ial - To recover the spent fuel pool

water level adequately to ensure some form of shielding above the top of active fuel.

- Fuel assemblies are covered by water ACHIEVED: CAUTION - to continue spraying and/or make

up - to continue monitoring of major

spent fuel pool parameters (temperature, water level)

- to improve radiation protection

- Cooling of spent fuel pool by external water injection (countermeasures 18, 22, 28)

- Circulation cooling of spent fuel pool (countermeasures 23, 24, 25, 27)

- Sampling of steam/pool water and measurement of radioactive materials (countermeasure 19)

- Reactor building cover (countermeasures 5, 50, 54, 55)

Lim

iting

eff

ects

of r

elea

ses

- Releases shall be only within the agreed allowed legal limits

- Radiation monitoring measurements shall be available

- Leakages from the spent fuel pool should prevented or controlled, or if not shall be confined

- Uncontrolled release are not expected so far - Radiation monitoring positions are available - According to a TEPCO statement, fuel

assemblies are assumed to be ‘intact’. However, further assessment by TEPCO of the latest sampling should confirm this statement

PARTIALLY ACHIEVED: CAUTION - Radioactive releases to be prevented

by sufficient water make-up - Reactor building cover to be pursued

- Nos 10, 12, 29–46 cooling of spent fuel pool by external water injection (countermeasures 18, 22, 28)

- Circulation cooling of spent fuel pool (countermeasures 23, 24, 25, 27)

- Sampling of steam/pool water and measurement of radioactive material (countermeasure 19)

- Reactor building cover (countermeasures 5, 50, 54, 55)