Science and Technology of Nuclear Installations

Review Article

The Fukushima Event: The Outline and the Technological Background

Table 2

(a) Cornerstone events during Fukushima accident, Reactor Core, Vessel and Containment, Units 1 to 3. (b) Cornerstone events during Fukushima accident, Spent Fuel Pool in Units 1 to 4.

(a)


No.	Event	Unit 1	Unit 2	Unit 3	Notes

1	Earthquake hits the NPP	time “0”			14.46 March 11, 2011.
2	Reactor scram	Within a few seconds after the time			Stop of fission reaction. RP trip. Possible LOCA. Isolation of RPV and of CO.
3	SBO event	Same as above			DBA conditions (expected).
4	Tsunami hits the NPP	55 minutes			Presumed failure of all DG.
5	SBO becomes LOOP	(1–N) hours, where “N” can be between a few and a few dozen.			Following “presumed” attempts to restart DG. SA starts. Mitigation measures needed.
6	Venting to the stack directly from D/W of CO	24 hrs.*	^{, }High pressure in RPV**	^{, }High pressure in RPV**	To protect containment.mitigation measures started.RCIC started.**most of the time.
7	Explosion (I) of RB	25 hrs.		^{, }High pressure in RPV**	15.36 March 12. Possible H2 explosion.
8	Venting to the stack directly from CO.	—		66 hrs.*	11.36 March 12. RCIC lost, HPCI initiated at 12.35 and lost at 02.42 on March 13. 09.20 March 13. CO venting
9	Explosion (II) of RB	—		68 hrs.	11.01 March 14. H2 flame.
10	Venting to CO from RPV.	—	80 hrs.	—	13.25 March 14 RCIC lost detected. 16.34 SRV1 opened, depressurization CO from RPV.
11	Explosion (III) of CO	—	86 hrs.	—	06.10 March 15. PSP damage.
12	Electricity restored in the Control Room	250–350 hrs.*			March 22–26 period. *CO venting to the stack for Unit 1.
13	Stable conditions reached for core cooling	~300 hrs.			March 25–26 period. Continued water injection into the vessel.
14	End of CO leakage	—	620 hrs.	—	April 6.Unit 2. Breakage not repaired yet.
15	Core status	Damaged*			End-of-the-Accident. (20 to 80) % core destroyed. continued steam release. **less in the case of Unit 2.
16	Containment status	**Undamaged	Repaired	**Undamaged
17	Reactor Building status	Damaged***

(b)


No.	Event	Units 1 to 3	Unit 4	Notes

1	Earthquake hits the NPP	time “0”		14.46 March 11, 2011.
2	Occurrence of LOOP	“N” hrs.		See Table 2(a). Mitigation measures.
3	Fire and Explosion (IV) occurred in RB	—	89 hrs.	09.38 March 15. H2 production and radioactivity to environment. SA starts.
4	Injection of water in the SFP	*	200 hrs.	*Started March 31st, 20^th, and 17th, for Units 1, 2, and 3, respectively. Various attempts after this time.
5	Electricity restored in the Control Room	*	~ 430 hrs.	March 29. *see Table 2(a). Connected with stable cooling conditions in SFP.
6	SFP status	Cooling recovered with damaged fuel*		End-of-the-Accident. presumed: majority of fuel bundles affected. *see Table 2(a).
7	Reactor Building status	**	Damaged