Research and Development Relating to the Accident at the TEPCO’s Fukushima Daiichi NPS

1-18 What’s the Point of Analyzing the Fukushima Accident?
－ Review of Five Investigation Reports on Fukushima Accident －from Reactor Building

FO

1F3: ca. 2.0 ca. 3.5 ca. 1.3
1F4: ca. 0.1 ca. 0.5 ca. 6.7
FO

FO: fail open
FAI: fail as isto Venting Lineto Stack(train A)
FOFAIFOFO① FAI
from Suppression Pool

Gravity Damper(NOT equipped at 1F4)SGTS Filters② to StackFOFOFAI
ca.18(1F3)

ca. 0.8 (1F4) ca. 0.7(1F3)
FAI
ca. 1.5

ca.1.0

1F3: ca. 1.6 ca. 3.2 ca. 3.1
1F4: ca. 0.1 ca. 0.5 ca. 5.5

FAI

FO(train B)
from Venting Line

Fig.1-39 Dosimetries at standby gas treatment system (SGTS) filters for 1F3 and 1F4 (unit: mSv/h)
Based on the dosimetries on the SGTS filters at 1F4, it is presumed that venting gases flowed from the 1F3 reactor building throughthe SGTS line(s) into the 1F4 reactor building. On the other hand, venting gas flow through the SGTS lines is assumed to be unlikely
because different trends were observed in the dosimetries on the SGTS filters at 1F3, and the dosimetry downstream of the valve
connected with the stack ( ② in the figure) was significantly low. However, the possibility of venting gas flow should be further examinedbecause the dosimetry of the filter in the center was the highest, and the containment pressure did not exceed the design pressure fora long time.

On March 11, 2011, an earthquake and subsequent tsunami
resulted in a severe accident at the TEPCO’s Fukushima
Daiichi NPS (1F). Four independent committees wereestablished by the Japanese government, the Diet of Japan,
the Rebuild Japan Initiative Foundation, and the TEPCO toinvestigate the accident and published their respective reports.
In addition, the Nuclear and Industrial Safety Agency analyzed
the causes of the accident to obtain the lessons learned from it.
This study reviews the five reports, clarifies the differencesin their positions, from a technological point of view, andidentifies undiscussed issues to obtain insights useful for nearterm regulatory activities, including the accident investigationby the Nuclear Regulation Authority. As a result, the different
positions identified are related to the reason of emergency
diesel generator trips, the reason of manual trip of theisolation condensers at 1F1, reactor depressurization at 1F1,containment integrity and venting operation, and the hydrogenleak path. For example, the hydrogen explosions at 1F1 and1F3 are assumed to be the result of hydrogen leaking directlyfrom the containments, and the explosion at 1F4 is assumed tobe caused by reverse hydrogen flow from 1F3 during venting.
This study pointed out that the contribution of venting gases
should be further examined based on the fact that containment

venting was executed shortly before the explosions at 1F1and 1F3 and on the measured dosimetries on the standby gas
treatment system (SGTS) filters (Fig.1-39).
In addition, the following undiscussed issues are identified.
(1) Adequacy of Operating Procedures Applied at 1F1: Indepth analysis of the basic concepts for preparing theoperating procedures by comparison with those at Tsuruga
1F1 is required.
(2) Design Concepts of SGTS Valves on Loss of Drive
Forces: The design concepts of “fail open” and “fail asis” when the drive force for the valves is lost should beclarified by examining the actual designs at other plants.
(3) Design Concepts of Valves Configured in Containment
Venting Lines: The design concepts for the venting lines,
which consist of one rupture disk and two normallyclosed valves placed in series, should be studied.
(4) Operating Basis of Startup of Containment Cooling
Systems: Although suppression pool cooling was initiatedafter the earthquake at 1F1 and 1F2, it was not startedup at 1F3 considering the effects of tsunami-inducedkataseism. It is necessary to identify the causes of thisdifference.

Reference
Watanabe, N., Tamaki, H. et al., Review of Five Investigation Committees’ Reports on the Fukushima Dai-Ichi Nuclear Power Plant Severe Accident, Nippon
Genshiryoku Gakkai Wabun Ronbunshi, vol.12, no.2, 2013, p.113-127 (in Japanese).
JAEA R&D Review 201329