Tepco handout (pdf), summary translation by Fukushima Diary, Jul 21, 2015 (emphasis added): Tepco announced Fukushima plant area has irregularly sunk since 311… The report reads Reactor 1 turbine building sank by 730 mm [2.40 ft], Reactor 2 by 725 mm, Reactor 3 by 710 mm, Reactor 4 by 712 mm.
IAEA Headquarters (pdf), 2015: We know that the buildings will decay and become less stable… there is the dilemma of 1) gathering more information… and 2) acting earlier and maybe not having enough information to make good decisions.
IAEA Nuclear Energy Series (pdf), 2014: The impact of the salt on the corrosion of structural materials had to be assessed and measures taken accordingly to retain integrity.
Lake Barrett, Tepco adviser (pdf): Reactor building structure has likely been degraded… Explosions Weaken RB Structure… Aftershock May Cause Building Failure… — Issues: … Aftershock Structural Integrity… — Safety Challenges: … Containment Degradation
US National Research Council, 2014: Substantial structural damage occurred… particularly Units 3 and 4… The explosions [were] extremely destructive. The complex structure of the lower part of the reactor buildings is well suited to cause flame acceleration… Ironically, having a strong structure with multiple compartments can greatly enhance the damage… this result, although not intuitive, is now well established.
Kazuhiro Suzuki, IRID managing director (pdf), 2014: Estimation of structural strength decline by sea water inflow; Evaluating device/structural integrity and remaining life…
Sugiura Machine Design Office: We obtained results [using a] flying robot. We already have started to work on plant deterioration investigation with major manufacturer.
IRID 2014 Annual Symposium (pdf):
- p. 94: Assessing structural integrity of RPV/PCV… data on corrosion rate will be collected… to evaluate aseismatic strength, taking into consideration long-term wall thinning by corrosion… stainless steel [components] may already be cracked
- p. 95: Overall structural integrity… Building behavior analysis (building damage simulation)… Influence of corrosion [and] high-temperature strength deterioration
- p. 98: Structural integrity of PCV structures… Corrosion wall thinning… Estimated thinning of Unit 1 dry well [and] suppression chamber… Generated stress… of the suppression chamber support structures was higher [than allowable]… reinforcement (such as burying the torus chamber with cement materials, etc.) will be studied
- p. 99: Structural integrity of RPV pedestal… influences of corrosion by molten fuel debris are not taken into account and further study is needed
- Part 6: “One more important point I need to cite is to assure the stability of the site… because of the presence of the ocean water, corrosion could take place… preventative measures against the corrosion need to be taken.“
- Part 85-87: “Next is assessing structural integrity of RPV and PCV [and] get qualitative data of corrosion rate. There is sea water injected so corrosion may gradually proceed… To be prepared against future possible earthquakes we have to evaluate whether this is tolerant or not… We must consider corrosion.”
- Part 91: “PCV [integrity] is generally alright, but in some parts — for instance the column support of the suppression chamber — it [doesn't meet standards].”
- Part 92: “This is the pedestal of RPV… The molten debris may be causing corrosion.”
Status of R&D Projects Related to Debris Fuel emoval