Experts Tell All On Fukushima Unit 1

The Nuclear Decommissioning Facilitation Corp (NDF) held their annual meeting recently. Presentations by various experts and those involved with the investigations at Fukushima Daiichi give some significant new admissions about what is now known about the disaster.
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Due to the amount of data, we are publishing on each unit separately. Quotes from the report are in italics, our commentary is in bold text below in each section. A conclusion section is available at the end of this report if you wish to jump to that information.

Estimates for the reactor cooling water system:
・A high dose around RCW piping [4]4-7
・Estimated that RCW piping in equipment drain sump was damaged and radioactive materials contaminated RCW system (estimation based on actual measurement)
・ The cause of high dose observed near RCW piping in the reactor building is estimated to be the debris damaging equipment drain sump in the pedestal, resulted in radioactive materials contaminating RCW system. Therefore, debris are estimated to have fallen in the pedestal [28]
・ Fallen debris are estimated to have eroded the pedestal floor and sump to a certain extent [22]

・ pedestal floor – The cause of high dose observed near RCW piping in the reactor building is estimated to be the debris damaging equipment drain sump in the pedestal, resulted in radioactive materials contaminating RCW system. Therefore, debris are estimated to have fallen in the pedestal [28] 
・Estimated that RCW piping in equipment drain sump was damaged and radioactive materials contaminated RCW system (estimation based on actual measurement)

It was noted that pressure and steam may have caused contamination to travel through the RCW system. The “RCW” system appears to either be the system used to spray water into the PCV (containment structure) and the suppression chamber or possibly a system that provided cooling to pump system motors. Our notes showed that the last action of the containment spray system at unit 1 was to turn it on and direct it at the suppression chamber (aka: the torus). This was before the tsunami hit, knocking out power. The loss of power may have made closing the system no longer possible. This would have left a route open to contaminate the piping. 

u1_RCW_coolingsystem_NDF_2017

The diagram (from the report) above shows various locations along the RCW system that were found to have elevated levels of radiation, highlighted in red.

u1_RCW_coolingsystem_NDF_2017_2

The explanation in the above diagram is the assumption for how the RCW system became contaminated. This finding may not indicate anything new and significant related to the meltdown itself. It does explain how certain pipes in the reactor building areas became highly contaminated. It does not explain, if the RCW system was spraying the suppression chamber rather than spraying containment, how it would replenish water.

 

Estimates for the containment top head & reactor well:

These two areas located at the top of the containment structure include the bolt on containment cap, the refueling reactor well area and the concrete cover pieces that are installed on top of the containment cap when the reactor is in operation. This containment cap is a known failure point during a meltdown scenario in a Mark 1 BWR reactor.

These statements were made about the components in this area:

・Estimated a high dose from the possibility of leakage at PCV top head
・Leakage points exist due to deteriorated sealing (steam/FP release)
reactor well
・The temperature immediately above the reactor is higher than those in other areas (10/2011)[20]. It is estimated to be an effect caused by heat generation of FP adhered around the shield plug
・Observed an uplift on the well plugs in the upper and middle stages of reactor [32]

This confirms that the gasket for the containment cap (PCV top head) is failed and allowed steam and fuel particles (FP) to escape the reactor. High levels of radiation near the containment cap were found and considered to confirm this. It is worth noting that in the days following the initial disaster little or no steam was seen coming out of unit 1’s refueling floor level, unlike units 2 and 3 that produced considerable amounts of steam.

The heat found directly above the reactor is assumed to be due to fuel particles stuck to the concrete shield plug. This would again confirm reactor fuel particles were released from containment and potentially to the wider environment.

They also note the displacement of the concrete shield plug, something that was confirmed by earlier visual investigations. All of this appears to confirm that to some extent fuel particles were ejected from unit 1. This failure also allowed the escape of radioactive gasses to the environment during the initial meltdowns. 


Heat Source May Indicate Fuel Mass:

These notes on a discovered heat source may provide hints where some of the melted fuel has deposited. The temperature sensors located in the HVH (Heat Ventilating Handling Units) units inside containment provided data to help estimate a possible fuel location. Based on temperature fluctuations they assume there may be some fuel in the north side of containment, or possibly just the north side in the pedestal area. When looking at the entire reactor building, this would be the side below the spent fuel pool (but inside containment). Fuel debris may be widely dispersed around the containment structure bottom and beyond. Unit 1’s curious lack of an obvious melted fuel mass location has been a long time cause for concern.

Estimated that there is a heat source near CRD piping from HVH temperature change
There is a correlation between the water injection flow rate of the feed water system and some HVH temperature behavior [11] (Estimated that heat sources exist near CRD piping in north) 
Estimates of fuel debris (corium):

The list below is considered to be the mix of melted materials in the reactor pressure vessel. Fuel debris aka: corium found further outside of the reactor vessel could include these material.  The materials list illustrates that the research agencies have established a better idea of what they expect to encounter.

・ Solidified B4C (boron carbide materials)
・ Molten core internals (various structures from inside the reactor vessel)
・ Clad residue (zirconium alloy fuel rod cladding)
・ Powder pellets
・ Pellets (this would likely be fuel pellets)
・ Heavy metal debris
・ Oxide debris (Oxide would be composed of nuclear fuel materials)
・ Fuel rod
・ Burning fuel
・ RPV damaged point Sediment (unknown material)
・ Sound shroud (shroud is a metal cylinder inside the reactor vessel that acts as a sort of liner)
・ CRD (debris inside)
(Control Rod Drive)
・ Sound CRD
・ Damaged CRGT
(Control Rod Guide Tube)
・ Sound CRGT
・ Concrete mixed debris
・ Granular debris
・Oxide debris (porous) (Oxide would be composed of nuclear fuel materials)

 

Estimates for the containment drywell area: 

Multiple locations have been found that indicate routes for water to leak to the torus room. This confirms another route of containment failure or potential routes for fuel to escape containment. There are assumptions that molten fuel exited the pedestal via the lower doorway and deposited on the drywell floor. It is also noted, the pedestal itself that holds up the reactor vessel may have suffered erosion at the base of the walls due to the molten fuel. While multiple comments are made about the possibility of fuel debris in the drywell area, they are still unable to confirm a fuel mass. The mention of “MCCI” refers to the molten corium concrete reaction. This is a process where the molten fuel mass burns and erodes concrete surfaces it comes into contact with. 

・Estimated that PCV had a damage due to water leakage from sand cushion drain pipe (actual measurement and analysis)

・Possibility of diffusion of debris to the D/W floor through the pedestal opening (general estimation/analysis)
・Possibility of part of fuel debris solidified without causing MCCI (general estimation)
・ Fuel debris that caused MCCI is mixed with concrete. (General estimation/analysis)
・Possibility of accumulation in stagnant spot when there are particulate debris (general estimation)
・If water is pooled on PCV floor, particulate debris will be formed.
・Estimated to be general oxide debris solidified with molten fuel (general estimation)
・The result image taken with the underwater CCD camera on D/W floor shows something like deposited sediments (actual measurement) 

・Lower pedestal walls near sump can be partially eroded by MCCI (General estimation/analysis)
・Camera investigation found deposits on the floor [14] 18-29. Second investigation found an increasing tendency [31]. Estimated that it is not a high dose because camera noise is small
・Analysis shows that debris diffused to D/W floor and reached PCV shell [22]
There are some deposits having a certain thickness at the bottom of containment vessel. Deposit seems not to be hard solids as it had a dent when the camera bumped it. Some deposits are bluish fragments. (Possibility of lead)
・Water level about 3 m (similar level of water; leakage from vacuum break tube bellows is observed) [14] 18-29
・From HVH temperature, it is estimated that debris exists near CRD (estimation based on actual measurement and analysis)
・Because a specific HVH thermometer shows high temperature rise when the FDW flow rate is decreased, debris can exist near CRD on the outer periphery (no distinction between adhesion to outer surface and intrusion into inside) or RPV can be damaged directly above it (estimation based on actual measurement)


u1_PCVinvestigation_NDF_2017

 

Estimates for the reactor vessel:

A significant admission in this area is that there may have been a failure in the upper reactor vessel. This could include the rupture of a main steam pipe. During the earthquake all of these lines are closed by MSIV valves to isolate the reactor piping from the turbine building. During the meltdowns the steam relief valves (SRV) were never activated. This series of valves are used to release pressure from the reactor vessel into containment via the torus. This lack of pressure build up in the reactor vessel seems to confirm the conclusion of a pipe break on the reactor vessel.

It is estimated that no fuel remains within the reactor vessel except for some small debris. The various systems inside the reactor vessel including the shroud and pump systems are assumed to be severely damaged.

・ A leak point is estimated to have been formed near the pressure vessel upper part (MS piping, etc.) [29]
・Possibility that bottom drain at the bottom of lower plenum is fragile and damaged (general estimation)
・Possibility that fuel fallen in the lower plenum remains at the bottom of RPV (general estimation)
・If heat transfer from high temperature fuel debris is small, CRGT can be unmelted and remains (general estimation)
・Possibility of forming molten pool in the reactor during accident (general estimation)
・Possibility of shroud breakage (general estimation)
・If the shroud is broken, there is a possibility of breakage of the jet pump resulting from intrusion of molten fuel into the downcomer (general estimation)
・For Unit 1, water injection from CS system started on December 10, 2011. Before that, however, most of the observation points in the containment vessel fell below 100 ℃. Therefore, estimated that almost no fuel remains in the core.
・Estimated that most of fuel melted and no fuel rod remains based on the muon measurement, analysis result, and the fact that a water level is not formed (actual measurement and analysis)
・Abundance of debris is estimated to be slight from the cooling achieved before starting CS injection (12/10/2011)
u1_coremelt_NDF_2017


Estimates for the reactor vessel internal parts :

These components along with parts such as the shroud and jet pumps that reside inside the reactor vessel are assumed to be heavily damaged. This will matter in relation to any decommissioning effort. These pieces could be in unpredictable configurations and conditions making cutting and removal more challenging.  

・ Analysis results pointed out downward shifting due to creep deformation and deformed support casting.
・ Estimated a large amount of FP adhered on separator and dryer
・ Possibility of cesium taken into the oxide layer of steel material
・ Possibility of cesium combined with molybdenum, boron, and silicon

 

Estimates for the torus room (suppression chamber):

The torus room and torus (suppression chamber) estimates give some insight into ongoing concerns about the state of this area. The “high Cs concentration” on building walls inside the torus room follow earlier findings of significant damage, scorching and high radiation.  A 2012 scope inspection of the torus room found what appeared to be fuel debris under the standing water. It is quite possible that some fuel was ejected into the torus room causing these findings. A clear pathway for this has not been found to date. Also mentioned is that the torus tube itself is mostly sound, it does appear to hold water and likely holds at least some fuel debris due to the gas generation and high radiation levels along the tube. 

・From the result of the torus room dose investigation, a high Cs concentration was estimated on the building walls and S/C walls or in accumulated water in S/C [24] 
・ S/C with nearly full water (gas generated in the early stage of the accident remains slightly under nitrogen sealing) [17]
・S/C is estimated to be almost sound

 

Estimates for the pedestal area in containment:

The assumptions for this area may be partially based on their findings from unit 2. Actual inspection inside the pedestal has not been done to date. It is assumed that all the fuel left the reactor vessel and would have melted the structures directly below in the pedestal as was found at unit 2. The larger cause for concern is the amount of erosion to the pedestal base. If this is found to be significant it could cause problems related to supporting the reactor vessel and other structures inside containment. There may need to be inspections specific to this before actual decommissioning efforts take place.

・ Estimated damages caused by fallen fuel debris on grating, TIP piping, and CRD exchanger 
・Water injected into RPV is estimated to be falling on the pedestal 
・ The upper and lower parts of RPV indicate almost the same temperature value [9]
・ A large amount of FP adhesion
・ The chemical form (water-soluble/water insoluble, etc.) of adhered FP is unknown.
・ The degree of re evaporation of adhered FP is unknown
・Lower pedestal walls near sump can be partially eroded by MCCI (General estimation/analysis)


Estimates for the vent/exhaust stack and related systems:

These findings appear to implicate unit 1 as the cause of the high radiation levels in the shared vent tower for units 1 and 2.  A 3 Sv/h reading was found in piping before the SGTS (standby gas treatment system) filters.  High contamination was found further along the pipe system into the tower. It is also claimed that high radiation was found on the down stream side of the SGTS filter banks in unit 2 and that a rupture disc in unit 2 never worked. If this is the case then unit 1 would likely be the cause of the high contamination in both the unit 1 system and the downstream side of the unit 2 system. Based on wind patterns during the days of the individual meltdowns it has been assumed that unit 1 caused most of the high contamination fallout zone that runs north and west of the plant. 

・ Significant contamination up to 3000 mSv/h on near side of SGTS room on the second floor (8/2011)[7]
・ High contamination exceeding 10 Sv/h on the exhaust stack (near SGTS piping joint) shared by Units 1 and 2 (8/2011)[8].Dose dropped to 2 Sv/h (2015/10)[26]
・ Confirmed a deformed fracture on a part of diagonal (support) of the exhaust pipe common to Units 1 and 2. Estimated it to be a damage due to hydrogen explosion [26]
・ Unit 2 SGTS contamination of several Sv/h is estimated to be derived from Unit 1 because the rupture disk of Unit 2 is not damaged
・ Detected Cs134 (826 kBq/cm3) Csl37 (51.9 kBq/cm3) from accumulated water in pit (9/12/2016)[33]

 

Estimates for the HPCI (High Pressure Coolant Injection) system:

These deposits were found on an inspection years ago. At the time this high level of radiation wasn’t mentioned. What the deposits could be isn’t clarified. The HPCI system operation wasn’t attempted at unit 1 during the meltdown response. This pipe is in a small room that includes a personnel air lock for containment. 

HPCI – White powdery deposits (sampled) around Hpa steam pipe pene X-53 (bellows cover, floor, and wall surface). Leakage marks are seen between HPCI piping and bellows cover as well as at the base of bellows cover and biological shielding wall. The highest dose around the base (up to 7 Sv/h) [l5]

 

Estimates for the turbine building:

These two findings confirm some significant events. The 5 Sv/h reading near the SGTS room further confirms that significant contamination left unit 1 via the vent tower. This room sits in a small building that connects the reactor building and the turbine building and was categorized as part of the turbine building reporting in the NDF report.

The relationship between the containment failures into the torus room and a migration path into the turbine building is further confirmed. This links the containment failure to the torus room water and then to the turbine building. Other reports have cited an opening in the lower levels of the turbine building that allowed highly contaminated water to flow out of the building towards the sea front. The groundwater contamination maps further confirm this, that unit 1 may have openly leaked into the groundwater and then to the sea for years before any effort was made to block this.

・Significant contamination up to 5000 mSv/h on near side of SGTS room on the second floor (5/2011)[7]
・ Estimated that high concentration contaminated water flowed from vacuum break pipe bellows and sand cushion
Estimates for decay heat:

The graph below shows the drop in decay heat after shutdown along with heat generated by the zirconium fuel cladding and the remaining water. 

u1_graph_meltdown_NDF_2017

 

Conclusions:
 

  • The reactor water cooling system appears to be contaminated.
  • Containment leaked via the reactor well and has a failed gasket, this likely allowed some amount of fuel particles and radioactive gasses to escape.
  • There may be some amount of fuel collected in the north side of containment.
  • The containment drywell failed into the torus room and torus tube to some extent.
  • The torus room and outer wall contains highly radioactive materials.
  • Contaminated water from the torus room leaked to the turbine building basements.
  • The reactor pedestal may have suffered erosion at the base causing instability.
  • There is some evidence of debris in the drywell but they can’t find a mass of fuel.
  • The reactor vessel likely suffered a pipe break type failure in the upper section, possibly on the steam pipes.
  • No significant amount of fuel remains in the reactor vessel.
  • Reactor vessel internal structures are likely destroyed or heavily damaged.
  • There is an assumption that equipment in the pedestal region is heavily damaged.
  • Unit 1’s vent piping systems are highly contaminated.
  • Unit 1 is the likely cause of the vent tower contamination.

We will continue this series here at SimplyInfo.org by reviewing the same documents for unit 2 and 3. 

 

 

 

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