TEPCO published a Roadmap document right before leaving for Golden Week vacation. In this document is a 30+ page section of new data for unit 2’s missing fuel.
TEPCO has given varying explanations for unit 2’s meltdown and fuel location. Two muon scans have been completed for unit 2. The first found no fuel remaining in the RPV. A second scan by TEPCO claimed to have found some fuel in the bottom of the RPV, our analysis of the scan found otherwise. It is likely that all of the fuel inside the reactor vessel melted and all of it except for some residues is no longer in the RPV.
The volume of fuel debris inside unit 2 is difficult to calculate due to a number of factors. The debris is spread between multiple areas including the floor grate level, the pedestal floor and whatever debris may have burned down into the pedestal floor. The total volume of the fuel core is known for unit 2 but the exact size of the pedestal diameter is not known.
A fuel debris volume estimate was made for unit 1 based on known data and meltdown events at that reactor. Unit 1 is smaller than unit 2 in both fuel core size and size of the reactor structures. The general reactor building sizes and the fuel core sizes should be something that could roughly scale up for unit 2. Unit 1 estimate showed a fuel volume of all of the fuel and related melted structural materials as 60-100 cm deep.
Inside unit 2 about 50% of the pedestal floor was found to be covered with 70 cm of fuel debris. Additional fuel debris in an unknown volume is on the floor grate level. An unknown amount is burned down into the pedestal concrete basemat. Further fuel debris may be in lower reactor piping systems or the outer drywell floor. Unit 2’s fuel debris volume would also be reduced as the control rod drive array and bottom head of the reactor vessel are still intact. That large amount of metal structural material is known to not be part of the melted fuel debris in unit 2.
What has been found on inspection may be all of the fuel debris for unit 2 if a portion of the material is burned down into the pedestal basemat concrete. In most meltdown scenarios that is a given assumption unless the containment structure was heavily and repeatedly flooded with water at the time the fuel first dropped into the pedestal. With unit 2 that is an unlikely scenario.
There is an alternative possibility that a large amount of the radioactive materials in the fuel vaporized during the meltdown and escaped containment. This concept requires more investigation to confirm vaporization but this possibility for unit 2 is not completely ruled out. Fused microparticles containing nuclear fuel and other meltdown related materials have been found over a wide swath of Fukushima and beyond. Unit 2’s refueling floor blow out panel and reactor well containment gasket are one escape path for micro materials, steam and other gasses. Unit 2’s venting attempts are another concern. TEPCO has claimed the direct drywell venting of unit 2 didn’t work and the rupture disc for this system did not break as intended. TEPCO has provided no conclusive proof of this claim such as photos, video or other tangible evidence for this claim. Due to this, there is still the possibility that unit 2’s venting released some of these fused microparticles of fuel.
The radiation levels found in unit 2’s pedestal including a reading close to the fuel debris pile were between 7-8 Sieverts/hour. The high reading found along the CRD rail in 2017 was between 200-300 Sieverts/hour. These pedestal readings are drastically lower than what would be expected near an unshielded large pile of fuel debris.
By comparison, radiation levels along the outer containment wall in 2012 were within a similar range of the lower readings found on the CRD rail in 2017.
The elephant’s foot at Chernobyl, measured within the first year of the disaster, converted to Sieverts was 100 Sievert/hour.
Underwater readings in unit 1’s torus room near what is suspected fuel debris, taken in 2012 were 100,000 to 1 million Sieverts/hour.
Radiation levels near the fuel debris indicate that the top layers of debris may be mostly metallic materials with little fuel.
Radiation levels indicate that fuel bearing debris is not in the visible layer in the pedestal. (other possible locations – vaporized/vented, beneath the metallic layer, sml amounts in piping).