As we work through refining what took place at Fukushima Daiichi, old data can have new meaning in light of additional information. The recent unit 2 inspection is again bringing additional insight.
The new information that leans towards unit 2 having either fuel debris buried in the building basemat concrete or below the building may have some old information to back that up.
Paul ran some calculations based on this that caused us to review old data with some surprising results.
First, the basic information for unit 2:
Weight of UO2: 107 t
Number of fuel assemblies (body) 548
Fuel assembly length (m) 4.47
Length of active fuel: 3710 mm
The amount of uranium loading (t) : 94 t
Averaged uranium enrichment (wt%): 3.80 wt%
Burnup (9×9(B)): averaged 45.0 GWD/t, maximum 55.0 GWD/
This data is from our reactor page for unit 2.
These are Paul’s calculations based on the total fuel in unit 2’s reactor core at the start of the disaster.
“I did some calculations on how long an ellipsoid geometrically correct molten blob of fuel could remain critical if it progressed below the concrete basement base upon enrichment average and approximate amount of U235 in blob assuming 80 % made it down. The result is about 485 to 500 days which could mean March 14, 2011 plus 1.4 years of sustained criticality before shutdown due to loss of U235. Does this seem to fit any data?”
Does it fit any data? Actually, it does.
About Paul’s quote above, the concept of a geometrically correct mass of fuel means that this fuel is in a shape that allows it to produce fission aka: criticality.
We have documented in previous articles and reports that molten nuclear fuel and the related melted structures from inside the reactor will stratify. The heavier materials will sink to the bottom. Nuclear fuel would be the heaviest, sinking to the bottom. The estimate that 80% of the actual uranium containing fuel would be below the reactor base mat concrete is based on this understanding. The heaviest materials sink to the bottom of the molten mass and would be the first to reach the pedestal floor then burn down into the base mat or further. For unit 2, it is now known that the failure hole in the reactor vessel is directly above one of the two sump pits in the pedestal floor. This finding further increases the odds of the behavior we assume took place inside unit 2. The sump pit acts as a container for molten fuel, keeping it in one location where it can burn down rather than spread out across the floor and cool.
Paul also cited this set of behaviors as adding to the stratification phenomenon and the potential for criticality or fissioning within the fuel debris:
“B4C (in stacked pellets in BWR control rod tubes) at temperatures over 1200 C can chemically fuse with metals found in stainless steel (control rod tubes). This new boron alloy has much lower densities than UO2 ceramic fuel pellets (including UO2 fused with Zr) and will tend to stratify and float on molten fuel mass (thus removing criticality control from fuel).
Density of boron material aprox 3.5 g/cm3. Density of fuel materials aprox 10 g/cm3.”
B4C is the boron control rod material that suppresses fission in the reactor core. The stratification process causes the nuclear fuel and the control rod materials to separate from each other. This leaves the nuclear fuel without anything to actively suppress fission. Nuclear fuel, if it is arranged into the right shape can begin fissioning again.
We went back and looked at our old data for unit 2. There were some events where fission was detected in the reactor containment along with some other data that showed some sort of activity was going on related to the molten fuel. That data fits within the 1.4 year time frame. Below we document those events.
We had detected a clear pattern of data changes within the reactors when strong aftershocks hit the plant. We documented some behaviors in all three units. Unit 2’s data showed the following:
Earthquake 1 on July 31 and earthquake 2 on August 12.
– #2:– temperature drop in CRD (control rod device) housing top (orange bubbles in plot):
— from 121.0 centigrades (7/30 23:00) to 110.0 centigrades (7/31 5:00) exactly on eq1
— from 98.0 centigrades (8/11 23:00) to 87.0 centigrades (8/12 5:30) exactly on eq2
— pressure decreasing since eq1:
— from constantly 135 kPa before 7/30 23:00 to 120 kPa now core pressure a (eq1)
— from constantly 134.325 kPa (101.325 + (0.033* 1000)) before 7/30 5:00 to 120.325 kPa + (0.019 * 1000) now torus pressure (eq1)
— radiation rise and sudden drop in drywell b:
— eq1: radiation slowly rising from 14.1 Sv/h before 7/28 11:00 (eq1) to 15.7 Sv/h on 8/11 23:00 (before eq2)
— eq2: sudden drop from 15.7 Sv/h (8/11 23:00) to 0.00207 Sv/h (8/12 5:30) exactly eq2
— rapidly rising again after eq2: 0.00207 – 0.00218 – 0.832 – 1.35 – 1.8 – 2.18 – 2.5 – 2.81 – 3.07 – 3.31 – 3.53 – 3.75 – 3.95 – 4.15 – 4.32 –
4.47 – 4.61 – 4.74 Sv/h now
The temperature, pressure and radiation fluctuations tied to the quakes show that the quakes appear to have disrupted the fuel mass.
A month later, a researcher at the University of California, San Diego documented additional data tied to these two events. He found increased radiation levels in Tokyo and Yokohama tied to these quake events. He also cites increases in iodine 131 found in sewage sludge in Tokyo and Kanagawa that appear to be tied to these events. This researcher clearly considers the evidence he found to be evidence of recriticality during these two earthquakes.
November 2, 2011
NHK reported that xenon and other fission products were detected at unit 2. TEPCO said they didn’t think the fuel had begun to re-melt. Boron was added to the water injection in hopes of suppressing this. The issue wouldn’t necessarily be the fuel re-melting, but the fissioning or recriticality in the fuel mass.
Unit 2 began losing containment pressure 4 days prior. Temperature readings at the reactor vessel support skirt sensor had gone up 5.6 degrees.
Reports later in the day TEPCO admits “it was undeniable that fission reaction had occurred.”
A NISA official makes a rather ominous statement around the same time:
“Senior official Yoshinori Moriyama said it is difficult to determine the amount of nuclear fuel remaining in the containment vessel of the No.2 reactor. He said his agency will assess the situation and judge if it is still possible to stably lower temperatures in the reactor and containment vessel.”
He didn’t say remaining in the reactor vessel, he states the containment vessel. He appears to admit that reactor fuel may have escaped containment. TEPCO still refuses to admit this is even a possibility.
A month prior, a citizen based radiation monitoring project found iodine 131 in local rice. To find iodine 131 so long after the meltdowns indicate it could have been new releases from the plant but that would have to be caused by recent fissioning. The only other possibility would be such a massive amount of iodine 131 that traces were still available to detect 8 months after the disaster. The half life of iodine 131 is 8 days.
November 3, 2011
TEPCO attempts to dismiss the problem as being only spontaneous fission, not a recriticality of the fuel. Insiders at TEPCO disputed this claim as did the nuclear regulator in Japan at the time (NISA). NISA demanded access to all the data so they could do their own analysis. Krypton 85 experienced a 100 fold increase from November 1 to 2 at unit 2. This wasn’t the general public that was concerned with what was going on, those dealing with the disaster had concerns that this was a serious problem.
February 6, 2012
Temperature increases are seen inside unit 2. TEPCO responds by injecting Boron. Boron injection is intended to suppress fission. Workers mentioned that some of this may have been kicked off by changes to the water injection system.
February 7, 2012
The temperature inside unit 2 reached a post meltdown record high of 72.2 Celsius. This caused TEPCO to begin sampling for evidence of fission byproducts at the blow out panel opening and the containment gas system for unit 2. Both of these actions indicate TEPCO had concerns there may be fissioning or a recriticality happening at unit 2.
By the end of the day temperature readings began to decrease. TEPCO claimed they could not detect any leaks of fission byproducts. As TEPCO was injecting boron, this is unsurprising that temperature levels began to go back down.
All of these events fit within the 1.4 year time frame and were evidence of fissioning or criticality events at unit 2.
This article would not be possible without the extensive efforts of the SimplyInfo research team
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