Status of the Spent Fuel At Fukushima Dai-ichi: How much was released to the environment?

By Jay T. Cullen

The purpose of this post is to answer the question posed in the title by summarizing a recently published peer reviewed study in the journal Nuclear Engineering and Design. The diary is part of an ongoing effort to communicate results from scientific studies aimed at understanding the impact of the Fukushima Dai-ichi meltdowns on the environment. The paper by Jäckel compares measured and predicted activities of reactor products 134-Cesium (134Cs half life ~2 years) and 137-Cs (137Cs half life ~30 years) in the reactor cores and spent fuel to measurements in the spent fuel pools (SFPs) of Units 1, 2, 3 and 4 at the site to determine how much spent fuel radiocesium was released after the accident. The comparison indicates that only very minor damage to the spent fuel occurred during the accident and subsequent clearing work such that at most about 1% of the Cs inventory from a single bundle (in Unit 2 SFP) was released to the cooling water. The short answer to the question is that not very much of the spent fuel was released at all and the bulk of releases originated from the reactor fuel in Units 1, 2 and 3 at the time of the accident. This is consistent with the results of measurements made of Fukushima derived radionuclides in air, soil and water worldwide since March 2011.

In the early days and weeks following the disaster which began with the earthquake and tsunami waves on March 11, 2011 a major release from the SFP of Unit 4 was assumed due to lack of data. However, the first evidence based investigations relying on models and measurements of 131-Iodine (131I half life ~8 days), 134Cs and 137Cs in releases supported the case that little damage to spent fuel in the reactor buildings had occurred (see studies by Wang et al. (2012) and Machiels et al. (2012) for details).

Jäckel (2015) uses the fact that reactor fuel that had recently been undergoing fission in March 2011 in Units 1, 2 and 3 had 134Cs/137Cs ratios close to 1 while fuel in the SFP would have ratios much lower than 1 given the relatively fast decay of 134Cs. Thus, change in the 134Cs/137Cs with time (and the presence of short lived 131I) in the SFPs water can be interpreted to determine if spent fuel contributes to the measured contamination.

Unit 1
A hydrogen explosion at Unit 1 occurred on March 12, 2011 at 15:36 local time resulting in significant releases of radionuclides to the environment. Activity measurements of radiocesium from Unit 1’s SFP over 18 months since March 2011 are shown in the figure below. Activities were determined on water samples taken from the pool but details of the measurements were not provided by TEPCO.


Radiocesium activities and ratio with time in Unit 1 SFP over time. The solid black line indicates the predicted change in the 134-Cs/137-Cs ratio owing to the decay of the isotopes.

Given that absolute activity does not increase in the SFP and that the 134Cs/137Cs ratio does not deviate markedly from the predicted decay, significant releases owing to damaged fuel rods in the pool is unlikely and almost of the activity in the pool is the result of fallout from the fuel in the melted reactor in Unit 1.

Unit 2
An explosion in the suppression chamber of Unit 2 occurred on March 15, 2011 at approximately 06:00. The evolution of 134Cs and 137Cs activities and their ratios are shown in the following figure.


Activities of radiocesium, their ratio and the solid line indicating predicted relative decay in Unit 2 SFP.

After roughly 240 days after the accident began most of the activity in the SFP of Unit 2 was removed by an ion exchange process followed by desalination to remove salt water from the SFP. The rapid drop in both 134Cs and 137Cs activity is indicative of this removal. After about 1 year post March 2011 the 134Cs/137Cs ratio began to decline more quickly than the theoretical decay curve indicating release of radiocesium from damaged spent fuel. Activities of the isotopes continued to decrease because of desalination of SFP water. The increase of activity from day 500 forward likely reflects washout from damaged fuel rods in the SFP after the desalination process was halted. The final activities measured of roughly 100 Bq mL-1 represents the upper bound for the release from the fuel rods. The increase in 137Cs and decrease in the radiocesium ratio can be explained by the release of about 1% of one 9 x 9 fuel assembly in the pool assuming a mean burn up of 55 MW d per kg.

Unit 3
Despite the explosion (March 14, 2011 at 11:01) and debris that fell into SFP3 the relative amounts of 134Cs and 137Cs compare very well to theoretical decay curve up to, including and after the period at 300 days where the cooling water was subjected to cleaning using ion exchange.


Radiocesium activities and ratio compared to theoretical decay curve for Unit 3 SFP

These results suggest that either the fuel sustained little in the way of mechanical damage or that the amount of radiocesium released was small compared to the ~103 Bq ml-1 remaining activity in the SFP water roughly 2 years after the disaster. Increases in activity (but in a ratio consistent with core fuel contamination) during the period after ion exchange reflect radioactive debris introduced to the pool during efforts to remove debris in and around the pool.

Unit 4
Unit 4 experienced a hydrogen explosion on March 15, 2011 at 06:00. Compared to other SPF pools the amount of contamination initially measured in the pool water was low (~ three orders of magnitude less, ~102 Bq ml-1 compared to ~105 Bq ml-1).

Ratio and activities of radiocesium compared to theoretical decay curve in SFP of Unit 4 with time after the disaster

The starting 134Cs/137Cs ratio in the pool was identical to the one observed in SFP of Unit 3 supporting the idea hydrogen and aerosols from Unit 3 had leaked to Unit 4 and were responsible for the explosion of the building and contamination of the Unit 4 SFP. The uncertainty and scatter of the data appears to be higher than for the other SFPs likely because of the smaller overall activities. About 7 months after the accident debris removal work started in earnest at Unit 4 and lasted for about a year. Less sampling was carried out during this time but increases in activities in the pool are consistent with contaminated debris being introduced to the SFP. Some slight damage to the spent fuel must have occurred to explain the lower than expected ratio of 134Cs/137Cs but this damage appears to be less than that experienced in Unit 2 given the relatively small increases in total activities of radiocesium.

Conclusions
Addressing the stability of and containment of the SFPs is important given the high activities of long lived radionuclides present in the pools compared to the inventories of the reactor cores. For an accounting of these inventories please consult the following resource. By comparing the activities of 134Cs and 137Cs and their ratio to one another the extent to which SFP inventories were mobilized during the accident can be assessed because spent fuel has fundamentally lower 134Cs/137Cs ratios than recently active core fuel given the shorter half life of 134Cs. Measurements of the activities in the SFPs show that:

  1. Units 1 and 3 do no show any significant deviations from ratio decay curves indicating that within measurement uncertainty little damage to spent fuel occurred and that releases resulted from their damaged reactor cores
  2. Unit 2 SFP had activities that indicated small releases of Cs from the spent fuel that amounted to ~100 Bq ml-1 corresponding to a small percentage from a single rod
  3. Contamination of Unit 4 SFP came from the reactor of Unit 3
  4. Very low activities were measured in Unit 4 SFP before fuel removal suggesting little spent fuel damage. The last 137Cs measurement in the pool was ~600 Bq L-1

Jäckel’s results are broadly consistent with measurements made outside the buildings of contamination in air, soil and water where radiocesium activity ratios were ~1 after the disaster. This ratio traces the great majority of contamination of the environment following Fukushima to the reactor cores of Units 1-3 rather than any significant mobilization of spent fuel at the site.

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3 thoughts on “Status of the Spent Fuel At Fukushima Dai-ichi: How much was released to the environment?”

  1. Jackel says there was an explosion in unit #2 on March 15, 2011, at about 6am. However, the sound heard from the unit #2 containment was concurrent with when unit #4 had its hydrogen explosion. Tepco has maintained that the unit #2 sound was a result of geologic sonic transmission (the four units are built on the underlying bedrock) causing the “Impulsive sound” heard by unit #2 staff. Subsequent investigations show no indication of an explosion in the unit #2 suppression area of its containment at 6am on 3/15/11.

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