Plutonium Inventories at Fukushima Dai-ichi Nuclear Power Plant and Mixed Oxide (MOX) Fuel at Unit 3

By Jay T. Cullen

The purpose of this post is to provide estimates of the plutonium (Pu) isotopes present at the Fukushima Dai-ichi nuclear power plant (NPP) at the beginning of the disaster in March 2011. The post is part of an ongoing effort to communicate facts about Fukushima obtained through scientific study of the impact of the meltdowns on the environment. Comments on this site and in other public forums highlight the fact that Unit 3 at the NPP was burning mixed oxide (MOX) fuel at the time of the accident which, because it is enriched in Pu, suggests that these releases are potentially more harmful. Here I report estimates of Pu present in the reactors (Units 1, 2 and 3) and spent fuel pools (Units 1-4) at the site based on burnup calculations. Because fission of low enriched uranium (LEU) fuel produces Pu isotopes during operation there was a significant amount of Pu on site in Units 1-4. During extended operation a MOX fuel burning reactor can produce multiple times the Pu of LEU but this was not so at the time of the Fukushima meltdowns. The amount of additional Pu present due to Unit 3’s MOX fuel is small compared to the other reactor cores and the inventory of the spent fuel pools. The differences between environmental impact of MOX versus LEU reactor core meltdown in this case are small. Estimates of the release of Pu isotopes from Fukushima, based on measurements of air, soil and water suggest 100,000 fold less was broadcast to the environment compared to Chernobyl and 5,000,000 fold lower than releases from nuclear weapons testing in the 20th century.


The longer lived isotopes of Pu are 238-Pu (half-life 87.74 years), 239-Pu (half-life 24,100 years), and 240-Pu (half-life 6,570 years). The isotopes are alpha-radiation emitters and can represent a significant radiological health risk if internalized. Understanding how much Pu was broadcast into the environment from the damaged units at the Fukushima NPP is therefore a high priority.

A summary of the reactor units 1-6 is provided in the table below:

Boiling water reactors (BWR) like those at the FNPP can burn both low enriched uranium (LEU) or mixed oxide (MOX) fuel. LEU refers to fuel where the content of 235-U has been increased to on the order of 3-5%. MOX is, as its name implies, a mixture of the oxides of U (UO2) and Pu (PuO2) where the presence of Pu at levels ~8% can facilitate fission in un-enriched U. Unit 3 was burning MOX at the time of the disaster while the other units were producing electricity from the fission of LEU.

Calculations of the inventories of the reactor cores (Units 1-3) and spent fuel pools (Units 1-4) were reported by:

Nishihara, K., H. Iwamoto, K. Suyama. 2012. Estimation of Fuel Compositions in Fukushima-Daiichi Nuclear Power Plant. Japan Atomic Energy Agency, Tokai. Japan.

Tables summarizing this information were reported in the very useful text:

Povinec, P.P, K. Hirose and M. Aoyama. Fukushima Accident: Radioactivity Impact on the Environment. 2013. Elsevier Amsterdam.

Here you can find tables that I have modified from the Povinec et al text which represent a resource for those interested in the inventories at FNPP.

Plutonium Isotopes at FNPP

During normal operation in every uranium burning reactor there is fission of uranium isotopes and the formation of heavier isotopes through neutron capture. Most of the fuel in the reactor is 238-U which through neutron capture and successive beta decays becomes 239-Pu which can can capture further neutrons to become 240-, 241- and 242-Pu. Beta decay from there can produce other transuranics and actinides. 238-Pu is formed in a similar series of reactions from 235-U. MOX fuel reactors start with Pu isotopes on the order of ~8%. The sum of Pu isotopes 238, 239 and 240 in the reactor cores of Units 1, 2 and 3 are presented in the pie chart below in PBq (where PBq = PetaBecquerel = 1015 Bq).

Inventories of Pu isotopes 238, 239 and 240 in the reactor cores of Units 1, 2 and 3 at the time of the disaster in March 2011. Total inventory of these Pu isotopes in the cores at the time was 20.6 PBq.

 

The timing of operation of the reactors leading up to the earthquake, tsunami and meltdowns led to conditions where there was approximately and conservatively 20% more Pu isotopes in the MOX burning Unit 3 compared to either Unit 1 or 2. Unit 3’s inventory of Pu accounted for about 38% of the total Pu in the reactor cores of 1, 2 and 3. The amounts of Pu in the spent fuel pools are generally larger than the core inventories in all cases. While the burning of MOX fuel does generate more of the shorter lived transuranics like 242-Curium (half-life 162.8 days) in the case of Fukushima a case can be made that the differences between the LEU’s and the MOX meltdowns are not of any real significance in terms of their environmental impact.

Under the conditions which the meltdowns took place measurements of air, soil and water indicate that very little of the Pu was mobilized from the cores and spent fuel pools at Fukushima relative to more volatile elements like 131-I and 137-Cs. The release of any Pu is a cause for concern and strong reason to monitor conditions at the site and levels in the environment. However, it appears that releases from Fukushima were not significant to this point to be detectable in the North Pacific Ocean where the bulk of radioisotopes broadcast to the environment ultimately ended up.

When one looks at this incident in particular the numbers and the release estimates the fact that Unit 3 was burning MOX fuel at Fukushima seems to have been of little consequence for determining the amount of Pu broadcast as a result of the disaster. While the significance and risk of using MOX fuel deserves debate the relevance of singling out MOX fuel at Unit 3 to the overall impact of Fukushima on the environment is questionable.

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