Tokyo Outsider

Analysis and translation by a tatami-chair observer of East Asian economics and security.

Fukushima Dai-ichi worst case measures (UK gov, Guardian FOIA request)

The Guardian has published documents today showing worst-case scenario calculations and safety measures for UK citizens in Tokyo following the crisis at Fukushima Dai-ichi (obtained under the Freedom of Information Act).

Transcript follows:

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Assumed Source Term

We now believe that 3450 assemblies in ponds is a figure across all the 6 units’ ponds (and not at each Unit)

Assuming little has been sent to the common pond since March 2010 – estimate circa 4000 spent fuel assemblies in the 6 ponds in March 2011

In addition, Unit 4 was on outage and had a full core offload (extra ~500 assemblies into the ponds)

Therefore, the assumption is 4500 spent fuel assemblies split across of 6 ponds (approx 8 cores in ponds). The working assumption for the worst case is that each reactor fuel pond release is the equivalent to 1/3 of the caesium inventory of a reactor core (lower because of cooling and decay of iodine-131). More detail is given in Annex A.

Dispersion Modelling (HPA – SEER model)

Dispersion modelling was performed by the UK Health Protection Agency (HPA) using a simplified single wind-speed, wind-direction model. Clearly this represents an extreme weather case and will add a significant safety margin to the predicted dose levels. More advanced dispersion modelling using the Met Office NAME model is currently being performed. Further data will be provided when available.

Doses in Tokyo in event of Worst Case Scenario release

The worst case release is taken to be the three reactors that had been operating prior to the earthquake and all six cooling ponds.

Release assumed involving reactors and cooling ponds
The worst case scenario underpinning this advice is as follows, as advised by NII.

  • The pressure vessels on Units 1, 2 and 3 at the Fukushima site rupture, releasing 10% of all the radioactive caesium-137 and iodine-131 in the core, as advised by NII to be 3.2 1016 Bq caesium-137 and 2.4 1017 Bq iodine-131 per reactor (NII advises these radionuclides will contribute 90% or more of the total dose from all the radion uclides that could be released from the ractors). This is a total release of 9.6 1016 Bq caesium-137 and 7.2 1017 Bq iodine-131.
  • The working assumption is that each of the fuel ponds has 1/3 of the radiological inventory of ar eactor core for caesium-137 with effectively no iodine-131 (because of decay). Then in the fuel ponds at Units 1, 2, 3, 4, 5 and 6 the amount of caesium-137 released, is 3.7 1017 Bq per pond (NII advises this radionuclide will contribute 90% or more of the total dose from all the radionuclides that could be released from the ponds). This is a total release of 2.2 1018 Bq of caesium-137.
  • The overall total release is 2.3 1018 Bq caesium-137 and 7.2 1017 Bq iodine-131.
  • All this radioactive material is blown in the direction of Tokyo under weather conditions that maximise the amount of material that reaches Tokyo, ie straight line trajectory, Pasquill Category D, windspeed 5m/s, no rain.

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Estimated Doses
Doses have been estimated integrated over the first two days. It is assumed that further protective measures could be implemented in the event they are required to protect against doses being received from longer term exposure to residual contamination on the ground and from ingesting contaminated food.

Doses have been estimated for a child outdoors continuously for the full two days.

More than 90% of dose over the first two days would be inhalation. Therefore, assuming rainfall occurring or not occurring over Tokyo would make little difference to the dose over this period.

Worst case doses (to a child outdoors when the plume passes) are estimated to be around 20mSv effective dose and 140 mSv thyroid dose.

Release assumed involving a single reactor
In discussion with NII, it has been agreed that a 10% inventory release from a single reactor may not be adequately ‘worst case’. This is because, in reality, a wide range of radionuclides would be released, not just caseium-137 and iodine-131. It is judged that the doses from these two radionuclides would contribute most of the dose, but there is considerable uncertainty surrounding the contribution that would be made by other radionuclides. This uncertainty arises from lack of knowledge of the likely amounts of these radionuclides in the core. Therefore for calculating the worst case doses in Tokyo from a release from a single reactor, releases of 10%, 50% and 100% of the likely inventory of caesium-137 and iodine-131 have been assumed. These are: 3.2 1016, 1.6 1017 and 3.2 1017 Bq caesium-137, and, 2.4 1017, 1.2 1018 and 2.4 1018 Bq iodine-131. The weather conditions are the same as before.

Estimated Doses
Worst case doses (to a child outdoors when the plume passes) are estimated to be around 5, 25 and 50mSv effective dose and 50, 250 and 500 mSv thyroid dose.

Release assumed involving a single cooling pond
For calculating the worst case doses in Tokyo from a release from a single cooling pond, a 100% release of the likely inventory of caesium-137 has been assumed. This is: 3.7 1017 Bq caesium-137. The weather conditions are the same as before.

Estimated Doses
Worst case doses (to a child outdoors when the plume passes) are estimated to be around 1mSv effective dose and 1mSv thyroid dose.

Protective action criteria
The UK has well-proven criteria for taking protective measures in the event of a serious radioactive release. The emergency protective measures that are most helpful in providing protection against exposures to a dispersing plume of radioactive material immediately before and during the first day following arrival of such a plume are:

  • evacuation (best carried out before the plume arrives)

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  • sheltering, in solid (ie brick/stone, not wooden), airtight buildings with ventilation systems turned off (provides protection against inhaling the plume and from external radiation exposure)
  • stable iodine tablets (provides protection against inhalation of radioactive iodine in the plume).

Evacuation avoids all dose, if it is carried out before the plume arrives. If this is not possible, it is better to shelter people while the plume is overhead and then, when the plume has passed, to consider whether evacuation should be implemented to protect against material deposited on the ground or potential further releases.

Sheltering in solidly constructed, air tight buildings can reduce external doses by around a factor of 10 or more, and inhalation doses by a factor of 2 to 3. It is important to shelter before the plume arrives and to ventilate buildings thoroughly after the plume has passed, in order to gain the maximum protection.

Stable iodine only protects against doses from inhaled radioactive iodine. If taken shortly before or shortly after the plume arrives, it has the potential to prevent virtually all of the dose. If taken within a few hours of the plume arriving, it still has the potential to prevent around half of the dose.

The criteria for taking these actions are specified in terms of the doses to young children as these are the most vulnerable group. They are specified as a range. This is because it is important to consider the harmful consequences of taking a countermeasure as well as the benefits, when deciding what is best to do. If doses are expected to be above the lower level, then the protective measure is advised if it is straightforward to implement (ie the dis-benefits of taking the measure are low). If doses are expected to be above the upper level, then the protective measure is advised, so long as the protective measure can be implemented in time to be effective. The dose ranges are:

Evacuation: 30mSv – 300mSv effective dose
Sheltering: 3mSv – 30mSv effective dose
Stable iodine tablets: 30mSv – 300mSv thyroid dose.

Conclusion
If the dose of 20mSv effective dose and 140mSv thyroid dose to a child, predicted for the worst case scenario involving releases from the reactors and fuel ponds occurring together, were predicted to be likely to received by a population group in the UK, then this would almost certainly trigger a decision to shelter them and issue stable iodine tablets to them.

If doses of 5-50mSv effective dose and 50-500mSv thyroid dose to a child, predicted for the worst case scenario involving a release from a single reactor occurred, were predicted to be likely to received by a population group in the UK, then this would almost certainly trigger a decision to shelter them and issue stable iodine tablets to them. A decision to evacuate a group as large as the probable number of British nations in Tokyo would probably not be taken to avoid an effective dose of 50mSv.

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If the assumption that iodine-131 is not present in significant quantities in the cooling ponds is correct, then any release from the cooling ponds would not cause a significant exposure.

Any decision on evacuation would need to take account of the likelihood of the release. It should be noted that the doses calculated here are for a worst case scenario, as advised by NII.

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Annnex A

Assemblies v Core size

Unit 1: 1380 Mwth
Unit 2- 5: 2381 Mwth (170% of Unit 1)
Unit 6: 3293 Mwth (239% of Unit 1)

Average number of assemblies per core from 16 Nov 2010 presentation = 576 or 570

Unit 1 – 400 assemblies
Units 2 to 5 – 548 assemblies
Unit 6 – 810 assemblies

Milestone Point Unit 1 – 400 assemblies per unit

Possible split of fuel across units based on thermal power: 10% of Unit 1, 17% Units 2, 3, 4 and 5, 23% Unit 6

Basis

Starting point is Tokyo Electric Power Company Presentation on the internet 16 Nov 2010 – quoting position March 2010

States that 3,450 assemblies is in the “spent fuel pool at each reactor unit”, 4098 in dry cask and 6291 in the common pool. 700 assemblies generated each year for storage.

The confusion is whether 3450 is per unit or across each all 6 units.

GRS feel 3450 is too large for one pond ~ around 8.5 cores worth if 400 assemblies per unit (Milestone Point Unit – similar 1970 design o Unit 1)

Calculation

Gesellschaft für Anlagen und Reaktorsicherheit (GRS) in Germany guess mass of uranium per assembly is around 200 kg
Milestone design figure: core loading at rated power – 78 200 kg U
Per assembly = 78200 / 400 = 195.5 kg U per assembly

Compare with figure in March 2010 presentation, 1760 ton U total on site.

1760 / 0.1955 = 9000 assemblies

Summary on March 2010 presentation says 10,149 assemblies total on site

██████████’s desktop calculation

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  • 3450 assemblies, at 560 per core, equates to just over 6 core loads
  • In the presentation, the stored capacity figure is added to the common pool total, and so seems to be the total amount. This (8310 assemblies) equates to just under 15 core loads, or 2,5 core loads.
  • A capacity of 2.5 core loads per reactor fuel pond seems an eminently sensible design principle. You would need enough room for 1 core load during refuelling outages, space for the cooling fuel less than 19 months old (about half a core – see next bullet) and room for manoeuvre when stacking (about 1 core load seems sensible).
  • Fuel is discharged at a rate of 700 assemblies per year across the site, i.e. 120 per reactor, which is 20% of the core. This seems reasonable based on UK PWR strategies. Based on a 13 month refuelling strategy, there would be 2-3 batches of 120 assemblies that are less than 19 months cooled in the pool at any one time, i.e. about half a core.
  • The spreadsheet suggests that the common pool is full to the gunnels. In March 2010 there wasn’t enough room for a full year’s site discharges (only room for 500 assemblies compared to 700/year required) – so presumably the site would have had to have had a storage strategy for older fuel utilising the space in the reactor ponds. This would explain some of the extra 0.5 cores per reactor that appear to be being stored if 3450 is interpreted to be the total figure.
  • Equally, the March 2010 snapshot will likely include assemblies in temporary storage during a refuelling outage at one of the reactors, which again would contribute to the 3450.

Other info – EDF have a figure of 514 assemblies in Unit 3 pond

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This entry was posted on June 21, 2011 by in English.

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