Minutes from SAGE update meeting 16 March 2011
Held in 70 Whitehall at 18.00
Attendees
Sir John Beddington GCSA
Nick Gent (HPA)
Jill Mearer (HPA)
Paul Howarth (NNL)
Hilary Walker (DH)
Mike Weightman (HSE/NII)
Mike Griffiths (RIMNET)
By phone
Arywn Davies (Defra)
Colin Potter (HSE)
Lesley Prosser (HPA)
John Cooper (HPA)
Mary Morrey (HPA)
David MaKay (DECC CSA)
Secretariat
Miles Elsden
Elizabeth Surkovic
Key discussion points
- The group agreed that the worst case scenario was for a simultaneous release from storage ponds containing the equivalent of six reactor cores, and the three at risk reactors at the Fukushima facility. Given the latest information of the content of the storage ponds, the group agreed that this would correspond to 9 reactors worth of material.
- The group agreed that the worst case weather conditions should be assumed which would carry radioactive material towards Tokyo. The group agreed that these worst case assumptions would give rise to predicted dose in Tokyo of 15 mSv for adults and less than 30 mSv for children.
- The group agreed that the maximum projected total dose of around 15-30 mSv resulting from the worst case release should be considered minor in health impact terms. The group agreed that the advice during any period of such exposure for British nationals in Tokyo should be to shelter, and children should receive Iodine prophylaxis.
- However, the group agreed that it was manifestly unlikely that all six ponds and all three reactors would fail simultaneously and that a cascade of failures was to be expected. The total cascade period was estimated to be up to 2 weeks.
- Consequently, the group agreed that a more reasonable worst case scenario was for multiple releases some larger from the ponds and smaller from the reactors. A typical sequence would be 9 smaller releases spaced over 1-2 weeks at a rough rate of 1 per day. However, the group agreed that failures would not be regularly spaced.
- The group agreed that in this reasonable worst case, the total amount of radioactive material released, and the maximum dose, would be the same as the first worst case scenario. However, the group agreed it was highly unlikely that the worst case assumption of continuous winds in the direction of Tokyo would hold for this extended period. The group agreed that this meant that the likely dose experienced on the ground in Tokyo would be lower than the estimate of 15-30 mSv for the worst case.
- The HPA told the group that levels for shelter and evacuation were internationally recognised values that balance the radiation hazard against the risk of evacuation. Therefore, while the maximum predicted levels are close to the lower range considered for evacuation, this would normally only be considered for a small number of people in a well controlled environment. Evacuation of a large, unprepared city population would only usually be considered at the top of the range (i.e 300 mSV).
