In my previous post, I showed a method of getting a rough idea about how much Cs-137 there is in spent fuel as it leaves the reactor. Once it leaves the reactor, the Cs-137 decays with a half-life of about 30 years. Therefore, the amount of Cs-137in the spent fuel removed on the first year of operation would have decayed to one quarter of that amount after 60 years of operation. That produced in the 30th year of operation would have decayed to one half that value when the reactor is stops operating.

We can just put this into a spreadsheet and add the cumulative amounts year by year.

I have already worked out how much spent fuel is/may be produced by Sizewell B and Sizewell C. Also, we know how the Cs-137 will decay (see Rates of Decay).

We can now calculate the number of Cs-137 atoms in the spent fuel. However, the figures usually quoted is the activity of the Cs-137 in spent fuel. To get this, we can just multiply the number of atoms by the decay constant.

Below is a plot of the Cs-137 activity for the proposed reactors at Sizewell C

The  total inventory at Chernobyl was about 280PBq (280 x 10¹⁵ Bq) although it is estimated that only 85PBq was released.

The Cs-137 at Chernobyl was all in the reactor core. Since the two reactors proposed for Sizewell have a combined power three times that of the Chernobyl plant and that about 1/3 of the fuel is replaced each year, we would expect that Sizewell would have the same amount of Cs-137 in its spent fuel ponds after about a year as that contained at Chernobyl.

To put this in perspective, the “operational criteria for evacuation” set by the IAEA¹ is 1MBq/m² . Let us say that my figure is a bit of an overestimate and that there will actually be a maximum of 1×10¹⁹Bq of Cs-137 at Sizewell C&D. If this was spread evenly over an area it could contaminate 1×10¹³m2 which is 1×10⁷Km² – or about 40 times the area of the UK.

If anyone wants to check my calculations of play about with the data, I have uploaded the spreadsheet here.

¹ IAEA says Fukushima fallout warrants more evacuation, New Scientist, 2011 (http://www.newscientist.com/article/dn20324-iaea-says-fukushima-fallout-warrants-more-evacuation.html)