# Peter Lux

## Cs-137 Inventories in Spent Fuel

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 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 over estimate 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)

### 2 Responses to “Cs-137 Inventories in Spent Fuel”

• Graham Ford says:

Dear Peter,

Thankyou for your calculations. My rather rougher computation agreed with yours (I did mine first and sought out yours to check).
With 331 PWRS, 23 CANDUs, 19 RMBKs aournd the world, with most of their fuel in storage (240,000 t) the Cs+Sr load of these fuel stores represents a continent or world killing boobytrap, waiting for a nuclear bomb to release. The levels of long term soil contamination would make food lethal. Most of northern humanity (the white folks) would die. How to prevent? No idea. Do you?

• Pete says:

It would take very high doses of radiation to make food ‘lethal’. However, it would very significantly increase the risk of cancer, heart disease and other illnesses.