A recent paper estimated krypton-85 activity in a cubic metre of air at 1.31 Bq. See: Variability of atmospheric krypton-85 activity concentrations observed close to the ITCZ in the southern hemisphere.

Measurements between August 2007 and May 2010 covered three wet seasons. The mean activity concentration of krypton-85 measured during this period was 1.31±0.02BqmThe measurement, done over 3 years found krypton-85 levels were stable. neither increasing nor decreasing by much. I'll assume that krypton-85 released in balanced by decay.^{-3}. A linear model fitted to the average monthly data, using month and monsoon as predictors, shows that krypton-85 activity concentration measured during the sampling period has declined by 0.01Bqm^{-3}per year.

##### How much krypton-85 could there be in the atmosphere?

The surface density of air = 1.217 kg/m^{3}

Total mass of the earth's atmosphere = 5.1 × 10^{18} kg

Let's assume 1.217 kg/m^{3} of surface air has a krypton-85 activity of 1.31 Bqm^{-3}

Let's next assume that krypton-85 activity is the same throughout the air. This will overestimate krypton-85 because its heavier than air. It's almost twice as heavy as carbon dioxide.

Proceeding with our over-estimation. Total activity of Kr-85 in all the atmosphere:

= (1.31 Bqm^{-3}) × (5.1 × 10^{18} kg) / 1.217 kg/m^{3}

= 5.49 × 10^{18} Bq

The Specific Activity of krypton-85:

= 400 (Ci/g) [Ci = Curie]

= 1.48 × 10^{13} Bq/g

So our over-estimation for the amount Kr-85 in earth's atmosphere:

= (5.49 × 10^{18} Bq) / (1.48 × 10^{13} Bq/g)

= 3.71 × 10^{5} g

= 371 kg

##### How much krypton-85 is made each year

Krypton-85 is about 0.3% of fission products. Let's assume there is 400 GWe of nuclear reactor capacity on earth. That a 1GWe NPP operating over a year produces just less than 1 ton of fission products. Let's call that 400 tons fission products per year for all the world's reactors. That's works out at 1200 kg of krypton-85 made each year.

But all of krypton-85 does not leak into the atmosphere. Most of it is in sealed casks. The amount that leaks must be about the same as the amount that decays.

t_{½}(Kr-85) = 10.7 years

λ(Kr-85) = 0.693 / 10.7 years = 0.064766355 years

^{-1}

##### How much Kr-85 is left after 1 year:

Fractional proportion at time t = N(t) / N(0) = e^{-λ t}

= e

^{-0.064766355 years¯¹ × 1 years}

= e

^{-0.064766355}

= 0.93728643

##### How much Kr-85 leaks?:

= 371 kg × (1 - 0.93728643)= 23.27 kg

##### Note: Estimating the amount of fission products made/year

This depends upon the total capacity of all the world's nuclear reactors. There are:

- 435 commercial nuclear power reactors operable in 31 countries, with about 375 GWe of total capacity.
- 180 nuclear reactors power some 140 ships and submarines.
- 240 research reactors

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