Textbox 1: Krypton-85 accumulation in the atmosphere
Krypton-85 is a long-lived radioactive isotope which is naturally released into the atmosphere in small quantities
(Harrison and Apsimon, 1994), approximately 5.2 1013 Bq/yr and, in larger quantities artificially (1017-1018 Bq/yr). It has
steadily accumulated in the atmosphere since 1945 (from <0.2 Bg/m3), when anthropogenic nuclear activities started, and
reaches 1.3 Bq/m3 nowadays.
Ion production
The principal concern with krypton-85 release is not a radiological/medical one, as population doses are small (Boeck,
1976), but the possible disturbance of the global electrical system (Legasov et al, 1984, Tertyshnik et al., 1977). It is
known from nuclear weapon testing (Huzita, 1966) that atmospheric radioactivity increases air?s natural conductivity.
The conductivity of air is proportional to the (small) ion concentration. These ions are formed naturally in atmospheric air
at a rate (near the surface) of about 10 ion-pairs cm-3 s-1 (Chalmers, 1967). There are three major sources of these ions:
airborne alpha radiation, cosmic rays and terrestrial gamma radiation. Near the Earth?s surface, gamma radiation from the
soil is the chief source of ionization, due to the nuclear decay in the Earth?s crust. This accounts for about 80% of the
ionization near the surface. The remaining ionization is caused by cosmic rays, whose intensity increases greatly with
height. Ionization over the oceans is considerably lower, since there is no gamma contribution and a greatly reduced
amount of airborne alpha radiation.
Removal
The removal of ions can take place through two mechanisms: ion-ion recombination and ion-aerosol attachment. In the
last case the particles become electrically charged (Fuchs, 1963). In the steady state, the bipolar ion production rate q per
unit volume and the ion loss rates are balanced, given by (Harrison and Apsimon, 1994):
q-αn2-βnZ=0 (1)
Where α is defined as the ion-ion recombination coefficient (1.6,10-6 cm3.s-1, e.g. Gringel et al, 1978) and β is the
attachment coefficient between an ion and aerosol particle. β depends on the aerosol particle radius and charge (Gunn,
1954). Z is aerosol particle number concentration per unit volume, and n is the average ion number concentration. At
higher aerosol concentration (i.e. 10 μg/m3 with 0.2 μm radius particles) n is dominated by aerosol-ion attachments. From
the formula it becomes clear that a change in conductivity can occur due to an increase in the production rate q (by, for
example the additional ionization caused by krypton-85) or a change in aerosol concentration (increase will decrease
conductivity).
Change in conductivity by krypton-85
The amount of extra ionization caused by the beta radiation can be found by using the average beta energy (0.249 MeV)
for krypton-85. For a krypton-85 concentration of Ckr Bq/m3 the ionization rate is:
qkr=(2.49.105/35).Ckr. (2)
Assuming a surface ionization rate qo of 10 ion-pairs cm-3.s-1 the change in ion production is:
dq/q0 = 7.11.10-4 Ckr. (3)
Over the oceans, where q0 is about one-fifth of its continental value, the fractional change will be corresponding larger.
The concentration of krypton falls with density (height) of air:
Ckr(z)= c(0)e-z/8561, where c(0) is the surface concentration. (4)
Combining ion production from the crust and cosmic ray, a maximum share of krypton-85 ion production can be expected
at a height of 500-1500m, about twice the value at the surface and at a surface concentration of 1.3 Bq/m3 , a change of
2? in ion concentration at 1000 m can be expected . Locally, near a nuclear waste processing plant, the share can
increase to approximately 20% (Clarke, 1979). Note that the conductivity above mountainous (remote) areas (Antarctic,
Himalaya, determines the Earths resistance and interaction with the ionsphere.
Consequence for the atmospheric system
? It is generally assumed, although surrounded with some uncertainty and controversial (Illingworth and Latham,
1975), that thunderstorms provide the earth with a small negative charge. The slight conductivity of the
atmosphere (see above) creates a small, opposite ?fair weather current? (E= + 100 V.m-1, J ~2 pA.m-2 at the
surface). Considering the earth as a spherical capacitor (with Ct ~2.8 Farads) it would lose it?s charge (τ ~667 s)
in about an hour. The earth needs therefore continuously be charged by approximately 2000 thunderstorms
(Schonland, 1953). A change of 0.1% could therefore be compared with the equivalent of two continually active
thunderstorms. The interaction between an increasing conductivity and thunderstorms remains unclear although
there are suggestions (Spangler and Rosenkilde, 1979) that it would weaken thunderstorm lighting.
? Recently there have been some suggestions that charged ions can, even at small concentrations, can have a
(substantial?) effect on the formation of certain type?s of clouds (Marsh and Svensmark; 2000, Harrison, 2000;
Carslaw et al., 2002) . If confirmed this would imply that a changing concentration of krypton-85 could affect to
some extent the earth?s climate.
? MNP 2007
Parts of this publication may be reproduced, on condition of acknowledgement: 'Netherlands Environmental Assessment Agency, the title of the publication and year of publication
