⁸⁵Kr (krypton-85)

Background

⁸⁵Kr is a radioactive isotope of the noble gas krypton. It is produced naturally, as well as by human activities, mainly the nuclear industry. Due to its chemical stability as a noble gas, it is an ideal tracer of environmental processes. Downsides of ⁸⁵Kr as tracer in hydrology are the large amount of water required for measurement and the difficult methodology (ultra low-level b -counting). There are only very few laboratories worldwide that can measure ⁸⁵Kr.

Natural ⁸⁵Kr production

⁸⁵Kr is produced by interaction of cosmic neutrons with ⁸⁴Kr in the atmosphere and by fission of U and Pu.
The nuclear reaction in the atmosphere is as follows:

⁸⁴Kr (n,g ) ⁸⁵Kr The (spontaneous) fission reactions include:

Isotope	% fraction of fissions producing ⁸⁵Kr
^{235 U}	1.3
^233U	6.9
^239Pu	2.1

Radioactive decay

⁸⁵Kr decays by b^- decay with a maximum energy of 0.675 MeV.

Its half life t is 10.76 years.

Natural global inventory

The global inventory of natural ⁸⁵Kr is about 10 Ci from the (n,g ) reaction and 4 Ci from fission.

Anthropogenic ⁸⁵Kr production

The main sources of ⁸⁵Kr are:

Nuclear weapons testing
Plutonium production
Nuclear power generation

In nuclear reactors, ⁸⁵Kr is produced by fission and contained in the fuel rods. Ca. 1 % of the ⁸⁵Kr escapes into the atmosphere from the fuel rods. The remaining 99% of the ⁸⁵Kr is released during the reprocessing of the fuel rods.

Atmospheric Inventory for 1976

Source	Estimated atmospheric inventory [Mci]
Natural	1.4 10^-5
Nuclear explosions	2-4*
Nuclear power plants	11-28*
Plutonium production	30-49

* Represents ⁸⁵Kr released; Significant amounts of ⁸⁵Kr remain in unprocessed fuel rods.

Atmospheric concentrations

Determined by

Releases (Fig.)
Vertical mixing (see vertical profile; Fig.)
Hemispheric and inter-hemispheric mixing (Fig.)
Local sources (Fig.)
Radioactive decay

Determined by measurements

Typically well known in clean air (Fig.)
Small hemispheric gradients (Fig.)
Inter-hemispheric gradients are of the order of xxxx percent

Dating with ⁸⁵Kr

If the ⁸⁵Kr/Kr ratio of a water sample is measured, it can be matched with the activity measured in the atmosphere as a function of time. A logarithmic plot of the atmospheric ⁸⁵Kr activity allows to graphically include the effect of radioactive decay by simple lines (Fig.). The formation age of groundwater can then be read off this graph.

Measurement

Technique: low-level b - counting using gas-filled proportional counters
Water sample size: ca. 100 to 250 liters
Detection limit: 0.5 dpm cm^-3 Kr; ca. 1 % of present atmospheric concentration.

There are only very few laboratories worldwide that can measure ⁸⁵Kr. Only very few ⁸⁵Kr measurements have been performed on groundwater samples.

Experiments are underway to explore new measurement techniques, specifically AMS (Accelerator Mass Spectrometry) and RIS (Resonance Ion Spectroscopy). If these methods are successful, ⁸⁵Kr will be one of the best tracers for groundwater flow studies in shallow aquifers.

85Kr (krypton-85)