Under radiolysis refers to the cleavage of chemical bonds under the influence of ionizing radiation , mainly radicals or ions formed. Usually the term is used to refer to the radiolysis of water .

The name is made up of the parts “radio” [from the Latin radius “ray”] and “lysis” [from the Greek λύειν (lýein) “(to) solve; to separate ”, λύσις (lýsis) “ to dissolve ”] together.

Radiolysis of water takes place in several steps, which are listed below.

Ionizing radiation (e.g. γ radiation ) can stimulate or ionize water molecules :

${\ displaystyle \ mathrm {H_ {2} O} \ {\ xrightarrow [{\ qquad}] {\ gamma}} \ \ mathrm {H_ {2} O} ^ {*}}$
${\ displaystyle \ mathrm {H_ {2} O} \ {\ xrightarrow [{\ qquad}] {\ gamma}} \ \ mathrm {H_ {2} O} ^ {+} \ {+} \ \ mathrm {e } ^ {-}}$

The H 2 O + ion reacts very quickly (within 10 −14  s ) with water:

${\ displaystyle \ mathrm {H_ {2} O} ^ {+} \ {+} \ \ mathrm {H_ {2} O} \ {\ xrightarrow [{\ qquad}] {}} \ {\ cdot} \ mathrm {OH} \ {+} \ \ mathrm {H_ {3} O} ^ {+}}$

The excited water molecules dissociate  to radicals within 10 −14 −10 −13 s:

${\ displaystyle \ mathrm {H_ {2} O} ^ {*} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H} {\ cdot} \ {+} \ {\ cdot} \ mathrm {OH}}$
${\ displaystyle \ mathrm {H_ {2} O} ^ {*} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H_ {2}} \ {+} \ \ mathrm {O} {\ cdot}}$

Provided they have enough energy, the electrons released during the ionization of water molecules can excite or ionize further water molecules. After they have largely lost their energy, they are first solvated . This process takes place within 10 −12  s.

Further follow-up reactions are:

${\ displaystyle 2 \; \ mathrm {e_ {aq} ^ {-}} \ {+} \ 2 \; \ mathrm {H_ {2} O} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H_ {2}} \ {+} \ 2 \; \ mathrm {OH} ^ {-}}$
${\ displaystyle \ mathrm {e_ {aq} ^ {-}} \ {+} \ {\ cdot} \ mathrm {OH} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {OH} ^ { -}}$
${\ displaystyle \ mathrm {e_ {aq} ^ {-}} \ {+} \ \ mathrm {H_ {3} O} ^ {+} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm { H} {\ cdot} \ {+} \ \ mathrm {H_ {2} O}}$
${\ displaystyle \ mathrm {e_ {aq} ^ {-}} \ {+} \ \ mathrm {H} {\ cdot} \ {+} \ \ mathrm {H_ {2} O} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H_ {2}} \ {+} \ \ mathrm {OH} ^ {-}}$
${\ displaystyle 2 \; \ mathrm {H} {\ cdot} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H_ {2}}}$
${\ displaystyle 2 \; {\ cdot} \ mathrm {OH} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H_ {2} O_ {2}}}$

Because of the large number of competing reactions, the yields of the individual products depend heavily on the reaction conditions. Typical values ​​for pure liquid water when irradiated with γ or β - radiation are listed in the following table:

Product yield when pure liquid water is irradiated with γ or β - radiation
product G in µmol / J
${\ displaystyle \ mathrm {H_ {2}}}$ 0.047
${\ displaystyle \ mathrm {H_ {2} O_ {2}}}$ 0.073
${\ displaystyle \ mathrm {e_ {aq} ^ {-}}}$ 0.28
${\ displaystyle \ mathrm {H} {\ cdot}}$ 0.062
${\ displaystyle {\ cdot} \ mathrm {OH}}$ 0.28

From the values results, for example, that during the irradiation of 1  l (water mass m  = 1  kg ) at a dose of D  = 1  Gy  = 1  J / kg , a hydrogen - amount of substance of n  = 0.047  .mu.mol arises:

${\ displaystyle n (\ mathrm {H_ {2}}) = m \ cdot D \ cdot G (\ mathrm {H_ {2}}) = 1 \; \ mathrm {kg} \ cdot 1 \; \ mathrm {J / kg} \ cdot 0 {,} 047 \; \ mathrm {\ mu mol / J} = 0 {,} 047 \; \ mathrm {\ mu mol}}$

Molecular oxygen (O 2 ) is not a primary product of radiolysis; however, it arises from the following reactions:

${\ displaystyle \ mathrm {H_ {2} O_ {2}} \ {+} \ {\ cdot} \ mathrm {OH} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {HO_ {2} } {\ cdot} \ {+} \ \ mathrm {H_ {2} O}}$
${\ displaystyle \ mathrm {HO_ {2}} {\ cdot} \ {+} \ {\ cdot} \ mathrm {OH} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H_ {2} O} \ {+} \ \ mathrm {O_ {2}}}$
${\ displaystyle 2 \; \ mathrm {HO_ {2}} {\ cdot} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H_ {2} O_ {2}} \ {+} \ \ mathrm {O_ {2}}}$
${\ displaystyle \ mathrm {H_ {2} O_ {2}} \ {+} \ \ mathrm {HO_ {2}} {\ cdot} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H_ {2} O} \ {+} \ {\ cdot} \ mathrm {OH} \ {+} \ \ mathrm {O_ {2}}}$

In addition, reverse reactions of the radiolysis products also occur, so that water is produced again:

${\ displaystyle \ mathrm {H_ {2}} \ {+} \ {\ cdot} \ mathrm {OH} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H} {\ cdot} \ { +} \ \ mathrm {H_ {2} O}}$
${\ displaystyle \ mathrm {H} {\ cdot} \ {+} \ \ mathrm {H_ {2} O_ {2}} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H_ {2} O} \ {+} \ {\ cdot} \ mathrm {OH}}$
${\ displaystyle \ mathrm {H} {\ cdot} \ {+} \ {\ cdot} \ mathrm {OH} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {H_ {2} O}}$
${\ displaystyle \ mathrm {H_ {3} O} ^ {+} \ {+} \ \ mathrm {OH} ^ {-} \ {\ xrightarrow [{\ qquad}] {}} \ 2 \; \ mathrm { H_ {2} O}}$

Therefore - if the radiolysis products are not removed (e.g. escape as a gas) or react with other substances - an equilibrium of the various reaction products is established with continuous irradiation.

As living organisms largely consist of water, the biological radiation effect of ionizing radiation is not only based on direct radiation effects, but also on the chemical reactions of the reactive oxygen species formed in the water by radiolysis . These reactive molecules can diffuse and thus indirectly damage the DNA of the cells through further reactions , which in turn can lead to cell death . This is used, for example, in radiation therapy with photons to fight tumors .

### Nuclear technology

Radiolysis of water already takes place in normal operation in all nuclear reactors moderated or cooled with water . For this reason, the live steam of a boiling water reactor also contains so-called "radiolysis gas " ( hydrogen and oxygen ). In contrast, in the pressurized water reactor, a small excess of hydrogen is metered into the reactor coolant in order to suppress the formation of corrosive oxidizing agents (especially OH , H 2 O 2 and O 2 ) according to the reverse reactions mentioned above .

An accumulation of ignitable radiolysis gas mixtures should be avoided as far as possible in order to exclude an explosive reaction of hydrogen with oxygen.

${\ displaystyle 2 \; \ mathrm {H_ {2}} \ {+} \ \ mathrm {O_ {2}} \ {\ xrightarrow [{\ qquad}] {}} \ 2 \; \ mathrm {H_ {2 } O}}$

On December 14, 2001, however, such a radiolysis gas reaction occurred in the Brunsbüttel nuclear power plant , which destroyed an approximately 2.7  m long section of the cover spray pipe.

Radiolysis of water must also be taken into account when considering design-basis accidents (e.g. hypothetical loss-of-coolant accidents). The following sources in particular must be taken into account for hydrogen formation:

• Radiolysis in the reactor core
• Radiolysis in the sump of the containment
• Radiolysis in the fuel storage pool

When calculating the hydrogen formation, a conservative G value of G (H 2 ) = 0.44 molecules / 100  eV must be assumed.

The radiolysis of water should not be confused with the formation of hydrogen through the exothermic reaction of zirconium with water vapor, which can occur in the event of serious accidents in nuclear reactors:

${\ displaystyle \ mathrm {Zr} \ {+} \ 2 \; \ mathrm {H_ {2} O} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {ZrO_ {2}} \ {+ } \ 2 \; \ mathrm {H_ {2}}}$

A well-known product of the radiation-chemical reaction of oxygen is ozone . As early as 1911, Samuel C. Lind described the radiation-chemical yield of ozone formation.

${\ displaystyle \ mathrm {O_ {2}} \ {\ xrightarrow [{\ qquad}] {\ gamma}} \ 2 \; \ mathrm {O}}$
${\ displaystyle \ mathrm {O} \ {+} \ \ mathrm {O_ {2}} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {O_ {3}}}$

When air or similar gas mixtures of nitrogen and oxygen are exposed to ionizing radiation , nitrogen oxides (mainly nitrogen dioxide ) are formed according to the following reactions:

${\ displaystyle \ mathrm {N_ {2}} \ {\ xrightarrow [{\ qquad}] {\ gamma}} \ 2 \; \ mathrm {N}}$
${\ displaystyle \ mathrm {O_ {2}} \ {\ xrightarrow [{\ qquad}] {\ gamma}} \ 2 \; \ mathrm {O}}$
${\ displaystyle \ mathrm {N} \ {+} \ \ mathrm {O_ {2}} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {NO} \ {+} \ \ mathrm {O} }$
${\ displaystyle 2 \; \ mathrm {NO} \ {+} \ \ mathrm {O_ {2}} \ {\ xrightarrow [{\ qquad}] {}} \ 2 \; \ mathrm {NO_ {2}}}$
${\ displaystyle \ mathrm {NO_ {2}} \ {+} \ \ mathrm {N} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {N_ {2} O} \ {+} \ \ mathrm {O}}$
${\ displaystyle \ mathrm {NO_ {2}} \ {\ xrightarrow [{\ qquad}] {\ gamma}} \ \ mathrm {NO} \ {+} \ \ mathrm {O}}$
${\ displaystyle \ mathrm {NO_ {2}} \ {\ xrightarrow [{\ qquad}] {\ gamma}} \ \ mathrm {N} \ {+} \ 2 \; \ mathrm {O}}$
${\ displaystyle \ mathrm {O} \ {+} \ \ mathrm {NO_ {2}} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {NO} \ {+} \ \ mathrm {O_ { 2}}}$
${\ displaystyle \ mathrm {NO_ {2}} \ {+} \ \ mathrm {N} \ {\ xrightarrow [{\ qquad}] {}} \ 2 \; \ mathrm {NO}}$
${\ displaystyle 2 \; \ mathrm {NO} \ {+} \ \ mathrm {O_ {2}} \ {\ xrightarrow [{\ qquad}] {}} \ 2 \; \ mathrm {NO_ {2}}}$
${\ displaystyle \ mathrm {NO_ {2}} \ {+} \ \ mathrm {N} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {N_ {2} O} \ {+} \ \ mathrm {O}}$
${\ displaystyle \ mathrm {N_ {2} O} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {N_ {2}} \ {+} \ \ mathrm {O}}$
${\ displaystyle \ mathrm {NO_ {2}} \ {+} \ \ mathrm {N} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {N_ {2}} \ {+} \ 2 \ ; \ mathrm {O}}$
${\ displaystyle 2 \; \ mathrm {O} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {O_ {2}}}$
${\ displaystyle \ mathrm {N_ {2} O} \ {\ xrightarrow [{\ qquad}] {\ gamma}} \ \ mathrm {N_ {2}} \ {+} \ \ mathrm {O}}$
${\ displaystyle \ mathrm {N_ {2} O} \ {\ xrightarrow [{\ qquad}] {\ gamma}} \ \ mathrm {N} \ {+} \ \ mathrm {NO}}$

These reactions are particularly important when nuclear reactors are cooled with air. Carbon dioxide is also often used to cool gas-cooled reactors , which is why its radiation-chemical reactions have been investigated:

${\ displaystyle \ mathrm {CO_ {2}} \ {\ xrightarrow [{\ qquad}] {\ gamma}} \ \ mathrm {CO} \ {+} \ \ mathrm {O}}$
${\ displaystyle \ mathrm {CO_ {2}} \ {\ xrightarrow [{\ qquad}] {\ gamma}} \ \ mathrm {C} \ {+} \ 2 \; \ mathrm {O}}$
${\ displaystyle \ mathrm {CO} \ + \ \ mathrm {C} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {C_ {2} O}}$
${\ displaystyle \ mathrm {C_ {2} O} \ + \ \ mathrm {CO} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {C_ {3} O_ {2}}}$
${\ displaystyle \ mathrm {C_ {3} O_ {2}} \ + \ \ mathrm {O} \ {\ xrightarrow [{\ qquad}] {}} \ \ mathrm {CO_ {2}} \ + \ \ mathrm {C_ {2} O}}$

Autoradiolysis is the radiolysis of a chemical compound by radiation from radioactive atoms in the substance itself. It therefore only occurs in substances with radioactive elements or radioactive isotopes of elements. One example is the conversion of organic compounds radioactively labeled with C-14 or S-35, which are used in research.

## Individual evidence

1. ^ Gregory R. Choppin, Jan-Olov Liljenzin, Jan Rydberg: Radiochemistry and Nuclear Chemistry . 3. Edition. Butterworth-Heinemann, 2001, ISBN 978-0-7506-7463-8 , pp. 175-179 .
2. ^ Karl Heinrich Lieser: Introduction to nuclear chemistry . 3. Edition. VCH, Weinheim 1991, ISBN 3-527-28329-3 , pp. 366 .
3. ^ Gregory R. Choppin, Jan-Olov Liljenzin, Jan Rydberg: Radiochemistry and Nuclear Chemistry . 3. Edition. Butterworth-Heinemann, 2001, ISBN 978-0-7506-7463-8 , pp. 176 .
4. Hans-Gerrit Vogt, Heinrich Schultz: Basic features of practical radiation protection . 6th edition. Carl Hanser Verlag GmbH & Co. KG, Munich 2011, ISBN 978-3-446-42593-4 .
5. EJ Hall, AJ Giaccia: Radiobiology for the Radiologist , 6th edition of 2006.
6. ^ Hans-Günter Heitmann: Practice of power plant chemistry . 2nd Edition. Vulkan-Verlag, Essen 1997, ISBN 978-3-8027-2179-3 , p. 280-281 .
7. Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU): Notifiable events in plants for the fission of nuclear fuels in the Federal Republic of Germany, Annual Report 2002 , BMU Bonn (2003), p. 16.
8. Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU): Safety Criteria for Nuclear Power Plants , Revision D, April 2009.
9. ^ Gregory R. Choppin, Jan-Olov Liljenzin, Jan Rydberg: Radiochemistry and Nuclear Chemistry . 3. Edition. Butterworth-Heinemann, 2001, ISBN 978-0-7506-7463-8 , pp. 550-551 .
10. ^ Karl-Heinz Neeb: The Radiochemistry of Nuclear Power Plants with Light Water Reactors . Walter de Gruyter, 1997, ISBN 978-3-11-013242-7 , p. 490 .
11. ^ Karl Heinrich Lieser: Introduction to nuclear chemistry . 3. Edition. VCH, Weinheim 1991, ISBN 3-527-28329-3 , pp. 630 .
12. ^ Gregory R. Choppin, Jan-Olov Liljenzin, Jan Rydberg: Radiochemistry and Nuclear Chemistry . 3. Edition. Butterworth-Heinemann, 2001, ISBN 978-0-7506-7463-8 , pp. 167 .
13. ^ Karl Heinrich Lieser: Introduction to nuclear chemistry . 3. Edition. VCH, Weinheim 1991, ISBN 3-527-28329-3 , pp. 364-365 .
14. Springer Environmental Lexicon . Springer-Verlag, 2013, ISBN 978-3-642-97335-2 , p. 124 ( books.google.de ).