Radiolysis
Under radiolysis refers to the cleavage of chemical bonds under the action of ionizing radiation, mainly, radicals or ions. Usually the radiolysis of water is addressed with the concept.
The designation is released from the parts "Radio" [ from Latin radius, " ray" ] and " lysis " [ from Greek λύειν ( lýein ) "( on ); separate " λύσις ( lysis ) " the ( On ) solve " ] together.
Radiolysis of water
Radiation Chemistry
The radiolysis of water takes place in several steps, which are listed below.
Ionizing radiation (such as γ - radiation), the water molecules can be excited or ionized:
The H2O ion reacts very quickly (within 10-14 s ) with water:
The excited water molecules dissociate within 10-14-10-13 s radicals:
If they have enough energy released during the ionization of water molecules, electrons can excite more water molecules or ionize. Once they have their energy largely lost, they are initially solvated. This process takes place within 10-12 s.
Episode reactions are:
Because of the variety of the competing reactions, the yields of the individual products depend strongly on the reaction conditions. Typical values for pure liquid water in the irradiation with γ - or β - rays are shown in the following table:
From the values resulting for example, that in the irradiation of 1 liter of water ( mass m = 1 kg) at a dose of D = 1 Gy = 1 J / kg produced a hydrogen mole of n = 0.047 micromol:
Molecular oxygen (O2) is not a primary product of the radiolysis; it arises, however, by the following reactions:
In addition, reverse reactions of the radiolysis products appear, so again is water:
Hence arises - if the radiolysis products are not removed (eg escape as gas) or react with other substances - under continuous irradiation, a balance of the various reaction products.
Radiobiology
Since living organisms are to a large part of water, based biological radiation effects of ionizing radiation not only on the direct effects of radiation, but also on the chemical reactions of reactive oxygen species formed by radiolysis in the water.
Nuclear Engineering
The radiolysis of water takes place at all or moderated water-cooled nuclear reactors already in normal operation. For this reason, the steam of a boiling water reactor and so-called " radiolysis " includes (hydrogen and oxygen). In contrast, the pressurized water reactor the reactor coolant a slight excess of hydrogen is added to suppress under the above reverse reactions, the formation of corrosive oxidizing agents (especially · OH, H2O2 and O2).
An accumulation of ignitable Radiolysegasgemischen should be avoided in order to prevent an explosion under certain circumstances reaction of hydrogen with oxygen.
On 14 December 2001, however, during nuclear power plant Brunsbuttel to such radiolysis, whereby an approximately 2.7 m long piece of the head spray was destroyed.
The radiolysis of water is also to be observed in the consideration of design basis accidents (eg hypothetical loss of coolant accidents ). The following sources in particular are considered for the formation of hydrogen:
- Radiolysis in the reactor core
- Radiolysis in the bottom of the containment
- Radiolysis in the fuel pool
When calculating the hydrogen formation while a conservative G- value of G (H2 ) = 0.44 is assumed Moleküle/100 eV.
The radiolysis of water is not to be confused with the formation of hydrogen by the exothermic reaction of zirconium with water vapor, which may occur in severe accidents of nuclear reactors:
Radiolysis of air
A well-known product of the radiation-chemical reaction of oxygen is ozone. In 1911, described Samuel C. Lind, the radiation-chemical yield of ozone formation.
By the action of ionizing radiation on the air or similar gas mixtures of nitrogen and oxygen, produces nitrogen oxides (chiefly nitrogen dioxide) in accordance with the following reactions:
These reactions are particularly important when reactors are cooled with air. For the cooling of gas-cooled reactors is often also used carbon dioxide, which is why the radiation-chemical reactions have been studied: