Nuclear fission product

As the fission products produced in nuclear fission nuclides are known. Fissions can be cause by neutron sources in the laboratory scale. On a large scale it will take place in nuclear reactors.

Physical Basics

The costs directly incurred (so-called primary) fission product nuclei have a high neutron excess; they are therefore unstable ( radioactive) and are transformed by successive beta-minus decay into other nuclei around. Since the beta decay the mass number of the nucleus remains the same, the succession resulting nuclides form so-called Isobarenketten. Each of these decay chains continues until a stable nuclide. Essentially, it is in these final products of nuclear fission to metals with mass numbers 90 ( in the periodic table of zircon to palladium) and some elements with mass numbers 140 ( tellurium to samarium, with the exception of tellurium, iodine and xenon also metals).

These radioactive decays mostly find rather short ( fractions of seconds to hours) after the fission, while the energy released contributes to several percent to power a nuclear reactor at ( decay heat ). However, no health importance especially those fission products, where the last (partly the penultimate ) of these decays has a half-life ranging from days to years, especially if they are easily transported because of their chemical properties and can enter the human body.

Some fission products have significant impact on the operation of a nuclear reactor. In particular, these are fission products, neutrons release only after a delay and in general make it possible only to regulate a reactor, as well as nuclides which strongly absorb neutrons, especially 135Xe.

Exact numerical values ​​for the yield of the various Isobarenketten in the cleavage are specified in a data collection of the International Atomic Energy Agency.

The initial composition of the resulting cleavage product mixture depends on the type of reactor ( fissile material, the neutron energy spectrum) and on the residence time of the cleavage products in the reactor (continuous further neutron exposure ). After removal from the reactor to determine half-life, and decay products of the individual cleavage products the temporal change of the composition.

Fission products may be gaseous (eg 133Xe, 85Kr ), volatile ( eg 131I ) or fixed (eg 137Cs, 90Sr ) be.

Nuclear fission

Example of a neutron -induced fission of uranium -235:

Decay chains of the primary cleavage products:

Through multiple beta decay caused the stable end products of Ru -101 and Cs- 133rd

The liberated in the fission neutrons - in the example two - are slowed down by collisions with atomic nuclei of the surrounding matter and eventually absorbed, ie in a nuclear reaction (usually a neutron ) is "consumed ".

Properties of selected fission products

The most common fission products from light-water reactors are isotopes of iodine, cesium, strontium, xenon and barium. Many fission products decay quickly into stable nuclides, but a significant residual has half-lives of more than a day, up to millions of years. Some nuclides with short to medium half-lives are used in medicine or industry. It is noteworthy that not a single generated during the fission nuclides has a half-life 100-200000 years. This gap means that the total activity of the mixture of fission products still starts to decline in the first centuries, mainly to the dominant in this period isotope 137Cs has disintegrated. Then the residual activity remains (which is of course orders of magnitude less than the activity of kürzerlebigen nuclides ) of historical periods (many millennia ) remained virtually unchanged. In spent fuel rods are found, however, in addition to the fission products and transuranic elements fill this " gap" in the half-lives.

Cesium

Among the fission products, there are three cesium isotopes. 134Cs ( half-life of about 2 years) is not created directly, because the decay chain of nuclei with mass number 134 already in the stable nuclide 134Xe ends, but indirectly by neutron capture from the stable fission product 133Cs. 135Cs has a very long half-life ( 2.3 million years ) and is therefore only moderately radioactive. The most important cesium 137Cs isotope with a half life of about 30 years. It's after the collapse of the short-lived isotopes for many centuries the most radiant nuclide in the mixture of fission products.

Technetium

After the decay of 137Cs is 99 Tc (half-life 211,100 years ) the most radiating isotope of the remaining fission products ( in total there are seven fission products with very long half-lives ). Technetium has no stable isotope and does not occur in nature.

99Tc is next 129I a prime candidate for transmutation, its elimination would ( after the collapse of kürzerlebigen isotopes) reduce the radiation in the far future by about 90%.

Krypton

Radioactive krypton produced in small amounts in nuclear reactors. During reprocessing spent fissile material, such as for the extraction of plutonium and uranium remaining, the relatively long-lived isotope of krypton 85Kr ( 10.756 years half-life ) is released, and escapes into the atmosphere. The amount of radioactive krypton in the atmosphere, therefore, is an indication of the amount of fissile material worked. The difference between the krypton resulting from known and measured machining allows the amount undisclosed edits (usually to produce nuclear weapons ) to estimate.

Strontium

Since strontium biologically similar to calcium acts, strontium isotope 90Sr is the radioactive one of the potentially harmful to human health fission products, because it is deposited upon entry into the organism in the bones and remains until its disintegration in the body. Since the half -life is about 30 years old, wearing the strontium for the rest of life in itself. The really dangerous thing 90Sr is the daughter nuclide 90Y, which also decays, where its beta radiation has four times the energy to that of 90Sr.

The preferential uptake of strontium in the bones is used therapeutically or palliative for bone cancer: The preferred storage in the bone may strontium 89Sr (half-life 50.5 days ) can be used to combat tumors daughter.

Iodine

On the fission occur in varying amounts, the iodine isotope 127I (0.12 % of all divisions in the fission of 235U in a thermal reactor ), 129I (0.7%) and 131I ( 2.9%). 127Iod is not radioactive (natural iodine is exclusively aus127I ); 129Iod is only weakly radioactive because of its long half-life ( 15.7 million years ). A particular danger is, however, of 131I, since iodine is very mobile due to its physico-chemical properties in the environment - it is very easy also released in an accident - and is also actively taken as an essential trace element the human organism. Especially the thyroid contains high levels of iodine.

131I was a half-life of 8 days. After 8 days, the radiation thus is decayed to half after 27 days after 80 days and one tenth to one thousandth. After the disaster of Chernobyl 131I presented in the first few days the dominant radioisotope dar. With early warning, some protection by the ingestion of stable iodine can be constructed in the form of potassium iodide tablets before a feared exposure. The organism becomes saturated with iodine and radioactive iodine increases the then in much smaller quantities ( iodine prophylaxis ).

Ruthenium, rhodium and palladium,

Ruthenium and rhodium, are among the most frequent elements in the mixture of cleavage products (11% or 3% in the cleavage of 235 U in a thermal reactor). Separation and recovery of valuable metals in the reprocessing is possible, but will not have been practiced. Ruthenium could be used only after a multi-year waiting period, since the mixture of isotopes produced in the reactor initially also contains the radioactive 106Ru (half-life 374 days ).

Palladium occurs in slightly lower volumes (1.6 % for fission of 235U ). In addition to the stable isotopes 105Pd, 106Pd, 108Pd and 110Pd It also contains the slightly radioactive 107Pd (half-life 6.5 million years ), which limits the usability greatly.