Help:Fission Products and Yields

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Nuclear Fission

Spontaneous Fission (sf)

The discovery of fission by neutrons is credited to Hahn and Strassmann (1938), and to Meitner and Frisch (1939) for their explanation of the phenomena and introduction of the term nuclear fission. Spontaneous fission was discovered in 1940 by Petrzak and Flerov.

Although the alpha emitting properties of 238U were well known by that time, the much less common spontaneous fission had been “masked” due to its very small branching ratio of about one sf in 2x106 alpha emissions. With the exception of 8Be (which decays into two alpha particles), sf has not been detected in any elements lighter than thorium. In the 1960s, sources of 252Cf became available and detailed measurement of the fissioning of this system contributed much to our understanding of the process.

Actinides and trans-actinides can undergo radioactive decay by spontaneous fission. In this process the nucleus splits into two fragment nuclei, with mass and charge roughly half that of the parent, and several neutrons.

The spontaneous fission decay process can be described qualitatively by:

Sf1.jpg

and in more detail by:

Sf2.jpg

A “parent” nuclide splits into two “daughter” nuclides and together with the release of υ prompt neutrons and energy E*. Typically υ ranges from 2 – 4 and E* is approximately 200 MeV. Additional, so-called delayed neutrons may be emitted by the primary fission products. The daughter nuclides or fission products have in general different mass numbers A and atomic numbers Z.


Since there are more than two decay products, the products and their energies cannot be uniquely identified. In the case of the spontaneous fission of fermium-256, one such reaction is:

Fm1.jpg

(this reaction represents only one of many fission product combinations). The kinetic energy release in this process, due mainly to large electrostatic repulsion of the fragments, is approximately 200 MeV.

Sf3.jpg

About 87% of the total energy is emitted promptly with the fission fragments. Most of the neutrons released are prompt neutrons and are released within 10–14 s of fissioning. Some neutrons are released on a much longer timescale and are associated with the fission decay chains.


Neutron Induced Fission

Neutron induced fission follows the same process as the above described spontaneous fission. A neutron is absorbed by a heavy nucleus forming a compound nucleus which splits into two fast moving lighter nuclides, called fission products, and free neutrons. Fission is basically a binary process. Much less frequently (about once every few hundred events), fission can lead to the formation of a third light particle (ternary fission). Fission reactions differ according to the mass and nuclear charge of the fissioning system and, typically, more than 900 fission products are produced as a result of the fission process. The amount of a specific nuclide produced is generally expressed as fission yield (given in units of production per unit fission or in percent). Various yields can be distinguished. The independent yield describes the fraction of fission of a specified nuclide produced directly after fission (after emission of prompt neutrons but excluding radioactive decay of precursors). The cumulative yield describes the fraction of fission of a specific nuclide produced directly AND after via decay of precursors. The chain yield is the fraction of fissions giving rise to nuclei with a specific mass number (isobar). Since the discovery of fission in 1938, experimental measurement of fission yields have been of significant importance from two standpoints: the fundamental understanding of the fission process and in the framework of nuclear technology (reactor technology, nuclear medicine, safeguards, etc.). The measurements of fission yields encompass a wide range of techniques from γ-ray spectroscopy of un-separated fission products to a very selective and fast separation of one element out of a mixture of fission products. In the early days radiochemical separation methods were widely used, while today these are replaced by more sophisticated physical methods. Only a small fraction of the yield distributions have been experimentally investigated, and hence most the yields must be estimated. Chain fission yields are most accessible with current measurement techniques. The reason for this is that the chain yield distribution of fission products does not change with time after the fission reaction. Evaluated thermal chain yields for the 235U and 239Pu, fissile material used in reactors, are given in the Karlsruhe Nuclide Chart (7th Edition 2006).


Definitions of Fission Yields

The daughter nuclei showing up right at scission of a fissioning mother nucleus are called primary fission “fragments”. In the large majority of the cases, the fragments will be sufficiently excited to evaporate neutrons in times less than 10−15s. This means that the nuclei detected in experiments are not the primary fragments, but instead secondary fragments having lost a varying number of neutrons. The secondary fragments are called the fission products (i.e. fission fragments after prompt neutron evaporation).

A fission product is denoted symbolically by the notation (A,Z,I) where A and Z are respectively the mass number and the atomic number, and I indicates the isomeric state (I = 0 for the ground state, I = 1, 2 . . . for the 1st, 2nd , . . . isomeric states). If a fission product has no isomers, or if one is referring to the sum of yields of all its isomers, the notation (A,Z) is used. Using this terminology, the following fission yield definitions are given:

Independent Fission Yield, IND(A,Z,I)

The independent fission yield, IND(A,Z,I), is the number of atoms of a specific nuclide produced directly (after emission of prompt neutrons but excluding radioactive decay) per 100 fission reactions.

Cumulative Fission Yield, CUMUL(A,Z,I)

The cumulative fission yield, CUMUL(A,Z,I), is the number of atoms of a specific nuclide produced directly and via decay of precursors per 100 fission reactions. If the nuclide is stable, the cumulative yield is the total number of atoms of that nuclide remaining per fission after the decay of all precursors (ignoring the effects of other nuclear reactions e.g. nuclear capture). Similarly, for a nuclide with a much longer half-life than any of its precursors, the cumulative yield is very nearly equal to the amount produced at a time short compared to its half-life but long compared to the half-life of its precursors. However, for a radioactive nuclide for which this is not the case, some atoms will have decayed before all have been produced. In such a case, at no time will there actually be present the reported cumulative yield of atoms per fission present.

Chain Yield, Y(A)

The chain yield is the number of isobars of a specific mass, produced in 100 fission reactions. In other terms, the chain yield Y(A) is equal to the sum of all stable or long-lived cumulative yields for a given mass chain. The cumulative yield of the last (stable or long-lived) chain member is generally2 identical to the chain yield. These chain yields apply to fission products after emission of prompt neutrons that takes place in a time of 10−14 s after scission.

The Fission Yield Module

To use the Fission Yield module within NUCLEONICA, the user must first select the "Fission Yields" from the Data Centre on the main portal page. In this module, the Element and Mass drop-down menus contain only nuclides for which fission yield data is available. The fission "parent" interest can then be selected using the drop down menus. Alternatively, the fissioning nuclide can be selected from the nuclide chart, and then with the right mouse button, the Fission Yield module can be selected. The same result can also be obtained from the Applications in the taskbar.

The resulting Fission Yield input interface is shown to the right. By clicking on the Fission Yields tab, the chain yield graph for the selected fissioning system (U235) is shown. The user can either accept the default input values shown or enter his input into the boxes. In the Select tab, the user can select the data source (the JEFF3.1, ENDF-BVI, and JENDL 3.2 libraries are available), the type of fission (spontaneous, thermal neutron, fast neutron and fusion neutron fission). A list of fission productes for the fissioning system can then be obtained by clicking on the Results button. U235 for example has almost 1000 fission products.

The Search can be further refined by selecting an element or mass from the drop-down menus. Selecting the element Cs for example will result only in Cs fission products from the selected fission nuclide. The search can be further refined by specifying the half-life range.

With Cs selected as Element, pressing the Results button leads to a list of Cs isotopes resulting from the fast neutron fission of U235 using the JEFF3.1 data library. The results are shown below. By default the results are ordered on the Cumulative Yield in descending order. Clicking on the header Cumulative Yield will re-order the results in ascending order. Similarly all of the results can be ordered by selecting a particular column header.

Fission Yield module user interface
Results of a search for Cs isotopes resulting from the fast neutron fission of U235 using the JEFF3.1 data library



Fission Product Yield Comparisons: Parents

In the above diagram, the first column contains "compare" hyperlinks. These links have been activated from the main Fission Yield page by activating the "Enable advanced comparisons" checkbox. By clicking on the compare link for a particiular isotope, a new wondow opens which allows one to compare the yields of this isotope from various fissioning systems (or fission parents) and from different data libraries. In the example below, the window results from clicking the "compare" link adjacent to Cs137.

FissionYield4.jpg

In the highlighted tab "Comparison of fission systems", the fast neutron fisison yields of Cs137 from the fission parent U235 (given in the second column) are compared with the yileds form the parents Th232, Pu239, U233, and U238. It can be seen that the Cs137 cumulative fission yields range from 5.9% to 6.5% depending on the fission parent. The independent yields show substantially greater variation. Results for other fissiong systems can be obtained by choosing the fission parents in the drop down menus.


Fission Product Yield Comparisons: Libraries

By clicking on the "library" tab in the baove window, the results are shown below. Here the Cs137 yields from the fast neutron fission of U235 from all three data libraries, JEFF3.1, ENDF-BVI, and JENDL3.2. Also shown are the relative differences from thr slected library (in this case JEFF3.1) to the ENDF-BVI and JENDL3.2 libraries.

Fission Yield5.jpg

References

1. J. Galy: Investigation of the fission yields of the fast neutron-induced fission of 233U. Thesis submitted for the degree of Doctor of Philosophy (September 1999)

2. H. O. Denschlag in: Experimental Techniques in Nuclear Physics, D.N. Poenaru, W. Greiner (eds.). Walter de Gruyter, Berlin 1997

3. F. Gönnenwein in: The Nuclear Fission Process, C.Wagemans (ed.). CRC Press, Boca Raton 1991

4. JEF-2/FPY (ref. AEA-TRS-1015, 1018 AND 1019, and M. F. James et al.: Progress in Nuclear Energy 26,1 1991). The evaluated data have been taken over from UKFY2, the UK fission-product yield data library by M. James and R. Mills. Brief summary: IAEANDS- 123 Rev. 1

5. JENDL-3.2/FPY The JENDL fission-product yield data library, has been compiled by T. Nakagawa in ENDF-6 format. The evaluated data have been taken from JNDC-FP2, a special format data library documented in the reports JAERI-M-89-204 (1989) and JAERI- 1320 (1990)

6. ENDF/B-6 fission-product yield data (ref. LA-UR-94-3106 ENDF349 1993) This is a separate ENDF/B-6 sublibrary which was released in September 1991 and updated in June 1993 and May 1995. It has two parts: one part for neutron induced fission, another part for spontaneous fission. Summary see document IAEA-NDS-106 Rev. 3.

7. R.W. Mills: Review of fission product yields and delayed neutron data for selected actinides. Report NEANDC-300. July 1990

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