Shaginyan R.A., Korobeinikova L.V.
A.I. Leypunsky Institute for Physics and Power Engineering, Obninsk, Russia
Most of the isotopes in demand are often activated by thermal neutrons. This fact determines the practice of developing target isotopes in thermal reactors. The irradiation time to achieve the parameters required is several years. At the same time, obtaining the same indicators in fast reactors turns out to be faster due to the neutron flux density exceeding by 2 orders of magnitude.
To carry out the effective production of isotopes (produced in the thermal spectrum) in fast reactors, it is necessary to use specialized irradiation devices (ID) with a moderating material. In addition, the operating time in a fast reactor may not be limited to several irradiating devices, but have a large-scale character (several dozen irradiating devices). Large-scale operating time with the replacement of a number of breeding blanket with irradiation devices definitely leads to unevenness, both azimuthal due to the distance from the center of the core, and axial due to the peculiarities of the distribution of the neutron flux embedded in the core design.
According to the results of calculations of unevenness (deviation of activity from the average), the difference between the maximum and the minimum in terms of the location of the irradiation device in the core is 1.9, according to the azimuth distribution inside the device 1.51, according to the altitude distribution 1.16. If we consider the altitude partition in a more detailed approximation, then the ratio of maximum to minimum will be 1.32.
To compensate for the irregularities, there are a number of measures that contribute to their reduction: rotation of the irradiation device around its axis, an increase of the irradiation duration, rearrangement of under-irradiated ID with over-irradiated ones.
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