The expanding network of nuclear power stations and fuel element reprocessing plants is causing an increase in the global environmental distribution of the radioactive isotopes of A(, Kr, Xe, and I, as well as 14C and T due to emission of these gases into the atmosphere. This results in additional irradiation of the population [1,2]. The main contribution to this global irradiation of the population is from 85Kr and T [2; 3]. It has been conjectured that by the year 2000 there will have been up to 5.108 curies of tritium released to the environment [2]. ~ Electric power production by thermonuclear reactors may also lead to a significant buildup of tritium in the biosphere, since it is assumed that a thermonuclear reactor will release 104-105 times more tritium than a nuclear power station of equivalent power [4][5].At present, methods and equipment for collecting Kr, Xe, and I have been developed [6][7][8]. Reducing the emission of tritium, whose potential danger is associated with its possible absorption into genetic material, is a serious problem which is practically unsolved at this time [9][10][11].T is formed in nuclear reactors directly in the fuel elements, and in the coolant (H20 , D20), moderator (graphite, D20), and boron co~rol rods. The average emission of tritium depends on the type of nuclear power plant and ranges from 2 curies/year to 10 curies/day, with a maximum of more than 100 curies/day [5,11]. The principal source of atmospheric T is fuel reprocessing plants which release 50-432 curies/day [5,12]. During accidents up to 2.9.105 curies/hour may be released [13]. Up to 25% gaseous tritium and 75~c HTO reaches the atmosphere [9]. The tritium at effluent stacks comes with waste gases (air, inert gases) from the active zone or the primary loop of reactors, or from the processing rooms (canyons) of fuel reprocessing plants. The concentration of T in the gases entering the ventilation stacks varies from 10, 5 to 20 curies/liter [11,13].
The effect of neutron absorption and scattering on the form of the boundary condition at the surface of a control rod is taken into account in calculatingthe neturon flux in the surrounding medium in the diffusion approximation. One-group effective boundary conditions (EBC) are found for a gray cylindrical rod containing a neutron source and located in a scattering and absorbing medium with a distributed source. A procedure for calculating the EBC from the neutron balance is based on the requirement of a correct description of neutron leakage from the medium. The source in the medium is taken to be uniform or having a radial variation described by a logarithmic function or a Besset function. The effect of the nonuniformity of a source distributed linearly or exponentially is analyzed in plane geometry.A number of approximate formulas are obtained for calculating the EBC. A comparison is made with the results of exact numerical calculations of other authors [1, 2]. We recommend the following approximate expression for the EBC at the surface of a black rod without an internal source located in a medium with a uniform source: 1 d (ln q~) ]-1 4 0.577 + 0.046Y.~/Y.t 7!: Zt dr .jr=r0~3" --r~ 0.225~, roY~t-~O,18(~s/Zt) ~ 'where ~p is the neutron flux, r 0 is the radius of the rod, Zt is the total macroscopic cross section, and Y's is the scattering cross section. This expression is not in error by more than 2%. For Y's = Zt it agrees with the Flatt [3] approximation and has the correct limits for r0~ t = 0;Zs Y~8 ro~ t ~ co, ~tt = 1; roZt--~ co, ~t = O.Our results are useful for refining calculations of control rod effectiveness in the multigroup diffusion approximation.LITERATURE CITED r 1.
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