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Letters to the Editors NUCLEAR SCIENCE AND ENGINEERING: 10, 362-366 (1961) Letters to the Editors Spatial Dependence of Thermal-Neutron cantly alter the experimental determination of the thermal utilization. Spectra and the Interpretation of Thermal Second, the pronounced spatial dependence of the Utilization Measurements thermal-neutron spectra will not usually permit equating the activation ratio to the flux ratio. Bigham has measured the temperature dependence of the ratio of the Pu239 to The thermal utilization, /, is usually defined as the U235 average fission cross sections in a Maxwellian spectrum number of thermal neutrons absorbed in the fuel per thermal (3). In light w^ater, at 50°C, a value of 1.44 was measured, 2b5 neutron absorbed in the lattice. For a low U enrichment whereas the value at 95°C was 1.51. Klein has made room lattice consisting of uranium fuel rods temperature measurements of Pu239 fission reaction rates relative to U235 in light-water uranium-metal lattices f <f>mVmNm*m(Tm) + 0cl Vol N0i (Tcl^cl)]"1 containing 1.3% U235 enrichment (4). The measured ratios ^ 1 0uFu[iV26 *26(ru) + iV28^28(Tu)] J of the Pu239 to U235 fission reaction rates below the cadmium where V is the volume of the material; N is the atomic cutoff is 2.00 for a 2.35:1 water-to-metal lattice, and 2.37 density; a (T) is the microscopic absorption cross section for a 1:1 water-to-metal lattice. These latter ratios are averaged over an appropriate thermal-neutron spectrum approximately proportional to the ratio of the average characterized by a neutron temperature, T; and <f> denotes Pu239 to U235 thermal-fission cross sections. The results of the neutron flux. The subscripts m, cl, u, 25, and 28 refer to Bigham's measurements have been extended analytically moderator, clad, uranium, U235, and U238, respectively. by averaging the measured microscopic fission cross sections The experimental determination of the thermal utiliza- of Pu239 and U235 at higher temperatures and comparing tion involves the introduction of foils, usually U235 or Dy, with Klein's lattice measurements. In terms of an effective in the different regions within the lattice. The resulting neutron temperature, these analyses show that the fuel activation of these foils determines the neutron density in regions in the lattices have a neutron temperature more each region. Since the activation in each region is propor- than 250°C greater than a room temperature spectrum tional to the product of the neutron flux and cross section, would predict. On the other hand, the neutron spectrum in it is not necessary to make any experimental correction for the moderator can deviate considerably from the spectrum different effective neutron temperatures in each lattice in the fuel rod as is shown in a calculation by Volpe and region provided the foils have the identical neutron absorp- Klein (5). Assuming a Maxwellian-type spectrum in both tion energy dependence as the other materials in the lattice. moderator and fuel regions but with each Maxwellian at a The fact that the foils do have a slightly different energy different neutron temperature, the average neutron velocity dependence introduces an extremely small correction (1,2). is different in the two regions and, consequently, the Besides the / value, additional quantities that are often average cross section of foils placed in the two regions is quoted as being measured are the neutron-flux ratios or different. Therefore, a significant correction factor should disadvantage factors, 0m/0u and <j>c\/4>m} which have been be applied to the activation measurements before they can assumed to be proportional to the activation ratio (1, 2). be considered proportional to the neutron fluxes in each These latter quantities are often used to verify the accuracy region. of theoretical methods that are used to calculate lattice To illustrate by means of an extreme example: if the neutron-flux distributions. effective neutron temperature is 200°C in the fuel and 20°C It may be worth while to point out that there are certain in the moderator, the cross sections of U235 and Dy foils are analytical simplifications in the experimental interpretation approximately , 30% greater in the moderator than in the of these activation measurements that may not be entirely fuel. A correction of this magnitude should be applied to justified. First, in these measurements, foils may not have the activation ratio to obtain the true disadvantage factor. been placed in all regions in the lattice but only in the fuel The large difference in neutron temperature between the and moderator regions. It is then necessary to make assump- fuel and moderator used in this example is probably not tions regarding the neutron spectrum existing in the typical of most lattices. However, this example does in- cladding and structural materials. If the cladding and dicate that the spatial dependence of neutron spectra may structural materials are weak neutron absorbers, e.g., not be slight and, furthermore, that the determination of aluminum, then only a small uncertainty in the measure- the disadvantage factor requires a substantial correction ment of thermal utilization will occur due to the assump- to the measured activation ratio. This correction should be tions made concerning the effective neutron temperature applied to presently quoted disadvantage factors. in these materials (2). However, if the cladding and struc- It would seem that any analytical method requiring that tural materials are strong absorbers, e.g., stainless steel, the predicted values of both the thermal utilization and then any uncertainties involved in the assumption of an activation ratios agree with experiment should include the effective neutron temperature for these regions can signifi- spatial energy dependence of neutrons. Recently, Honeck 400 LETTERS TO THE EDITORS 401 and Kaplan have presented a method for performing such TABLE I an analysis together with the experimental comparison (6). PLUTONIUM DEPOSIT ENRICHMENTS REFERENCES Deposit Pu239(%) Pu240(%) Pu241(%) Pu242(%) 1. H. KOUTS, R. SHER, J. R. BROWN, D. KLEIN, S. STEIN, "Pu239" 99.36 0.63 0.01 R. L. HELLENS, H. ARNOLD, R. M. BALL, AND P. W. "Pu240" 18.64 80.08 1.14 0.14 DAVISON, Proc. 2nd Intern. Con}. Peaceful Uses Atomic "Pu241" 9.02 Energy, Geneva 12, 446 (1958). 31.01 57.96 2.01 2. D. KLEIN, A. Z. KRANZ, G. G. SMITH, W. BAER, AND J. DEJUREP^, Nuclear Sci. and Eng. 3, 403 (1958). activation integrals are denoted by the symbol 7; i.e., 3. C. B. BIGHAM, Nuclear Sci. and Eng. 6, 379 (1959). 4. D. KLEIN, Nuclear Sci. and Eng. 8, 405 (1960). /»oo r 00 i i 5. J. J. VOLPE AND D. KLEIN, Nuclear Sci. and Eng. 8, P = J a/ {E)cf>(E) dE and 7e = J <r/(E)4>(E) dE 416 (1960). 6. H. C. HONECK AND I. KAPLAN, Nuclear Sci. and Eng. 8, where e is the Cd cutoff energy (~0.45 ev). The Pu242 con- 203 (1960). tribution was neglected. If the above expression is solved for the ratio of the Pu240 ROBERT W. DEUTSCH to Pu239 fission integrals, the result is the following: General Nuclear Engineering Corporation Dunedin, Florida Received March 17, 1961 af(E)4>{E) dE Jo Nil mi r 00 = p«>: / <jf(E)4>(E) dE (i) Jo Nil Measurements of Relative Pu Fission Rates in - P(i)S Ntlf'R1 Slightly Enriched Uranium-Water Lattices where Ri is the measured Cd ratio of the ^th isotope and P(t)m{ is the ratio u(t)i/u(t)m and accounts for the relative The fission activation of Pu240 relative to Pu239 and Pu241 gross fission product decay rates of isotopes i and m. has been measured for the TRX facility, a slightly enriched In a similar manner, the following is obtained: uranium-light water moderated reactor. The lattices in which the experiments were performed were composed of u(t)i0NtUi0 + uity'Nill? + UWNZI? 1.3 wt % enriched uranium metal fuel rods, 0.387 in. in T«)f 40 40 41 4 1 4 diameter, with a water to uranium volume ratio of either u«) iO + M(i) JVS J . + u(.t) °Nl\l? 2.35:1 or 1:1. The experiments described here are an exten- sion of previously reported measurements for these lat- <Tf\E)4>(E) dE Jo tices (1,2). = P«4J The Pu was in the form of 1 mg/cm2 deposits on highly a,\E)<t>(E) dE pure nickel foils 0.010 in. thick. The isotopic concentrations FJo of the deposits are given in Table I. (2) Nti The presence of approximately 19% Pu239 in the "Pu240" r mi . p^'T 7(<) + p{thi 7(<)4 deposit introduced the problem that the Pu240 isotope con- law ~ "J inr L^ " 'J X tributes only of the order of 3-4% to the total fission ac- jvsn tivity of this deposit irradiated in these lattices. This relative contribution was increased to roughly 30% by ir- radiating this deposit Cd covered, thereby suppressing the The experimental procedure is similar to the previously thermal activation contribution of the Pu239. reported measurements (1, 2). The quantity y(t) was ob- The ratio of the measured total fission product activity tained by irradiating the deposits in a split fuel rod and of a Cd covered "Pu240" deposit to a bare "Pu239" deposit counting the resultant fission product activity with a is given by: scintillation counter biased to reject pulses having equiva- lent energies less than 400 kev. The quantities designated u(t)A°Nto /4° + unrNtin1 + uim Ua: as P(t) were obtained from a separate experiment utilizing 7(^)49 = u(t)49Nlll49 a double fission chamber technique (2).
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