AE-19
Sources of gamma radiation in a reactor core
Matts Roas
AKTIEBOLAGET ATOMENERGI
STOCKHOLM • S\\ HDJtN • 1959
AE-19
ERRATUM
The spectrum in Fig. 3 has erroneously been normalized to 7. 4 MeV/capture. The correct spectrum can be found by mul- tiplying the ordinate by 0. 64.
AE-19
Sources of gamma radiation in a reactor core
Matts Roos
Summary: -
In a thermal reactor the gamma ray sources of importance for shielding calculations and related aspects are 1) fission, 2) decay of fission products, 3) capture processes in fuel, poison and other materials, 4) inelastic scattering in the fuel and 5) decay of capture products. The energy release and the gamma ray spectra of these sources have been compiled or estimated from the latest information available, and the results are presented in a general way to permit 235 application to any thermal reactor, fueled with a mixture of U and 238 U • As an example the total spectrum and the spectrum of radiation escaping from a fuel rod in the Swedish R3-reactor are presented.
Completion of manuscript April 1959 Printed Maj 1959
LIST OF CONTENTS
Page
Introduction ...... 1 1. Prompt fis sion gamma rays i 2. Fission product gamma rays 2 3. Uranium capture gamma rays 4 O -2 Q 4. U inelastic scattering gamma rays 5 5. Gamma rays from capture in poison, construction materials and moderator .....*•»..•...... 8 6. Gamma rays from disintegration of capture products. . 8 7. Total gamma spectra. Application to the Swedish R3 -reactor 9
SOURCES OF GAMMA RADIATION IN A REACTOR CORE.
INTRODUCTION
In reactor shielding studies and related aspects it is of importance to know the energy released as gamma radiation and its spectral distribution. So far detailed calculations of the total spectrum of gamma radiation from a reactor core have been hampered by a very limited knowledge of the sources. However, the large volume of relevant information published during 1958, especially concerning the main sources, now facilitates an esti- mate based on fewer guesses than before. The gamma ray spectra of different sources can most con- veniently be compared when expressed in units of energy release per fission per energy interval^ or MeV/f. MeV, and the integrated spectra thus in MeV/f. However, only the spectra of prompt fission and fission product radiations can without loss of generality be ex- pressed in these units, whereas in capture processes, for instance, the number of captures cannot be related to the number of fissions without reference to a specific reactor. In order to show the rela- tive importance of the different sources, we therefore in the last section apply the general results to a particular reactor, the Swedish R3 (MARGEN & al. 1958), for which we give the total spectrum and the spectrum of radiation escaping from a fuel rod.
1. PROMPT FISSION GAMMA RAYS
The spectrum of y-rays emitted within 5» 10 s of fission has been measured in the energy ranges from 0.3 MeV to about 7.3 MeV by MAIENSCHEIN & aL(i958), from 0.015 MeV to 0.800 MeV by VOITOVETSKII & aL(l957) and from about 0.020 MeV to about 0.260 MeV by SKLIAREVSKII & aL (1957). It seems possible to join the spectra of MAIENSCHEIN and SKLIAREVSKII in the region between 0.26 MeV and 0.30 MeV, whereas the spectrum of VOITOVETSKII matches the spectrum of SKLIAREVSKII only at the softest gamma line (0. 03 MeV)s falling a factor 5 below at 0.20 MeV and a factor 10 lower than MAIENSCHEIN ' s spectrum in the region from 0. 30 MeV to 0. 60 MeV. _ Q The energy released within 5*10 s of fission and within the energy range 0.3 - 10 MeV (extrapolated from 7,3 MeV to 10 MeV) is reported by MAIENSCHEIN to be 7.2 ± 0. 8 MeV/f, and within the range 0. 015 - 0.260 MeV by SKLIAREVSKII to be about 0.24 ± 0. 05 MeV/f. In addition MAIENSCHEIN has found delayed gamma rays in Q L the region between 5*10 s and 10 s after fission8 and in the energy range 0.1-2 MeV. The intensity is reported to be (5. 7 ± 0. 3) % of the prompt radiation, or about 0, 4 MeV/f. Thus the total energy released within 10 s is about 7.9 MeV/f (1.1) The spectrum shown in Fig.l is obtained by joining the spectra of MAIENSCHEIN and SKLIAREVSKII, and is, to account for the unknown spectrum of delayed gamma raysj normalized to 7.9 MeV/f.
2. FISSION PRODUCT GAMMA RAYS Several reports ( BLOMEKE and TODD 1957, KNABE and PUTNAM 1958, MAIENSCHEIN & al. 1958, MILLER 1957, PERKINS and KING 1958, PRAWITZ & al. 1958, SCOLES 1958 a,b, STEHN and CLANCY 195 8 ) have recently been published on the gamma-radiation of products of thermal fission of U at various cooling times after irradiation times of various durations. The reports of BLOMEKE and TODD, MILLER, SCOLES, PRAWITZ & al. , and PERKINS and KING are based on available chemical data and thus do not ex- tend to very short cooling times. As the most short-lived isotopes emit comparatively hard v-radiationj it is recognized that the extra- polation of decay curves down to zero cooling time might give results which are too small by a factor of 4. It could be possible, however, to obtain better agreement by taking into account new nuclear data on very short-lived isotopes^ as reported for instance by O'KELLEY & al. (1958). MAIENSCHEIN 8t al. have measured the Y~ray spectra at various cooling times down to about 1 second after irradiation for time intervals sufficiently short to be considered instantaneous. These values appear to give the best starting point at present in an effort to evaluate the spectrum at zero cooling time and infinite irradiation time. (There is essentially no change in the spectrum between an irradiation time of a few hundred days and infinity, for most reactors.) The total energy above 0.3 MeV emitted be- Q tween 1 s and 10 s after fission is reported to be 5.9 i 0.7 MeV/f (MAIENSCHEIN has used the chemical data of PERKINS and KING 3 8 for extrapolation from 1.8» 10 s to 1 0 s). Extrapolating in MAIENSCHEIN ' s curve from 1 s down to zero, one obtains an increment of about 0.3 MeV/f. An estimate of the contribution from energies < 0. 3 MeV can be based on the lowest energy group of PERKINS and KING which extends down to 0.1 MeV. This gives an additional increment of about 0. 3 MeV/fa bringing the total energy release up to 6. 5 +. 0.7 MeV/f. STEHN and CLANCY have made an extensive survey over several measurements on (3- and v-activities at very short cooling times (this survey includes some of the results reported by
MAIENSCHEIN)8 and they conclude that a reasonable value for the total v -energy released would be about 7.0 MeV/f. From the standpoint of shielding this value is slightly more conservative than the value of MAIENSCHEIN. It thus seems reason- able to adopt it at present. The fission product v-ray spectrum at zero cooling time and infinite irradiation timej Fig. 2, has been constructed in the follow- ing way: The energy release in each of the 6 energy groups is obtained by extrapolating the photon-intensity time distributions of MAIEN- SCHEIN from 1 s to zero, integrating from zero to 1800 s, adding a contribution for the time between 1800 s and 5* 10 s (it was necessary to calculate this separately from the data of BLOMEKE and TODD, for each isotope present and each gamma line, because the similar calculations published by PERKINS and KING have an energy grouping different from MAIENSCHEIN and are thus difficult
1) KNABE and PUTNAM give 6. 6 MeV/f for photons of energy > 0. 1 MeV released between Is and 10^ s. to compare), multiplying by the group width and by the average energy of the group. The average energy was estimated from the continuous belt measurements of MAIENSCHEIN. The sum of the 6 groups, covering the energy range 0.3 - 5.0 MeV is found to be 6.5 MeV/f. This figure can be compared with the figure 5.9 i 0.7 MeV/f of MAIENSCHEIN plus the estimated contribution of energy released within i s of fission, 0.3 MeV/f. The difference 6.5-5.9 0.3 = 0.3 MeV/f is probably attributable to the fact that the photon intensity time-distributions are uncorrected for the spectrometer response function, whereas the total energy release curve has an approximate correction. To the histogram of the 6 energy groups we finally add the estimated 0.3 MeV/f in the region < 0.3 MeV. The spectrum is obtained by fitting the histogram with a continuous curve in such a way, that within each group the shape of the spectrum resembles that of the corresponding part of the spectrum from the continuous belt measurements, and the curve is normalized to 7.0 MeV/f.
3. URANIUM CAPTURE GAMMA RAYS 238 The (n, Y)~sPectrum of U has been investigated by BARTHOLOMEW and HIGGS (1958). In the low energy region SCHULTZ & al. (1957) have presented measurements on natural uranium9 but their paper gives the intensity only on a relative Z38 scale, and is therefore difficult to relate to the U spectrum of BARTHOLOMEW and HIGGS. In Fig. 3 we have reproduced the spectrum of BARTHOLOMEW and HIGGS, after normalizing it 239 to the last-neutron binding energy of U f
4.70 MeV/capture . (3.1) 235 The gamma ray spectrum from capture in U has nots to our knowledge, been measured. It has been conjectured by BERTINI & al. (195 6) to use the same spectral distribution as for prompt fission gamma rays. On the other hand GROSHEV & al. (1958) have investigated the general shape of the unresolved part of (n, v)-spectra for different compound nuclei of the same proton- neutron parity. For even-even nuclei like U , they show that the continuous spectra start roughly at 1,5 MeV below the binding
energys increase to a maximum at about 2 or 3 MeV and then decrease to zero. On this very approximate basisone can construct O O c O O £ a spectrum for U (^1*'Y) U and normalize it to the bindning energy 6.42 MeV/capture. (3.2)
7 ~\R 4. U INELASTIC SCATTERING GAMMA RAYS
The differential inelastic scattering cross-sections O T Q cr(E t E , 9) of the energy levels E g. 1.75 MeV of U have o been measured at 9 =90 for neutrons of energies E <. 2 MeV by CRANBERG and LEVIN (1958). The integral inelastic scattering
cross-sections cr(E , E ) of the energy levels E s which for most energies E are equal to 4 7T 1) In his recent compilation HOWERTON (1958) suggests about 2.5 barns. When this value is inserted in (4. 8), the figure 0. 8 MeV/f in table 2 changes to 0. 7 MeV/f. a MejyieVv E \ N(E )P [rS(E , E )] dE , (4.1) LV n n Vjn'cO Y where v • N(E ) = uncollided fission neutron spectrum» P [rS(E ) E )] = probability that a neutron of energy E 23 8 collides with a U -atom and excites the E -level, and r = radius of the fuel element. The intensity is obtained in MeV/f j however, both S(E ,E ) and r are dependent on the choice of reactor and fuel elements, and the integration will therefore be left to the last section. The hardest gamma line of CRANBERG and LEVIN is actually not one line but a number of lines arising from several energy levels between 1.4 and 1.75 MeV. At still higher energies the level density increases and the statistical theory should become applicable. From the review article of KINSEY (1957), assuming a constant cross- section cr(E ) = a" in the statistical region^ the spectrum of inelastical ly scattered neutrons can be expressed as w(En) ,Q n e/ F(e,En)de = const. ^E +<^ n co(E ) = const. E ~ ' e 9 the level density at the bombard- ing energy E , n E d n G =-T=p—In w(E ) = s the nuclear temperature n aVE +5/4 introduced by Weisskopf and 238 — 1 /2 a is a constant^ which for U has the value 5.25 MeV ' < If we normalize the spectrum (4. 2) by the requirement E r n' F(e,Ej de = E^ , (4.3) we obtain the fraction F(eaE )dedE of all bombarding neutrons of energ6y7 between En and E n + dEn which are scattered into the energy interval between e and e + de , n FfcjEjdedE)de dE^ = - -_— . (4.4) n n .2 j _ En \ _- En /'e 9 n \ n' The total energy released as gamma radiation is then given by an integral like (4. 1), where E - e is substituted for E , and where P (r2) now is a constant, since ar was assumed constant in the C region of interest (E > 2 MeV): E f dEn C" 2 MeV n 0 Substitution of (4.4) in (4.5), and integration over the integral in e gives -E /9 n (2+En/9)-(2+3En/9)e v -P (rS) \ N(E )• 9 . = 2 ^_ dE . (4. 6) cv—v / j -v~x n# -p E \ -E/e x ; ' 21 Me Vn ' 1- (/ 1--^E - \ J e -E nTB n ' In our case E /9 >5>i (with a minimum value = 8. 67 at 2 MeV) so n that (4. 6) can be simplified to oo vP (rS) \ N(E ) (29 + E )dE . (4.7) cx ' J v n' x n' n 2 MeV The integral has the value 1.61 MeV per inelastically scattered neutron. When v =2.47 neutrons/f, the expression (4.7) takes me value 4.0 P (rS) MeV/f- (4.8) The spectral distribution of this radiation is unknown. If all the excitation energy were radiated as ground-state transitions the spectrum would be given by the integrand in (4.7). Since this is not the case, the spectrum is considerably softer, with a peak somewhere between 1 and 2 MeV. As a rough estimate we assume that 3 Pc(r£) MeV/f has the distribution of the integrand in (4.7), and the remaining quarter is distributed in the region < 2 MeV in such a way that the spectrum becomes zero at zero energy and is continuous at 2 MeV. 5. GAMMA RAYS FROM CAPTURE IN POISON» CONSTRUCTION MATERIALS AND MODERATOR After short irradiation times almost all the poison in the reactor 135 fuel is Xe 3 for which both the capture gamma spectrum and the total energy release per capture (the binding energy of Xe ) are 12 14 unknown. After a year ' s irradiation time in a flux of 1 0 - 1 0 n/cm s the Xe -fraction in the posion is still over 50 %, so that it is not worthwhile to investigate the spectra of the other poison components in any detail (BLOMEKE and TODD, 1957). Jo/ The binding energy of the two last neutrons in Xe is 14. 5 MeV» which suggests that we adopt the figure 8 MeV/capture (5.1) as the approximate total energy release. The capture gamma spectra of other absorbers present in the construction materials, the coolant or the moderator can be found in the recent compilations of GROSHEV & al. (1959)* BARTHOLOMEW and HIGGS (1958) or DELOUME (1958). 6. GAMMA RAYS FROM DISINTEGRATION OF CAPTURE PRODUCTS _ . t , , TT239 TT236 . __ 136 , The main capture products are U * U and Xe t of 239 which only U is radioactive. In addition there might be radio- active capture products in the construction materials, the coolant 239 239 and the moderator. U disintegrates to Np by emission of one 239 239 photon of 0. 074 MeV. Np disintegrates in turn to Pu in a complicated way, by emission of soft (< 0.35 MeV) gamma rays. From the decay schemes suggested by STROMINGER & al. (195 8 a, b) and by DZEPELOV and PEKER (1957) one obtains a total energy release of approximately 0.4 MeV per disintegration, including 239 the single line of U . At saturation (times large compared with the half-lives 239 239 of U t 23.5 m and Np , 2.33 d) every neutron capture in 238 2 39 U is followed by on23e 9disintegration of a U nucleus and one disintegration of a Np nucleus, so that the net energy release is 238 0.4 MeV/capture in U (6.1) The approximate spectrum is given in table 1 below. Table 1. 239 Gamma energy release from disintegration of U Energy range Energy release MeV MeV/capture 0.05 - 0,10 0.109 0.10 - 0.15 0.046 0.15 - 0.20 0 0.20 - 0.25 0.091 0.25 - 0.30 0.125 0.30 - 0.35 0.027 7. TOTAL GAMMA SPECTRA. APPLICATION TO THE SWEDISH R3-REACTOR The R3-reactor is a UO9 -fueled, D-, O-mode rated and D,O- Lt Li Lt cooled reactor to be operated at 125 MW. The fuel elements contain 43. 5 % UO2 by volume, 46.4 % D£O at about 220° C and 10. 1 % Zr, and they are arranged in the moderator on a square lattice with a lattice pitch of 27 cm (MARGEN & al. 1958). In natural uranium, there are 238 0.66 thermal captures in U per fission» and a=0.i9 " " " U " " Tin The number of neutrons captured in U -resonances per fission is given (see for instance WEINBERG and WIGNER, 1958, p. 179) by 10 ve PfPr(i-p) , (7.1) where v = Z.47 fission neutrons produced per fission 8=1. 03, the fast fission factor p = 0.89» the resonance escape probability Pf = the fast non-leakage probability P = the resonance non-leakage probability 2 -4 -2 B a 2.4 • 10 cm 3 the buckling of the reactor core* and T = 160 cm , the Fermi age. With these values, the expression (7.1) becomes 0.27 captures/f. 238 239 The total number of capture reactions U (n, v) U per fission is thus 0.66 + 0.27 = 0.93 captures/f. This factor and the figures in (3.1) and (6.1) give 238 4. 37 MeV/f released as U capture gamma radiation, and 0.37 MeV/f " " U239 decay " " a times the figure in (3.2) gives -p o r 1.22 MeV/f released as U capture gamma radiation. The number of neutrons absorbed in poison* construction materials and moderator per fission is given by v 1 - f n f - <7'2> where — =s 1.85 neutrons absorbed in U per fission, and f = 0.94, the thermal utilization factor. With these values, the expression (7.2) becomes 0.12 captures/f. In order to find the proportions of neutrons absorbed in Xe, Zr and D?O respectively, we weight the components with the product of neutron flux and macroscopic cross-section in the homogenized lattice cells. The cross-section of Xe is given in terms of barns per 2 o c original number of U atoms, by BLOMEKE and TODD (1957). We find 0.079 captures in Xe per fission 0.029 " " Zr " " 0.012 " " D " " Ii Using the binding energies 8 MeV for Xe, 6.97 MeV for Zr and 6.24 MeV for D, the energy release becomes Xe: 0. 63 MeV/f Zr: 0.20 " D,O: 0.07 it 238 The contribution from inelastic scattering in U is found by calculating the collision probability P (r2), which was introduced C in (4.1). P (r2) has been computed for uniform source strength distributions and different source geometries by PLACZEK & al. (1953). For a homogenized fuel element of radius 5. 62 cm the total energy released in inelastic scattering is found to be 0.8 MeV/f. In table 2 we collect all gamma sources together and compare them with a previous calculation by BRAUN (1957) on a similar J238 reactor. The largest difference is found to arise from the u capture, where BRAUN has used a binding energy of 7.5 MeV instead of 4.7 MeVj influenced by the value of the average binding energy per nucleon (which is approximately 7.5 MeV for U ) and by a gamma line at 7.5 MeV reported by KENNEY and MATTINGLY (1956), but not found later (BARTHOLOMEW and HIGGS, 1958). Table 2. Total gamma energy release. Energy release (MeV/f) Source Present BRAUN Prompt fission 7.9 7.8 Fission products 7.0 7.2 238 UTT capture 4.37 2 UTT 35 capture 1.22 j 8.2 Inelastic scattering 0.83> 135 0.9 Xv e capture 0.63 2 Other capture 0.271) i.o ) U decay 0.37 2 Other decay Ö.4 ) Total 22.6 25.5 1) Zr and D2O 2) Al 3) Cf footnote on p. 5. 12 The resulting total spectrum is shown in Fig. 5. The spectrum of radiation escaping from a fuel rod can be found by multiplying the total spectrum by the energy-dependent escape probability P (E) for photons of energy E. P (E) has been investigated by STORY 6SC (1957) for a uniform source strength distribution and for a step- function approximation to the real source strength distribution in cylindrical fuel rods. However, it can be shown in the energy range covered by STORY» that a simplified calculation taking into account only the first absorbing collision and assuming a uniform source strength distribution gives a result which lays well between the maximum and minimum curves of STORY. The escape probability would thus be given by where P is the collision probability of PLACZEK, and \i (E) is the energy absorption coefficient for photons of energy E in the homogenized rod of radius r. The integral of the spectrum of escaping radiation, shown in Fig. 5, is found to be 7. 6 MeV/f, which can be compared with the figure 8. 6 MeV/f of BRAUN. The fraction of gamma energy escaping from the fuel rod is thus approximately i/3. In table 3 we give the integrals over 8 energy groups of the spectrum of escaping radiation, S(E)» and the average energies of these groups, defined as E. E. E. = \ S(E) E d E / \ S(E) d E. il i 13 Table 3. Energy groups of the spectrum of escaping radiation. i Energy interval Ei Released energy MeV MeV MeV/f 1 0 - 1 0.7 1.30 2 1 - 2 1.5 2.92 3 2-3 2.4 1.91 4 3-4 3.4 0.88 5 4-5 4.3 0.36 6 5 - 6 5.4 0.88 7 6 - 7 6.2 0.12 8 7-8 7.4 0.02 Total 0-8 2.1 7.59 ACKNOWLEDGEMENTS For valuable discussions and helpful suggestions the author is indebted to Messrs. J.S. Story* Harwell, J. Braun and N. G. Sjöstrand, AB Atomenergi. REFERENCES BARTHOLOMEW G. A. and HIGGS L.A. (1958) Compilation of thermal neutron capture gamma rays. AECL-669. BERTINI H.W., COPENHAVERC.M., PERRY A.M. and STEVENSON R.B. (1956) ORNL-2113. BLOMEKE J.O. and TODD M.F. (1957) Uranium-235 fission- product production as a function of thermal neutron flux, irradiation time, and decay time. ORNL-2127, pt I-II. BRAUN J. (1957) Gamma volume-sources in the reactor core. AEF 88, AB Atomenergi-report in Swedish. CRANBERG L. and LEVIN J.S. (1958) Inelastic neutron scattering by U238. Phys. Rev. U>9, 6, 2063-2070. DELOUME F.E. (1958) Gamma ray energy spectra from thermal neutron capture. APEX 407. DZEPELOV V.S. andPEKER L.K. (1957) Decay schemes of radio- active isotopes. AECL-457. FETISOV N.I. (1957) Spectra of neutrons inelastically scattered on U238. Atomn. Energ. 3, 9» 211. GROSHEV L.V., DEMIDOV A.M., LUTSENKO V.N. and PELEKHOV V.I. (1958) Thermal-neutron capture gamma-ray spectra and nuclear level distribution. Presented as P/2029 in Geneva 1958. GROSHEV L.V., DEMIDOV A.M., LUTSENKO V.N. and PELEKHOV V.I. (1959) Atlas of spectra of gamma rays produced by thermal neutron capture. Pergamon Press. HOWERTON R. J. (1958) Semi-empirical neutron cross sections. 0.5-15 MeV. Part II Vol. I. UCRL-5351. HUGHES D. J. and SCHWARTZ R.B. (1958) Neutron cross sections. BNL-325, Second Edition. KENNEY R.W. and MATTINGLY J.T. (1956) Thermal neutron- capture gamma-ray spectrum from U238. UCRL-4735. KINSEY B.B. (1957) Nuclear reactions, levels and spectra of heavy nuclei. Encycl. of phys. Vol. XL (Nuclear reactions I), 202-449, Springer, Berlin. KNABE W.E. and PUTNAM G.E. (1958) The activity of the fission products of U235. APEX-448» MAIENSCHEIN F.C., PEELLE R.W., LOVE T.A. and ZOBEL W.(1958) Energy spectra of fission-associated gamma radiation. Presented at the EAES Shielding Symposium in Cambridge (1958), as P/670 in Geneva (1958) and in ORNL-2609, pp 45-51. MANDEVILLE C.E. and KAVANAGH D.L. (1958) The scattering of neutrons by U238. CWR-4028. 15 MARGEN P.H., CARRUTHERS H., HARGÖB., LINDBERG G. and PERSHAGEN B. (1958) R3 - A natural uranium fuel heavy water moderated reactor for combined electricity production and district heating. AB Atomenergi-report R3-100 (a modification of Geneva Conference Paper 135). MILLER CF. (1957) Gamma decay of fission products from the slow-neutron fission of U235. USNRDL-TR-i 87. O'KELLEY G.D., EICHLER E. and JOHNSON N. R. (1958) Studies of short-lived fission products and their importance to reactor technology. Presented at Geneva as P/672t 1958. PERKINS J.F. and KING R. W. (1958) Energy release from the decay of fission products. Nucl. Sci. and Eng. 3, 726-746. PLACZEK G.j CASE K. M. and de HOFFMANN F. (1953) Introduction to the theory of neutron diffusion. Vol. I, Los Alamos. PRAWITZ J., LOW K. and BJÖRNERSTEDT R. (1958) Gamma spectra of gross fission products from thermal reactors. Presented at Geneva as p/149, 1958. SCHULTZ H.L. , BOCKELMAN C.K., DRAPER J., FENSTERMACHER C.A. and ROS LER L. (1957) Gamma rays following resonant neutron capture. TID-7547. SCOLES J.F. (1958) Fission product gamma ray spectra. FZM-1042, Convair-Fort Worth. Presented at the EAES Shiel- ding Symposium in Cambridge» 1958. 235 SCOLES J.F. (1958) Calculated gamma ray spectra from U fission products. NARF-58-37T. SKLIAREVSKII V.V., FOMENKOD.E. and STEPANOV E. P. (1957) Investigation of 11^35 fission y rays in the energy region up to 250 keV. Soviet Physics 5, 2, 220-225. (Russian reference: JETP, ~ 32, 256-262). STEHNJ.R. and CLANCY E.F. (1958) Fission-product radio- activity and heat generation. Presented at Geneva as P/1071, 1958. STORM E., GILBERT E. and ISRAEL H. (1958) Gamma ray absorption coefficients for elements 1 through 100 derived from the theoretical values of the NBS. LA-2237. STORY J.S. (1957) Escape of gamma radiation from uranium rods in a pile heat evolved in the moderator. AERE T/R 2218. STROMINGER D. andHOLLANDER J.M. (1958) Decay schemes. UCRL-8289. STROMINGER D., HOLLANDER J.M. and SEABORG G. T. (1958) Table of isotopes. Rev. Mod. Phys. 30, 2, II. SULLIVAN W.H. (1957) Trilinear chart of nuclides. Oak Ridge. 16 WEINBERG A.M. and WIGNER E.P. (1958) The physical theory of neutron chain reactors. Univ. of Chicago Press. VOITOVETSKII V.K. , LEVIN B.A. and MARCHENKO E.V. (1957) Soft 15-800 keV radiation accompanying the thermal neutron fission of uranium. Soviet Physics 5, 2, 184-188 (Russian reference: JETP, 32, 263-268. ~" MeV f.MtV J._ i 4.. 3.. \ 2 . \V \ 1 . \ X 0 f 3 4 S 6 7 6 MeV Fig. 1. Spectrum of gamma rays emitted promptly (within i O s) after fission. Normalized to 7.9 MeV/fission. 'eV 3 r\ \ 9 / \ / \ \ 1 / / -^ 0 / HeV Fig. 2. Spectrum of gamma rays emitted by U thermal fission products, after 5»iO s irradiation time and zero cooling time. Normalized to 7.0 MeV/fission. 3 AfeV Z38 Z39 Fig. 3 U (n»v) U gamma ray spectrurtij normalized to 4.70 MeV/capture. 2p MeV Fig. 4. Total inelastic scattering cross-sections cr and inelastic scattering cross-sections MeV MeV f.MeV 11 å 10 9 I A 3 8 7 \ \ f, \ \ Z / \\ s / 4- j J v \ t ? 1 / 0 jj • 0 MeV Fig. 5. A. Spectrum of total gamma energy released in the R3 core (scale on left). B. Spectrum of gamma radiation escaped from the R3 fuel elements (scale on right). The peaks at 4 MeV and 6.25 MeV Z38 are due to capture in U and Ds respectively. Price Sw. cr. 4: — Additional copies available at the library of AB ATOMENERGI Stockholm - Sweden Affärstryck Offset 1959