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Sources of Gamma Radiation in a Reactor Core Matts Roas

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Sources of Gamma Radiation in a Reactor Core Matts Roas 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 .
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