Recommendation on Decay Heat Power in Nuclear Reactorst

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1 Journal of NUCLEAR SCIENCE and TECHNOLOGY, 28[12], pp. 1134~1142 (December 1991). SUMMARY REPORT Recommendation on Decay Heat Power in Nuclear Reactorst Kanji TASAKAt1, Toshio KATOHt1, Junichi KATAKURAt2, Tadashi YOSHIDAt3, Shungo IIJIMAt4, Ryuzo NAKASIMAt5 and Satoshi NAGAYAMAt6 t1 Nagoya University, t2 Japan Atomic Energy Research Institute t3 Toshiba Corp., t5 Hosei University, t6 Japan Atomic Power Co. Received August 6, 1991 A recommendation on the decay heat power in nuclear reactors was issued from the Research Committee on Standardization of the Decay Heat Power in Nuclear Reactors of Atomic Energy Society of Japan (AESJ). The recommendation is based on the summation calculation results with the Japanese Nuclear Data Committee (JNDC) Fission Product (FP) Nuclear Data Library, Version 2 (JNDC-V2) and the recommended data are consisted of tabulated values of decay heat power, its b and g-ray components, and their accuracies for five fissionable nuclides; 235U, 238U, 239Pu, 240Pu, and 241Pu. A set of 33-term exponential polynomials are given to help the users calculating the decay heat power for any irradiation history and cooling time. The time evolution of the energy spectra of the g-ray component is also given. A realistic estimation of the neutron capture effect on the decay heat power can be obtained by the interpolation of the tabulated data obtained by the summation calculation. KEYWORDS: recommendations, Japan, fission products, decay heat power, standard, assessment, summation calculation, gross theory, beta decay, gamma-ray spectrum, neutron capture effect, exponential fitting, uranium 238, uranium 235, plutonium 239, plutonium 240, plutonium 241 I. INTRODUCTION There are a number of important design and operating criteria which require a knowledge of decay heat power of fission products (FPs) irrespective of which reactor system one may consider. The FP decay heat power plays an important role in predicting the heatup of the nuclear fuel rod during a loss-of-coolant accident (LOCA) of a nuclear reactor. The decay heat power is also important in designing the heat removal system of a reactor and also spent fuel handling equipments. Decay heat power may be determined by either integral measurements on mixed fission products or by calculations. Although the calculation may be preferred, by virtue of its generality, in the earlier years experimental results were required to fill the gaps at short cooling times because decay data for shortlived FPs were sparse to calculate the decay heat power accurately at short cooling times. Therefore, the predictions of decay heat power became necessary with the aid of standard curves and equations. These standards were devised to provide conservative estimates of decay heat power, and some ones have remained in favor up to the present time because the use of standards is more expedient t This article is a summary of the 23rd AESJ Technical Award, No. 2305, (Mar. 28, 1991). Furo_cho, Chikusa-ku, Nagoya t1 2 Tokai -mura, Ibaraki-ken t Ukishima-cho, Kawasaki-ku, t3 Kawasaki Deceased on November 14th, t 5 Fujimi, Chiyoda-ku, Tokyo 102. t 6 Tokai _mura, Ibaraki-ken t. 68

2 Vol. 28, No. 12 (Dec. 1991) Summary Report (K. Tasaka et al.) 1135 in many cases than a summation calculation for the evaluation of decay heat power. The summation calculation consists of solving the balance equations for individual FP nuclides during reactor operation and after shutdown. The concentration of each nuclide and its decay constant give its decay rate at any time and multiplying the decay rate by the recoverable energy per decay gives the decay heat power contributed by that nuclide. Then the decay heat power from FPs is obtained by summing all contributions, hence the terminology "summation calculation ". The ANS-5.1 standard proposed in 1971 and revised in 1973(1) has been used extensively all over the world primarily for evaluation of Emergency Core Cooling System (ECCS) performance in a hypothetical LOCA of a light water reactor (LWR). The standard had a large uncertainty of +20%, -40% for short cooling times less than 103 s. New experimental research programs(2)-(13) were initiated in 1974 to better quantify decay heat power and its uncertainty at short cooling times. The standard was revised in 1979(14) utilizing these new results. In Japan, the Decay Heat Evaluation Working Group (DHEWG) was established by the Japanese Nuclear Data Committee (JNDC) in the early 1970's. The DHEWG completed the first version of the FP nuclear data library (INDC-V1)(15) in 1980 for the summation calculation of the decay heat power. The gross theory(16)~(20) of 9 decay was extensively used to estimate the decay data of short-lived FP nuclides, and the agreement between calculated and measured decay heat powers was remarkably improved at short cooling times less than 103 s(15). The library was revised in 1989 as the second version (JNDC-V2)(21) after a long period of elaboration, and the discrepancy was removed which remained for the cooling times longer than a few hundreds seconds between measurement and calculation with the first version (JNDC-V1)(15). Along with theoretical studies a series of experimental studies(22)~(25) were conducted by Akiyama et al. at the University of Tokyo, Tokai (UTT). Beta- and g-ray energy releases 69 and their spectra from aggregate FPs were measured in their experiments for the fissionable nuclides from 232Th to 239Pu. The high precision measurements provided excellent benchmarks for verifying the reliability of calculations. The Research Committe on "Standardization of Decay Heat Power in Nuclear Reactors" was organized in 1987 by the Atomic Energy Society of Japan to establish the recommendation for decay heat power by fully utilizing the Japanese recent works of the accurate calculation results(21) and high precision measurements(22)~(25). The Committee completed the recommendation(26)(27) after the evaluation work for three years. The recommendation can give b- and g-ray decay heat power separately for the fissions of 235U, 238U, 239Pu, 240Pu 241Pu with the consideration of neutron capture effects in FPs. The g-ray spectrum can be also obtained by the recommendation. The details are described in the following chapters. II. SUMMATION CALCULATION OF FP DECAY HEAT POWER AND ITS ASSESSMENT WITH MEASURED DATA The nuclear data of FPs have been compiled for summation calculation of the FP decay heat power. The nuclear data included in the JNDC-V2 library(21) are decay data, fission yield data, and neutron capture cross section data. Decay data are consisted of decay constants, Q-values, excitation energy of isomeric state, energies and intensities of b -rays, positrons, g-rays and internal conversion electrons, and branching ratio of each decay mode including delayed neutron emission. The JNDC-V2 nuclear-data library has been prepared for 1,227 FPs, of which 147 are stable and the others are unstable. The library contains the decay data and the fission yield data for all of the 1,227 FP nuclides and the neutron capture cross section data for 166 FP nuclides. There are difficulties in constructing a complete decay scheme of a nuclide with high Qb-values from experimental information based on g-ray spectrum measurements by Ge-detec-

3 1136 Recommendation on Decay Heat Power in Nuclear Reactors J. Nucl. Sci. Technol., tors, because many high energy g-rays may remain unobserved and also because an appreciable amount of detected g-rays may fail to be placed in right positions in the decay scheme. In that case some g-rays are often excluded from the decay scheme. For that reason average g energy Eg per b decay could be underestimated and the average b energy Eb per b decay overestimated, when we use the decay scheme constructed from the g-ray spectroscopic data for the nuclides with high Qb-value in order to derive those values(28). On the other hand, the gross theory can be applied satisfactorily to the high Qb-value nuclides for which the strength of b decay can be described by the continuous b strength function. The introduction of the theoretical values of Eb and Eg explained the overestimation of the b decay heat power and the underestimation of the g decay heat power in the existing libraries and brought about the remarkable improvement in the accuracy of summation calculation of FP decay heat power with the JNDC-V2 library. Decay heat powers of FPs were calculated by the DCHAIN(29)(30) code, using the JNDC- V2 library, for the thermal neutron fissions of 233U, 235U, 239Pu and 241Pu and for the fast neutron fissions of 232Th, 233U, 235U, 238U and 239Pu. The results were compared with the recent decay power experiments at UTT(22)~, ORNL(8)~(14) and LANL(3)(4). (25) Akiyama et al.(22)-(25) at UTT measured the decay heat power of FPs for the fast neutron fissions of 232Th, 223U, 235U, 238U and 239Pu. Samples of approximately 2 mg were irradiated for 10 to 300 s in the core of fast neutron source reactor "YAYOI" of the University of Tokyo. The b and g-rays emitted from the sample were measured for times-after-fission of 11 to 26,000 s. The data were obtained for b- and g-rays separately as spectral distributions. The b-ray spectra were obtained by using a plastic scintillation detector combined with a transmission type proportional counter to eliminate g-ray effects ; and the g-ray spectra were obtained by using an NaI(Tl) scintillation detector. These spectra were integrated over full energy range of the b- or g-rays to provide the total yields and the energy integrated values as a function of time after a fission pulse in the range of 19 to 24,000 s. The calculated decay heat powers by JNDC-V2 are in satisfactory agreement with the ones measured at YAYOI as shown for 225U, 239Pu and 238U in Fig. 1 for an example. The calculated ones also show reasonable agreement with the measured ones at ORNL 8)~(14) and LANL(3)(4). Therefore, it is ( determined to adopt the calculated values of decay heat powers by JNDC-V2 as the recommendation, in stead of the simultaneous fitting values of the calculated and measured data as in 1979 ANS-5.1 standard. III. RECOMMENDED DECAY HEAT POWER 1. Decay Heat Power As described in Chap. II, the recommended values(26)(27) of FP decay heat powers were obtained by the summation calculation with the JNDC-V2 nuclear data library(21). This means that every quantity, such as the time evolution for the FP inventories, their activities and g-ray spectra, are available strictly on the same basis as the recommended decay heat power itself. Table 1 is an example of the tabulated data of recommendation for the decay heat powers after a fission. The recommendation is applicable, even after quite a long cooling time, say, 1013 s (320,000 yr). Varieties of the fissile nuclides are extended from the existing standards or recommendations. Five fissionable nuclides 235U, 238U, 239Pu, 240Pu and 241Pu are included in the present recommendation. The differences are considered small between decay heat powers by the thermal-neutron fission and by the FBR-neutron fission from the comparison of the ORNL experiments(8)~ (14) for the thermal-neutron fission and the UTT experiments(22)~(25) for the FBR-neutron fission. The fission-pulse function fi(t), where t and i stand for the cooling time and the fissionable nuclide index, can also be reproduced by a 33-term exponential polynomial, 70

4 Vol. 28, No. 12 (Dec. 1991) Summary Report (K. Tasaka et al.) 1137 Fig. 1 Comparison of calculated total sensible decay heat power (multiplied by cooling time t) with measured results at the YAYOI reactor of the University of Tokyo for the burst fission of 235U, 239Pu and 238U by fast neutrons 71

5 1138 Recommendation on Decay Heat Power in Nuclear Reactors J. Nucl. See. Technol., Table 1 Recommended decay heat power ( MeV / s / fission ) after one burst fission with constants aij and lj given in Table 2. Furthermore, by integrating Eq. (1), we obtain the decay heat power F(T, t) at time t after the end of operating (or irradiation) time T as (1) (2) where wi stands for the contribution from the i-th fissionable nuclide. As is exemplified in the ANS-5.1 standard(14), Eq. (2) can easily be extended to cases where the operation history is more complicated, Not only the total decay heat power, but its b- and g-ray components are provided in the recommendation. 2. Uncertainties of Recommended Decay Heat Powers The uncertainties or the standard deviations of the recommended values were calculated from the error in the basic nuclear data by using the relative sensitivity coefficients of the fission-pulse function of the FP decay heat power to parameters, such as fission yields, decay constants, and average decay energies(31). The present recommended value has high precision. As an example, the error (on 1-s level) associated with the 235U decay heat power at 1 s after an infinite irradiation is estimated to be 2.1 %. 3. Gamma-ray Spectra The time evolution for the g-ray energy spectra are also given in the recommendation. The normalized g-ray energy spectra of 235U, and 239Pu fissions were prepared for 238U each decade cooling time with an energy bin of 1 MeV from 0 to 10 MeV by summation calculation with the use of JNDC-V2 and a separate file containing the g-ray spectrum data of each FP nuclide. The theoretically estimated spectra were applied for the nuclides with no or insufficient g-ray transition data(32)(33). The gross theory of b decay and El-cascade model for g transition were used for the theoretical estimation. By introducing the estimated spectra, it becomes possible to have satisfactory agreement of the calculated g-ray energy spectra of aggregate FPs with the measured ones at ORNL(8)~(14) and UTT(25) even at short cooling times as shown for an example in Fig. 2(33). Table 3 shows an example of the recommended spectra for 235U after a 3 yr irradiation. The g-ray spectrum of 134Cs, which is produced by neutron capture in 133Cs, is included as well. A spectrum at any cooling time is obtained by an interpolation. The tabulated spectra should be used for a fuel irradiated for more than 1 yr. 4. Neutron Capture Effect Those of long-life or stable FP nuclides which have large neutron-capture cross sections, transform themselves to heavier nuclides 72

6 Vol. 28, No. 12 (Dec. 1991) Summary Report (K. Tasaka et al.) 1139 Table 2 Constants l(1/s) and a(mev/s/fission) in exponential formula fitted to recommended FP decay heat power Fig. 2 Comparison of calculated g-ray energy spectrum with measured results at ORNL at 2.7 s after thermal neutron fission of 235U (tirrad=1.0 S, twait=1.7 s, tcount=1.0 s) 73

7 1140 Recommendation on Decay Heat Power in Nuclear Reactors J. Nucl. Sci. Technol., Table 3 Normalized g-ray spectra (g/mev) for 235U after 3 yr irradiation and affect the decay heat power of FPs as a result. Especially the stable FP nuclide 133Cs is transformed by neutron capture into 134Cs which emits strong g-rays and increases the decay heat power considerably for the long cooling time period up to 10 yr. The neutron capture effect can be expressed as the ratio of decay heat powers with and without consideration for the neutron capture effect(34). The neutron capture cross section data in JNDC-V2 are derived from the FP nuclear data library of JENDL-3, the Japanese Evaluated Nuclear Data Library, Version 3(35). In the present work, the neutron capture effect were calculated for the following variables. Three reactor types PWR, BWR and FBR were considered. Considered fissiles were 235U and 239Pu. The 239Pu data were recommended to use for other nuclides 238U, 240Pu and 241Pu because of similarities of fission yield data. The neutron flux p was varied as 0.0, 0.1, 0.5, 1.0, 2, 5 and 10 times the typical value of each reactor type. Irradiation time T and cooling time t were extended up to 5 yr and 1 x1013 s, respectively. The neutron capture effect for the actual irradiation condition (p, T, t) is obtained by a linear interpolation of the calculated results after selecting the reactor type or the neutron spectrum. 5. Comparison with Existing Standards The contents of the present work is compared with the 1979 ANS-5.1 standard and the ISO (International Standard Organization) standard in Table 4. It is seen in Table 4 that the present standard is extended considerably compared to the other standards in Table 4 Comparison of standards of decay heat power 74

8 Vol. 28, No. 12 (Dec. 1991) Summary Report (K. Tasaka et al.) 1141 number of fissionable nuclides, data contents, cooling time and so on. Figure 3 compares the present recommendation with the ANS-5.1 for the burst fission case, where the difference is larger than any other irradiation and easier to see. The differences between the AESJ and the ANS-5.1 standards are within +-6% for 235U, +-8% for 239Pu and +-16% for 238U. (3) Presentation of the delayed (or FP) g- ray energy spectrum data. The present recommendation is more inclusive and has wider scope than the existing recommendations or standards, therefore, it is applicable to a wide range of problems, related to decay heat power in PWR, BWR, FBR and other nuclear facilities. In future the b-ray energy-spectrum data will be included as well as the data for the fissionable nuclides 232Th and 233U. ACKNOWLEDGMENT The authors are grateful to the members of the AESJ Research Committee on Standardization of Decay Heat Power in Nuclear Reactors for the discussion and various kinds of inputs to the standards. They are also indebted to the members of Decay Heat Evaluation Working Group (DHEWG) of Japanese Nuclear Data Committee (JNDC) for the discussion and encouragement to the present work. REFERENCES Fig. 3 Ratio of AESJ Recommendation to ANS-5.1 IV. CONCLUDING REMARKS The present recommendation is based on the summation calculation using the JNDC FP Nuclear Data Library, Version 2, and recommended values are reproduced by the simple fitting formula given in the text. The recommendation has the following new features in addition to an intention of improving the reliability when being applied to the problems related to planned or accidental shutdown of LWRs : (1) Refined treatment for the FP neutroncapture effect, which guarantees the reasonable corrections even in very long cooling-time ranges. (2) Explicit inclusion of 240Pu and 241Pu leading to more proper application to FBRs. (1) ANS Proposed Standard, ANS-5.1 "Decay energy release rates following shutdown of uraniumfueled thermal reactors", (1971, revised 1973). (2) YARNELL, J. L., BENDT, P. J.: Decay heat from products of 235U thermal fission by fast-response boil-off calorimetry, LA-NUREG-6713, (1977). (3) idem : Calorimetric fission product decay heat measurements for 239Pu, 233U and 235U, LA MS, NUREG/CR-0349, (1978). (4) SCHROCK, V. E., et al.: A calorimetric measurement of decay heat from 235U fission products from 10 to 105 seconds, EPRI Rep., NP 616, Vol. 1, (1978). (5) FRIESENHAHN, S. J., et al.: U-235 fission product decay heat from 1 to 100,000 seconds, EPRI- NP-180, (1976). (6) FRIESENHAHN, S. J., LURIE, N. A. : Measurements of fission-product decay heat from 235U and 239Pu, IRT , (1977). (7) DICKENS, J. K., et al.: Fission product energy release for times following thermal neutron fission of 235U between 2 and 14,000 seconds, ORNL/NUREG-14, (1977). (8) DICKENS, J. K., et al.: Fission product energy release for times following thermal neutron fission of 239Pu between 2 and 14,000 seconds, ORNL/NUREG-34, (1978). (9) DICKENS, J. K., et al.: Beta and gamma-ray production due to thermal fission of 235U, spectral distributions for times after fission between 2 and 14,000 seconds : tabular and graphical data, NUREG/CR-0162, ORNL/NUREG-39, (1978). (10) DICKENS, J. K., et al.: Fission product energy 75

9 1142 Recommendation on Decay Heat Power in Nuclear Reactors J. Nucl. Sci. Technol., release for times following thermal neutron fission of 241Pu between 2 and 14,000 seconds, NUREG/CR-0171, ORNL/NUREG-47, (1978). (11) DICKENS, J. K., et al.: Delayed beta- and gamma-ray production due to thermal-neutron fission of 239Pu : tabular and graphical spectral distributions for times after fission between 2 and 14,000 sec, NUREG/CR-1172, ORNL/NUREG- 66, (1980). (12) DICKENS, J. K., et al.: Nucl. Sci. Eng., 74, 106 (1980). (13) DICKENS, J. K., et al.: ibid., 78, 126 (1981). (14) American National Standard, ANSI/ANS "Decay heat power in light water reactor", (1979). (15) TASAKA, K., et al.: JNDC nuclear data library of fission products, JAERI-1287, (1983). (16) TAKAHASHI, K., YAMADA, M.: Progr. Theor. Phys., 41, 1470 (1969). (17) KOYAMA, S., et al.: ibid., 44, 663 (1970). (18) TAKAHASHI, K., et al.: At. Data Nucl. Data Tables, 12, 101 (1973). KONDOH, T., et (19) al.: Progr. Theor. Phys., 74, 708 (1985). (20) TACHIBANA, T., et al.: ibid., 84, 641 (1990). (21) TASAKA, K., et al.: JNDC nuclear data library of fission products second version, JAERI-1320, (1990). (22) AKIYAMA, M., et al.: Measurements of g-ray decay heat of fission products for fast neutron fission of 235U, 239Pu and 233U, J. At. Energy Soc. Jpn., (in Japanese), 24, 709 (1982). 3) AKIYAMA, M., et al.: Measurements (2 of betaray decay heat of fission products for fast neutron fission of 235U, 239Pu and 233U, ibid., (in Japanese), 24, 803 (1982). (24) AKIYAMA, M., AN, S.: Measurement of fission product decay heat for fast reactor, Proc. Int. Conf. on Nucl. Data for Science and Technology, Antwerp, Belgium, 237, (1982). AKIYAMA, M., KATAKURA, (25) J.: Measured data of delayed gamma-ray spectra from fissions of 232Th, 233U, 235U, 238U and 239Pu by fast neutrons; tabular data, JAERI-M , (1988). (26) AESJ Research Committee on Standardization of Decay Heat Power in Nuclear Reactors : Reactor Decay Heat and Recommended Values, (in Japanese), (1989). TASAKA, (27) K., et al.: Recommended values of decay heat power and method to utilize the data, JAERI-M , (1991). (28) YOSHIDA, T., NAKASIMA, R.: J. Nucl. Sci. Technol., 18, 393 (1981). (29) TASAKA, K.: DCHAIN-code for analysis of build-up and decay of nuclides, JAERI-1250, (in Japanese), (1977). (30) idem: DCHAIN2 ; a computer code for calculation of transmutation of nuclides, JAERI-M 8727, (1980). (31) KATAKURA, J., IIJIMA, S.: Analysis of uncertainties in the summation calculation of decay heat with JNDC FP nuclear data library, J. Nucl. Sci. Technol., 29[1] (1992). (32) YOSHIDA, T., KATAKURA, J.: Nucl. Sci. Eng., 93, 193 (1986). (33) KATAKURA, J., YOSHIDA, T.: Gamma-ray spectrum data library of fission product nuclides and its assessment, JAERI-1311, (1988). (34) TASAKA, K. : Nucl. Sci. Eng., 62, 167 (1977). (35) NAKAGAWA, T., et al.: FP nuclear data library of JENDL-3, presented at Int. Conf. Nucl. Data for Sci. and Technol., May 1991, Juelich, C-42 ; See also NAKAGAWA, T., et al.: Preprint 1990 Fall Mtg. of AESJ, Sendai, (in Japanese), 18. B14 ~ 76