This is a progress report on activities of the Special Committee on Reactor Physics for the period from April 1979 to March 1982.
For these three years, the Committee continued to make efforts for the objectives, keeping the close relation with the NEA Committee on Reactor Physics (NEACRP). The activities were preparation of reviews on reactor physics activities in Japan, discussion on papers to be presented at the meetings of the NEACRP and Specialists' Meetings, and participation in international benchmark tests such as LMFBR burnup and shielding calculations. The Subcommittees on Reactor System, Fusion Reactor and Shielding discussed topics of relevance to each field. For the Third Specialists' Meeting on Reactor Noise (SMORN-III), the Preparatory Committee was set up under the Committee, which made preparations for the Reactor Noise Analysis Benchmark Test and contributed to the success of SMORN-III. For the Six International Conference on Radiation Shielding in 1983, the Preparatory Committee worked in the fiscal year of 1981.

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Gamma decay heat released from fission products (FPs) has been measured for fast-neutron fissions of ²³⁵U, ²³⁹Pu and ²³³U using the radiation spectrometry method. The sample irradiations were for 10 and 100s in the fast neutron source reactor "YAYOI" of the University of Tokyo. Spectral data for γ-ray were obtained at post-irradiation time intervals ranging 11-25, 950s using a NaI (Tl) scintillation detector. The data were processed to the form of γ-ray energy release rates per fission for each set of time-interval parameters. The standard representation of the decay heat following fission pulse (in cooling times ranging 19-24, 000s) was provided from the γ-ray energy release rates. The uncertainties (1σ) of the decay heat data were about 5% for ²³⁵U and ²³⁹Pu, and about 7% for ²³³U.
The data obtained from the present experiments are compared with three summation calculations using JNDC, TASAKA and ENDF/B-IV decay data libraries, and with other experimental results. The summation calculation results using the JNDC FP decay data library are in better agreement with the present data than the other calculations.

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The analytical results of blowdown characteristics and its thrust force were compared with the experiment, which were performed as pipe whip tests under the PWR·LOCA conditions on the hypothetical accident of guillotine break of pipes. The blowdown thrust force was obtained by the integral momentum equation about single-phase flow, homogeneous and segarated two-phase flow, assuming critical pressure at the exit if critical flow condition was satisfied. The following results are obtained:
(1) The node-junction method is useful for the analysis of water hammer phenomena and of the blowdown thrust force.
(2) The Henry-Fauske model for subcooled critical flow is effective for the analysis of the maximum thrust force under the PWR·LOCA conditions. The jet thrust parameter of analysis and experiment is 1.08.
(3) The thrust parameter of saturated blowdown has the same one with the value under pressurized condition when the stagnant pressure is chosen as the saturated one.
(4) The dominant terms of the blowdown thrust force in the momentum equation are the pressure and momentum terms, while the acceleration term is contributive just after break.

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