MEASUREMENT OF SPENT FUEL ASSEMBLIES IN SPRR-300

Transcription

1 MEASUREMENT OF SPENT FUEL ASSEMBLIES IN SPRR-300 CHEN Wei, HU Zhiyong, YANG Rui Institute of Nuclear Physics and Chemistry, Sichuan, China 1 Preface SPRR-300 is a pool-typed research reactor which uses light water as both coolant and moderator. Its fuels are rods made of uranium dioxide. The enrichment of uranium-235 is 10 percent. Fifteen or sixteen rods are fabricated into an assembly. The highest power of the reactor is 3.5 mega-watts. The planar map of its upper reactor hall (e.g. Hall 309) is showed in figure 1. The reactor was designed in 1965 and started to be built in It reached criticality in June, From 1996 to 2002 it operated at a maintenance state because the fuel rods were used up. Having purchased 300 new fuel rods, the reactor resumed normal operation in Until 2005, the reactor has been operated for about 3330 mega-watt-day. Fig. 1 Hall 309 of SPRR Reactor pool; 2-Shield wall of upper reactor room; 3-Storage pool; 4-Dry well; 5-Peep window SPRR-300 will sure be decommissioned in the near future after more than 25 years operation. SPRR-300, a research reactor, starts and stops frequently. So usually an assembly in the reactor will undergo about 3000 times operation with power [1, 2]. Because the records of the reactor operation and the neutron flux are not only uncompleted but also inexact, some of the assemblies should be measured to ensure the source term calculation method using those records is feasible. So two kinds of experiments are designed, which are gamma irradiation rate measurement experiment and gamma spectrum measurement experiment. 2 Gamma irradiation measurement experiment The data of gamma irradiation rates, which are very important for radiation protection of the assemblies, are the most concerned data by the authority of the reactor. So gamma irradiation rates measurement experiment is first designed. Gamma irradiation rates of three spent fuel assemblies are measured at different distances. As part of the data calculated, the gamma radioactivity data can be converted to gamma irradiation rates at same distance so as to be compared with those measured [3]. 2.1 Experiment conditions a) Site: Hall 309 of SPRR-300; b) Shielding: Shield wall of upper reactor room;

2 Hall 309 North Hall 121 c) Equipments: Portable gamma detectors of FJ-302BG1 and FJ-317C; Neutron and gamma detector of FJ Experiment steps a) As showed in figure 2, the experiment site is arranged. The probe is fixed on a handcart; b) The range from the point where the spent fuel assembly is craned to the north wall of Hall 309 is labeled on the floor of the hall using a band tape. The interval is one meter; c) The handcart is taken into the first place. A spent fuel assembly is craned from the temporary storage shelf in the reactor pool until it is about one meter high to the floor of Hall 309. The spent fuel assembly should be lowered into the pool when the measuring at the distance is finished; d) The spent fuel assembly is craned or lowered again and again while the handcart is pushed forward one meter every time. The gamma irradiation rates at five distances, which are ten, eight, six, five, and four meter, will be measured from far to near if possible. If the data measured at any distance is large than Sv/h, the measuring at the following distances will be canceled; e) The spent fuel assembly is put back to the original position in the temporary storage shelf; f) According to step c, d, and e, other two spent fuel assemblies are measured, too; g) The experiment site is restored. 2.3 Experiment data analysis Fig. 2 On-site arrangement of gamma irradiation rates measurement experiment 1-Scale line; 2-Handcart; 3-Probe; 4-Spent fuel assemblies; 5-Shield wall of upper reactor room ; 6-Reactor pool Table 1 Gamma irradiation rates Distance Average gamma irradiation rates Assembly No. Load date Unload date m 10-2 Sv/h A070 Feb. 10, 1982 Oct. 17, A058 Jan. 19, 1980 Mar. 17, A103 Jan. 19, 1980 Feb. 17,

3 Actually gamma irradiation rates at three distances of the spent fuel assemblies are measured, which are showed in table 1. Eighteen groups of gamma activities of the same assemblies are also calculated. These data can be converted to gamma irradiation rates at different distances considering the total effects of eighteen groups of gamma rays. Those two kinds of gamma irradiation rates are compared in table 2. It can be seen that the data calculated are larger than those measured. So the data calculated are enough safe for the shield reckoning during the decommissioning. The maximum relative error is 31.56% while the average is 22.98%. The error is acceptable when it is taken into account that the error of absolute neutron flux measurement is 4%, the error of relative neutron flux measurement is 5%, the input masses of the impurity substance are all the largest of their analytical values which can influence the accuracy of calculation input data, and the error of measurement for gamma irradiation rates is 15%, low-energy gamma cannot be measured in distance which might influence the accuracy of experiment data. Table 2 Gamma irradiation aates (at 1cm distance) Assembly No. A058 A070 A103 Irradiation rates Calculated Sv/h Measured Error (%) Gamma spectrum measurement experiment To further verify the credibility of the data calculated, gamma spectrum at different distances of three spent fuel assemblies are measured. 3.1 Experiment conditions a) Site: Hall 309 of SPRR-300; b) Shielding: Water of the reactor pool; Fig. 3 On-site arrangement of gamma spectrum measurement experiment 1-Loop plate; 2-Container (fuel position); 3-Reactor pool; 4-Crane rope; 5-HP-Ge probe; 6-Main unit c) Equipments: High-purity germanium (HP-Ge) detector of ORTEC TEM-13185; portable gamma detectors of FJ-302BG1 and FJ-317C

4 3.2 Experiment steps a) As showed in figure 3, the experiment site is arranged. A square aluminum tube is used to form a container for the spent fuel assemblies. The probe of the HP-Ge detector is fastened above the reactor pool. The major unit of the HP-Ge detector is arranged in Hall 309; b) The container for the assemblies is put at the fuel position showed in figure 3 when the crane rope is calibrated and labeled; c) The container is put on the loop plate in the reactor pool while a spent fuel assembly is lay into it. The container is first craned to six meters. Gamma irradiation rates at every work post are always measured using other detectors to ensure that they are all below Sv/h; d) The container will then be craned steadily while gamma spectrums of the assembly at different distances, which may be five, four, and three meter, is measured from far to near. The suitable distance for measuring will be confirmed during the experiment; e) The container is put back on the loop plate when the assembly is taken back to the original position in the temporary storage shelf; f) Gamma Spectrums of other two assemblies are also measured following step c, d, and e; g) The experiment site is restored. 3.3 Experiment data analysis Fig. 4 Gamma spectrum at three distances of a assembly During the experiment, the HP-Ge probe is settled 1.5 meter away from the surface of the water. As showed in figure 4, in fact, the gamma spectrums at 1.2m, 1.3m, and 1.5m of three assemblies can be measured. The detecting efficiency of the experiment is calculated by a program called LabSOCS. Fission nuclides likes 134 Cs, 137Cs, and 154 Eu are found when those gamma spectrums are analyzed. Activities of those nuclides can be deduced from the spectrums. The data caused by background nuclides likes 60 Co are discarded. These data of activities are compared with those calculated, which are shown in table 3. The two kinds of data are in the same order. But the errors are great, while the maximum relative error is 69.96%, and the average one is 43.59%. First, in the experiment, the distances from the assembly to the surface of the water cannot be measured precisely while the error is in the order of centimeter, which makes experimental errors

5 up to 4%. Second, because of the feature of the spent fuel, we cannot calibrate the detecting efficiency using standard source. According to experience, the experimental errors arising thereof it should be larger than 30%. Last, but not the least, a spent fuel assembly actually consists of many activated and fission products. Gamma beam of any energy is emitted by more than one nuclide. But during experiment, the detector identifies that gamma beam of specific energy emitted by specific nuclide. For example, all kev gamma beam should be emitted by only 137 Cs. So, naturally, the data measured will be greater than those of calculated. Considering all reasons mentioned above, the error in table 3 can be accepted. This means that the data calculated are feasible. 134 Cs 137 Cs 154 Eu Table 3 Activities of nuclides Assembly No. A003 A062 A070 Activity measured (Bq) 5 E10±1.1E10 7 E10±4.6E E10±2.7E9 Activity calculated (Bq) 1.502E E E9 Error (%) Activity measured (Bq) 5 E12±1.2E E12±9.43E10 5 E12±1.1E12 Activity calculated (Bq) 3.661E E E12 Error (%) Activity measured (Bq) 5 E10±1.1E E10±3.93E9 3.3 E10±9.5E9 Activity calculated (Bq) 2.355E E E10 Error (%) Unload date Mar. 3, 1992 Dec. 6, 1995 Oct. 17, Conclusion It is to ensure the calculation method of the source term in SPRR-300 is reliable that two kinds of experiments, which are gamma irradiation rates measurement experiment and gamma spectrum measurement experiment, are arranged using equipments and conditions available. The measurements are yet in the rough and they may be improved in the future. Despite of that, it is verified that the data calculated are basically believable by comparing the data measured with those calculated. And it is also confirmed that the data calculated are safe enough to be used for shield calculating when the spent fuel assemblies are to be transported out of the reactor site during decommissioning. 5 References [1] Wei CHEN, Zhiyong HU, Rui YANG, Estimation of Source Term for Spent Fuel Elements in SPRR-300, PARTICIPATE WTO AND SUSTAINED DEVELOPMENT OF CHINESE SCIENCE & TECHNOLOGY (Edition 1), BEIJING: China Science & Technology Press, 2002, pp , [in Chinese] [2] Wei CHEN, Zhiyong HU, Rui YANG, Calculating Radioactivity of Spent Fuel Elements of No. 300 Reactor with ORIGEN2, THE 13TH PACIFIC BASIN NUCLEAR CONFERENCE (Edition 1), BEIJING: Atom Energy Press, 2002, pp. 303 [3] CHEN Wei, DAI Junlong, HU Zhiyong, Preliminary Measuring of Spent Fuels in SPRR-300, Nuclear Electronics & Detection Technology, Vol. 24, Iss. 5, 2004, pp , [in Chinese] Tel.: , Fax: chenwei_roy@sina.com