SETTING OF THE APPARATUS FOR IRRADIATION OF SAMPLES WITH FAST NEUTRONS IN THE EXPOSURE ROOM OF TRIGA MARK II REACTOR IN LJUBLJANA

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1 International Conference Nuclear Energy in Central Europe 2001 Hoteli Bernardin, Portorož, Slovenia, September 10-13, 2001 www: tel.: , fax: Nuclear Society of Slovenia, PORT2001, Jamova 39, SI-1000 Ljubljana, Slovenia SETTING OF THE APPARATUS FOR IRRADIATION OF SAMPLES WITH FAST NEUTRONS IN THE EXPOSURE ROOM OF TRIGA MARK II REACTOR IN LJUBLJANA ABSTRACT Edvard S. Krištof Jožef Stefan Institute Reactor Physics Division Jamova 39, SI-1000 Ljubljana, Slovenia edvard.kristof@ijs.si In this paper the arising of a device for irradiation of samples with fission neutrons is described. Also fast neutron spectra in several irradiation positions are given. 1 INTRODUCTION Apparatus is positioned in the exposure room of the reactor. The essential piece of the device is a fission plate with the diameter of 25 cm. It contains 294 grams of the isotope 235 U. Construction materials of the exposed parts of the device were chosen so that after 40 years of operation it will not be classified as a radioactive matter. 2 IRRADIATION SET-UP A simplified vertical cross section through the most important part of the device is shown in Fig.1. The irradiation position S A is 20cm ahead of the outlet aperture and 38cm from the fission plate. This position is convenient for irradiation with moderated neutrons. For the time being, the device has been provisionally used also for irradiation of materials sensitive to fast neutrons. Before the irradiation, the fission plate must be lifted from its shielding (Fig.2 and Fig.3) into the beam of moderated neutrons, which enter the exposure room through the empty horizontal shaft. These neutrons induce fission of 235 U nuclei in the plate. Irradiation of the sample begins by lowering the sledge (Fig.4) into the room and finishes by lifting it back. Lifting and lowering of the sample and of the fission plate are performed by use of two manipulators on the top of reactor (Fig.4). The construction materials of the support for the fission plate and for the guiding frame (Fig.5) are aluminium castings AlMg3 and AlSi1Mg [1]. Our reactor produces yearly 250MWh of thermal energy. By the use of the reevaluated data from [2], the conservative upper limits of the specific activity of the exposed parts were estimated. Hereby it was supposed the reactor operation through the period of 40 years under conditions of the time being (Table 1). All radioisotopes are belonging to third or even to the fourth group of radio toxicity [3]. The sum of upper limits does not exceed the boundary value of 7400 Bq/g for the 614.1

2 614.2 third group. Hence, the device with the removed fission plate might be brought in the usual human surrounding at any time. Table 1: The upper limit of the specific activity in the exposed parts of the guiding frame with the support. Nuclides with the half life shorter then 3 weeks are not taken into account. Mother isotope 50 Cr 54 Fe 54 Fe 58 Fe 63 Cu 64 Zn 92 Zr 94 Zr Radioactive nuclide Half life Upper limit of the specific activity after 40 years of operation [Bq/g] Upper limit of the specific activity after six months of cooling down [Bq/g] 51 Cr 27,70 d 150 1,7 54 Mn 312,50 d 0,067 0, Fe 2,70 a Fe 45,10 d 1,0 0, Ni 100,04 a 0,52 0,52 65 Zn 244,30 d Zr 1, a 5, , Zr 92 Nb 63,98 d 35,15 d 0,69 0,69 0,098 0,19 3 FAST NEUTRON SPECTRA A preliminary result of the fast neutron spectra is shown in Fig.6. The curve M belongs to the neutron spectrum measured in absence of the fission plate in the irradiation position S A (Fig.7). The presence of the fission plate enhances the fast neutron flux. The strength of this additional source was determined from the measured augment of the saturation activity 115 In(n,n ) in the irradiation point S A [7]. Here the Leachman approximation of the fission spectrum was used. The curve A is the flux spectrum M to which is added the contribution from the fission plate. After that this contribution was calculated to the point S B and added to the data M (curve B). Finally, the fission augment was calculated to the point S C above the neutron beam entering the exposure room. It is presented by the curve C. Unknown background is not taken into account. Neutrons from the fission plate enhance the fast neutron flux in the point S A for good 20%. However, they do not fill the dip in the resonance region of carbon. 4 CONCLUSION The irradiation point S A is characterized by the neutrons that enter the exposure room through the horizontal shaft. To obtain more fission like spectrum an additional irradiation position S B or S C should be built in. A way to complete the apparatus is schematically shown in Fig.7.

3 614.3 Figure 1: Vertical cross-section through the exposure room. Letters L, S, FP, Z, C and DS denote the shutter, the sledge, the fission plate, the shielding for the fission plate, the steel cable with the housing, and both manipulators, respectively. Figure 2: The interior of the exposure room. The fission plate is lowered into the shielding

4 614.4 Figure 3: The interior of the exposure room. The fission plate is lifted into the operating position. Figure 4: Our manipulators on the top of the reactor.

5 614.5 Figure 5: Support KV&PO for the fission plate FP and the guiding frame VO. Letter Z denotes the shielding of lead. Flux spectra [ cm -2 s -1 ev -1 ] Energy [ev] B A M C Figure 6: Fast neutron flux spectra in dependency of energy. The curve M is the measured spectrum in the point S A in absence of the fission plate. The curve A is a provisional evaluation of the data measured in the point S A in the presence of the fission plate. The curves B and C are the predicted neutron spectrum in the irradiation positions S B and S C in the presence of the fission plate. Integrals of the flux spectra B and C from 100 kev to 20 MeV are nearly cm -2 s -1 and cm -2 s -1, respectively.

6 614.6 Figure 7: The proposed additional irradiation position S B and S C. Letter SW denotes the mechanical switch for diverting the sledge. ACKNOWLEDGMENTS I would like to thank Bojan Huzjan, Janez Jezeršek, Darko Kavšek, Bojan Oman, Gvido Pregl, Matjaž Ravnik, Marko Rosman and Dušan Rudman for kind support. REFERENCES [1] Bojan Kraut, Strojniški priročnik, stran 404, Tehniška založba Slovenije, 1994, Ljubljana. [2] Edvard S. Krištof, Neutron Flux in the Exposure room of the TRIGA MARK II Reactor in Ljubljana. International Conference "Nuclear Energy in Central Europe 2000", Bled, Slovenia, September 11-14, [COBISS-ID ]. [3] M. Trampuž, Varstvo okolja pred ionizirajočimi sevanji, strani 144 in 145, 1989, Ljubljana. [4] W. Seelman-Eggelbert et al, Karlsruher Nuklidkarte, 6 th edition, 1995, Kernforschungszentrum Karlsruhe, Germany. [5] JEF-PC, A personal Computer program for Displaying Nuclear Data from the Joint Evaluated File Library, 1997, OECD Nuclear Energy Agency, Issy-les-Moulineaux, France. [6] Table of the Nuclides, [7] J. L. Lamarsh, Introduction to nuclear engineering, page 412, Addison-Wesley Publishing Company, 1975, Reading, USA.