A GERMAN RESEARCH PROJECT ABOUT APPLICABLE GRAPHITE CUTTING TECHNIQUES

Transcription

1 A GERMAN RESEARCH PROJECT ABOUT APPLICABLE GRAPHITE CUTTING TECHNIQUES D. HOLLAND, U. QUADE Siempelkamp Nuklear- und Umwelttechnik GmbH & Co., Krefeld, Germany F.W. BACH, P. WILK University of Dortmund, Institute of Materials Engineering, Dortmund, Germany Abstract. In Germany, too, quite large quantities of irradiated nuclear graphite, used in research and prototype reactors, are waiting for an environmental way of disposal. While incineration of nuclear graphite does not seem to be a publically acceptable way, cutting and packaging into ductile cast iron containers could be a suitable way of disposal in Germany. Nevertheless, the cutting of graphite is also a very difficult technique by which a large amount of secondary waste or dust might occur. An applicable gaphite cutting technique is needed. Therefore, a group of 13 German partners, consisting of one university, six research reactor operators, one technical inspection authority, three engineering companies, one industrial cutting specialist and one commercial dismantling company, decided in 1999 to start a research project to develop an applicable technique for cutting irradiated nuclear graphite. Aim of the project is to find the most suitable cutting techniques for the existing shapes of graphite blocks with a minimum of waste production rate. At the same time it will be learned how to sample the dust and collect it in a filter system. The following techniques will be tested and evaluated: thermal cutting water jet cutting mechanical cutting with a saw plasma arc cutting drilling. The subsequent evaluation will concentrate on dust production, possible irradiation of staff, time and practicability under different constraints. This research project is funded by the German Minister of Education and Research under the number 02 S 7849 for a period of two years. A brief overview about the work to be carried out in the project will be given. 1. INTRODUCTION Also in Germany quite large quantities of irradiated nuclear graphite and carbonstone, used in research and prototype reactors, are waiting for an environmentally applicable disposal. The most acceptable way in Germany seems to be the direct disposal after packaging in ductile cast iron containers. To keep the final storage costs low an optimal packaging concept is needed. In 1995 a syndicate of German companies made a disposal concept for the green field solution of the Jülich AVR-reactor [1]. Result of that study was that an amount of 2,511 MOSAIK II-Containers or 364 Containers type VII 1

2 are needed for the final storage of the irradiated nuclear graphite and carbonstone. But for the filling of those two different types of containers, which are a common style in Germany, a big percentage of the graphite-stones must be cut. Unfortunately, no applicable cutting technique for irradiated graphite is available yet. Because of the public interest the German Federal Minister of Education and Research decided to fund this project from 1999 till 2001 under the name: "Trennen von graphitischen Reaktorbauteilen - Cutting of graphite reactor components" The partners are: Acon automation & construction GmbH, Lübeck Alba Industries GmbH, Lübeck Research Center Jülich - FZJ German Cancer Research Center, Heidelberg, DKFZ AVR GmbH, Jülich ISOT GmbH, Dortmund Institute of Materials Engineering, University of Dortmund - LWT Medical University Hannover PTB, Braunschweig RWTÜV, Essen Siempelkamp Nuklear- und Umwelttechnik GmbH & Co., Krefeld VKTA, Dresden WTI GmbH, Titz Aim of the project is to develop a cutting technique with a significantly reduced amount of secondary waste and dust. The following main steps are planned: 1) Data collection of irradiated graphite and carbon-stone (geometry, volume, radioactivity, experience with cutting techniques, cutting constraints). 2) Influence of graphite irradiation on cutting (Wigner-energy). 3) Development of applicable cutting techniques. 4) Development of dust collecting techniques, filter systems. 5) Evaluation of selected cutting techniques using irradiated graphite up to a licensable state. 2. MASSES OF RADIOACTIVE GRAPHITE IN GERMANY 2.1 Prototype Reactors In Germany the two high-temperature prototype reactors AVR in Jülich and THTR 300 in Hamm-Uentrop (Schmeehausen) with quite a big amount of radioactive graphite and carbonstone inside are waiting for an applicable disposal. The main reactor data are given in Table I. 2

3 TABLE I: CHARACTERISTICS OF THE GERMAN PROTOTYPE HIGH-TEMPERATURE REACTORS AVR THTR 300 Characteristics Location 1 st criticality date Final shutdown Power Thermal (MW) Net electric (MW Graphite/Carbonstone Mass (Mg) Radioactive inventory (Bq) Jülich /158 2,4 E+15 (1996) Hamm-Uentrop ? In 1995 Siempelkamp and WTI made a concept study for the disposal of the irradiated graphite and carbonstone of the AVR-reactor [1]. The cross section of the AVR-reactor is given in Fig. 1. As a result of that study it became clear that 67 Mg of graphite and 158 Mg of carbonstone must be packed either in 2,511 MOSAIK II-containers or in 364 containers type VII. For the packaging in MOSAIK II-containers nearly 90% of the graphite and carbonstone pieces and for packaging in containers type VII still 15% must be cut. The main dimensions of the MOSAIK II-container are given in Fig. 2 and the dimensions of container type VII are shown in Fig. 3. Taking into account that many graphite blocks and carbonstones have a length of more than 2.20 m and a cross section of more than 0.60 m x 0.60 m it is clear that a cutting is indispensable either because of the dimensions or of the radioactive inventory. Leading nuclides are Tritium, C 14 and Co 60. The radioactive inventory is given in Table II, based on neutron flux calculations and assumed impurities of cobalt. Sampling of graphite and carbonstone has been done and measurements are under work to confirm the activity inventories. The THTR 300-reactor will not be discussed here in detail. 2.2 Research Reactors Five typical German research reactors with graphite reflectors and/or carbonstones are VKTA, PTB, MHH, DKFZ and FRJ-I. The total graphite mass of those reactors is 37 Mg altogether. These main data of the research reactors are listed in Table III. As an example for research reactors the FMRB-reactor (PTB) will be discussed a little bit more in detail. In Fig. 4 the cross section of the FMBR-reactor including the thermal column built of graphite stones is shown. More detailed information of the column construction is given in Fig. 5, where the sectioning of the graphite blocks can be imagined. The total length of the column is 2.5 m with a graphite weight of 12 Mg and a total activity of 3

4 FIG. 1. Cross section of AVR-reactor. 4

5 Height 1500 mm Diameter 1060 mm Gross volume 1,3 m³ FIG. 2. Main dimension of Mosaik II-ductile cast iron-container. 5

6 FIG. 3. Main dimensions of container type VII. 6

7 TABLE II: AVR-REACTOR, ACTIVITY DISTRIBUTION IN GRAPHITE AND CARBONSTONE (1996) Bq H 3 C14 Co 60 Sum Graphite 1.0 E E E E15 Carbonstone 4.5 E E E E15 Sum 1.45 E E E E15 TABLE III: CHARACTERISTICS OF FIVE TYPICAL GERMAN RESEARCH REACTORS 5.34 E+8 Bq. The activity distribution in comparison with the graphite mass is given in Fig. 6 as a function of the distance from the reactor core. Most of the activity is concentrated in one third of the graphite mass. The specific activity decreases rapidly within the first meter from 780 Bq/g down to 30 Bq/g. The leading nuclide is Co 60. Following the latest estimations approximately one third of the whole FMBR-graphites is below the shallow land disposal release value of 4 Bq/g [2]. 3. POSSIBLE GRAPHITE CUTTING TECHNIQUES TO BE INVESTIGATED The trials to qualify the cutting techniques will be mainly carried out at the Institute of Materials Engineering, University of Dortmund. Within this project, the hydraulic technique water jet cutting and the thermal cutting technique plasma arc cutting are applied as references because of their narrow cutting kerf, flexible operability, lightweight cutting tools, small tool sizes and neglectible recoil forces. Thus, minimization of personell s radiation-exposition as well as minimized secondary waste production during dismanting of graphitic nuclear components can be achieved [3, 4]. 7

8 FIG. 4. Cross section FMRB ( PTB) reactor [2, example 8

9 FIG. 5. Construction of thermal column of FMRB (PTB) - reactor [2]. Besides these, different mechanical cutting techniques (e.g. wire sawing and drilling) will be examined. 4. DUST FILTER AND COLLECTING SYSTEMS During evaluation of the different cutting techniques it is important to focus on the minimization of dust and secondary waste production. The exposure of the service staff and the environment must be kept as low as possible. Therefore it is essential to collect the dust directly at the source using suitable suction and filter systems. As the project is right at the beginning no test results can be reported yet. 9

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11 REFERENCES [1] PRINTZ, R.-J.; QUADE, U.; WAHL, J.: Packaging requirements for graphite and carbon form the decommissioning of the AVR in consideration of the german final disposal regulations, IAEA-TECDOC-1043, (Proc. Technical Committee meeting in Jülich, Germany, 1997), [2] HAJEK, W.: Physikalisch-Technische Bundesanstalt PTB, Braunschweig, personal communication, [3] BACH, FR.-W.; LINDEMAIER J.: State of the art of thermal and hydraulic cutting techniques for decommissioning tasks in the nuclear Industry, Proc. Waste Management 1998, Tucson, Az, March, 01-05, [4] BACH, FR.-W.; VERSEMANN R.: State of the art of cutting techniques IIW Doc. No. IE267-97,