Disposal Of Spent Fuel In Salt Using Borehole Technology: BSK 3 Concept

Transcription

1 IYNC 2008 Interlaken, Switzerland, September 2008 Paper No. 154 Disposal Of Spent Fuel In Salt Using Borehole Technology: BSK 3 Concept Stefan Fopp 1, Reinhold Graf 1, Wolfgang Filbert 2 1 GNS Gesellschaft für Nuklear-Service mbh, Hollestrasse 7A, D Essen, Germany; 2 DBE TECHNOLOGY GmbH, Eschenstrasse 55, D Peine, Germany ABSTRACT Stefan.Fopp@gns.de The BSK 3 concept was developed for the direct disposal of spent fuel in rock salt. It is based on the conditioning of fuel assemblies and inserting fuel rods into a steel canister which can be placed in vertical boreholes. The BSK 3 canister is suitable for spent fuel rods from 3 PWR or 9 BWR fuel assemblies. The emplacement system developed for the handling and disposal of BSK 3 canisters comprises a transfer cask which provides appropriate shielding during the transport and emplacement process, a transport cart, and an emplacement device. Using the emplacement device the transfer cask will be positioned onto the top of the borehole lock. The presentation describes the development and the design of the transfer cask and the borehole lock. A technically feasible and safe design for the transfer cask and the borehole lock was found regarding the existing safety requirements for radiation shielding, heat dissipation and handling procedure. 1 INTRODUCTION Within the scope of the project ESDRED promoted by the EU, and supported by the BMWi, DBE TECHNOLOGY GmbH and GNS mbh have further developed the concept of the emplacement technique for vertical boreholes. In analogy to the reference emplacement concept for POLLUX casks, an above ground test-programme is being developed for demonstrating the technical feasibility. The concept envisages final emplacement of fuel rods of spent fuel assemblies in fuel rod canisters (BSK 3) in vertical boreholes in a rock salt formation. For the intra-plant transport of a BSK 3 canister in a final repository from above ground to underground as well as for the underground transport to the final emplacement location, a transfer cask is utilised for radioactive shielding of the BSK 3 canister. A borehole lock constitutes the link between the emplacement borehole and the transfer cask. Furthermore, the borehole lock closes the emplacement borehole during the filling phase and shields the last BSK 3 canister brought into the borehole. This article presents the design of the transfer cask and of the borehole lock and shows their feasibility. In addition to this, the design is assessed with regard to the radioactive, mechanical and thermal design, taking into consideration the defined design requirements for appropriate operation and for hypothetical accident conditions of transport

2 2 COMPONENT DESCRIPTIONS 2.1 Description of the handling procedures In an emplacement process the transfer cask is transported in horizontal orientation on a transport cart into the disposal drift. At the emplacement location the transfer cask is lifted with an emplacement device and swivelled to the vertical orientation after retraction of the transport cart. The transfer cask is placed into a borehole lock, which constitutes the link to the emplacement borehole, with the subsequent lowering movement of the lifting device. The borehole lock is in a borehole cellar underneath the floor level of the disposal drift. To reduce the load stress level in the assumed case of dropping the transfer cask, two shock absorbers are positioned around the borehole lock in the axial borehole cellar direction. The arrangement of the components for underground emplacement is depicted in Figure 1. Cask lock (top end) Cask lock (bottom end) Transfer cask Trunnions Shock absorber Shock absorber Borehole lock Borehole Figure 1: Arrangement of the components transfer cask and borehole lock for an emplacement process. In the following work step the transfer cask is opened at the top end, and the BSK 3 canister is lifted with the grapple of the cask lifting system. Then follows the simultaneous opening of the slider of the borehole lock and the bottom end transfer cask lock, and the BSK 3 canister is lowered into the borehole and deposited there. All slides close after moving out the grapple and the empty transfer cask is lifted from the borehole lock. The closed borehole lock shields the last BSK 3 canister brought into the borehole

3 2.2 Description of the transfer cask The transfer cask consists of a cask body made of ductile cast iron with spheroidal graphite and two face cask locks made of stainless steel, which are designed as almost mirror images with regard to the geometry and the lock system (see Figure 2). The wall thickness and the wall design of the cask body comply with the requirements with regard to the mechanical strength as well the neutron and gamma radiation shielding. In the cask wall there are two rows of boreholes on different circles that are filled with polyethylene rods for neutron moderation. The neutron shielding in the region of the cask locks is provided in each case by plateshaped neutron moderators. Four trunnions are provided on the cask body for handling the transfer cask. Cask lock (bottom end) Cask lock (top end) M56-bolts Cask body Slider housing Trunnions Slider housing Lock slider Figure 2: Design of the transfer cask. Lock sliders utilised in the cask locks operate according to the drawer principle and are guided in a spring-groove system. The lock sliders are secured by two locking studs each to prevent random opening. The transfer cask has no setting devices of its own for unlocking and actuating the lock sliders. Opening and closing procedures of the lock sliders are executed on the bottom end with the drive units of the borehole lock. 2.3 Description of the borehole lock The borehole lock consists of two parts. The upper part with the take-up surface for the transfer cask is constituted by a lock system, whereas the lower part of the borehole lock consists of an exhaust air flange. Both components are made of stainless steel. The structure of the borehole lock is shown in Figure 3. The lock system consists of a pot-shaped slider housing for taking up the transfer cask with integrated slider and slider drive and the fittings for unlatching the bottom end transfer cask lock slider as well as the fittings for positioning the transfer cask. The exhaust air flange is offset in flange form on the underside for connecting to the adapter pipe in the borehole. In the upper part of the exhaust air flange, segmented break-throughs guide the exhaust air out of the borehole into an annular channel and from there to the connector of an exhaust ventilation system

4 Positioning device Releasing device Lock slider Slider drive Slider housing Exhaust air flange Figure 3: Design of the borehole lock. 3 DESIGN REQUIREMENTS 3.1 Inventory The transfer cask is charged with one BSK 3 canister. The BSK 3 canister is a steel cylinder having an outer diameter of 440 mm and a height of 4945 mm, into which either the fuel rods of 3 PWR or of 9 BWR fuel assemblies (FA) can be inserted. The desired comparability with respect to the reference concept of POLLUX is given on the assumption of the BSK 3 canister being loaded with 3 PWR-UO2-FA with 4 % initial enrichment, 50 GWd/t HM burn-up, 1.63 t HM per canister and 6 kw decay heat power at most. For the calculations the fuel rods are modelled as homogenised fuel assuming 300 fuel rods per FA. Only the active zone with a length of 390 cm is modelled. 3.2 Shielding The limit values for intra-plant transport are based on the regulations of IAEA [1]. For appropriate operation a maximum admissible dose rate of 2 msv/h is defined as the sum of neutron and gamma dose rate at any point on the cask surface, and 0.1 msv/h at a distance of 2 m from the BSK 3 canister. For the proper emplacement process, the maximum admissible dose rate on the track level (top edge of the borehole cellar) at a distance of 2 m from the cask surface is 0.1 msv/h. For accident conditions of transport the maximum admissible dose rate at a distance of 1 m from the cask surface is 10 msv/h. The programme MCNP5 [2] is utilised for the shielding calculations. The underground transport of the transfer cask on a transport cart in a rock salt mine shaft was identified in numerical advance calculations as covering load case for the appropriate operation and in accident conditions of transport. The calculations take into account reflected radiation from the surrounding atmosphere and from the salt mine shaft. The transport cart is not modelled in the calculations

5 3.3 Thermal design For appropriate operation it must be verified that the maximum decay heat power of the BSK 3 inventory of 6 kw can be dissipated. With constant maximum air temperature of 30 C for underground transport, the directly accessible surface of the transfer cask without consideration of insolation and with unimpeded heat dissipation, irrespective of a specified time period, must not become hotter than 85 C. In numerical advance calculations with the FEM programme ANSYS [3], the case of the transfer cask inserted vertically in the borehole lock was found to be the covering load case for appropriate operation. In the 3D calculation model an eccentric arrangement of the BSK 3 canister inside the transfer cask is considered conservatively. 3.4 Mechanical design The loads acting on the transfer cask in appropriate operation result from the accelerations involved in handling and transport. A value of 2 g is assumed for these accelerations [4]. The trunnions including the trunnion bolts are designed according to KTA3905 [5]. The stress loads imposed on the trunnions and trunnion bolts were determined with the FEM programme ANSYS [3]. In advance analytical and numerical analyses a bottom flat drop of the transfer cask from a height of 0.6 m onto the slider housing of the borehole lock during handling with the emplacement device was found to be the design determinant accident condition of transport. The attachments of the borehole lock were not taken into consideration in the calculation. The assessment criterion for the integrity of the transfer cask is the safety objective "retention of the shielding effect" under accident conditions of transport. Thus, it must be verified that the components of the transfer cask are not stressed to the tensile strength limit. The design-relevant bottom flat drop of the transfer cask onto the borehole lock was calculated with the explicit FEM programme LS-DYNA [6]. In the 3-dimensional calculation model, the BSK 3 canister is positioned centred in the cask cavity and it is assumed that before dropping the BSK 3 canister is at the maximum possible distance from the slider of the bottom end transfer cask lock. 4 RESULTS 4.1 Shielding calculations The numerical calculations have shown that for the underground transport of the transfer cask the actual dose rates are less than the admissible limits. The maximum dose rate on the surface of the transfer cask is 0.26 msv/h and occurs in the bottom part of the active zone. Up to about 83 % of the dose rate consists of neutron radiation. A maximum dose rate of 0.07 msv/h is reached at a distance of 2 m from the transfer cask (see Figure 4). Under accident conditions of transport the maximum dose rate at a distance of 1 m from the cask is approx. 1.6 msv/h. The dose rate values are about twice as high compared with transport above ground, due to radiation reflection from the salt

6 Dose rate msv/h MCNP TM GNS-Gruppe Figure 4: Total dose rate distribution in the vicinity of the transfer cask at the position of the axial maximum for underground transport. 4.2 Thermal calculations When the transfer cask is inserted in the borehole lock, the maximum temperature on the surfaces of the cask body is 74 C and on the cask locks 76 C. These values are significantly below the maximum admissible value of 85 C for directly accessible surfaces (see Figure 5). Temperatur, C Figure 5: Temperature distribution in the cask body with cask locks for appropriate operation with the transfer cask placed inside the borehole lock Mechanical calculations For the trunnions and trunnion bolts, the general stress proof showed that the maximum admissible stress values are not exceeded. Within the scope of the proof of operational stability, adequate fatigue resistance of the trunnion bolts for 1100 cask transports was verified. The admissible number of cask transports for the trunnions is For essential transfer cask components, adequate load bearing capacity was verified by analytical means for load with an acceleration of 2 g. The results for the design-relevant bottom flat drop of the transfer cask onto the borehole lock show that the form-fit groove-spring connection of the lock slider system has adequate shear strength. For example, the maximum shear stress in the shear plane of the lock slider spring is about 304 MPa (see Figure 6), which is significantly less than the shear strength limit of 471 MPa. Locally confined 154.6

7 Figure 6: Shear stresses in the shearing section of the bottom end slider in the case of a 0.6 m bottom flat drop. plastic elongations on the surface of the contact places between the groove and the spring do not impair safety. The locking studs subjected to bending stress during the impact as a result of shock waves are subjected to stress below the yield point. All M56-bolts at the connection between cask locks and cask body have an adequate reserve factor with respect to the yield stress. 5 CONCLUSIONS The transfer cask is intended for utilisation as an intra-plant transport cask for a fuel rod canister (BSK 3). It consists of a cylindrical cast iron cask body with spheroidal graphite with let-in polyethylene rods and with locks made of stainless steel on the face ends. The borehole lock constitutes the link between the emplacement borehole and the transfer cask, and for this purpose is positioned in a borehole cellar underneath the floor level of the disposal drift. Furthermore, the borehole lock closes the emplacement borehole during the filling phase and shields the last BSK 3 canister brought into the borehole with respect to the disposal drift. In accordance with the POLLUX reference concept, it is assumed that the BSK 3 canister is loaded with the fuel rods of 3 PWR-UO2-FA with 4 % initial enrichment, 50 GWd/t HM burn-up, 1.63 t HM per canister and 6 kw maximum decay heat power. In this document, the design of the transfer cask and of the borehole lock is presented and assessed with regard to the radioactive, mechanical and thermal design, taking into consideration the defined design requirements for appropriate operation and for hypothetical accident conditions of transport. The results of the executed examinations show that all design requirements imposed on the transfer cask and the borehole lock are met for the emplacement of BSK 3 canisters in vertical boreholes in a rock salt formation

8 REFERENCES [1] Regulations for the Safe Transport of Radioactive Material, 2005 Edition, International Atomic Energy Agency (IAEA), No. TS-R-1 [2] MCNP A General Monte-Carlo N-Particle Transport Code, Version 5.1.4, LA-UR , March 2005 [3] ANSYS Release 10.0A1 UP , 2005 SAS IP Inc. [4] D. Pujet, P. Malesys, Measurement of the Acceleration Undergone by the Trunnions of Irradiated Fuel Transport Flasks during Normal Use, PATRAM 1989, Washington, Proceedings, Vol. II, pp [5] KTA 3905, Lastanschlagpunkte an Lasten in Kernkraftwerken, Fassung Juni 1999 [6] Livermore Software Technology Corporation, LS-DYNA Explicit Finite Element Code, Version 971, Revision