Dynamic crushing characteristics of spruce wood under large deformations
An extensive series of large deformation crushing tests with spruce wood specimens was conducted. Material orientation, lateral constraint and loading rate were varied. Regarding material orientation, a reduction in the softening effect and the general force level was observed with a higher fiber-load angle. A comparison with characteristics gained by application of Hankinson’s formula showed discrepancies in compression strength and the beginning of the hardening effect. Lateral constraint of the specimens caused a multiaxial stress state in the specimens, which was quantified with the applied measuring method. Further, a higher force level compared to specimens without lateral constraint and significant hardening effect at large deformations resulted. Thus, the influence of a multiaxial stress state on the force level could be determined. An increase in the loading rate led to higher force levels at any displacement value and material orientation.
A SURVEY ON 35 YEARS OF PACKAGING , TRANSPORT AND STORAGE OF RADIOACTIVE MATERIAL
This will is a report from an expert working in this field since 1978, participating in PATRAM conferences since 1980. Exactly in that year 1978 the development and testing of the CASTOR casks started in Germany, with the intention to use these transport casks for interim storage of spent fuel. After involvement in the first full-scale drop tests of those large spent fuel casks for transport package design approval, the official assessment as storage cask started. At the beginning of that new decommissioning technology a series of storage demonstration programs were conducted; most interesting were cases where a cask was loaded in a NPP with fuel assemblies instrumented with thermocouples, equipped with a first lid penetrated by instrumentation cables, closed by a second lid, transported to a research institute and stored there for 2 years, with verification of maximum cladding temperatures and leak-tight closure. In 1982 the basis BAM expert reports for the licensing of CASTOR casks in away-from-reactor storage sites Gorleben and Ahaus were finished. The first cask loading for interim storage was performed in Switzerland in 1983; that CASTOR Ic-Diorit cask, 30 years of age, is the “grandfather” of all dual purpose casks. In 1984, a remarkable transport accident happened, the sinking of the “Mont-Louis” with 30 48-Y-UF6 containers; BAM shortly before had issued the approval for a tank container that could be used for the recovery of some these UF6 containers. The 90s saw the full-scale drop tests with the POLLUX multipurpose cask design ( for transport, interim storage, final disposal), drop test series with new transport packages for fresh fuel assemblies including the “French puncture drop”, finally a spectacular BLEVE blast onto a CASTOR cask. 2004 we were proud to host the PATRAM conference, the second time in Berlin. For the Technical Tours BAM performed drop tests with two full scale SNF casks (the largest ones tested world-wide) at our new 200 t drop test facility. In the meantime a lot of other assessment experience, e.g. in ductile iron brittle fracture, numerical calculations, radwaste disposal container testing etc. was stacked. Changes in safety assessment methodology over the last decades will briefly be addressed. PACKAGE DESIGN DEVELOPMENT Dual Purpose Casks for Spent Nuclear Fuel (SNF) In 1978 the BAM testing of the German CASTOR ® casks (designer GNS, Germany) started. After a series of drop tests with a 1:2 scale CASTOR Ia model, a full scale prototype of that design was drop tested [1]. The justification for requesting a full scale model was the impossibility of “scaling” the material properties of thick-walled ductile iron correctly. In 1980 the first dual purpose cask (DPC) versions of CASTOR IIa (1:2 scale model) and TN 1300 (1:3 scale model; designer Transnuklear, Germany) had been drop and fire tested. 1 In Germany very early the dry SNF storage in DPCs was chosen to serve as an alternative SNF decommissioning option. The design of these DPC is focused on a double lid closure system of a monolithic cask body made of ductile iron or forged steel. The two lid barriers are sealed with long-term resistant metal seals, and the interspace pressure between the lids is monitored by a pressure switch indicating a drop of overpressure [2]. BAM was not only involved in mechanical and thermal testing of the transport package design, but also had been contracted with the DPC safety assessment in the storage site licensing procedures [3, 4]. Between 1982 and 1984 a series of dry storage demonstration programs with different cask designs was performed [5], see Figure 1. The DPC development in Germany spread rapidly, and 1986 CASTOR V/21 casks were delivered to the USA (in use by VEPCO, Surry, Va.), and a demonstration program with one of that casks was performed at INL [6]. Figure 1. SNF dry storage demonstration program; CASTOR Ia cask with thermocouple penetration through the closure lid There were numerous different CASTOR DPC designs developed for use in Germany and other countries. The first loading of a DPC took place 1983 at Paul-Scherrer-Institute in Switzerland. There the grandfather of all SNF DPCs, the CASTOR Ic-DIORIT is still in storage since 30 years (Figure 2). A comprehensive overview representing the status of package designs that had been approved around 1995 is given in [7]. For decommissioning actually German NPPs CASTOR V/19 (for 19 PWR assemblies) and CASTOR V/52 (for 52 BWR assemblies) casks are used. These DPCs are stored since 2005 in storage sites located at the NPP sites (316 casks at the end of December 2012). Other important SNF DPCs are CASTOR 440/84 casks for decommissioning of the shut-down WWER 440 NPPs of the former German Democratic Republic and Czech Republic. 65 of these casks are stored in “Zwischenlager Nord (ZLN)” in Lubmin. In Germany the Thorium-High-Temperature Reactor technology was developed, and for the transport and storage of these “pebble-bed balls” SNF the CASTOR THTR/AVR design was developed [8]. 305 of these casks with THTR SNF are stored in central storage site Ahaus, and 152 of these casks with AVR SNF at research center Juelich. 2 Figure 2. CASTOR IcFigure 3. POLLUX cask for transport, storage and DIORIT since 30 years disposal of SNF; left: 5-m drop onto concrete, right: in operation (Foto: preparation for 9-m drop with lid corner onto unyielding Paul-Scherrer-Institute) target (BAM Lehre, 1994) Around 1994 there was an interesting development of a multipurpose cask design, the POLLUX cask for transport, storage and disposal of SNF [9] from which a full scale prototype underwent a BAM drop test program including regulatory 9-m drop tests as well as on-site handling accidental drops from 5 m onto concrete (Figure 3). Dual Purpose Casks for High Level Waste (HLW) Based on the same principle as the DPCs for SNF, appropriate cask designs have been developed to transport and storage of vitrified HLW. Till 2005 most of the spent fuel from German NPPs went to reprocessing in La Hague, France, and a smaller part to Sellafield, UK. The HLW residues are vitrified and filled into stainless steel canisters. 28 of these canisters are loaded into one DPC, and in several transport campaigns the casks had been transported from France to central transport cask storage site Gorleben. Currently all HLW from France is stored there in 108 casks (1 TS 28 V, 74 CASTOR HAW 20/28 CG, 12 TN 85, 21 CASTOR HAW 28M). The transport of 21 CASTOR HAW 28M from Sellafield back to Germany is planned for near future. In an interesting joint USA-Germany cooperation a CASTOR VHLW transport cask for a canister of vitrified HLW, provided by GNSI/DOE, had been tested in a series of regulatory drop tests (from 9 m, equipped with impact limiters) where Sandia accompanied with measurements BAM drop testing [35]. The cask was also investigated in a joint Sandia/BAM research project on safety of ductile iron casks; here the cask with a large machined artificial failure was dropped from heights up to 14 m onto steel rails to demonstrate safety margins. Package Designs for LLW/MLW Important for NPP decommissioning are also all those different waste packages designs. In Germany the ductile iron technology was also applied on compact thick-walled cylindrical waste containers (MOSAIK ® casks, GNS, Germany). With the perspective of storage and disposal in a repository (which will be in Germany the finally licensed KONRAD site, a former iron ore mine), also cubically shaped containers ( “Gußcontainer”) were developed. The assessment of these cubic ductile iron containers was concentrated on the complex structural analysis of flat bottom drops onto a target simulating a real underground, and the fire impact assessment for containers filled with ion exchange resins [10] 3 Transport Packages for fresh Fuel Assemblies Around the year 2000 it was identified that some older designs of packages for fresh fuel assemblies had deficiencies, specifically concerning puncture resistance. This resulted in series of BAM drop tests with an existing package design [32], and with new developed package designs [33], see Figure 4. Figure 4. Testing of packages for transport of fresh fuel assemblies; 1-m puncture bar drop with ANF-10 package (left); internals of opened RA-3D package after a drop test sequence DEVELOPMENT OF PACKAGE ASSESSMENT METHODS The structural analysis was at first very much concentrated on performance of drop tests. From the beginning in Germany large scale tests have been preferred to avoid problems in transfer of small scale model test results to real size packages. The older cask designs incorporated rather large margins of safety; they did not required large impact limiters, and had been tested sometimes without any impact limiter. With growing cask capacities, what increases cask weight and diameter, and with shrinking margins of safety, with consideration of more specific design criteria, like limitation of maximum allowable local strain, stress intensity factors for brittle fracture failure prevention etc., the character of mechanical assessment methods changed consequently (Figure 5). Also the available methods improved. Since about 10 years the Finite Element Analysis (FEA), even for dynamic impacts, is able to provide reliable results, if modeling is correctly done on basis of appropriate chosen material laws and contact parameters. For thermal analysis this method is used more or less solely a bit longer. With respect to dynamic stress analysis, combined with brittle fracture safety analysis, and thermo-mechanical assessment, we on the other hand made the experience that the application of FEA is accompanied with a significant increase in physical testing. FEA of drop test impacts needs to be verified by experimental package drop tests; here is the only justified use of small scale models, besides containment leakage rate testing. If the impact loads are close to the limits allowed, F
Failure analysis for poor solderability of Au surface of PCB
Nonwetting is one of the common problems and is always rejected in electronic assemblies. ENIG pads of a PCBA exhibited nonwetting after lead-free reflow soldering. With the aid of stereomicroscope, SEM&EDS and XRF, a series of analysis was performed. And after visual inspection, surface and cross section analysis, thickness measurement and solderability test, it can be inferred that oxidation of plating surface was the main reason for nonwetting, and corrosion in Ni plating caused an adverse impact on solderalibity.