Earthquake Analysis for Cryogenic Components of NPDG Liquid Hydrogen Target

Transcription

1 Earthquake Analysis for Cryogenic Components of NPDG Liquid Hydrogen Target at SNS, Oak Ridge. TN Prepared by: Walt Fox, CEEM, Indiana University, (Revised , 2 nd Revision ) Checked by: Jack Thomison, SNS, NFDD, BL 13, Overview: Analysis: The purpose of this analysis is to examine the loading and reactions for an acceleration of 0.44g due to an earthquake on the cryogenic assemblies inside the NPDG target. This load condition has been determined for the SNS site and NPDGamma experiment by the SNS staff as shown in Attachment 1. The NPDG target consist of an approximately 20 liter welded aluminum vessel that is cryogenically cooled to about 17K using a pair of commercial cryo refrigerators and is filled with liquid hydrogen (approximately 2.6lbs) for capturing neutrons. The vessel is filled and vented thru a 1.5in OD pipe. Liquefaction initially occurs in a small volume directly attached to the second stage of the upper cryo, a Pulse Tube refrigerator Cryomech PT405. As the gas is liquefied, it drains to a small chamber attached to the second stage of the lower Cryo, a CGR 511. Inside this small chamber is a labyrinth filled with catalyst to convert the LH2 from the Ortho state to the Para state. Consistent with minimizing conductive thermal loads, the primary support points are the cryo refrigerators themselves. Axial constraint is completely provided by the cryos and radial constraint is provided by the cryos and concentric G10 CR rings with nubs to minimize thermal contact. The inner and outer rings are designed to provide slight interference as they are slid towards each along the cold shield cylinder so that ultimately they are tightly fixed in place. The nub sides of the rings are sized such that they are a loose fit on the OD of the LH2 vessel and on the OD of the cylindrical section of the vacuum chamber (see Fig. 3). Fig 1 and 2 show the location of the CG and total weights as calculated by the3d model (the complete assembly is modeled in Autodesk Inventor) for the 20K and 80K assemblies respectively. Axially the 20K assembly and 80K assembly are constrained independently and load the cryos separately. Using this info and geometry, the

2 reactions at the cryo mount points are calculated for an axial acceleration of 0.44g due to an earthquake. Fig 3 is the complete cryogenic assembly and the transverse reactions are calculated assuming the worst case of the acceleration due to earthquake acting in the same direction as gravity. Fig 4 is the basic FEA analysis of the 34.6lb load on the single outer G10 CR ring. The assumed load is distributed in the downward direction on the lower half of the ring ID. The single nub at the bottom is constrained to take the full load. Based on manufacture s data supplied to us by Michigan State, in the range of T=50K 80K, G 10CR has an elastic modulus of about 4 x10^6psi and a minimum compressive strength of 70,000psi and min tensile strength of 50,000psi. The maximum stress in this result is about 8,000psi. Note in the real case the circular shape would actually be reinforced by the cold shield cylinder and for any significant deflection adjacent nubs would make contact and help take up the load. Fig 5: In order to estimate bending stresses on the first and second stage tubes of the cryos, we are required to make conservative assumptions about unknowns; specifically about tube wall thicknesses in each section. I feel safe in assuming all wall sections are equal to or greater than 0.03in in both refrigerators. An additional conservative assumption I am going to make is that the entire bending load in the upper cryo is taken up by the largest OD stainless tube in each stage. This assumption will greatly over estimate the actual bending stress due to the imposed loading from the cryogenic assemblies inside the NPDG target. Note that there will be additional stresses in the stainless tubes due to the self weight excitation of the cryo modules themselves, but detailed design information of the cryo modules is not readily available. These additional stresses can conservatively be assumed to be of the same order of magnitude and we can include them by doubling the imposed load calculated. Under these assumptions we have the following section modulii and bending stresses for each tube section (where I/z = pi*(r^4 r^4)/4r and sig = Mz/I): Mount point PT410; (D=2.30 & d=2.24): I/z = 0.120in^3, sig = 129in lbs/0.120in^3 = 1080psi, or doubling = 2160psi Stage one PT410; (D=1.18 & d=1.12): I/z = in^3, sig = 51in lbs/0.0304in^3 = 1680psi, or doubling = 3360psi Mount point CGR511; (D=2.96 & d=2.90): I/z = 0.200in^3, sig = 192in lbs/0.0220in^3 = 959psi or, doubling = 1918psi

3 Conclusion: Stage one CGR511; (D=1.01 & d=0.95): I/z = in^3, sig = 62.4in lbs/0.0220in^3 = 2840psi, or doubling = 5680psi The load on the lower cryo results in a compressive stress on the small tube of F/A = 61.2lbs/.0924in^2 = 663psi but we also note this vertical load is in reality shared by several other elements of larger cross section. The small tube between the vessel and catalyst cup experiences tensile/compressive stress of about 9.1lbs/0.046in^2 = 198psi Characteristically 304 and 316 stainless steels have significant increases in yield and tensile strengths at cryogenic temperatures. In his book Alloy Steels for Liquid Helium Service Joseph R. Davis lists mechanical properties of several stainless steels at cryogenic temperatures in Table 2 page 497. At 20K the yield strength of both 304 and 316 are greater than 50kpsi, significantly stronger than at room temperatures. Based on the conservative assumptions made and elements examined we can identify no load bearing element inside the vacuum chamber that will exceed the normal allowable stress under a 0.44g acceleration.

4 Figure 1 20K Assembly; CG, Weight and Reactions Calc Figure 2 80K Assembly; CG, Weight and Reactions Calc

5 Figure 3 Complete Cryogenic Assembly; CG, Weight and Vertical Reactions Figure 4 FEA of G10 CR Support Ring

6 Figure 5 Loading of Upper and Lower Cryo Refrigerators

7 Attachment 1: Design Input of seismic acceleration values.