Modifications and Improvements to the NPDGamma Liquid Hydrogen Target System and Safety

Transcription

1 Modifications and Improvements to the NPDGamma Liquid Hydrogen Target System and Safety Prepared: H. Nann, Indiana University, November 01, 2011 Checked: S. Penttila, ORNL, November 01, 2011 Approved: M. Snow, Indiana University, October 2x, SCOPE The scope of this document is to record changes to the structure of the liquid hydrogen target system, to its safety, and to its operational baseline established at LANSCE when the NPDGamma Liquid Hydrogen System was approved and successfully operated in FP12 in Changes were made for the thickness of the beam entrance windows, the target alignment inside the isolation vacuum chamber, the copper radiation shield, to the two cryo-cooler flanges and the chimney flange, the thermal conduction between the cryo-coolers and the LH 2 vessel, the thermometry and heaters, the ortho-to-para converter (OPC), and the neutron shielding inside the isolation vacuum chamber. 2. THICKNESS OF THE BEAM ENTRANCE WINDOWS During the operation in 2006, it was found that the neutron beam created too much background signal in the gamma detectors due to interactions in the entrance windows to the target vessel and the isolation vacuum chamber. To reduce this background contribution, the thicknesses of entrance windows to the target vessel and the isolation vacuum chamber (cryostat) seen by the neutron beam were reduced. 2.1 New Target Vessel A new target vessel was designed and built meeting the safety requirements of Section VIII of the ASME Boiler and Pressure Vessel Code. Figures 1 and 2 show the liquid hydrogen target vessel and its entrance window, the thickness of which was reduced from to in a diameter of 9. A Finite Element Analysis (FEA) was performed by Claire Luttrell at ORNL to see if the new target vessel can have a large enough Maximum Allowable Working Pressure (MAWP). The results of the FEA analysis (see

2 show that the vessel can withstand a maximum internal pressure of 137 psi. Based on this new design, the target vessel was fabricated from 6061-T6 aluminum and U stamped by the ASME authorized vendor Ability Engineering Technology, Inc., of South Holland, IL. The FEA analysis, performed by Ability Engineering, yielded a target vessel MAWP of psi ( see selreport%20copy.pdf). Consequently, it was pressure tested at 72 psi. AbilityVesselReport.pdf gives an overall vendor report on the design, QA, and testing/certification of the target vessel. Fig. 1 The LH 2 target vessel assembly drawing.

3 Fig. 2 The thinner beam entry window in the target vessel. 2.2 New Thinner Entrance Windows for the Cryostat The target cryostat has a two-layer entrance window, i.e. the windows are formed by two parallel windows with a small gap between them. The windows are bolted with 36 1/4-20 size bolts on the bolt circle of for the inner window and 15.0 for the outer window giving an axial sealing force of 2300 lb/inch and 2100 lb/inch on these bolt circles, respectively. The thickness of each window was reduced from to A Finite Element Analysis (FEA) was performed by Walt Fox at Indiana University to see if the two new windows can withstand the Maximum Allowable Working Pressure (MAWP) set by two parallel rupture disks, each with a set point of 7.0 psid. The results of the FEA analysis (see show that the cryostat can withstand a maximum internal pressure of 31 psi. The FEA analysis was performed with 6061-T0 aluminum, the weakest temper of 6061 aluminum. The new windows were fabricated by hydro-forming from 6061-T6 aluminum by the ASME authorized vendor Ability Engineering Technology, Inc., of South Holland, IL.

4 3. TARGET ALIGNMENT INSIDE THE ISOLATION VACUUM CHAMBER The relative location of the neutron beam and the hydrogen vessel has to be well known for the NPDGamma experiment. Unfortunately, during the 2006 run at LANSCE, the LH2 vessel alignment with respect to the neutron beam was not enough well known. Thus the precision of the mechanical interfaces between the target vessel, the G-10 spacers (Fig. 3), and the radiation shields was improved. These improvements have no effect on the target operation and safety. Fig. 3 The G-10 spacer to align the target vessel. 4. COPPER RADIATION SHIELD AND ITS UPSTREAM COVER LID During the 2006 LANSCE run it was found that the thermal and mechanical contact between the upstream lid on the copper radiation shield and the cylindrical portion of the shield was not sufficient. This led to an increased heat load on and thermal gradient in the target vessel. The copper radiation shield was redesigned to improve this contact area and the ease of installation. Bolts were added to improve the thermal contact of the radiation shield by mechanical force. This will result in a better temperature distribution in the target and less heat flow to the LH2 target. These improvements have no effect on the target safety.

5 5. EXTRA BOLTS FOR THE INDIUM SEALS IN THE TWO CRYO-COOLER FLANGES AND THE FILL/VENT LINE FLANGE The existing 6 bolts on the flanges for the cryo-refrigerators and the fill/vent line adaptor were not enough to reliably supply enough axial force to ensure that the indium o-rings sealed reliably. Thus the number of bolts in the flanges was approximately tripled (see Fig. 4). Fig. 4 Added bolt holes on the top of the isolation vacuum chamber. In addition, a bridge clamp and some toe clamps for the upper refrigerator flange were constructed according to the design shown in Fig. 5. This will provide more axial force to the joint and make it more reliable and decrease the probability for leaks.

6 Fig. 5 Bridge and toe clamps for the upper refrigerator cryo-refrigerator flange. 6. IMPROVEMENT OF THE THERMAL CONDUCTION BETWEEN CRYO- COOLERS AND THE LH 2 VESSEL The thermal contact between the cryo-refrigerators and the LH 2 target vessel was smaller than desired. Thus it was redesigned to increase the thermal conductance as well to increase the surface area of contact between the refrigerators and the target vessel. The ring clamp used previously has been replaced by a tab on the end of the target vessel with a flat surface (see Fig. 6) that can be directly bolted to the new bar (shown in Fig. 7) with an increased cross sectional area. This will improve the cooling of the target vessel, but has no effect on safety.

7 Fig. 6 Flat surface on the end of the target vessel for the thermal link. Fig. 7 Thermal connection bar between the target vessel and the upper cryo-cooler

8 7. THERMOMETRY AND HEATERS The thermometry of the LH 2 target as it was used at LANSCE did not give enough information on the status of the system. There were too few heaters to implement some desired target recirculation modes. Thus more heaters and thermometers were added, especially to the upper region of the target and the fill/vent line. A new electrical feed-through was added to the chimney to accommodate the new wires. This will improve the knowledge of the target state, but has no effect on safety. 8. ORTHO-TO-PARA CONVERTER AND ITS CONFLAT FLANGE There were four problems with the lower OPC during the runs at LANSCE: its volume was too small, the hard soldered CF flange was distorted, its knife edge was warped, and the stainless steel mesh on the bottom of the cup was glued in with the epoxy partially covering the flow channel. Thus a new, larger volume OPC with a new CF flange was designed and built (see Fig.8). The heat exchange area between the catalyst and the body of the OPC was also increased. Fig. 8 The assembly of the new ortho-to-para converter.

9 This new design of the OPC improves the seal, makes the installation easier, reduces stresses in other parts of the target, and increases the reliability of the joint. 9. NEUTRON SHIELDING AROUND THE TARGET VESSEL During the operation in 2006 at LANSCE, it was found out that there were several gaps in the 6 Li neutron shielding inside the isolation vacuum chamber. The escaped neutrons produced background γ-radiation from their capture in the walls of the isolation vacuum chamber and the CsI detectors. Thus the 6 Li neutron shielding was redesigned to reduce the number of escaped neutrons and thus the γ-radiation background in the CsI detectors. In addition, the thermal connection bar and the ortho-to-para converter were covered with shielding. Figures 9 and 10 show the new neutron shielding. These improvements have no effect on the target safety. Fig. 9 A cut view of the cryostat, the improved neutron shielding shown in blue.

10 Fig. 10. A model view of the target vessel and interconnected piping. White shells are 6 Li loaded neutron absorber.