Challenges in the Development and Operation of the MW-class SNS Mercury Target Bernie Riemer SNS Upgrades Office J-PARC Symposium 2019 September 23-27, 2019 ORNL is managed by UT-Battelle, LLC for the US Department of Energy
SNS mission is focused on neutron scattering science • 1.4 MW accelerator – 1 GeV protons – Linac & accumulator ring – 0.7 µs pulses at 60 Hz • Liquid mercury target • Groundbreaking in Dec. 1999 • First protons on target in April 2006 Riemer – SNS Challenges 2 J-PARC Symposium 2019 September 23-27, 2019
The SNS mercury target is a first of a kind target design: mega-watt class, liquid metal and short-pulse Replaceable target module consists of mercury vessel surrounded by a water-cooled shroud with leak sensors in interstitial space Inflatable metal seal to core vessel Mercury vessel Water-cooled shroud Center flow baffle Bulk mercury Bulk mercury return inlet (1 of 2) / spallation region Module material AISI 316L Design basis beam power 1.4 MW Module mass 1130 kg Beam repetition rate 60 Hz Length 2.1 m Beam pulse length 0.7 µs Mercury mass inside module 794 kg Energy per pulse 23 kJ The target is not safety credited Mercury mass flow rate 19.4 t/min Proton energy 1 GeV Cost: ca. $1.5M Radiation damage limit 12 dpa Riemer – SNS Challenges 3 J-PARC Symposium 2019 September 23-27, 2019
Early SNS target systems R&D was directed at foreseen challenges • Time-averaged heat removal from target – One water and two mercury test loops for thermal-hydraulics • Isochoric energy deposition which initiates pressure waves – Vessel beam pulse vessel cyclic stress needed estimation method – Mercury cavitation occurring under tensile pressure conditions • Target vessel material issues – Compatibility with mercury – High-cycle fatigue strength (10 8 ~10 9 pulses) – Radiation damage limits Riemer – SNS Challenges 4 J-PARC Symposium 2019 September 23-27, 2019
SNS Target Development used 3 major facilities at ORNL as well as accelerator facilities in Los Alamos and Brookhaven Water Thermal Mercury Thermal Hydraulic Loop Hydraulic Loop • Flow visualization • Mercury-to-steel wettability & • Recirculation zone heat transfer • Flow stability • Design data for target window • CFD benchmark • Corrosion / erosion tests Mercury target tests in proton beams • Vessel response to beam pulse Target Test Facility SNS full-scale mercury test loop • Final CFD benchmark • Verify mercury process equipment • Operational experience Riemer – SNS Challenges 5 J-PARC Symposium 2019 September 23-27, 2019
Water Thermal Hydraulic Loop (WTHL) flow visualization was used to develop and benchmark CFD simulations Poor shear ~stagnation flow at wall Riemer – SNS Challenges 6 J-PARC Symposium 2019 September 23-27, 2019
The Mercury Thermal Hydraulic Test Loop (MTHL) became operational in 1999 Test Hg-Water Section Heat Exchanger Key heat transfer data for design of target beam window cooling passage EM Pump 27 L/min Riemer – SNS Challenges 7 J-PARC Symposium 2019 September 23-27, 2019
Target Test Facility (TTF) – a SNS full-scale mercury test loop • Final CFD benchmark – Ultrasonic Velocity Profilometry • Verified mercury full-scale process equipment • First use of EPICS control software for SNS • Now used for target gas injection R&D Riemer – SNS Challenges 8 J-PARC Symposium 2019 September 23-27, 2019
In-beam mercury target tests provided data to calibrate modeling of target vessel stress needed for fatigue life estimation Large Effects Target Energy Deposition Target Prototypical Shape Target Riemer – SNS Challenges Axisymmetric Target 9 J-PARC Symposium 2019 September 23-27, 2019
Thermal shock R&D • Peak energy deposition in mercury for a single pulse = 13 MJ/m 3 * – Peak temperature rise is only ~ 7 K for a single pulse, but rate of rise is 10 7 K/s! • This is an isochoric (constant volume) process – Beam deposition time (0.7 µ s) << time required for mercury to expand – Beam size / sound speed ~ 30 ms • Local pressure rise is 38 MPa (380 atm compared to static pressure of 3 atm!)* • Mercury expansion and wave reflection at the vessel interface lead to tension and cavitation of the mercury Riemer – SNS Challenges 10 * SNS @ 2 MW J-PARC Symposium 2019 September 23-27, 2019
Concerns about pulse cavitation erosion grew with findings by Kogawa et al. (J-PARC) in 2000 • Studies on mercury wave speed employing an offline device with prototypic pressure magnitude and rise time produced pitting damage with few impacts! Only 200 beam pulses! • SNS team conducted in-beam mercury target tests using Los Alamos accelerator in 2001 – Pitting damaged confirmed – Projections to 10 8 ~10 9 pulses 1 mm Riemer – SNS Challenges 11 J-PARC Symposium 2019 September 23-27, 2019
22 SNS targets since operations began in 2006 • Average power: 1 MW • Average energy: 2,180 MWh • 8 targets leaked – First leak cause unconfirmed – Fatigue at welds : 4 – Cavitation damage : 2 – Latest issue is under assessment Gas injection is key to reducing fatigue and cavitation damage Riemer – SNS Challenges 12 J-PARC Symposium 2019 September 23-27, 2019
The pressure wave / cavitation damage issue has been and remains an area of strong collaboration with J-PARC • Combined efforts Material damage resistance studies – Gas injection techniques – Off-line and in-beam experimentation – Engagement with experts in academia and industry – Numerical studies – Post Irradiation and Examination and analysis – electro-Magnetic IMpact Test Machine (Offline mercury cavitation - JPARC) In-beam test of gas injection Riemer – SNS Challenges 13 Flowing mercury test loop J-PARC Symposium 2019 September 23-27, 2019
Post Irradiation Examination (PIE) of SNS targets is regularly conducted to understand damage after use • Samples are cut from beam entrance window, cleaned, photographed, and erosion damage precisely measured • Target interiors are photographed • When leaks occur – samples extracted if possible and analyzed Target 9 (4,195 MWh) Sample diameter: 60 mm Riemer – SNS Challenges 14 J-PARC Symposium 2019 September 23-27, 2019
SNS target gas injection started in 2017 - Cavitation damage is substantially reduced! Cavitation damage on center of inner wall 1.2 MW 1.3 MW 1.4 MW Target 20 Target 18 Target 19 Target 17 Q avg = 0. 57 SLPM Q avg = 0.31 SLPM Q avg = 0.45 SLPM Q avg – no gas injection P avg = 1287 hW P avg = 1121 kW P avg = 1187 kW P avg = 1127 kW E total = 2231 MWh E total = 1260 MWh E total = 1987 MWh E total = 1936 MWh T17 – no gas injection T18 – T20 targets used gas injection Riemer – SNS Challenges 15 J-PARC Symposium 2019 September 23-27, 2019
Gas bubble injection also improves fatigue life by reducing pulse stress Measurements show clear target vessel strain reductions with gas bubble injection in operating SNS targets 40% strain reduction with gas Gas off strain ( µε ) Gas on time (ms) • First gas bubble injection: Nov. 2017 • Improvements are repeatable • Improvements are independent of power levels • Improvements are proportional to gas injection level Riemer – SNS Challenges 16 J-PARC Symposium 2019 September 23-27, 2019
2 MW operation of the SNS mercury target is coming with the Proton Power Upgrade (PPU) project • PPU doubles accelerator power to 2.8 MW to support a Second Target Station and increase power to the First Target Station • The First Target Station was mostly designed for 2MW but at 1.0 GeV • Reliable target operation is required … with an acceptable replacement frequency • Fatigue life and cavitation damage rate dependence on beam power are both aggressively non-linear Riemer – SNS Challenges 17 J-PARC Symposium 2019 September 23-27, 2019
Reliable 2MW operation of the mercury target will be achieved with design changes and high-flow gas injection • Design changes that reduce pulse fatigue stress • Gas injection for cavitation damage and pulse stress reduction – Swirl bubblers for small gas bubble generation (J-PARC technology) – Gas wall / bubbly curtain for localized cavitation protection – Gas rates up to ~1% of mercury flow (vs. current levels of ~0.05%) • Robust weld designs that ease inspections 1.4 MW Target PPU 2MW mercury vessel Mercury vessel and preliminary design Water-cooled shroud Riemer – SNS Challenges 18 J-PARC Symposium 2019 September 23-27, 2019
High power targets are challenging • Development and collaboration with J-PARC are helping SNS achieve unprecedented power and reliability • The target design for 2MW operation incorporates lessons learned from R&D and operating experience Riemer – SNS Challenges 19 J-PARC Symposium 2019 September 23-27, 2019
We are very grateful for the open and fruitful collaboration with J-PARC Neutron Source Section over these years, and look forward to future years working together! Riemer – SNS Challenges 20 J-PARC Symposium 2019 September 23-27, 2019
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