In previous posts on 24 May 2015 and 28 June 2016, I reported on the TOP500 rankings of the world’s supercomputers.
In June 2013, China’s Tianhe-2 supercomputer at the National Supercomputer Center in Guangzho topped this this worldwide ranking with an Rmax Linpack score of 33 petaflops (1 petaflops = 1015 floating-point operations per second) and retained the first place position for two years. In June 2016, the new leader was another Chinese supercomputer, the Sunway TaihuLight at the National Supercomputer Center in Wuxi. TaihuLight delivered an Rmax Linpack score of 93 petaflops and remained at the top of the worldwide ranking for two years, until it was eclipsed in June 2018 by the US Summit supercomputer, then with an Rmax rating of 122.3 petaflops.
In the latest TOP500 ranking, the new leaders are two US supercomputers: Summit (#1) and Sierra (#2).
Summit supercomputer. Source: NVIDIA
The IBM Summit improved its past Linpack score to achieve an Rmax of 143.5 petaflops in the current ranking. Summit is located at the Department of Energy (DOE) Oak Ridge National Laboratory (ORNL) in Tennessee.
873 megawatts peak power
Sierra supercomputer. Source: Lawrence Livermore National Laboratory / Randy Wong
The IBM Sierra also improved its past Linpack score to achieve an Rmax of 94.64 petaflops / second and move into second place, marginally ahead of China’s TaihuLight. Sierra is located at the DOE Lawrence Livermore National Laboratory (LLNL) in California.
438 megawatts peak power
The Summit and Sierra supercomputer cores are IBM POWER9 central processing units (CPUs) and NVIDIA V100 graphic processing units (GPUs). NVIDIA claims that its GPUs are delivering 95% of Summit’s performance. Both supercomputers use a Linux operating system.
China’s Sunway TaihuLight was ranked 3rd, and Tianhe-2A was ranked 4th. A total of five DOE supercomputers were in the top 10 positions.
You’ll find the complete 52ndedition (November 2018) TOP500 ranking here:
20 February 2019 Update: Los Alamos National Laboratory (LANL) plans new supercomputer
The TOP500 ranking places LANL’s Trinity supercomputer (a Cray XC40) as the #6 fastest supercomputer in the world, but its performance (Rmax of 20.16 petaflops) is far below that of the #1 Summit supercomputer at Oak Ridge national Laboratory and the #2 Sierra supercomputer at Lawrence Livermore National Laboratory.
Not to be outdone, LANL issued a request for proposal (RFP) in February 2019 for a new supercomputer, to be named Crossroads, to support the lab’s missions for the National Nuclear Security Administration (NNSA). A LANL spokesperson reported that, “High performance computing across the NNSA complex is used to assure the safety, security and effectiveness of the U.S. nuclear deterrent; to analyze and predict the performance, safety, and reliability of nuclear weapons and certify their functionality.” Responses to the RFP are due by 18 March 2019. Crossroads is expected to go online in 2021.
Radioisotope Thermoelectric Generators (RTG), also called Radioisotope Power Systems (RTS), commonly use non-weapons grade Plutonium 238 (Pu-238) to generate electric power and heat for National Aeronautics and Space Administration (NASA) spacecraft when solar energy and batteries are not adequate for the intended mission. In comparison to other RTG heat sources (Strontium-90, Cesium-137), Pu-238 has a relatively long half-life of 87.75 years, which is a desirable property for a long-life RTG.
Approximately 300 kg (661 lb) of Pu-238 was produced by the Department of Energy (DOE) at the Savannah River Site between 1959 – 1988. After U.S production stopped, the U.S. purchased Pu-238 from Russia until that source of supply ended in 2010.
Limited production of new Pu-238 in the U.S re-started in 2013 using the process shown below. This effort is partially funded by NASA. Eventually, production capacity will be about 1.5 kg (3.3 lb) Pu-238 per year. The roles of the DOE national laboratories involved in this production process are as follows:
Idaho National Engineering Lab (INEL):
Store the Neptunium dioxide (NpO2) feed stock
Deliver feed stock as needed to ORNL
Irradiate targets provided by ORNL in the Advanced Test Reactor (ATR)
Return irradiated targets to ORNL for processing
Oak Ridge National Lab (ORNL):
Ship some targets to INEL for irradiation
Irradiate the remaining targets in the High Flux Isotope Reactor (HFIR)
Process all irradiated targets to recover and purify Pu-238
Convert Pu-238 to oxide and deliver as needed to LANL
Los Alamos National Lab (LANL):
Manufacture the Pu-238 fuel pellets for use in RTGs
Source: Ralph L McNutt, Jr, Johns Hopkins University APL, 2014
In 2015, the U.S. had an existing inventory of about 35 kg (77 lb) of Pu-238 of various ages. About half was young enough to meet the power specifications of planned NASA spacecraft. The remaining stock was more than 20 years old, has decayed significantly since it was produced, and did not meet specifications. The existing inventory will be blended with newly produced Pu-238 to extend the usable inventory. To get the energy density needed for space missions while extending the supply of Pu-238, DOE and NASA plan to blend “old” Pu-238 with newly produced Pu-238 in 2:1 proportions.
Since 2010, NASA’s RTG for spacecraft missions has been the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), which It is based on the SNAP-19 RTG flown on the two Viking Mars landers (circa 1975) and the Pioneer 10 and 11 deep space probes (circa 1972). At beginning of life, the current MMRTG can provide about 2,000 watts of thermal power and 110 watts of electrical power from eight General Purpose Heat Source (GPHS) modules that contain a total of 10.6 pounds (4.8 kilograms) of plutonium dioxide fuel. Electric conversion efficiency is about 6%.
Assembled MMRTG on a transport dolly. Source: NASA
NASA had a program to develop an Advanced Stirling Radioisotope Generator (ASRG), which was designed to produce about four times the power of the MMRTG per unit of Pu-238. Electric conversion efficiency was about 26%. The ASRG required a total of 2.7 pounds (1.2 kilograms) of plutonium dioxide in two GPHS modules. However, the ASRG would produce less waste heat, which can be used productively to warm electronics in the interior of a spacecraft, such as the Mars rover Curiosity. In November 2013, NASA announced that ASRG development had been discontinued because of budget cuts. You’ll find a NASA ASRG Fact Sheet at the following link: https://rps.nasa.gov/resources/65/advanced-stirling-radioisotope-generator-asrg/
On 22 December 2015, DOE reported the first U.S. production in nearly 30 years of Pu-238. This production demonstration, which was partially funded by the NASA, was performed at ORNL and yielded 50 grams of Pu-238. The last U.S. production of Pu-238 occurred in the late 1980s at the Savannah River Plant in South Carolina.
DOE reported that it plans to set an initial production target of 300 – 400 grams (about 12 ounces) of Pu-238 per year. After implementing greater automation and scaling up the process, ORNL expects to reach the the production target of 1.5 kg (3.3 lb) Pu-238 per year.
The next NASA mission that will use an RTG is the Mars 2020 rover, which will use an MMRTG, as used on NASA’s Mars rover Curiosity.
In the past three years, ORNL has made scant progress in producing Pu-238. In a 13 December 2018 article, “NASA Doesn’t Have Enough Nuclear Fuel For Its Deep Space Missions,”author Ethan Siegel reports: “Although current production (at ORNL) yields only a few hundred grams per year (less than a pound), the laboratory has the eventual goal of ramping up to 1.5 kilograms (3.3 pounds) per year by 2023, at the earliest. Ontario Power Generation in Canada has also begun producing Pu-238, with the goal of using it as a supplemental source for NASA.” You can read the complete article on the Forbes website at the following link: https://www.forbes.com/sites/startswithabang/2018/12/13/nasa-doesnt-have-enough-nuclear-fuel-for-its-deep-space-missions/#1a73d47e1c18
The Canadian plans for becoming a source of Pu-238 was announced on 1 March 2017: “Ontario Power Generation (OPG) and its venture arm, Canadian Nuclear Partners, are participating in a project to produce isotopes in support of deep space exploration. Under the agreement, OPG would help create isotopes at the Darlington nuclear station east of Toronto that will help power space probes.” You can read the complete OPG press release here: https://www.opg.com/news-and-media/news-releases/Documents/20170301_DeepSpace.pdf
The NASA Mars rover, Perseverance, was launched from Cape Canaveral on 30 July 2020, with an expected landing date of 18 February 2021 in the Jezero crater on Mars. Once on the surface, Perseverance will be powered by an MMRTG.
The Pu-238 and some other special materials for the Perseverence MMRTG were produced in the U.S. at ORNL, as described in the following short (2:03 minutes) video, “ORNL-produced tech fuels NASA’s Perseverance mission to Mars”:
At the planned U.S. production rate for Pu-238, NASA should be able to conduct an MMRTG mission at about four-year intervals. If NASA MMRTG missions will be more frequent than this, the U.S. will need to purchase additional Pu-238 from another source, perhaps Canada.
5 March 2021 Update:
The Perseverance rover landed on Mars on 18 February2021, in the planned target area in Jezero Crater. Power from the MMRTG was nominal after landing. Perseverance will spend at least one Mars year (two Earth years) exploring the landing site region.
The next NASA mission with an MMRTG-powered spacecraft is the Dragonfly mission to Saturn’s moon Titan, which will launch in 2026 and arrive on Titan in 2034.
The Voyager 1 and 2 spacecraft were launched in 1977, each with three RTGs delivering a maximum of 470 watts of electrical power at the beginning of the mission. Both spacecraft have left the solar system (Voyager 1 in 2013 and Voyager 2 in 2018) and continue to transmit from interstellar space in 2021 with their RTGs operating at a reduced power level of about 331 watts after 44 years of Pu-238 decay during the mission. NASA plans to continue the Voyager missions until at least 2025.
A muon is an unstable elementary subatomic particle in the same class as an electron (they’re both leptons), but with a much greater mass (207 times greater). A useful property of muons is that they can penetrate matter much further than X-rays with the added benefit of causing essentially zero damage to the matter it passes through. Muons scatter and lose energy as they pass through matter, slowing down and eventually decaying, typically into three particles: an electron and two types of neutrinos. The higher the average density of the matter encountered along the muon’s flight path, the more quickly the muon slows down.
Muons are created by the interaction of high-energy cosmic rays with the upper regions of Earth’s atmosphere and they account for much of the cosmic radiation that reaches the Earth’s surface. This means that the existing flux of muon radiation at the Earth’s surface (about 10,000 muons/square meter/sec) is a free resource for clever researchers.
Muon tomography uses this free muon flux and the muon’s characteristic of slowing down more quickly in denser matter to create a density map of the field-of-view available to a muon detector. There are two types of muon imaging, transmission and scattering. The differences are addressed in LA-UR-15-24802, listed below. In both types of muon imaging, denser objects and structures in the detectors field of view appear as shadows (muon shadows) that are darker (fewer muons getting thru to the detectors) than less dense areas.
Muon tomography at the Fukushima Nuclear Power Plant
Tokyo Electric Power Company (TEPCO) supported the use of muon tomography at the Fukushima Nuclear Power Plant to help determine what damage was done to the reactor cores at Units 1, 2 and 3 during the 11 March 2011 accident, which was precipitated by a 9.0 magnitude earthquake followed by a 15-meter (49.2-ft) tsunami.
A 2013 muon tomography feasibility study (Hauro Miyadera, et al.) reported: “Muon scattering imaging has high sensitivity for detecting uranium fuel and debris even through thick concrete walls and a reactor pressure vessel. Technical demonstrations using a reactor mockup, a detector radiation test at Fukushima Daiichi, and simulation studies have been carried out. These studies establish feasibility for the reactor imaging. A few months of measurement will reveal the spatial distribution of the reactor fuel.”
At Reactor #1, two 22 ton (20 metric ton), 21-foot by 21-foot (6.4 m by 6.4 m) muon detectors were installed and used to collect data over periods of months to develop high-resolution images of the damaged reactor core and surrounding areas. Placement of the muon detectors and the general scan geometry is shown in the following diagram.
Reactor #1 muon scan results
In March 2015, TEPCO announced that its muon tomography scanning efforts at Fukushima were successful, and confirmed that the nuclear plant’s Reactor #1 suffered a complete meltdown. The muon scans showed no corium (i.e., the lava-like product of a reactor core meltdown containing the melted nuclear fuel, fission products, control rods, and structural materials) remained in the reactor pressure vessel (RPV). The muon scans did not show the distribution of the corium that flowed out of the bottom of the reactor vessel into the primary containment vessel (PCV).
Reactor #2 muon scan results
World Nuclear News (WNN) reported (2016 & 2017), “TEPCO said analysis of muon examinations of the fuel debris shows that most of the fuel has melted and dropped from its original position within the core (and resolidified)…..Measurements taken between March and July 2016 at unit 2 showed high-density materials, considered to be fuel debris, in the lower area of the RPV.”
Reactor #3 muon scan results
In 2017, WNN reported, “Some of the fuel in the damaged unit 3 of the Fukushima Daiichi plant has melted and dropped into the primary containment vessel, initial results from using a muon detection system indicate. Part of the fuel, however, is believed to remain in the reactor pressure vessel.”
Summary of fuel debris status at Fukushima
Based on the results of the muon tomography program and other means of investigation, TEPCO created the following graphic summary showing the estimated distribution of core and containment vessel fuel debris in Fukushima Units 1, 2 & 3.