I recently was browsing around the Department of Energy's Office of Nuclear Energy website and came across something I'd not yet heard about, but which I find very exciting... coal-to-nuclear transitions. This is where a coal power plant (either operating or decommissioned) is retrofitted to operate with nuclear power generation. This would provide the benefit of already having a large amount of infrastructure on site (turbines, distribution wiring, generators, piping, etc.) which could be fed steam (and thereby electricity) from something like a small modular reactor (SMR) or advanced reactor (AR). As a greater emphasis is placed on moving away from dirty fossil fuels like coal and oil to "cleaner" fossil fuels like natural gas or green/renewable energy generation, many utilities are looking to retire their coal power plants (CPPs). According to the US Energy Information Administration (EIA), 28% of operating CPPs will be retired by 2035. As of September 2021, there were no new coal power plants scheduled to be built in the US [source]. In fact, at Iowa State University (where I graduated from), I took a tour of their university power plant which had been converted from coal to natural gas-fired generation. This makes me wonder what percentage of CPPs are retired versus how many are converted to something like natural gas. Converting to natural gas certainly can reduce emissions and, in the case of the ISU power plant, using cogeneration can substantially increase the efficiency of a power plant as well. This is an improvement, but we can do better. We can fully eliminate generating station emissions (and provide other economic benefits) by transitioning over to nuclear power generation at these sites. The US Department of Energy estimates that to meet the 2050 net-zero carbon emissions goal, the US will need to construct 200 gigawatts (GW) of additional nuclear power capacity. Some of this can be installed at already operating or recently retired nuclear sites (around 60-95 GW of capacity), but the rest could find a home at the sites of CPPs that are being phased out across the country [source].
The Department of Energy released a great report in 2022 titled "Investigating Benefits and Challenges of Converting Retiring Coal Plants into Nuclear Plants" [PDF]. In it, the authors look at the percentage of both retired and operational CPPs that would be candidates for nuclear power plants (NPPs). They find that a wide majority of retired CPPs considered, 80%, are suited to being fitted with ARs and some of them, around 20%, are even suitable candidates for larger light water reactors (LWRs). When considering operating CPPs, the number of amenable sites for an AR refit is the same (80%), but the number of suitable sites for an LWR jumps up to 40%. This presents a considerable amount of opportunity to expand nuclear power generation while also avoiding much of the complicated aspects of constructing a nuclear site from scratch (also called "greenfield" construction). The potential cost savings for overnight constuction (where factors like interest, capacity factor, and financing costs are neglected) are in the range of 15% to 35% as compared to overnight greenfield construction. Additionally, due to nuclear power plants having the highest capacity factor (i.e., time spent on and generating power) of any form of power generation as well as the introduction of other support activities, it is projected that a nuclear refit could provide around 650 more permanent jobs to the community. This does not include the additional jobs that could be created outside of the region as a result of more demand for nuclear fuel and parts. Nuclear jobs pay more, so the local community would see an increase in wages and could see in increase of economic activity up to $275 million. The environment would be substantially improved by a coal-to-nuclear transition, seeing as much as an 86% decrease in steady-state greenhouse gas (GHG) emissions. As for waste concerns, the ash waste from coal-fired generation would still need remediation, but the waste developed by the nuclear power plant (NPP) would be comparatively negligible and could be stored at existing dry cask storage sites away from the community. The waste itself would present no danger to the surrounding community if stored on-site, but by storing it at existing facilities (say, those already being managed and supervised by the utility provider) the possibility of local debate impeding construction or long-term operation could be avoided. Ultimately, people will need to understand how miniscule the risk is from nuclear waste and not allow it to impede the installation of NPPs in their area, but that is a topic for another time.
The company TerraPower has been undertaking the construction of a new NPP based on its Natrium reactor in Kemmerer, Wyoming near the site of a retired CPP. They will utilize the existing utilities infrastructure and skill base from those who have been working at the plant. The Natrium reactor concept can be seen below in a rendering from TerraPower.
This reactor is not only one of the first advanced reactor designs (Generation IV) to see construction in America, but the NPP will also be a great demonstration for the refitting of CPPs with nuclear technology. The DOE is clearly excited about the opportunity as they've invested $2 billion in getting the plant going. It is a sodium-cooled fast reactor (SFR), meaning it is capable of burning spent fuel from the current fleet of operating reactors and produces much less transuranic waste (plutonium and other actinides). This could be a major selling point for convincing communities to welcome SFRs to their local utilities.
While it may be more costly than natural gas, I am certainly hoping that more utilities will consider the idea of installing NPPs at retired coal generating sites. Especially given that the DOE is so keen on getting more reactors online in the coming decades. Programs do exist to push these options over the typical fossil fuel plants, such as the DOE's Gateway for Accelerated Innovation in Nuclear (GAIN) program through the Idaho National Laboratory. They offer hundreds of thousands to millions of dollars in funding to kickstart nuclear projects. An aggressive transition to nuclear energy can allow us to break our dependence on fossil fuels, especially if the US embraces more electric transportation options (not just cars, but more electric trains!).
The M65 atomic cannon, a 280 mm gun, is a nuclear-capable artillery piece from the early days of the Cold War (like 1950s early). It rested on a massive steel structure supported by a turnstyle that could be used to make major adjustments to the azimuthal angle of the gun. Given it's huge size, it required two large trucks to move, as shown below (don't let the perspective fool you, I took this on 0.5x. See my pictures page for a pic of me in front of the trunion area).
One of these guns, nicknamed "Atomic Annie" (above) was used to deploy a nuclear artillery shell in the Grable shot of the Upshot-Knothole series of nuclear tests at the Nevada Test Site (formerly Nevada Proving Ground) in 1953. The artillery shell produced a yield of approximately 15 kilotons. This was accomplished by the W9 warhead, which was one of the few highly enriched uranium (HEU) gun type weapons maintained by the US. We later moved over to implosion-type plutonium weapons exclusively (although, these still involve the use of HEU, just not as the primary fissile material). A video of the test can be found here. Another pic of the gun is below.
Something I find particularly fascinating about this weapon system is that the US was able to go from the crude Mk-1 "Little Boy" device used in 1945 against Japan, to a miniaturized warhead deployable on artillery shells with the same yield in the matter of a few years. General Douglas MacArthur actually pressed President Truman to approve the use of nuclear weapons against the North Korean forces during the Korean war, with 8 of these 280 mm guns being deployed to theater with a complement of nuclear shells [I will need to come back with a specific source, but I believe this is discussed in 15 Minutes: General Curtis LeMay and the Countdown to Nuclear Annihilation]. You can read more about MacArthur's crazy plan to wage nuclear warfare against the Koreans and Chinese here. What I found particularly wild from this article was his plan to use "enhanced radiation weapons" (a.k.a. cobalt bombs or salted nukes) to deny the enemy entry through the northern border between Korea and the USSR and China.
Upshot-Knothole Grable
Operation Upshot-Knothole would serve many purposes, from studying the effects of nuclear weapons on combat operations and demonstrations of weapons platforms to research on the radiation implosion mechanism and enhancing civil defense preparations. The DoD was highly involved in Upshot-Knothole, with around 20,100 DoD personnel being involved in the test. One thing I found notable was that the Marine Corps performed operational tests of helicopters in the presence of a nuclear detonation which were "designed to investigate the capability of helicopters and their crews to withstand a nuclear burst and its effects" [Defense Threat Reduction Agency, Upshot-Knothole Factsheet].
The Grable shot itself took place at 0830 PDT on May 25, 1953 at Area 5 of the Nevada Test Site, known as Frenchman Flat (shown below). The device was delivered by the 280 mm M65 atomic cannon with an airburst height of 524 feet. The cannon was manned by the Artillery Test Unit out of Ft. Sill in Lawton, OK (the gun itself now sits outside the US Army Field Artillery Museum at Ft. Sill). In addition to the usual DOE testing, more than two and a half thousand troops and 700 observers were placed in trenches 2.8 miles from ground zero to see the detonation and then navigate into the affected zone to survey the effects on equipment placed around ground zero. This was part of the Desert Rock exercises, specifically Desert Rock V. They got within 0.9 miles of ground zero before stopping due to a dust storm. Following this, they were to attack two locations as part of an exercise. One was 1.5 miles SE of ground zero and the other was 1.7 miles ESE of ground zero. Some of these troops got within 0.4 miles of ground zero and were able to view destroyed equipment 0.1 miles away. Again, these oberservers had to abort their planned exercise due to dust storms.
At this point, you may be wondering what the hell people were doing within a half mile radius of a fresh nuclear burst. The Army's Office, Chief of Army Field Forces (OCAFF) set blast overpressure and ionizing and thermal radiation exposure limits for the military observers. From this, they determined minimum distances to ground zero. OCAFF set the gamma exposure limit of these troops to 6.0 roengten (R), with only 3.0 R allowed to be absorbed from prompt radiation (radiation given off directly from the nuclear burst) [Defense Threat Reduction Agency, Upshot-Knothole Factbook]. For comparison, the dosage limits of a radiation worker in the US are 0.3 roengten equivalent man per week (rem/wk) or 5.0 rem/yr. The dosage limit for a typical citizen is 0.100 rem/yr. The NRC guidelines are shown below.
While I don't think these units (R and rem) are entirely equivalent, I think they illustrate the very lax safety standards around radiation exposure, especially in the form of fallout as part of nuclear tests. According to Nuclear Weapons Archive, 3,000 soldiers exceeded these maximum dosages with the highest exposure being recorded at 26.6 rem [Nuclear Weapon Archive]. Even more alarmingly, a group of volunteers were granted maximum exposure limits of 10 R of gamma radiation (with 5 R allowed to come from prompt radiation) and a total limit of 25 R for the entire exercise! These volunteers were also given a maximum blast overpressure of 8 pounds per square inch (psi). An overpressure of 5 psi is usually enough to demolish most wood framed structures.
The W9 Warhead
Manufactured from April 1952 until November 1953, 80 W9 warheads were produced for use in the US Army's T-124 280 mm shell. The Los Alamos Scientific Laboratory (LASL, later renamed Los Alamos National Laboratory or LANL) designed the device. These shells were made to be used with the M65 cannon, of which only 20 were produced [Nuclear Weapon Archive]. The warhead used approximately 110 lbs of HEU to produce a yield of 15 kilotons (fixed yield). The device had fuzing options for setting the burst height. Eventually, the W9 would be retired in May 1957 in favor of the W19 warhead. The old W9s were converted into T4 Atomic Demolition Munitions (ADMs) [Nuclear Weapon Archive].
The W19 Warhead
Assigned to LANL in 1953, the W19 warhead was developed to replace the W9 and entered the US stockpile in 1956 (a year before the retirement of the W9) [Nuclear Weapons Databook Vol. I, Cochran, Arkin, and Hoenig]. It had a yield of 15-20 kilotons and would end up being retired in 1963 as the 155 mm based W48 warhead entered the stockpile. I imagine that the logistical ease of deploying 155 mm munitions over 280 mm was a contributing factor in the transition away from these large atomic guns.
More About US Gun Configuration Warheads
So, while the W9 of Upshot-Knothole Grable and Little Boy are the two most notable gun-type warhead detonations (especially since they were used in their "battle configuration" with the W9 being fired from the M65 and Little Boy used in combat), there were other gun configuration warhead tests. These tests were of the W33 warhead, which would be deployed in a 203 mm artillery shell. The two tests were shots Laplace of Operation Plumbbob and Aardvark of Operation Nougat, occuring in 1957 and 1962, respectively.
First, we will look at Plumbbob/Laplace. The Laplace shot was an airburst test using a balloon to suspend the device at 750 feet. The detonation occurred at 0600 PDT on September 8th, 1957 at the Nevada Test Site. The yield was comparably small at 1 kiloton. The test was used as an opportunity to see how well US Army radars could be employed to measure nuclear detonations, with teams detecting the detonation and tracking fireball and cloud growth. Using this data, they tested their ability to measure the weapon yield and predict fallout patterns. One can imagine how this information would be critical in the case of tactical nuclear weapons employment during a Soviet invasion of Western Europe. The shot was also used as an opportunity to understand more about nuclear weapons effects, with the Army, Navy, and Air Force all conducting their own research into the effects of high neutron-flux and gamma radiation [Defense Threat Reduction Agency].
For Nougat/Aardvark, the shot took place on May 12th, 1962 at 1900 Zulu-time (1200 local time). It was an underground test of considerably higher yield. The device was a TX-33 (experimental version of the W33) yielding 40 kilotons. It was buried 1424 feet (434 m) underground at Nevada Test Site, Area 3 (coords: 37° 7′ 39.14″ N, 116° 2′ 57.01″ W (sat view shown below)). This would be the United States' final test of a gun type fission weapon.
