Hydronuclear Testing

Some time ago, via a certain orange website, I came across a report about a mission to recover nuclear material from a former Soviet test site. I don't know what you're doing here, go read that instead. But it brought up a topic that I have only known very little about: Hydronuclear testing.

One of the key reasons for the nonproliferation concern at Semipalatinsk was the presence of a large quantity of weapons grade material. This created a substantial risk that someone would recover the material and either use it directly or sell it---either way giving a significant leg up on the construction of a nuclear weapon. That's a bit odd, though, isn't it? Material refined for use in weapons in scarce and valuable, and besides that rather dangerous. It's uncommon to just leave it lying around, especially not hundreds of kilograms of it.

This material was abandoned in place because the nature of the testing performed required that a lot of weapons-grade material be present, and made it very difficult to remove. As the Semipalatinsk document mentions in brief, similar tests were conducted in the US and led to a similar abandonment of special nuclear material at Los Alamos's TA-49. Today, I would like to give the background on hydronuclear testing---the what and why. Then we'll look specifically at LANL's TA-49 and the impact of the testing performed there.

First we have to discuss the boosted fission weapon. Especially in the 21st century, we tend to talk about "nuclear weapons" as one big category. The distinction between an "A-bomb" and an "H-bomb," for example, or between a conventional nuclear weapon and a thermonuclear weapon, is mostly forgotten. That's no big surprise: thermonuclear weapons have been around since the 1950s, so it's no longer a great innovation or escalation in weapons design.

The thermonuclear weapon was not the only post-WWII design innovation. At around the same time, Los Alamos developed a related concept: the boosted weapon. Boosted weapons were essentially an improvement in the efficiency of nuclear weapons. When the core of a weapon goes supercritical, the fission produces a powerful pulse of neutrons. Those neutrons cause more fission, the chain reaction that makes up the basic principle of the atomic bomb. The problem is that the whole process isn't fast enough: the energy produced blows the core apart before it's been sufficiently "saturated" with neutrons to completely fission. That leads to a lot of the fuel in the core being scattered, rather than actually contributing to the explosive energy.

In boosted weapons, a material that will fusion is added to the mix, typically tritium and deuterium gas. The immense heat of the beginning of the supercritical stage causes the gas to undergo fusion, and it emits far more neutrons than the fissioning fuel does alone. The additional neutrons cause more fission to occur, improving the efficiency of the weapon. Even better, despite the theoretical complexity of driving a gas into fusion¸ the mechanics of this mechanism are actually simpler than the techniques used to improve yield in non-boosted weapons (pushers and tampers).

The result is that boosted weapons produce a more powerful yield in comparison to the amount of fuel, and the non-nuclear components can be made simpler and more compact as well. This was a pretty big advance in weapons design and boosting is now a ubiquitous technique.

It came with some downsides, though. The big one is that whole property of making supercriticality easier to achieve. Early implosion weapons were remarkably difficult to detonate, requiring an extremely precisely timed detonation of the high explosive shell. While an inconvenience from an engineering perspective, the inherent difficulty of achieving a nuclear yield also provided a safety factor. If the high explosives detonated for some unintended reason, like being struck by canon fire as a bomber was intercepted, or impacting the ground following an accidental release, it wouldn't "work right." Uneven detonation of the shell would scatter the core, rather than driving it into supercriticality.

This property was referred to as "one point safety:" a detonation at one point on the high explosive assembly should not produce a nuclear yield. While it has its limitations, it became one of the key safety principles of weapon design.

The design of boosted weapons complicated this story. Just a small fission yield, from a small fragment of the core, could potentially start the fusion process and trigger the rest of the core to detonate as well. In other words, weapon designers became concerned that boosted weapons would not have one point safety. As it turns out, two-stage thermonuclear weapons, which were being fielded around the same time, posed a similar set of problems.

The safety problems around more advanced weapon designs came to a head in the late '50s. Incidentally, so did something else: shifts in Soviet politics had given Khrushchev extensive power over Soviet military planning, and he was no fan of nuclear weapons. After some on-again, off-again dialog between the time's nuclear powers, the US and UK agreed to a voluntary moratorium on nuclear testing which began in late 1958.

For weapons designers this was, of course, a problem. They had planned to address the safety of advanced weapon designs through a testing campaign, and that was now off the table for the indefinite future. An alternative had to be developed, and quickly.

In 1959, the Hydronuclear Safety Program was initiated. By reducing the amount of material in otherwise real weapon cores, physicists realized they could run a complete test of the high explosive system and observe its effects on the core without producing a meaningful nuclear yield. These tests were dubbed "hydronuclear," because of the desire to observe the behavior of the core as it flowed like water under the immense explosive force. While the test devices were in some ways real nuclear weapons, the nuclear yield would be vastly smaller than the high explosive yield, practically nill.

Weapons designers seemed to agree that these experiments complied with the spirit of the moratorium, being far from actual nuclear tests, but there was enough concern that Los Alamos went to the AEC and President Eisenhower for approval. They evidently agreed, and work started immediately to identify a suitable site for hydronuclear testing.

While hydronuclear tests do not create a nuclear yield, they do involve a lot of high explosives and radioactive material. The plan was to conduct the tests underground, where the materials cast off by the explosion would be trapped. This would solve the immediate problem of scattering nuclear material, but it would obviously be impractical to recover the dangerous material once it was mixed with unstable soil deep below the surface. The material would stay, and it had to stay put!

The US Army Corps of Engineers, a center of expertise in hydrology because of their reclamation work, arrived in October 1959 to begin an extensive set of studies on the Frijoles Mesa site. This was an unused area near a good road but far on the east edge of the laboratory, well separated from the town of Los Alamos and pretty much anything else. More importantly, it was a classic example of northern New Mexican geology: high up on a mesa built of tuff and volcanic sediments, well-drained and extremely dry soil in an area that received little rain.

One of the main migration paths for underground contaminants is their interaction with water, and specifically the tendency of many materials to dissolve into groundwater and flow with it towards aquifers. The Corps of Engineers drilled test wells, about 1,500' deep, and a series of 400' core samples. They found that on the Frijoles Mesa, ground water was over 1,000' below the surface, and that everything above was far from saturation. That means no mobility of the water, which is trapped in the soil. It's just about the ideal situation for putting something underground and having it stay.

Incidentally, this study would lead to the development of a series of new water wells for Los Alamos's domestic water supply. It also gave the green light for hydronuclear testing, and Frijoles Mesa was dubbed Technical Area 49 and subdivided into a set of test areas. Over the following three years, these test areas would see about 35 hydronuclear detonations carried out in the bottom of shafts that were about 200' deep and 3-6' wide.

It seems that for most tests, the hole was excavated and lined with a ladder installed to reach the bottom. Technicians worked at the bottom of the hole to prepare the test device, which was connected by extensive cabling to instrumentation trailers on the surface. When the "shot" was ready, the hole was backfilled with sand and sealed at the top with a heavy plate. The material on top of the device held everything down, preventing migration of nuclear material to the surface. The high explosives did, of course, destroy the test device and the cabling, but not before the instrumentation trailers had recorded a vast amount of data.

If you read these kinds of articles, you must know that the 1958 moratorium did not last. Soviet politics shifted again, France began nuclear testing, negotiations over a more formal test ban faltered. US intelligence suspected that the Soviet Union had operated their nuclear weapons program at full tilt during the test ban, and the military suspected clandestine tests, although there was no evidence they had violated the treaty. Of course, that they continued their research efforts is guaranteed, we did as well. Physicist Edward Teller, ever the nuclear weapons hawk, opposed the moratorium and pushed to resume testing.

In 1961, the Soviet Union resumed testing, culminating in the test of the record-holding "Tsar Bomba," a 50 megaton device. The US resumed testing as well. The arms race was back on.

US hydronuclear testing largely ended with the resumption of full-scale testing. The same safety studies could be completed on real weapons, and those tests would serve other purposes in weapons development as well. Although post-moratorium testing included atmospheric detonations, the focus had shifted towards underground tests and the 1963 Partial Test Ban Treaty restricted the US and USSR to underground tests only.

One wonders about the relationship between hydronuclear testing at TA-49 and the full-scale underground tests extensively performed at the NTS. Underground testing began in 1951 with Buster-Jangle Uncle, a test to determine how big of a crater could be produced by a ground-penetrating weapon. Uncle wasn't really an underground test in the modern sense, the device was emplaced only 17 feet deep and still produced a huge cloud of fallout. It started a trend, though: a similar 1955 test was set 67 feet deep, producing a spectacular crater, before the 1957 Plumbbob Pascal-A was detonated at 486 feet and produced radically less fallout.

1957's Plumbbob Rainier was the first fully-contained underground test, set at the end of a tunnel excavated far into a hillside. This test emitted no fallout at all, proving the possibility of containment. Thus both the idea of emplacing a test device in a deep hole, and the fact that testing underground could contain all of the fallout, were known when the moratorium began in 1959.

What's very interesting about the hydronuclear tests is the fact that technicians actually worked "downhole," at the bottom of the excavation. Later underground tests were prepared by assembling the test device at the surface, as part of a rocket-like "rack," and then lowering it to the bottom just before detonation. These techniques hadn't yet been developed in the '50s, thus the use of a horizontal tunnel for the first fully-contained test.

Many of the racks used for underground testing were designed and built by LANL, but others (called "canisters" in an example of the tendency of the labs to not totally agree on things) were built by Lawrence Livermore. I'm not actually sure which of the two labs started building them first, a question for future research. It does seem likely that the hydronuclear testing at LANL advanced the state of the art in remote instrumentation and underground test design, facilitating the adoption of fully-contained underground tests in the following years.

During the three years of hydronuclear testing, shafts were excavated in four testing areas. It's estimated that the test program at TA-49 left about 40kg of plutonium and 93kg of enriched uranium underground, along with 92kg of depleted uranium and 13kg of beryllium (both toxic contaminants). Because of the lack of a nuclear yield, these tests did not create the caverns associated with underground testing. Material from the weapons likely spread within just a 10-20' area, as holes were drilled on a 25' grid and contamination from previous neighboring tests was encountered only once.

The tests also produced quite a bit of ancillary waste: things like laboratory equipment, handling gear, cables and tubing, that are not directly radioactive but were contaminated with radioactive or toxic materials. In the fashion typical of the time, this waste was buried on site, often as part of the backfilling of the test shafts.

During the excavation of one of the test shafts, 2-M in December 1960, contamination was detected at the surface. It seems that the geology allowed plutonium from a previous test to spread through cracks into the area where 2-M was being drilled. The surface soil contaminated by drill cuttings was buried back in hole 2-M, but this incident made area 2 the most heavily contaminated part of TA-49. When hydronuclear testing ended in 1961, area 2 was covered by a 6' of gravel and 4-6" of asphalt to better contain any contaminated soil.

Several support buildings on the surface were also contaminated, most notably a building used as a radiochemistry laboratory to support the tests. An underground calibration facility that allowed for exposure of test equipment to a contained source in an underground chamber was also built at TA-49 and similarly contaminated by use with radioisotopes.

The Corps of Engineers continued to monitor the hydrology of the site from 1961 to 1970, and test wells and soil samples showed no indication that any contamination was spreading. In 1971, LANL established a new environmental surveillance department that assumed responsibility for legacy sites like TA-49. That department continued to sample wells, soil, and added air sampling. Monitoring of stream sediment downhill from the site was added in the '70s, as many of the contaminants involved can bind to silt and travel with surface water. This monitoring has not found any spread either.

That's not to say that everything is perfect. In 1975, a section of the asphalt pad over Area 2 collapsed, leaving a three foot deep depression. Rainwater pooled in the depression and then flowed through the gravel into hole 2-M itself, collecting in the bottom of the lining of the former experimental shaft. In 1976, the asphalt cover was replaced, but concerns remained about the water that had already entered 2-M. It could potentially travel out of the hole, continue downwards, and carry contamination into the aquifer around 800' below. Worse, a nearby core sample hole had picked up some water too, suggesting that the water was flowing out of 2-M through cracks and into nearby features. Since the core hole had a slotted liner, it would be easier for water to leave it and soak into the ground below.

In 1980, the water that had accumulated in 2-M was removed by lifting about 24 gallons to the surface. While the water was plutonium contaminated, it fell within acceptable levels for controlled laboratory areas. Further inspections through 1986 did not find additional water in the hole, suggesting that the asphalt pad was continuing to function correctly. Several other investigations were conducted, including the drilling of some additional sample wells and examination of other shafts in the area, to determine if there were other routes for water to enter the Area 2 shafts. Fortunately no evidence of ongoing water ingress was found.

In 1986, TA-49 was designated a hazardous waste site under the Resource Conservation and Recovery Act. Shortly after, the site was evaluated under CERCLA to prioritize remediation. Scoring using the Hazard Ranking System determined a fairly low risk for the site, due to the lack of spread of the contamination and evidence suggesting that it was well contained by the geology.

Still, TA-49 remains an environmental remediation site and now falls under a license granted by the New Mexico Environment Department. This license requires ongoing monitoring and remediation of any problems with the containment. For example, in 1991 the asphalt cover of Area 2 was found to have cracked and allowed more water to enter the sample wells. The covering was repaired once again, and investigations made every few years from 1991 to 2015 to check for further contamination. Ongoing monitoring continues today. So far, Area 2 has not been found to pose an unacceptable risk to human health or a risk to the environment.

NMED permitting also covers the former radiological laboratory and calibration facility, and infrastructure related to them like a leach field from drains. Sampling found some surface contamination, so the affected soil was removed and disposed of at a hazardous waste landfill where it will be better contained.

TA-49 was reused for other purposes after hydronuclear testing. These activities included high explosive experiments contained in metal "bottles," carried out in a metal-lined pit under a small structure called the "bottle house." Part of the bottle house site was later reused to build a huge hydraulic ram used to test steel cables at their failure strength. I am not sure of the exact purpose of this "Cable Test Facility," but given the timeline of its use during the peak of underground testing and the design I suspect LANL used it as a quality control measure for the cable assemblies used in lowering underground test racks into their shafts. No radioactive materials were involved in either of these activities, but high explosives and hydraulic oil can both be toxic, so both were investigated and received some surface soil cleanup.

Finally, the NMED permit covers the actual test shafts. These have received numerous investigations over the sixty years since the original tests, and significant contamination is present as expected. However, that contamination does not seem to be spreading, and modeling suggests that it will stay that way.

In 2022, the NMED issued Certificates of Completion releasing most of the TA-49 remediation sites without further environmental controls. The test shafts themselves, known to NMED by the punchy name of Solid Waste Management Unit 49-001(e), received a certificate of completion that requires ongoing controls to ensure that the land is used only for industrial purposes. Environmental monitoring of the TA-49 site continues under LANL's environmental management program and federal regulation, but TA-49 is no longer an active remediation project. The plutonium and uranium is just down there, and it'll have to stay.