Active nuclear reactor that required californium for its startup

Californium: The Behind-the-Scenes Element of Nuclear Plants and Mining

by kirkcoburn
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Radioactive materials have a variety of uses. One such material is californium, a synthetic element named for the state of California, where scientists first produced it.

What Is Californium?

Californium is a chemical element with atomic number 98. The symbol is Cf. It is a radioactive metal; scientists also classify it as an actinide.

Californium does not naturally exist on Earth. It was first synthesized in 1950 at the University of California by bombarding elements of curium-242 with helium ions in a cyclotron. It was the sixth transuranium element that scientists discovered. 

There is evidence that supernovae may produce californium isotopes. But those isotopes probably decay due to a relatively short half-life before traveling far from the source of the explosion. Thus, it is highly unlikely that californium will be found in nature in a usable state.

Even in the scientific community, scientists didn’t provide californium in usable quantities until 1958, when they synthesized a larger amount through prolonged neutron irradiation of plutonium-239. Twenty isotopes have since been reported, although californium-237 and 238 have not been fully confirmed. Most have a short half-life, with the longest being californium-251, which has a half-life of 898 years and decays into curium-247.

How Do We Make Californium Today?

Californium is now made through nucleosynthesis. Californium-250 is made from berkelium-249 after it captures a neutron to form berkelium-250, which has no commercial use.

Californium-252, the most useful isotope with a half-life of 2.6 years, is made through a complex process that starts by fabricating curium oxide pellets in long aluminum tubes. A neutron flux then bombards them to trigger beta decay, which creates a probabilistic mix of Cf-252 and other actinide elements. Producers can then purify and separate the Cf-252.

However, all of this has to be done in hot cells with extreme protection. It also generates a lot of waste and by-products. Because it’s so complicated, companies are researching ways to create Cf more efficiently.

Is Californium Dangerous?

Yes! Californium is dangerous because it is a strong neutron emitter, and some isotopes also decay by emitting alpha radiation. This means that even low doses of californium can be carcinogenic. It is important to use proper safeguards if handling and working with californium.

How Is Californium Used?

Example of a mine that was first explored with californium-252 testing

Californium has a variety of uses at present:

In Nuclear Reactors

The Oak Ridge National Laboratory produces californium-252 in east Tennessee, the site of the world’s first continuously operating reactor. It plays a vital role in modern nuclear power plants, but it might not be what you think.

When nuclear fuel goes to a new site, it does not have enough neutron flux to safely initiate the chain reaction required for sustained power. This is where californium-252 comes in. As a strong neutron emitter, it acts as a primer to start the reaction. Once the reaction starts, there is no further need for Cf-252.

Without Cf-252 or some other strong neutron emitter, it is not possible to bring a new plant online. Any expansion of nuclear power thus requires a supply of these elements. The navy also uses Cf-252 to start up naval reactors on ships and submarines. 

Dormant reactors also require a neutron source to restart them, which is fairly common in naval uses. This may also lead to uses for larger nuclear reactors used in space applications, such as to power settlements on the moon and Mars.

Mars, in particular, may require extensive nuclear energy infrastructure since long-term use of solar power has proven problematic. Current space applications use plutonium-238-based RTGs, which essentially act like nuclear batteries.

It also helps to activate fission in a small sample of nuclear fuel for quality testing. Finally, it can help scan fuel rods for cracks, which may compromise the safety of a reactor. This means that californium plays a key role in the ongoing maintenance of reactors in small quantities. However, be careful not to confuse it with nuclear fuel (although it is a fissionable material). It is too difficult to produce or store and has too short a half-life to be a practical fuel.

In Medicine

Doctors use Cf-252 in brachytherapy to treat certain cancers, such as cervical and brain cancer. It is less commonly used than other isotopes because it is more challenging to make and synthesize. However, neutron brachytherapy has fewer side effects and complications, so it is likely to expand in use in the future. But more research needs to be done, and the expense and complexity of creating californium is an ongoing obstacle.

Right now, doctors only turn to californium if regular radiation therapy has failed. But the medical industry is continuing to research its efficacy for the treatment of other radiation-resistant cancers. Treatment consists of placing a small amount of Cf-252 close to the cancer. But it is very important to ensure that patients do not ingest the californium is not ingested. The bone marrow may hold onto it and reduce red blood cell production. Even worse, it is a potential carcinogen.

In Mining

Californium is a particularly efficient and reliable neutron emitter. This makes it of particular use in the mining industry. Specifically, mining professionals often use californium in neutron activation analysis to identify gold and silver ores. Most neutron activation analysis requires a research reactor. But because Cf-252 is such a good isotopic neutron source, it can help with in situ exploration.

Californium is now also being used for well logging. Cf-252 has a number of advantages, such as a very small physical size, low gamma and heat emissions, and (hopefully) lower cost. It produces an accurate log for clay logging and can identify sodium-24 and manganese-56. The industry is investigating other potential applications, including stable tracers and temporary depth markers. 

This is useful when exploring for both oil and water. Basically, Californium is a unique, portable neutron source. Sending it down a borehole can give an idea as to the well’s potential productivity. Neutron moisture gauges, which can detect and measure the water- and oil-bearing layers in a well, also use californium.

It has even helped analyze copper mill materials and rock cores, so it may be useful for monitoring extraction processes. In the coal and cement industry, it helps perform bulk material analysis. It can identify the composition of coal and assist with raw material quality control in cement making. It is also used to measure the mineral content level in coal ash. Californium is extremely useful for mining, which is causing increased demand that will likely necessitate further facilities capable of producing it in some quantity.

Other Uses

While californium is vital to nuclear reactors and extremely useful in mining, it has also developed a number of other uses over the years.

These include:

  • Some high-end portable metal detectors. Hobby metal detectors typically work through induction balance and do not contain californium. Manufacturers reserve californium for extremely accurate commercial-grade devices.
  • Detecting metal fatigue and stress in airplanes to manage safety concerns. In addition to metal fatigue, it can detect trapped moisture and bad welds.
  • Measuring the contents of explosive devices, that is to say, detecting bombs and finding landmines. This makes californium useful for cleanup in combat zones across the world. Security professionals use these devices in national security applications to detect potential terrorist attacks.
  • Calibration of instrumentation designed to detect neutrons.
  • Synthesizing oganesson (Og), which is atomic number 118. This is still suited only to research applications. Manufacturers also use it to produce lawrencium, another transuranic element (103).
  • Measuring moisture content in soil and monitoring the movement of groundwater.

Californium is a dangerous element to handle, so technicians have to handle it very carefully. But it does have a number of practical uses, with scientists discovering more all the time. It is one of the most valuable of the transuranic elements and is likely to find more uses as time goes on.

Where You Won’t Find Californium

It is not, however, true that californium is used in nuclear weapons. This rumor stems from the fact that at one point, the Soviet Union experimented with nuclear “bullets” made with californium (due to its low critical mass). While they did indeed make these bullets, they turned out to have a number of drawbacks. 

First of all, they had to be stored in a liquid ammonia refrigerator and fired within 30 minutes. But if they were not fired or returned to storage, they would become useless and dangerous. They produced enough heat to jam in the barrel or explode in your face. Firing three of them would equal your lifetime limit of radiation. …And they didn’t explode if fired into a tank full of water.

Needless to say, the experiment did not go very far. Californium’s short half-life and generation of heat make it not particularly suitable for the creation of weapons. However, it is sometimes reported as an element used in weapons, often alongside discussion of its role in nuclear power generation.

Californium is one of the most useful transuranic elements, and it’s likely to see more use in the future. While it is complicated and expensive to produce, it can reliably be produced in reasonable and usable quantities.

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