physics

Special nuclear material

Special nuclear material is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Special nuclear material rather than just read about it. In short: Special nuclear material (SNM) is a term used by the United States Nuclear Regulatory Commission to classify fissile materials. The NRC divides special nuclear material into three main categories, according to the risk and potential for its direct use in a clandestine nuclear weapon or for its use in the production of nuclear material for use in a nuclear weapon.

Key takeaways

Special nuclear material belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Special nuclear material to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Special nuclear material from memory before moving on to harder problems.

Reference excerpt

Special nuclear material (SNM) is a term used by the United States Nuclear Regulatory Commission to classify fissile materials. The NRC divides special nuclear material into three main categories, according to the risk and potential for its direct use in a clandestine nuclear weapon or for its use in the production of nuclear material for use in a nuclear weapon.

History The Atomic Energy Act of 1946 gave the newly-formed Atomic Energy Commission ownership over all 'Fissionable Materials', explicitly including uranium-235 and plutonium. The AEC was given authority to classify materials as fissionable, as well as to control access to such material, along with access to Restricted Data. Under the amended version of the Atomic Energy Act of 1954, such materials were redefined as Special Nuclear Material, as well as updated to include uranium-233. After the creation of the Nuclear Regulatory Commission by the Energy Reorganization Act, it took over the responsibility of classifying and controlling access to SNM.

Materials Special Nuclear Material refers only to uranium-235, uranium-233, and plutonium. The term Strategic Special Nuclear Material (SSNM) refers to uranium-235 contained in uranium enriched above 20 percent (highly-enriched uranium), as well as any concentration of uranium-233 or plutonium. The distinction between SNM and SSNM is due to the fact that uranium-235 is typically found mixed with other isotopes such as uranium-238. Plutonium-239 is made in a nuclear reactor by irradiating uranium-238 with neutrons, and uranium-233 is made the same way using thorium-232. Since they are different elements than the source material, they can be separated relatively easily through chemical processes. However, uranium-235 is produced from uranium ore, which contains 0.7% uranium-235 with most of the rest consisting of uranium-238. Since they are the same element, they behave in similar ways and must be separated by their slightly different atomic masses. This process is far more difficult than chemical separation. Since highly-enriched uranium is required for nuclear weapons, but low-enriched uranium is commonly used in nuclear power plants, it is classified both by its quantity and enrichment percentage.

Categories The NRC defines the three categories of SNM.

Category I Category I (Strategic SNM) is defined as SSNM in any combination in a quantity of

2 kilograms (4.4 pounds) or more of Pu-239; or 5 kilograms or more of U-235 (11 pounds; contained in uranium enriched to 20 percent or more in the U-235 isotope); or 2 kilograms (4.4 pounds) or more of U-233; or 5 kilograms (11 pounds) or more in any combination computed by the equation grams = (grams contained U-235) + 2.5 (grams U-233 + grams Pu-239). These combinations are referred to as a formula quantity.

Category II Category II (Special nuclear material of moderate strategic significance) is defined as

Less than a formula quantity of strategic special nuclear material but more than 1,000 grams of uranium-235 (contained in uranium enriched to 20 percent or more in the U-235 isotope) or more than 500 grams of uranium-233 or plutonium-239, or in a combined quantity of more than 1,000 grams (2.2 pounds) when computed by the equation grams = (grams contained U-235) + 2 (grams U-233 + grams Pu-239); or 10,000 grams (22 pounds) or more of uranium-235 (contained in uranium enriched to 10 percent or more but less than 20 percent in the U-235 isotope).

Category III Category III (Special nuclear material of low strategic significance) is:

Less than an amount of special nuclear material of moderate strategic significance (see category II above) but more than 15 grams (0.5 oz) of uranium-235 (contained in uranium enriched to 20 percent or more in U-235 isotope) or 15 grams of uranium-233 or 15 grams of plutonium-239 or the combination of 15 grams when computed by the equation grams = (grams contained U-235) + (grams Pu-239) + (grams U-233); or Less than 10,000 grams but more than 1,000 grams of uranium-235 (contained in uranium enriched to 10 percent or more but less than 20 percent in the U-235 isotope); or 10,000 grams or more of uranium-235 (contained in uranium enriched above natural but less than 10 percent in the U-235 isotope).

Access Individuals with access to special nuclear material require an access authorization (security clearance) from the NRC or DOE. The NRC defines two levels of Special Nuclear Material Access Authorization, NRC-U and NRC-R, in addition to the standard Department of Energy Access Authorizations L and Q.

Individuals with Q access authorization are permitted access to all three categories of SNM, while L access authorization only allows access to categories II and III. The NRC SNM access authorization levels (U and R) are given to individuals who are employed by an NRC contractor, licensee, or contractor of a licensee and who requires access to SNM, while NRC employees are given either Q or L depending on their position sensitivity. NRC-R requires the same Tier 3 background investigation as L, and permits access to protected areas in nuclear facilities. NRC-U requires a Tier 5 investigation, similar to Q, and allows access to all three categories of nuclear material. All individuals responsible for the transport of SNM are required to possess NRC-U.

References

Illustrations

Worked examples

Example 1 — a first encounter with Special nuclear material

Start with the simplest possible case. Write down what Special nuclear material claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Special nuclear material before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Special nuclear material ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Special nuclear material

In research: Special nuclear material appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Special nuclear material in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Special nuclear material is common in secondary-school and first-year university syllabi. It links to neighbouring topics Nuclear proliferation, Nuclear weapon design, Special nuclear materials, so understanding it makes those chapters shorter.
In everyday life: Look for Special nuclear material outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Special nuclear material in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Special nuclear material means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Special nuclear material out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Special nuclear material in simple terms?

Why does Special nuclear material matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Special nuclear material?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Special nuclear material.