I found the following expression in New York Times:

On the International Nuclear Event Scale, a Level 7 nuclear accident involves "widespread health and environmental effects" and the "external release of a significant fraction of the reactor core inventory."

In this sentence, I couldn't understand what "a significant fraction of the reactor core inventory" means. I think this refers to radioactive materials. Is that right? But if so, I don't know why such an expression is used.

According to Longman dictionary, "inventory" means "a list of all the things in a place". And "fraction" means "a very small amount of something".

Then, it follows that the phrase in question means "a small significant amount of a list of all the things in the reactor core". Is it correct to interpret this means "radioactive materials". If not, could you tell me "a significant fraction of the reactor core inventory"?

Also, significant modifies not "its amount" but "its quality". Is that right?

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    '"fraction" means "a very small amount of something"' 99/100 is a fraction & that is not a very small amount of the original, but a significant amount. – Andrew Thompson Jun 5 '12 at 17:46
  • One half is not a very small amount of something, but it is a fraction as well. Fraction means "one of several equal parts" – horatio Jun 6 '12 at 14:31

Significant fraction of the reactor core inventory means a not too small part of the things found in the reactor core.

  • Thank you. I understand what "significant fraction" means. "Inventory" here means "the things found in something". I got it. – foolnloof Jun 6 '12 at 11:22

Perhaps the Longman dictionary you refer to shows other senses than the one you mention. For example, the inventory entry in wiktionary shows "The stock of an item on hand at a particular location or business" as its first sense, and it is this sense that applies here. Thus, the quote says a Level 7 nuclear accident involves external release of a large part of the reactor core stock. Apparently besides materials in the reactor core itself, the quote refers to formerly-used core materials, which are often stored on-site in a cooling pond, and remain a part of the reactor's core materials inventory.


This phrase is best described as euphemistic technical jargon. The U.S. Nuclear Regulatory Commission defines reactor core to mean "the central portion of a nuclear reactor, which contains the fuel assemblies, moderator, neutron poisons, control rods, and support structures. The reactor core is where fission takes place." The fuel is radioactive and supports a controlled (ideally!) fission chain reaction that generates a variety of additional radioactive isotopes and other products, such as Iodine-131 and Caesium-137. Together, all this different radioactive stuff is contained in the reactor and known in nuclear power jargon as the reactor core inventory.

These products are very nasty even in tiny doses. Here's a statement (PDF) from an engineering company that works with nukes: "Very small fractions of a reactor core’s inventory would yield a major radiological hazard to the public if released off-site, e.g. typically a release of about one-millionth of the I-131 inventory in a reactor would equate to the Emergency Reference Level." Fraction in this context basically means "not everything." A scientific paper about the Chernobyl accident described the release "expressed as a fraction of the estimated reactor core inventory, of approximately 15-20% of the iodine, tellurium and caesium isotopes, approximately 1% of the ruthenium and lesser amounts of the other fission products and actinides, together with an implied major fraction of the krypton and xenon noble gases."

So while release of a significant fraction sounds like a small quantity of something, in this context the phrase means that dangerously high levels of highly radioactive particles and gasses have been set loose in the environment.

  • 1
    I don't see how this is euphemistic. – Bobbi Bennett Jun 5 '12 at 18:31
  • Merriam Webster on euphemism: "the substitution of an agreeable or inoffensive expression for one that may offend or suggest something unpleasant." Compare "external release of a significant fraction of the reactor core inventory" to "spewing lots of deadly radiation into the environment." The former clearly is a euphemism for the latter. – Bill Lefurgy Jun 5 '12 at 22:35
  • One man's euphemism is another man's accurate representation. Radiation is never spewed. Radiation emitting materials are. – horatio Jun 6 '12 at 14:26
  • The fact that the OP states "I couldn't understand what 'a significant fraction of the reactor core inventory" means'" indicates the bland opacity of the phrase. It may by an accurate representation as far as it goes, but it's meaningless to most people who read that NYT article. And it's technically right to specify "radiation emitting materials" but it's the radiation that causes harm. – Bill Lefurgy Jun 6 '12 at 15:31

When a nuclear reactor runs, it produces new isotopes of new elements in its core. A google search on the terms 'reactor core isotope inventory' will bring up many articles with more information on just what all these isotopes are.

'Reactor core inventory' is industry jargon for the chemical composition of the reactor core.

Fraction, in this use, might come from industry jargon as well. For instance, when distilling, the different substances distilled are called 'fractions'.

So it -might- mean 'significant chemicals (dangerous radioactive elements) are released from the core'. But I expect they intend 'significant fraction' to mean 'a dangerous amount'.


According to Merriam-Webster Unabridged, the primary definition of fraction is simply "a part of a whole;" whether it's a large part or small part is neither stated nor implied.

The Web site for the The International Nuclear and Radiological Event Scale provides a handbook explaining the different levels of the scale. In Section 2.2.2., "Definition of levels based on activity released," the full definition of Level 7 is given:

“An event resulting in an environmental release corresponding to a quantity of radioactivity radiologically equivalent to a release to the atmosphere of more than several tens of thousands of terabecquerels of I-131.”

This corresponds to a large fraction of the core inventory of a power reactor, typically involving a mixture of short and long lived radionuclides.

Radionuclides are simply radioactive elements; Level 7 means that the reactor released a large amount of radioactive material from its core.

The New York Times probably should have quoted the INRES definition of Level 7 directly; its paraphrasal adds neither clarity nor technical correctness.


Fraction is this sense doesn't mean "a small amount," it means more the mathematical sense of the word fraction and also "a measurable portion of the total." So a "significant fraction" is taken to mean something other than a small amount. It might be 1/4, but certainly not 1/64.

Note that it is possible that the amount is small but its impact is significant. I would not typically consider this when I think of the phrase "significant fraction," but the context is one where the materials matter.

  • 1
    I would consider even 1/100 of a reactorful to be a significant fraction, but opinions may vary. The point is that even an ounce is technically a fraction of the reactor's load (perhaps 1/1000000), but not a significant fraction. – TimLymington Jun 5 '12 at 16:38
  • I agree, and that is what I wrote. If one said "he ate a significant fraction of the box of corn flakes" one probably does not mean a spoonful, and one never thinks about the environmental impact of an individual flake. Even within the context of the OP, I still think the intention is to impress upon people that the amounts released would be a large portion of the total. – horatio Jun 5 '12 at 16:43

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