Geoffrey Sea’s Nuclear Bulletin #18 – Nuclear Basics

It has been requested that I review some physics and chemistry basics to help sort out what’s what.

Typical light-water reactor fuel is 3% enriched meaning that it is comprised of 3% Uranium-235 and 97% Uranium-238, whereas natural uranium is only .7% U-235. Hence the requirement for uranium enrichment, which was, for the Japanese reactors in question, done in the United States by and large. (Some recent fuel may include old Soviet weapons uranium that was downblended in Russia and sold through the American company USEC.)

In typical nuclear fuel only the U-235 is fissionable, meaning capable of splitting when hit by a neutron. When U-238 is hit by a neutron, it tends to absorb the neutron rather than split, transmuting into Plutonium-239, the source of most plutonium in reactors.

Since 97% of typical fuel is U-238, there is a lot of plutonium accumulation. Plutonium is much more radioactive than either isotope of uranium, and the fuel therefore becomes more radioactive over time. Since most reactors are not designed to operate when the fuel gets very radioactive, the fuel must be replaced with fresh fuel at varying frequency. The Spent Nuclear Fuel (SNF) comprised largely of plutonium is actually many times “hotter” than fresh fuel, contrary to the suggestion of the word “spent.” This is the problem in the SNF pools at Fukushima and why their exposure with loss of coolant is potentially much more problematic than a core breach at the formerly operating reactors themselves.

The SNF pools have no thick containment vessels as do the reactors, a result of the fact that on-site storage was designed as a makeshift solution for lack of a permanent disposal site. The inadequacy of on-site SNF storage pools will be one of the chief lessons of Fukushima. Another problem with SNF is that plutonium spontaneously ignites when exposed to air, whereas uranium does not.

Because Japan lacked even a candidate site for permanent SNF disposal, like the failed US candidate site at Yucca Mountain, Japan long ago turned to reprocessing which means chemical extraction of the plutonium and U-235 from spent fuel. This mixture extracted from spent fuel is then formulated into an alternative reactor fuel called MOX for “Mixed Oxide.” The United States has experimented with MOX at the Savannah River Site in South Carolina.

There has long been a debate about the safety of MOX fuel since most reactor components were designed for pure uranium fuel and might not behave as expected with high levels of plutonium mixed in. That is another debate resolved by the current accident, since Unit 3 was the only unit running on MOX fuel, and it’s the Unit 3 reactor now suspected of breaching its containment vessel. Presumably that is not just a coincidence – the containment appears to have been inadequately engineered to contain a meltdown of MOX fuel.

Now let’s go back to the U-235 atoms inside typical nuclear fuel, the atoms that undergo fission. These atoms, when hit by a neutron, tend to split into two pieces of slightly unequal mass. One piece tends to have an atomic weight of about 138 and the other about 95 (plus 2 neutrons = 235). The immediate fission products are very short-lived and rapidly decay into more long-lived fission products that have slightly lower atomic weights. Hence we get the most problematic fission products: Cesium-137, Iodine-131, Strontium-90, and Krypton-85.

The first three of these have the unfortunate property of mimicking minerals essential for good health. Cesium mimics potassium, radioiodine is treated just like stable iodine, and strontium mimics calcium. The body cannot distinguish between the nuclear fission products and stable minerals.

That is why the best overall protection against fallout and food contamination is to take mineral supplements and potassium iodide pills (as directed by health authorities), or foods very rich in minerals like seaweed. The point is to load the body with non-radioactive minerals so that fewer of the fallout particles are absorbed.

In judging reports of fallout as are now coming fast and furious, it’s important to distinguish a plume that may consist of mainly light gasses like krypton, xenon, and hydrogen, versus a plume of heaver particles like cesium, strontium and radioiodine. Unfortunately, most news reports do not make a distinction and only give “radiation levels” – which is almost useless information.

In general – the heavier fallout particles will “fall out” over shorter distances and will be less subject to transient wind patterns. So a sudden cloud of “radiation” over California or Montana is likely to be light gasses, which is not a major concern at the levels in question. If there are big cesium and iodine plumes, we are likely to have some warning, as they would first be detected over Pacific Islands, if not in Japan.

Light gasses will produce radiation readings that rise and fall rapidly, whereas heavy fallout particles will leave residues that persist at least for days and weeks.

In areas affected by heavier fallout particles, the principal danger is not from air exposure or even inhalation but from ingestion of particles from food or from contamination of clothing, hands, etc. If you are in such an affected area, you should minimize any contact with surface dust by wearing a filter mask, washing hands regularly, vacuuming often with a HEPA filter, and avoiding contact with vegetation or outdoor surfaces. Surfaces exposed to open air during fallout periods should be cleaned or discarded.

Basic practices to reduce internal deposition of dust particles can drastically reduce exposure and reduce risk of long-term cancer, so panic is really counter-productive. Even if you are in a fallout cloud, it need not mean a long-term impact on your health if you act wisely.

Geoffrey Sea holds a bachelor’s degree in History and Science from Harvard. He did graduate work in Science, Technology, and Society at MIT and in radiological health physics at San Jose State University. He is co-founder of Southern Ohio Neighbors Group, which successfully defeated plans for the centralized storage of spent nuclear fuel at Piketon, Ohio. He has published in the American Scholar, the Columbia Journalism Review, the Bulletin of the Atomic Scientists, and many newspapers. He can be contacted via email at

— Geoffrey Sea