Answer to Question #10097 Submitted to "Ask the Experts"

Category: Radiation Basics — Fission, Fusion

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Q

Which fission products are present in a nuclear reactor? Various sources say "many" or hundreds and then refer to 131I, 137Cs, and 90Sr as important products in the case of exposure.

However, I cannot find any diagrams or anything that show me how uranium fuel and its products could eventually end up as one of these three.

I did come across the actinium series, which I guess is what happens to 235U in rocks in the Earth's crust undergoing very, very slow decay. Is it correct to assume that? And then is it correct that in a nuclear reactor in a power plant, the neutrons zooming around, or other particles/conditions, initiate other types of decay so that these three products are made?

Is there any kind of diagram showing the variety of reactions to lead to the most common products in a nuclear reactor and what the waste constituents are?

A

There are indeed many hundreds of possible different fission products produced as nuclear fuel undergoes the fission process in nuclear reactors. Most reactors in the world are fueled with uranium, with 235U being the most common isotope associated with most of the fission events induced by low-energy neutrons, so-called thermal neutrons. The only significance the actinium series that you mention has is that the parent nuclide in the series is 235U, the uranium isotope relied on in thermal fission reactors. When the 235U captures a neutron, the excitation energy associated with the capture is sufficient to cause the compound nucleus, 236U, to break (fission) into two major fragments, usually accompanied by a few neutrons and some gamma radiation.

The two major fragments, which we refer to as fission products, may have a wide spectrum of nuclear masses, characterized by the fact that the fission process does not favor an equal mass distribution between the two fragments. The preferred mass distribution of the products is actually bimodal—i.e., if you plot the yields of fission products of given masses versus mass number, you will generate a curve with two peaks. The first peak expectedly will lie between about mass numbers 90 and 100, and the second peak will fall between about mass numbers 130 and 140. You can find this mass distribution on several Internet sites. The Wikipedia description of the fission process and the fission product yield curve is helpful. Look at the red curve for the fission of 235U, and you can see the favored mass distribution. The table below the curve shows some of the expected fission products, arranged from favored fission products to less likely products.

By "favored" I mean those whose fission yields are relatively high. You can see from that table that 137Cs has about a 6.1 percent yield, meaning that for every 100 fission events a bit more than six, on average, yield 137Cs as one fission product. A 6 percent yield is a high yield, and 137Cs is a favored fission product. As it turns out, if we were to look more closely at just where the 137Cs is coming from, we would find that most comes from the direct fission of 235U, and a small amount comes from indirect processes. By indirect processes we mean that the 137Cs comes from the decay of precursor radionuclides that were produced in the fission process. For example, some of the 137Cs is produced by the sequential beta decays of products that come from the initial production by fission of an isotope of tellurium, 137Te. The137Te decays to 137I, which decays to 137Xe, which decays to 137Cs. Additional 137Cs comes from the decays of 137I and 137Xe produced directly in the fission process.

You can find more detailed lists of direct and indirect fission yields on the Internet; a Lawrence Berkeley National Laboratory site shows these yields for a large number of fission products. The 131I and 90Sr radionuclides also fall in the mass number regions that are favored in the fission process, and their yields are also relatively high. The high yields explain part of why these three radionuclides get a lot of attention, but there are other reasons as well.

The 131I is important because it is volatile at elevated temperatures, and in an accident situation in which the fuel overheats and the cladding fails, the iodine may be released to air and subsequently be breathed in or deposited on vegetation. It is a concern because when breathed in or ingested it may be taken up by the thyroid gland and produce significant radiation dose to the thyroid.

Cesium is also volatile at elevated temperatures (temperatures higher than those at which iodine is volatile), and it may get released under accident situations. It has a much longer half-life than 131I and may remain in the environment for many years following a release; it emits characteristic gamma radiation when it decays and is relatively easy to measure.

The 90Sr is not released as easily from damaged fuel as are the iodine and cesium, but it is a radionuclide of significant biological concern because it behaves biochemically similarly to calcium, accumulating in the bone when ingested. This is a particular concern if children, who drink considerable milk that is susceptible to 90Sr contamination, ingest significant amounts. The public became aware of the potential threat from 90Sr decades ago when this country was involved in atmospheric atomic bomb testing, and one of the major concerns regarding public exposure related to the intake of 90Sr in contaminated milk.

The waste products that accumulate in the reactor fuel are the fission products, with the longer-lived ones, such as 137Cs, 99Tc, and 90Sr, being of most long-term concern. Among the longest-lived radionuclides and the ones often considered as most limiting in terms of how long they must be stored before they would be considered acceptable from a potential exposure point of view, are the transuranic species, radionuclides that lie beyond uranium in the periodic table. You have most likely heard of plutonium, especially 239Pu, which has a 24,000-year half-life and has a high radiotoxicity if taken into the body, especially if inhaled. Another that you might have heard of is 241Am, with a 433-year half-life. These and other transuranics are not fission products, but rather are produced by the capture of neutrons by heavy nuclides, often beginning with 238U or 235U in the reactor, frequently with the subsequent radioactive decay of the product and sometimes sequential steps involving additional neutron capture and more decay.

There is much more that could be said about the fission process and the generation and characteristics of the products produced by reactor operations, but I believe the above should be sufficient to answer your questions. There is a wealth of information available on the internet if you search under "fission," "fission products," "fission product yields," "transuranics," etc. Good luck.

George Chabot, PhD

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