Deseret News: ‘No thanks’ to uranium waste plans
One of the fears about the waste from FMRI Inc. is over thorium, found naturally in the earth’s crust and contained in the waste from Oklahoma. As thorium decays, it produces the radioactive gas radon. The concern about radon is that it could cause lung cancer in humans.
The radon isotope produced in the decay of thorium-232 is radon-220. It has a half-life of 55 seconds. Which means that in 1-2 minutes this gas has to leave the ore it came from, drift into your lungs, and decay into polonium-216. Possible? Perhaps if you’re snorting thorium ore. Likely? Not even remotely.
Uranium-238, on the other hand, produces radon-222, which has a half-life of 3.82 days, which lasts about 6000 times longer than the radon produced by thorium. So where do you think the radon comes from?
Check out thorium and uranium decay chains on the Radioactive Decay Applet.
The last few days I have been working on a new Java tool to help visualize fission product decay chains. Specifically, I want to help people learn quickly how many fission product decay chains decay to stable isotopes in very short order, so you can see that fission product waste isn’t radioactive forever.
Basically what you will see when the simulation comes up is a graph of the radioactive isotopes produced by fission, arranged vertically by what element they are, and horizontally by their atomic mass.
When a nucleus fissions, it produces two fission products. You might think they they would roughly be the same size, but that’s not what ends up happening. One is bigger and one is smaller. The distribution of atomic masses is shown by the gray bars. As you can see, there are two “humps” to the curve.
Since heavy nuclei like uranium and plutonium have many more neutrons than protons for atomic stability, when they fission, their fission-products end up neutron-rich. Nature corrects this through the process of beta-decay, where a neutron turns into a proton and emits a beta particle (electron). Beta decay doesn’t really change the atomic mass of a nucleus (since the emitted electron has very little mass) but it does change its atomic number, which governs what type of element it is.
Thus, when a pair of fission products are generated, they will march “upward” through beta decay, converting their neutrons into protons, until they achieve nuclear stability.
You can see this process happen graphically by moving the time slider on the bottom of the simulation. As you move it left-to-right, you increase the amount of time the fission products are allowed to decay, and you see their yield bars (the gray bars) fading out. The “fade” factor of the bar is given by a mathematical calculation of the amount of the longest-lived radioactive isotope in a given mass chain that remains.
For instance, on the chain with a mass number of 90, the longest-lived radioactive isotope is strontium-90. As you move the decay time, nothing much changes til you get to about 30 years (the first half-life of Sr-90) and then you see the bar begin to fade out, according to the radioactive decay of the element.
One of the things to notice from this simulation is how that after about 300 years, most of the chains have decayed to stability.
I’ve often found myself trying to find a good source of microscopic cross-sections for nuclear isotopes, and today I found a nice source:
On here, I found the thermal cross-sections I’ve been looking for that would allow me to compare the behavior of protactinium and uranium in the blanket salts to thorium. Here are the thermal (2200 m/s) cross-sections for several blanket isotopes, with units in barns:
TOTAL ELASTIC FISSION CAPTURE half-life
Li-7 1.015 0.97 - 0.045 stable
Be-9 6.1586 6.1510 - 0.0076 stable
F-19 3.643 3.652 - 0.0096 stable
Th-230 32.32 9.774 0.0 22.55 75,400 yr
Th-232 21.11 13.70 0.0 7.40 14e9 yr
Th-233 1478. 13.0 15.0 1450.0 22.3 min
Pa-231 210.69 9.954 0.02 200.72 32,700 yr
Pa-232 1176.2 12.23 700.0 464.0 1.31 day
Pa-233 53.051 13.021 0.0 40.031 27.1 day
U-232 162.3 10.79 76.66 74.88 69.8 yr
U-233 588.38 11.97 531.16 45.25 159,000 yr
U-234 119.2 19.41 0.0062 99.75 245,000 yr
U-235 698.2 15.03 584.4 98.81 704e6 yr
From these cross-sections, you can see that thorium-232 has a moderate cross-section for absorption, but there’s so much of it in the blanket that it does almost all the neutron-absorbing (as we would want). After absorbing a neutron, the Th-232 becomes Th-233, which has a monster absorption cross-section (almost 200x that of Th-232) but its half-life is so short (22 min) that it isn’t around very long to absorb a neutron. Once it turns into Pa-233, the absorption cross-section is still over 5 times greater than the Th-232. That is one of the basic reasons why it’s so important to isolate the Pa-233 from the blanket–in order to prevent another neutron absorption. This is a key step that you just can’t do in a solid-core reactor that’s trying to “burn” thorium (and achieve a conversion ratio of > 1.0). Finally, the Pa-233 decays to U-233 in 27 days. The U-233 has a huge cross-section, mostly for fission (531 barns) but with a lot of absorption (45 barns). Thus, uranium-233 left in the blanket will really want to gobble up blanket neutrons and cause fission. That leads to even more trouble, because that will deposit fission products in the blanket, complicating reprocessing and making the blanket “hot” with radiation from fission products. All of these factors argue for getting protactinium of out the blanket and letting it decay to U-233 outside of the neutron flux. The U-233 can then be removed by fluorination to UF6 and adding it back to the core salt by reduction to UF4. Continuous refueling of the core means that excess reactivity in the core can be held to almost nothing, an extremely important consideration for safe operation that is very difficult to achieve in a solid-core reactor.
The nominal blanket composition would be as follows:
From these cross-sections, you can see that thorium-232 has a moderate cross-section for absorption, but there’s so much of it in the blanket that it does almost all the neutron-absorbing (as we would want).
After absorbing a neutron, the Th-232 becomes Th-233, which has a monster absorption cross-section (almost 200x that of Th-232) but its half-life is so short (22 min) that it isn’t around very long to absorb a neutron.
Once it turns into Pa-233, the absorption cross-section is still over 5 times greater than the Th-232. That is one of the basic reasons why it’s so important to isolate the Pa-233 from the blanket–in order to prevent another neutron absorption. This is a key step that you just can’t do in a solid-core reactor that’s trying to “burn” thorium (and achieve a conversion ratio of > 1.0).
Finally, the Pa-233 decays to U-233 in 27 days. The U-233 has a huge cross-section, mostly for fission (531 barns) but with a lot of absorption (45 barns). Thus, uranium-233 left in the blanket will really want to gobble up blanket neutrons and cause fission. That leads to even more trouble, because that will deposit fission products in the blanket, complicating reprocessing and making the blanket “hot” with radiation from fission products.
All of these factors argue for getting protactinium of out the blanket and letting it decay to U-233 outside of the neutron flux. The U-233 can then be removed by fluorination to UF6 and adding it back to the core salt by reduction to UF4. Continuous refueling of the core means that excess reactivity in the core can be held to almost nothing, an extremely important consideration for safe operation that is very difficult to achieve in a solid-core reactor.
Some people have asked about the issues and potential dangers of thorium, especially relative to uranium. I have extracted this section from an AEC book published on thorium in the late 1950s. To sum it up, thorium, like other heavy metals, is toxic in the body. It is radioactive but with an exceptionally long half-life of 14 billion years, which means individual thorium-232 nuclei decay at an incredibly slow rate. Nevertheless, once a Th-232 nuclei decays, it proceeds through its decay chain relatively quickly, arriving at its final stable form of lead-208 with a minimum of long-lived intermediate nuclei.
So don’t eat thorium, don’t breathe thorium, just put it in the reactor and make energy.
HEALTH AND SAFETY ASPECTS OF THORIUM PRODUCTION
Prior to 1945, industrial interest in thorium was confined almost entirely to its use in the manufacture of gas mantles. Consequently, there was little interest in the health and safety of thorium operations, and little published information about the hazards of its use in industry. But with the advent of the atomic age, attention focused on thorium as a possible source of nuclear energy. As a result, interest increased greatly in the toxicity of thorium and the inherent radiological hazard in processing thorium materials.
Considerable research on thorium toxicity was done at the University of Rochester under USAEC contract. Published reports describe in detail this work and the findings. Others reported on the radiological problems in thorium operations. Standards were recommended for air, water and body concentrations of thorium and its daughter products. Both chemical and radioactivity toxicity will be discussed, but since the chemical toxicity of thorium is lower, radiation toxicity is the controlling hazard.
HEALTH HAZARDS OF THORIUM
9-2 Radiological toxicity. Of more importance to the management and workers in thorium plants than thorium’s chemical toxicity is its radiological hazard. To recognize exposure problems that may be encountered and to install the controls necessary to protect personnel, a thorough understanding of the radioactive decay of thorium is necessary.
The thorium decay series begins with Th-232, which has a radioactive half-life of 13.9 billion years, and ends with the formation of Pb-208, a stable isotope. In the transmutation of the parent thorium to the stable lead, a total of 10 other radioisotopes are formed. Four of these intermediate isotopes decay through beta emission and five through alpha emission. One isotope, Bi-222 (thorium C), may decay by either alpha or beta emission. Figure 9-1 shows the decay series of thorium. Also shown are the half-lives for the radioisotopes in the series as well as the energies of the alpha, beta and gamma radiations.
It can be seen from this figure that the thorium decay series differs considerably from the uranium decay series in that the half-lives are all relatively short, the longest being the 6.7-yr radium-228. Thorium-228, which exists in equilibrium with Th-232 in thorium which is either unseparated or freshly separated from other materials in the series, has a half-life of 1.9 years. The half-lives of other isotopes in the series range from 0.3 microsecond to 3.64 days.
In the processing of thorium, its ores, and its salts, it must be recognized that these decay products fall into several columns of the periodic table and therefore behave chemically in different ways. Because of the short half-lives of many of the decay products, the chemical problem is essentially one of handling two elements, thorium and radium. However, the short half-lives mean also that after any separation into thorium and non-thorium fractions, the composition of the fractions changes rapidly. The non-thorium fraction contains Ra-228, which has as one of its decay products the strongly active Th-228. Because the non-thorium fraction may have considerably less physical bulk than the thorium fraction, the percentage of Th-228 in it may become relatively high, even higher than it is in the thorium fraction. The content of Th-228 can be controlled by separating thorium from the non-thorium fraction at proper intervals. Additional Th-228 is thus not introduced by the decomposition of Ra-228 while the Th-228 level gradually decreases as a result of decay into non-thorium decay products. The Ra-224 in the non-thorium fraction decays to Rn-220, a short-lived radioactive gas that may build up high levels of radioactivity in working areas.
When thorium is separated from other isotopes in the decay series, the thorium fraction has only a slight alpha activity. There is no beta radiation and only a slight amount of gamma radiation (from the 0.09-MeV gamma rays which emanate from Th-228 decay). However, the activity from the Th-228 side of the chain is quickly re-established. A first equilibrium state is reached in about 36 days (10 half-lives of Ra-224). Activity then declines, as Th-228 decays faster than it is replenished by decaying Ac-228. About 3 years after separation, the activity is lower than at any other time except immediately after separation. From this point, activity increases until the second equilibrium state is reached in about 60 years. The initial buildup, decline, and second buildup of alpha and beta activity are shown by activity curves in Fig. 9-2. Since the alpha particle causes the highest ionization per unit length of path, it is this radiation which is of greatest concern in considering radiation inside the human body. Externally only the beta and gamma radiations are cause for concern.
The above indicates that the radiological hazards in processing thorium vary with the degree of separation, the concentration of the toxic members of the chain, and the interval between the processing steps. In general, the non-thorium fraction poses the more serious handling, storage and disposal problems.
Illustrations of specific situations may be of value. In the sulfuric acid treatment of monazite, most of the non-thorium fraction remains in the silica sludge. Another source of radioactivity connected with treatment of ore is the Rn-220 that is released. Radon-220 is not only hazardous itself, but decays rapidly into radioactive products; unless it is removed by a ventilating system, dangerous levels of radioactivity may be built up in working areas. The Rn-220 problem exists also in inadequately ventilated storage spaces containing thorium or its compounds.
The aqueous raffinate solution from the tributyl phosphate purification process also has relatively high concentrations of the radium isotopes. In precipitating thorium oxalate from a thorium nitrate solution on a production scale, approximately 70 percent of the Ra-228 usually remains in the filtrate. In the vacuum casting of thorium metal, a good share of the residual radium isotopes escape from the metal and deposit on the interior of the furnace.
If radioactive material enters the body, the thorium-type substances tend to settle in the liver, kidneys, spleen, lymph nodes and bone marrow, whereas the radium-type substances are more likely to be in the bone. Hence, the body receives radiation in both bone and tissue.
An example of the acute radiotoxicity of some of the
isotopes involved can be found in the work on Th-228 by Finkel and Hirsch. These authors state that Ra-228 is as toxic as Po-210, Pu-239 or U-233. The 20- to 30-day LD50 for these alpha emitters fall between 36 and 58 microcuries/kg and indicate a toxicity within the first month after injection twenty times as great as that for Ra-226. The 20- to 30-day LD50 value for Ra-224 was estimated at about 1000 microcuries/kg of body weight. On the activity basis, these LD50 values suggest that when thorium is separated from the radium isotopes, Ra-228 constitutes the main health hazard. This has been substantiated by other investigators.
The health hazard from the radioactive daughters of thorium was studied by Evans and Goodman and later by Albert. The tolerance levels were studied by them, by Morgan, and by German and Bouton. The tolerance levels for radiation exposure have been set by both the Bureau of Standards and the USAEC. One of the more recent derivations of tolerance levels for thorium and thorium daughter products (Table 9-1) is given by Healy.
9-3 Comparison with uranium. For purposes of better defining radiological hazards, thorium is often compared with uranium. The latter is better known, and there has been much more information developed for it. While the two are considered to be about the same order of hazard, they are dissimilar in many respects. Here, their radiological hazards will be compared.
Considering only parent or natural uranium and thorium freshly separated from its daughters, uranium has about three times the alpha activity of thorium. The U-238 parent, through alpha emission, becomes Th-234, a beta-emitter with a half-life of 24.5 days. Through beta decay, Th-234 becomes Pa-234, which decays, by beta-emission (half-life, 1.13 min) to U-234. After isolation of U-238, the equilibrium beta activity from Th-234 and Pa-234 is re-established in less than 1 year. At this stage of equilibrium, there are two alpha and two beta particles emitted per U-238 disintegration. A relative stability which exists for thousands of years is thus achieved, and equilibrium with the remainder of the uranium series is not re-established until about 10 half-lives of Th-230 have elapsed. This takes 800,000 years. While the decay rate of Th-232 is only one-third that of U-238, the number of alphas emitted per disintegration of Th-232 is, except for brief periods of time after separation, three times that of U-238. The total energy of the alpha radiation from thorium is 36.2 MeV. The total energy from U-238 plus U-234 is 9 MeV. Considering all factors (specific activities, total energies and percent of equilibrium attained), thorium deposited in the body appears to be a slightly greater radiological hazard than an equal quantity of uranium. Its slow clearance from the body makes it a potentially hazardous material.
It is the Th-234 and Pa-234 found with the uranium which account for the beta activity in natural uranium. The beta radiation from uranium in equilibrium with Th-234 and Pa-234 exceeds that from thorium in equilibrium with all of its daughters. Beta dose rates in contact with infinite sources would be approximately 240 and 115 mrem/hr for uranium and thorium, respectively. On the other hand, thorium gives considerably more gamma radiation than does uranium. Uranium dose rates from infinite sources have been estimated at about 10 mr/hr in comparison with 50 for thorium. Dose rates from the time of separation until the time equilibrium is achieved are lower, being related to, but not in direct proportion to, the percent or fraction of equilibrium activity achieved.
Here is a recent article I found that talks about the disposition of 3200 tonnes of thorium that was in the US stockpile. The thorium from Curtis Bay alone (2700 tonnes) would produce all the electricity the US needs for eight years, if used in a liquid-fluoride reactor. My own comments are in paretheses and italics.
Curtis Bay Thorium Nitrate Now in Nevada
By John Reinders
Defense National Stockpile Center
Public Affairs Office
The Defense National Stockpile Center’s thorium nitrate disposition project achieved a significant milestone in May when the last shipment of the radioactive material departed the Curtis Bay Depot. All material from the Curtis Bay, Md., storage site is now in Nevada and has been buried at the Department of Energy’s Nevada Test Site.
At the onset of the project, DNSC managed an inventory of over seven million pounds of thorium nitrate, stored at two DNSC depots – approximately five million pounds at Curtis Bay and another two million pounds at a depot in Hammond, Ind.
The thorium nitrate was originally acquired during the period 1957 to 1964. Ironically, the material was acquired on behalf of the Department of Energy’s predecessor agency – the Atomic Energy Commission. The thorium was acquired because of its potential use as a nuclear reactor fuel.
(It’s clear that they understood the value of thorium much better back in the 50s and 60s than we do now…back when Oak Ridge was building fluid-fueled thorium reactors to use this marvelous resource.)
According to Cornel Holder, DNSC administrator, the thorium nitrate relocation project has been a major success story.
“Had arrangements not been made to bury the material at the Nevada Test Site, we would have had to pay a lot of money to convert the material to a form for eventual disposition,” Holder said. “Conversion could have added another $40 million to the project.”
(Or we could have used the thorium to produce enough energy to run the United States for a decade…)
The project did not materialize overnight. Realizing in 1999 that there was no longer a Department of Defense need for the material and no viable commercial market, the project got its start with assistance of the Oak Ridge National Laboratory, private industry, academia and various government sources. In the first phase of the project, conversion, disposal and long-term storage of the material were considered.
(Perhaps it would be more accurate to say that you forgot what thorium could do, so you threw it away.)
A second project phase in 2002 was accomplished by private contractors with the support of ORNL. Material samples and containers were analyzed and tested to determine if the material met the waste acceptance criteria required for burial at NTS.
The third phase for actual transfer of the material began in August 2004 with an on-site audit by DOE’s National Nuclear Security Administration and a follow-up inspection by the Nuclear Regulatory Commission.
(Intense…they loaded the barrels on new trailers, drove to Nevada, uncoupled the trailers, and drove away. Intense.)
“This was the most complex project undertaken by DNSC in my nearly 28 years with the organization,” said Pecullan. “It involved many cooperative efforts among NNSA, NRC, DOE, the National Transportation Research Center, and private research agencies.”
(Sounds like bureaucratic intensity rather than technical intensity. I’m sure all the bureaucrats were making sure their hands would be clean from this filthy radioactive material.)
ORNL managed the thorium shipments under the auspices of the DNSC environmental office and with oversight from a contingency of stockpile employees from depots located at Binghamton and Scotia, N.Y., Hammond and New Haven, Ind., Point Pleasant W.Va., Warren, Ohio, Clearfield, Utah and Somerville N.J.
Pecullan pointed out that safe handling of the radioactive material has been another success story. “A total of 30,000 ‘no lost time’ work hours were expended on the Curtis Bay project,” he said. “More than 19,000 drums containing thorium nitrate were placed in DOE-approved containers and then transported by a total of 205 flatbed trucks to NTS.”
Upon arrival at NTS, the containers were placed in specially designated pits and were buried under 21 feet of top cover; with DOE accepting ownership and responsibility for the material.
(Could they at least tell us where the pit is so we can go dig it out when the time comes?)
The project cost, including packaging, transportation and disposal at NTS, is approximately $17 million.
(Money that could have been spent developing the thorium reactor technology that would free this nation from foreign energy–wasted on paperwork, meetings, trailer trucks and backhoe work… We probably spend that much money in five minutes on Saudi oil.)
As we approach the fourth of July and reflect on the declaration of America’s independence 230 years ago, it is not hard to see the shackles of dependence in many parts of our country. We are addicted to foreign oil, Chinese consumer goods, Japanese cars (which I love!), inexpensive Mexican labor, and foreign investors who underwrite our national debt at low interest rates.
In turn, these dependencies alter our course of action as a nation. I remember reading recently how Secretary of State Condoleeza Rice said that one of her biggest surprises of her new job was how much the need for foreign energy interferes with diplomatic activities. In other words–we can’t treat bad countries like bad countries because they have so much oil.
I am looking forward to a new “Independence Day”, a day marked by the independence of the United States on foreign countries (many of whom are hostile to our nation and its values) to provide the energy supplies we need. I truly believe that thorium is the fastest, safest, and most effective way to achieve this goal. I say this because thorium energy (through the fluoride reactor) can replace the baseload electrical generation currently provided by coal, gas, and solid-core uranium reactors. But thorium and the fluoride reactor can go further: we can use the high operating temperatures of the fluoride reactor to thermochemically generate hydrogen, which can then be used to hydrogenate coal (or other carbon-rich materials) to generate synthetic hydrocarbons. Eventually we can transition to an electric-car economy, or at least plug-in hybrid cars where the electricity is provided by thorium.
As I have noted in other posts, the thorium reserves of the United States are sufficient to provide for itself not only for tens of thousands of years, but for the entire world as well. And they have their own thorium should they desire it.
The alternative is rather bleak. An excellent text on the future we should expect if we proceed along the “business-as-usual” course is “Resource Wars” by Michael Klare. In this well-researched book, Klare describes the wars over petroleum, fresh water, and natural resources that we can expect in the next century. The lands of conflict probably won’t come as a surprise: the Persian Gulf, the Caspian Basin, the South China Sea, the Congo delta, and so forth. What they all have in common is that these conflicts flow out of continued dependence on petroleum and an ever-shrinking ability to produce it.
True independence is not found by increasing the US production of oil. Even if we produced all the oil we needed, oil is a globally-traded commodity, and we will always be dependent on those who are large producers (Saudi Arabia, Iran, Venezuela) to keep prices down. If any one of them turns off the spigot, so to speak, the global price skyrockets, and any US oil production goes overseas in search of better prices. That is why I take a dim view on drilling in ANWR or on the coasts–it’s not a solution, and it may not ever be much of a bandaid.
We have to get off the commodity altogether to free ourselves from dangerous oil-producing nations. And that will require access to an energy source of equal or greater magnitude. There are only a few options, and I think thorium is the best. Let us look forward to that day of energy independence.
As the realities of Kyoto emissions targets and the need for reliable energy continue to sink in, we will probably see more of this in Europe.
But I certainly don’t want to see more of this:
I find it rather disturbing that the “acceptable” alternative to nuclear is coal, and that these coal plants are “exempted” from the Kyoto restrictions.
I wish we had some thorium-fueled, municipal power submarines to sell them, rather than have them build these filthy coal plants.
I have found some old documents from the early 1970s that seem to indicate that Indian nuclear scientists were working with ORNL personnel on liquid-fluoride reactors. What initiated this interest or ultimately terminated it I do not know. Perhaps someone who knows more about the Indian nuclear program might offer some insight into what is in these documents.
Considering the Indian reserves of thorium, it would not be hard to understand why they might be interested in a reactor that could fully consume them. What perplexes me in the documents is their focus on starting the fluoride reactors with plutonium, which as I have noted previously, is a rather “dirty” way to start the reactor (leads to a lot more transuranic production than the alternatives).