In recent years, a confluence of factors in Canada and in the Netherlands resulted in a world wide shortage of radioisotopes for medical imaging and treatment.
In Canada, the Chalk River reactor was shut down for repairs and upgrades. The reactor is 52 years old.
Medical Isotopes from this reactor typically produce radioactive isotopes used in the treatment of 27 million patients per year providing roughly half of the medical isotopes used in the entire world.
The reactor was found to be leaking radioactive liquid on August 25, 2009 and it was shut for repairs until August 19, 2010, when it was restarted.
The effects of the closure for more than one year will be felt for many years to come, since the effective result of the shortage is that many cancers and heart syndromes were not diagnosed in the outage period and as a result, many people who might have been saved will, um, die.
One of the more important isotopes produced at Chalk River is technetium-99m.
Technetium is one of two elements in the first 82 elements that is not normally found one earth except in extremely tiny concentrations in natural uranium ores, where it is formed by spontaneous fission of U-238 and U-235. (The other such element is promethium.)
All isotopes of technetium are radioactive (as are all isotopes of promethium.)
(I have written in this space at some length on technetium in this space, including in a diary that I am most proud of writing here: The Deformed Nucleus: Neptunium and the Rain, a diary about Lise Meitner.)
The most commonly available isotope of technetium today is Tc-99, which has a half-life of around 211,100 years. I have estimated that the world supply of this interesting and potentially extremely valuable element, obtained chiefly from the fission of uranium in commercial nuclear reactors, is on the order of 80 tons.
This is not the isotope, however, that is used in medical treatment and diagnostics. That isotope is Tc-99m, which is a nuclear isomer of the Tc-99 obtained by nuclear fission.
A nuclear isomer is a nucleus that has the same atomic number and the same mass number as another nucleus, but which differs very slightly, according to the famous Einstein equation E = mc2 in actual mass because it has a slightly different arrangement, according to the laws of quantum mechanics, of neutrons and protons in energy levels.
(Those who are familiar with quantum mechanics should note that the statistics for some nuclei are Bose-Einstein statistics and not Fermi statistics, and thus nuclei are not compelled to obey the Pauli exclusion principle that applies to electrons in electronic shells. Technetium-99 isotopes are, however, fermionic, but Tc-98, with a half-life of 4.2 million years - it is not a fission product and does not accumulate in nuclear reactors - is a boson. Still, nuclear shells are a whole other ball of wax than electrons. Specifically, it is possible for many bosons to occupy an orbital of the same energy, whereas fermion orbitals can be occupied only by zero, one, or two orbitals. Thus nuclear transitions can be, um, sloppy compared to the electronic transitions that all chemists know and love.)
Unlike, technetium-99, Tc-99, technetium-99m, Tc-99m has a relatively short half-life, just 6.01 hours.
Tc-99m decays via an isomeric transition to Tc-99. A very, very tiny fraction of the Tc-99 found on earth actually derives from medical treatment. A physician or a technetian working for a physician injects a patient with Tc-99m which, because of its short half-life is a very prodigious producer of gamma rays. These gamma rays can be used for things like killing cancer cells, or producing an image of a blood vessel or an organ for a detector.
After a few days, all of the Tc-99m has decayed to Tc-99 (or in very small amounts to ruthenium-99) whereupon it ends up in patients toilet bowls when they pee it away.
Isn't that nice?
Many, many, many lives have been saved using Tc-99m, and apparently some were lost when there was a shortage of this isotope. The number of lives lost is almost certainly infinitely greater than the number of lives lost at Three Mile Island, the 1979 nuclear accident that anti-nukes like to talk about endlessly, usually with massive dollops of ignorance and superstition. (One cannot actually produce this ratio, since the laws of mathematics do not allow one to divide by zero.)
The paper from the primary scientific literature I will reference today comes from Pakistani scientists working at the Pakistan Atomic Energy Commission, PINSTECH. The reference is Nuclear Engineering and Design 239 (2009) 521–525. The scientists in question are Atta Mohammad, Tayyab Mahmood∗, Masood Iqbal.
Tc-99m is not often obtained directly, since one would have to ship it from a reactor to a patient and realistically this process would involve times much longer than the half-life of the isotope.
Instead, the isotope obtained is radioactive Mo-99, which is a fission product. This isotope has a half-life of 65.94 hours, a little under three days, making it possible to ship it all around the world by aircraft. Mo-99 decays radioactively to Tc-99m and as a result comes into equilibrium with it. It can be shown that after about 22 hours after the isolation of Mo-99, the amount of Tc-99m will reach a maximum value, which is about 4.38% of the Mo-99m that exists at that point.
Scientists can therefore "milk" Mo-99 loaded on a column for the Tc-99m present, do a little other chemistry in the next hours, inject a patient before it decays, and thus accomplish the treatment.
The Pakistani scientists in question have worked on this project.
Here is some excerpts from the paper in which they discuss this work:
99mTc radioisotope is widely used radioisotope in nuclear medicine. Currently it is being produced from the decay of 99Mo. The isomeric state of 99mTc is used in nuclear medicine procedures. This isomeric transition state is shown in decay scheme of 99Mo (Fig. 1) (Subramania et al., 2003). 99Mo can be produced in research reactors by (n,γ) reaction with 98Mo or by fission of 235U(n,γ) reaction. Activation method is rather simple and inexpensive but gives low specific activity. On the other hand, fission method is complex and expensive but gives high specific activity. At present nearly the entire demand for 99Mo is fulfilled by means of thermal neutron induced fission of highly enriched uranium (IAEA, 1999). However, activities are underway throughout the world to switch over from HEU to LEU fuel. Ahmad et al. (2008), have performed calculations for 99Mo production at Pakistan Research Reactor-1 (PARR-1) using a cylindrical target based on the LEU 235U foil. In that study it was concluded that with annular target, 99Mo can be produced safely at PARR-1. In this paper, Studies have been carried out for the production of 99Mo utilizing plate type LEU target (Figs. 2 and 3). In addition activity calculations have also been performed in the spent fuel of the target plates. In the current study, neutronic analysis (reactivity effect, neutron flux, heat flux) and activity calculations of 99Mo along with radioactive waste have been carried out. These calculations have been performed to evaluate the safety of LEU plate type target and estimate the time of irradiation for 100 Ci production of 99Mo. Present country demand of 99Mo is 100 Ci.
Present country demand?!? Oh well then...
Here's some more technical mumbo jumbo that you probably couldn't care less about, but the issue is the effect of placing the target in the reactor on the reactor's power level, to wit:
Reactivity effect of irradiation was studied by placing the target at five different axial positions in C-7 location. Excess reactivity in the core for these caseswas calculated and tabulated in Table 2. It is clear from this table that maximum excess reactivity of 278.32pcm is present in the core when target is placed at the fourth plane from top of the core. This is due to maximum thermal neutron flux at this location of the reactor core. Transient analysis of the core indicates that reactor is safe under the transient of 500pcm (FSAR, 1999). Therefore reactor remains safe even if the transient is introduced by rapid insertion of target at its most sensitive position. It has been observed that location of maximum power density in the Fig. 7. Average axial thermal neutron flux distribution in irradiation locations at PARR-1. Fig. 8. Thermal neutron flux at fourth plane from top in C-7 location. core remains unchanged by placing the target at 1st axial plane. Location of peak power density has been computed through CITATION and is found to be at 41cm from top of the core in B-6 element, which is the fuel element located at the corner of centralwater box facility. However, maximum power density shifts in the target, if irradiation is performed at any of the other four axial positions of centralwater box facility. This effect is due to high thermal neutron flux availability at these positions but control rods effect is dominant at 1st axial plane. Depression in thermal neutron flux due to control rods effect at upper part of the core can be seen in Fig. 7.
The business end is here:
The time required for the production of about 100 Ci 99Mo at PARR-1 at fourth position from top in irradiation location (C-7) would be 7.1 h. This time of irradiation Fig. 9. Activity of 99Mo and waste produced as a result of fission in target. Routine full power operation schedule of PARR-1 related to radiopharmaceutical production is fortnightly for 18 h. Fig. 9 shows the fission product, 99Mo and actinide activity produced during irradiation and decay with time. It can be seen from figure that the total activity will be less than 1 Ci after 100 days.
Well that's good.
Now I feel better. Pakistan can produce radioisotopes if I need them, not that I do.
While the Chalk River Reactor in Canada was down, by the way, the other major medical isotope producing reactor, the Petten reactor in the Netherlands was also experiencing problems. It is also now repaired and now functioning normally. We're, um, saved.
Have a nice day.