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RESEARCH PROFILES Detection of
36
Cl in nuclear reactor waste
JUPITERIMAGES
normal waste, it will be quite cheap,” alkali fusion at high temperatures with When most people think of radioactive says Hou. NaOH and Na2CO3 to a fused cake, waste, elements such as uranium, plutoOf the materials tested in this paper, which is then leached with water to exnium, or even americium come to mind. only graphite has a previously reported tract the chloride. Very few would immediately name chlomethod for extraction of 36Cl. In that For all the materials, the solutions rine as a problematic element for racontaining the chloride are precipitated dioactive waste disposal. But one of case, the chloride was leached from the as AgCl. At this point, the scichlorine’s radioactive iso36 entists faced another chaltopes, Cl, has a particularly long half-life (hundreds of lenge. Hou explains, “In most thousands of years) and is papers, they do the measure36 extremely water-soluble, ment of the Cl by just dissolving the silver chloride in a making it a potential longvery high concentration of term threat to the environammonium solution and then ment if it is not disposed of 36 doing the measurements, but properly. For Cl content to be determined accurately, . . . if the chloride content is it must first be separated high, you need quite a lot of from its matrix and from ammonium to dissolve it.” In other radionuclides, which addition, the dissolved silver is can be a difficult task if the not very stable over the time 36 Cl is embedded in the period necessary for the liquid components of a nuclear rescintillation counting process actor. In the April 15 issue and tends to precipitate. “If of Analytical Chemistry (pp there are particles formed in When this nuclear reactor is decommissioned, Hou and colleagues’ 3126 –3134), Xiaolin Hou the solution, it will change the methods could help authorities safely dispose of the construction and colleagues at the Techcounting efficiency so you materials. nical University of Denmark cannot get a very good represent a new set of methsult,” says Hou. 36 graphite by acid, but Hou’s group had ods for extracting and quantitating Cl The investigators solved this problem difficulty reproducing the results. “We from the steel, heavy concrete (concrete by separating the chloride from the silfound that very little of the chloride can ver by ion exchange chromatography. that contains BaSO4), aluminum, lead, and graphite of a decommissioned nube leached out from the graphite [beThis process had the added bonus of clear reactor. cause] the chloride exists inside of the removing other radionuclides from the 36 Cl is not a direct part of the energraphite grains,” Hou explains. mixture, and it improved the counting gy-generating nuclear reaction. Instead, Instead, the authors describe a proce- efficiency by 2–33. “We get a very high counting efficiency—almost 100%. This it is formed primarily when stray neudure for the complete decomposition of is impossible if you just use the ammotrons bombard the stable isotope 35Cl graphite by heating it with a mixture of nium to dissolve the silver chloride,” H2SO4, HNO3, and HClO4 until the that is present as an impurity in the Hou says. solid completely dissolves. Hou says, graphite rods, cooling water, and conSince these procedures were devel“The important thing is to use this mixstruction materials of the reactor. The oped, Hou and colleagues have begun majority of the 36Cl remains in the reac- ture of acids, because if you just use the to use their methods on samples from HNO3 or the H2SO4 or a mixture of tor until it is taken off-line and decommissioned. At that point, authorities these two acids without the HClO4, the two decommissioned Danish nuclear reactors. They have found that 60–80% of must decide whether to classify its comgraphite cannot be dissolved.” the concrete waste can be treated as ponents as radioactive or normal waste, For the other materials described in nonradioactive and disposed of cheaply. so accurate methods for measuring 36Cl the current work, the researchers broke Their methods will likely become more and other radionuclides in a solid matrix new ground with their extraction techand more important in the future as nuare critical. “This is very important, beniques. The metals are dissolved by clear reactors are shut down and decause if the waste is estimated as raacidic heating in a method very similar commissioned worldwide. a dioactive waste, it costs quite a lot to to the one for graphite. For the heavy dispose of it. [But] if it is classified as concrete, the material is converted by —Jennifer Griffiths M A Y 1 , 2 0 0 7 / A N A LY T I C A L C H E M I S T R Y
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