Process for the conditioning of radioactive iodine, particularly

Details Process for the conditioning of radioactive iodine, particularly Process for the conditioning of radioactive iodine, particularly

1996-10-24

1998-01-20

Mai, Ngoclan

Hazardous or toxic waste destruction or containment

Destruction or containment of radioactive waste

By fixation in stable solid media

588 16, 588252, 588 15, 588 14, 252625, 264 60, 423240R, G21F 900

Patent

active

057110166

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to the conditioning or packaging of radioactive iodine, particularly iodine 129, which is a .beta. and .gamma. emitting fission product having a decay period of 1.6.10.sup.7 years.
Radioactive iodine is present in irradiated fuels from nuclear reactors. This iodine is released when said fuels are reprocessed. Thus, gaseous iodine occurs in the gases emitted by the irradiated fuel dissolving solution and iodine traces appear in aqueous effluents. As iodine 129 is toxic for humans due to its strong affinity for the thyroid gland, it is necessary to eliminate said iodine and store it on a definitive basis for a long time due to its very high period, although the specific radioactivity of iodine 129 is very low, because a high iodine 129 concentration would be dangerous to health. It is therefore vital to condition and store iodine 129 in a reliable matrix.
Existing methods for the trapping of iodine 129 lead to the obtaining of silver iodide, copper iodide, lead iodide or barium iodate. For storing the thus trapped iodine, several procedures have been studied and consideration has been given to the storage thereof in ceramic phases or in low melting point glasses, but a stable phase is still being sought for long term storage purposes.
The present invention relates to a block for the conditioning of radioactive iodine, particularly iodine 129, which uses as the confinement matrix a material having properties particularly appropriate for long term storage.
According to the invention, the radioactive iodine conditioning block comprises an iodoapatite of formula: iodine to be conditioned and x is such that 0.ltoreq.x<1.
In this block, the iodine is chemically trapped in an apatite structure, which has very advantageous properties for a long term conditioning.
Thus, apatites have the very interesting property of being able to integrate into their structure other elements and in particular different halogens such as iodine. Moreover, apatites have the following remarkable properties: decreases when the temperature increases, and that they are able to receive the non-radioactive xenon produced by the radioactive disintegration of iodine 129 without embrittlement or increasing the porosity of the conditioning matrix.
Natural fluoapatite complies with the following formula:
In this structure, numerous substitutions can be made and in particular the calcium can be replaced by various divalent cations such as cadmium, strontium, barium, lead, etc., the phosphate ions can be substituted by vanadate or arsenate ions and the F.sup.- anions can be substituted by monovalent anions such as I.sup.-. Due to the size of the I.sup.- anion, it is only possible to replace F.sup.- by I.sup.- in apatites complying with general formula I in which M is Cd or Pb, X is V or As and 0.ltoreq.x<1.
Thus, in the block according to the invention, the replacement of the phosphate groups of natural apatite by more voluminous VO.sub.4 or AsO.sub.4 groups leads to a significant increase in the lattice constants. This leads to an increase in the section of the tunnels of the apatite, because said section is directly linked with the value of the lattice constant a and this makes it possible to introduce into the tunnels an iodide ion, whose ion radius (2.20 .ANG.) is much larger than that of the F.sup.- or Cl.sup.- ions (1.33 and 1.81 .ANG. respectively) present in the natural apatite.
In the same way, the substitution of the Ca.sup.2+ cation of the natural apatite by a more voluminous cation such as Pb, leads to an increase in the lattice constants and facilitates the introduction of I.sup.- into the tunnels.
In the case of Cd.sup.2+, which has an ion radius (0.95 .ANG.) smaller than that of Ca.sup.2+ (1.00 .ANG.), the possibility exists to introduce I.sup.- in place of F.sup.- or Cl.sup.-, which may be explainable by the strong polarizability of the Cd.sup.2+ ion and also the presence of XO.sub.4.sup.3- ions, which are more voluminous than PO.sub.4.sup.3-.
As will be shown hereinafter, the block according to th

REFERENCES:
patent: 4088737 (1978-05-01), Thomas et al.
patent: 4229317 (1980-10-01), Babad et al.
patent: 4274976 (1981-06-01), Ringwood
patent: 5075084 (1991-12-01), Jurgen et al.
patent: 5193936 (1993-03-01), Pal et al.
patent: 5512702 (1996-04-01), Ryan et al.
Chemical Abstracts, vol. 108, No. 6, 8 Feb. 1988, Columbus, Ohio, US: Abstract No. 48173, Miyake "Fixation of Iodine Ions in Lead(2+)-Silver(1+) Exchanged Hydroxyapatites", p. 709.

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