EP0029875B1 - Process for fixation of radioactive krypton in zeolites for ultimate disposal - Google Patents

Description

The invention relates to a process for solidification of radioactive krypton ready for disposal, in which the noble gas is pressed into a zeolite matrix at elevated temperature and under high pressure and is kept in the cavities by cooling the matrix while maintaining the pressure.

During the reprocessing of irradiated nuclear reactor fuel elements, certain amounts of radioactive noble gases occur in the exhaust gases, in particular from the stripping of the fuel elements and from the subsequent dissolution of the nuclear fuel. So far, these have, as far as they have been separated from the exhaust gases, been filled into steel pressure bottles for transport to a storage facility that allows temporary storage. The self-heating, radioactive gas is in the pressure bottle at high pressure (e.g. more than 100 bar) and depending on the radioactivity inventory or cooling mode (e.g. natural air convection) at elevated temperature, e.g. B. 393 K. Thus, the container wall is constantly exposed to considerable tension. In the event of a cooling accident or failure, a container rupture or crack is possible, with the result that the entire radioactive noble gas inventory that is stored or in transit is released. The daughter nuclide of krypton (the fission noble gas consists mainly of krypton and xenon isotopes) is rubidium, a reactive alkali metal that can cause corrosion damage. The rubidium and certain impurities possibly present in the noble gas, such as e.g. As oxygen, water, etc., react with each other and form products such. B. Rb ₂ 0, RbOH, etc., which are considerably more corrosive than the molten alkali metal itself at the storage temperatures.

The major disadvantages of storing radioactive noble gases in compressed gas cylinders and the large krypton 85 inventories that will have to be stored for a long period of time in the future made it necessary to look for further methods for improved long-term storage of highly radioactive noble gases. For some years now, the solidification of radioactive noble gases in zeolites or molecular sieves has been investigated (certain artificial zeolites are called molecular sieves). Zeolites or molecular sieves have been used, for example, in the separation of mixtures of substances by means of gas chromatography (frequently repeated changes in adsorption and desorption processes). When solidifying radioactive noble gases in zeolites, however, desorption must be largely avoided because an increase in safety during transport and storage can only be guaranteed if the gas diffusion from the loaded zeolite is very low. The gas diffusion is essentially determined by the type of zeolite, which has a certain pore diameter, and the temperature. The temperature in the zeolite structure in turn depends on the zeolite loading with the radioactive gas and the heat transport through the inorganic matrix / gas phase. A large number of investigations were therefore applied to the selection of suitable zeolites and to the process conditions. Typically, molecules larger in diameter than the channels or pores of a given zeolite are not sorbed by them. However, it was found that by increasing the temperature from room temperature, for example to 770 K, the pores of certain zeolites, such as. B. 3 A zeolite or sodalite can be expanded and krypton at very high pressure, for. B. 2000 bar, can be pressed into these cavities of the crystal structure. If cooling is then carried out while maintaining the high pressure, the gas is enclosed in the cavities. The enclosed gas is then, in contrast to adsorption, not in equilibrium with the gas phase.

Under the title "An evaluation of methods for immobilizing Krypton-85" (ICP-1125, July 1977, Idaho Chemical Programs, especially page 13), D. A. Knecht u. a. also on the use of zeolite 3A for the immobilization of Kr-85. Based on experimental measurements, a net leakage of the sorbed Kr was predicted to be 35% in about 8 years when stored at 323 K. The desorption of Kr from loaded zeolite 3A is greater than that of sodalite.

• a) für Proben mit niedrigem Gehalt gegenüber Proben mit hohem Gehalt an adsorbiertem H₂0,
• b) für Proben mit hoher ursprünglicher Krypton-Beladung gegenüber Proben mit niedrigerer Belastung,
• c) für unausgelaugten gegenüber ausgelaugten Sodalith.

A number of differently produced, leached and unleached sodalite varieties were examined for their ability to include krypton or krypton-xenon mixtures (RW Benedict, AB Christensen, JA Del Debbio, JH Keller, DA Knecht: Technical and Economic Feasibility of Zeolite Encapsulation for Krypton-85 Storage; DOE Report No.ENICO-1011, Sept. 1979). The authors used temperatures between 670 K and 850 K and pressures between 1200 and 2000 bar in their work on encapsulating krypton. In order to assess the zeolites which can best be used for the encapsulation of krypton, the respective maximum loads and the respective temperature and radiation resistances with respect to gas diffusion from the were treated on untreated K-exchanged, Cs-exchanged, Rb-exchanged zeolites A and on different types of sodalite loaded zeolites (krypton leakages) and the result values compared. Krypton loads of 20 to 40 normal cm ³ / g of sodalite or zeolite A were found. The load values for leached Sodalite was higher than for unleached sodalite. Krypton leakage measurements were carried out at temperatures between 570 K and 775 K for short times (approx. 2 to 24 hours) and at a temperature of 423 K for long times (approx. 1 to 12 months). The leakage rates were lower:

• a) for samples with a low content compared to samples with a high content of adsorbed H ₂ 0,
• b) for samples with a high original krypton load compared to samples with a lower load,
• c) for unleached versus leached sodalite.

Benedikt et al. concluded from the investigation results that for unleached sodalite with a krypton loading of approximately 20 cm ³ / g and with low amounts of adsorbed water, a 10-year leakage of krypton at a final storage temperature of 423 K of less than 0.1 % can be predicted.

Taking into account the Kr-85 decay heat, sodalite (with the formula Na ₂ 0 x A1 ₂ 0 ₃ x2 Si02 x 2.5 H ₂ 0) seemed to be thermally sufficiently stable after loading with noble gas in order to be technically possible without using one To ensure the immobilization of Krypton-85 over a period of 100 years, only very complex pore closure with a radiation-resistant resin that can still be found out, possibly to be accomplished in the roll-off or fluidized bed process. However, the long-term thermal stability at temperatures above 423 K, which was theoretically determined on the basis of the activation energy for gas diffusion from the zeolite by extrapolation, could not be confirmed experimentally. Experiments with sodalite samples which were loaded with argon (instead of krypton, the cheaper argon was used for the experiments, since the kinetic atomic diameters are very similar: krypton 0.39 nm and argon 0.37 nm) have shown that Even at 473 K the thermal stability of the loaded sodalite samples was insufficient. Sodalite loaded at 30.5 normal cm ³ Ar / g had already released 52% of the enclosed gas at 473 K after 1080 hours. This undesirable effect for final storage can only be counteracted by restricting the loading or using a pore-closing resin. A low load is associated with a loss of economy and increased waste volumes. The homogeneous embedding of highly active, hot compacts in a resin is a technically difficult undertaking. In addition, the loading conditions recommended as optimal, for example at a temperature of 773 K and a pressure of 2000 bar, are unpleasant when working with high inventories of radioactive gases. Since the use of at least one compressor is required, the effort to keep operational leaks at the devices low is considerable. A high-pressure system that is complicated in terms of safety is a requirement.

The invention is therefore based on the object of providing a method with which large inventories of radioactive noble gases which are produced in the future can be solidified in such a way that they are not released from the repository matrix containing them even at temperatures of 473 K and above. Furthermore, it is an object of the invention to fix the largest possible amount of noble gas per unit weight of repository matrix. Of course, all disadvantages of the previously known methods for solidifying noble gases are to be avoided at the same time using the method according to the invention.

wobei M ein Metall aus der Gruppe Mg, Ca, Ba oder Sr bedeutet, eingesetzt wird und die Matrix zum irreversiblen Einschluß des Kryptons während des Einpressens auf eine Temperatur über 720 K erhitzt wird.The object on which the invention is based is achieved in a surprisingly simple manner according to the invention in that an alkaline earth-substituted 5 A zeolite of the general composition is used as the matrix

where M is a metal from the group Mg, Ca, Ba or Sr, and the matrix for the irreversible inclusion of the krypton is heated to a temperature above 720 K during pressing.

• a) den Zeolithen in ein Behältnis bringt, das Behältnis verschließt,
• b) das verschlossene Behältnis auf einen Druck unter 1 mbar evakuiert,
• c) danach das Edelgas in das evakuierte Behältnis einbringt und bei einer Temperatur im Bereich von 720 K bis 870 K und unter einem Druck im Bereich von 200 bar bis ca. 2000 bar in den Hohlräumen des Zeolithen fixiert und
• d) schließlich den mit dem Edelgas beladenen Zeolithen in an sich bekannter Weise abkühlt.

Using the zeolite selected according to the invention, the implementation of the method according to the invention is characterized in that

• a) puts the zeolite in a container that closes the container,
• b) the closed container is evacuated to a pressure below 1 mbar,
• c) then introduces the noble gas into the evacuated container and fixes it at a temperature in the range from 720 K to 870 K and under a pressure in the range from 200 bar to approx. 2000 bar in the zeolite cavities and
• d) finally cooling the zeolite loaded with the noble gas in a manner known per se.

The fixing action of the alkaline earth-substituted zeolites of type 5 A, as used in the process according to the invention, for the noble gases argon and krypton was surprising, because these zeolites have x-cavities with pore openings of around 0.5 nm, which are considerably wider than the kinetic ones Diameter of the noble gas atoms. However, these zeolites have ß cavities with pore openings around 0.22 nm, which have not previously been considered for the noble gas inclusion. It is therefore assumed that the noble gas atoms pass through the large pores while the process according to the invention is being carried out a-cavities get into the ß-cavities and are retained there thermally stable. With the loading tests carried out to date (up to 2000 bar), argon loads of up to 57 Ncm ³ / g (based on the loaded zeolite mass) of zeolite could be achieved. Examination of the thermal resistance of alkaline earth zeolites of type 5 A, which were loaded with krypton, showed that within the experimental accuracy after 2520 hours at 473 K and after 3500 hours at 673 K no gas was released. The loading of the zeolites was determined before and after the heat treatment. The accuracy of these determinations was ± 5Q / o. Additional tests with a relatively high heating rate (at the beginning about 50 K / min, from 870 K sinking to about 20 K / min) showed that the diffusion of the krypton out of the zeolite only began at about 1080 K. Between 1080 K and 1180 K, however, only about 1 to 3% of the total load was released (after about 16 to 20 minutes). The majority of the enclosed gas only escaped from the crystal structure in the temperature range between 1180 K and 1380 K (after 20 to 29 minutes). A comparative sample of leached sodalite loaded with krypton was subjected to the same temperature treatment. Degassing began after only 7 minutes, ie at a temperature of 675 K. The majority of the noble gas escaping under these conditions was released between 775 K and 1180 K.

The alkaline earth zeolites which can be used in the process according to the invention are resistant to y-radiation. Samples containing fixed argon and exposed to a y dose of 10 ⁶ J / kg showed no significant changes. Likewise, loaded samples that had been stored in water for several days were found to be stable with regard to gas fixation.

• a) eine Verminderung der Gasdiffusion aus der beladenen Zeolith-Matrix, so daß auch bei hohen Beladungen auf porenschließende Verfahren (Harz, Glas usw.) verzichtet werden kann und gleichzeitig noch eine Zunahme der Sicherheit sowohl während des Transportes des verfestigten radioaktiven Gases als auch während der Lagerung gewährleistet ist;
• b) das Einpressen des Edelgases bei relativ niedrigem Druck (beispielsweise bei weniger als 600 bar);
• c) die Fixierung ohne Kompressor bzw. unter Anwendung einer Kombination Kryo-/Hochdruckautoklav und demzufolge Reduzierung potentieller Leckagequellen und Verminderung des freien Inventars an radioaktivem Edelgas;
• d) die Rückgewinnung des nach der Einpressung nicht fixierten, beispielsweise in den Leitungen der Vorrichtung sich befindenden Spaltedelgases durch Kryo-Pumpwirkung.

Experiments were also carried out to determine the distribution of the amount of noble gas injected into a zeolite over the α and β cavities. The loading was carried out in a temperature range from about 710 K to 810 K, the maximum loading at 770 K being about 50 Ncm ³ / g. It has been found that the loading of the a-cavities begins at 710 K, soon increases steeply, and then slowly drops again at about 730 K to a temperature of about 780 K, at which the loading of the a-cavities is practically zero is. Although the loading of the ß cavities at this temperature is slightly below the maximum (approx. 43 Ncm ³ / g, based on the loaded zeolite), this temperature can be regarded as the optimal loading temperature under the given test conditions. The only loading of the ß cavities in zeolites creates the conditions for

• a) a reduction in gas diffusion from the loaded zeolite matrix, so that even at high loads pore-closing processes (resin, glass, etc.) can be dispensed with and at the same time an increase in safety both during the transport of the solidified radioactive gas and during storage is guaranteed;
• b) the injection of the noble gas at a relatively low pressure (for example less than 600 bar);
• c) the fixation without a compressor or using a combination cryo- / high pressure autoclave and consequently reduction of potential leakage sources and reduction of the free inventory of radioactive noble gas;
• d) the recovery of the split noble gas, which is not fixed after the injection, for example located in the lines of the device, by means of a cryogenic pump effect.

Instead of the mechanical compression previously required, the gas in an autoclave can either be brought up to a pressure that is 3.3 times higher by simply increasing the temperature or by using the cryogenic pump principle to even higher pressures. Another advantage of the method according to the invention is the reduction in material stress, which is achieved by reducing the pressure in the method according to the invention compared to the prior art method.

In the following the invention is illustrated by some examples and experiments. However, the invention is not restricted to the examples given. The alkaline earth zeolites mentioned in the examples are commercially available products from various manufacturers or distributors (but all 5 A zeolites), the product names of which do not indicate the chemical composition. For this reason, the designations Z 1 to Z 6 were used for the zeolites usable in the process according to the invention without reference to the manufacturer or distributor.

example 1

Zeolite Z 3 was loaded with krypton at a temperature around 823 K and a pressure of 210 bar. The loading achievable under these conditions was 17.2 Ncm ³ / g loaded zeolite. To determine the thermal stability, the loaded zeolite was stored at a temperature of 673 K for 3500 hours.

The subsequent repeated determination of the krypton loading showed that no gas had escaped under these conditions.

Example 2

After pretreatment at 420 K to 470 K, several samples of zeolite Z 5 were loaded with argon at approx. 620 bar and 823 K in vacuo. The thermal stability of the loaded zeolite samples was examined after different service lives at two different storage temperatures by determining the load again. The samples, which were exposed to a storage temperature of 473 K, showed a practically unchanged argon loading after a standing time of 1080 hours and after a standing time of 2520 hours. The differences in the result values obtained were within the experimental accuracy. Even the samples, which had to withstand a storage temperature of 673 K, showed no argon losses after a standing time of 160 or 763 hours.

Example 3

Samples of zeolite Z 6, which were loaded with argon at 260 bar and 773 K, showed neither a reduction in the noble gas loading after 1080 hours at 473 K nor after 160 hours at 683 K.

In contrast, a zeolite of the designation 3 A, which cannot be used in the process according to the invention, had an argon loss of 57% of the original loading after a storage time of 1080 hours and a storage temperature of 473 K with a loading of 42.6 Ncm ³ / g zeolite. A sample of the zeolite 3A with the same load (42.6) was exposed to a storage temperature of 673 K for 17.5 hours. The argon loss determined afterwards by determining the load again was 88%. Sodalite samples showed a similar behavior with a loading of 30.5 Ncm ³ / g loaded zeolite: after 1080 hours at a storage temperature of 473 K, 52% of the argon had escaped and after 15 hours at a storage temperature of 673 K even 96% of the original loading.

Example 4

• Z 1 - 49,0 Ncm³/g beladener Zeolith
• Z 2 - 44,3 Ncm³/g beladener Zeolith
• Z 3 - 38,4 Ncm³/g beladener Zeolith
• Z4 - 37,4 Ncm³/g beladener Zeolith
• Z 5 - 36,0 Ncm³/g beladener Zeolith
• Z 6 - 29,0 N_CM ³/g beladener Zeolith

Zeolites of different origins were examined under the same conditions and their loading values measured. After pretreatment at 425 K to 475 K in vacuo, krypton was pressed into the zeolite samples under a pressure of 1000 bar at a loading temperature of 770 to 795 K. The following load values resulted:

• Z 1 - 49.0 Ncm ³ / g loaded zeolite
• Z 2 - 44.3 Ncm ³ / g loaded zeolite
• Z 3 - 38.4 Ncm ³ / g loaded zeolite
• Z4 - 37.4 Ncm ³ / g loaded zeolite
• Z 5 - 36.0 Ncm ³ / g loaded zeolite
• Z 6-29.0 N _CM ³ / g loaded zeolite

The loading values become higher with increasing loading if they are related to the unloaded zeolite. While the value 20 Ncm ³ / g, based on the loaded zeolite, gives the value 21.6 Ncm ³ / g, based on the unloaded zeolite, the loading value increases from 60 Ncm ³ / g loaded zeolite to 77.4 Ncm ³ / g unloaded zeolite. The last stated value was obtained at a pressure of 2500 bar.

Comparing the Benedict et al. working conditions recommended for the application of their process, namely at temperatures of 850 K or above and at pressures of 1660 bar or above to achieve a load of 20 cm ³ krypton per gram of zeolite, with the operating conditions of the process according to the invention which are required to achieve a To obtain a loading of 20 Ncm ³ krypton per gram of unloaded zeolite, the serious advantages of the process according to the invention become clear: All that is required is a loading pressure of approximately 300 bar at a temperature of 793 K.

Trial 1

Zeolite Z 3 loaded with 37.2 Ncm ³ krypton per gram was exposed to a y-radiation dose of 1.75 by 10 ^g rad. The loaded zeolite was irradiated in neon, the irradiation time was about 2 months. The analysis of the gas phase after the irradiation showed that only a very small amount of krypton (0.009%) had escaped from the zeolite matrix, which is presumably due to non-optimal loading conditions (slight x-void contribution). The determination of the krypton loading of the zeolite after the irradiation showed no discernible krypton loss, the value was within the experimental accuracy.

Trial 2

• Ein mit 37,4 Ncm³ Krypton pro Gramm beladener Zeolith Z4 wurde bei Raumtemperatur ungefähr 750 Stunden in Wasser gelagert. Nach einer 12 Stunden dauernden Trocknung in einem Ofen bei 423 K ergab die erneute Bestimmung der Beladung 36,9 Ncm³ Kr/g, d. h. der Beladungswert blieb innerhalb der experimentellen Genauigkeit, ein Kryptonverlust konnte nicht nachgewiesen werden.

Investigation of the influence of water storage on gas diffusion from the loaded zeolite:

• A zeolite Z4 loaded with 37.4 Ncm ³ krypton per gram was stored in water at room temperature for about 750 hours. After drying in an oven at 423 K for 12 hours, the determination of the load again resulted in 36.9 Ncm ³ Kr / g, ie the load value remained within the experimental accuracy, a loss of krypton could not be detected.

• Wird in eine 50-I-Druckgasflasche 1 Normalkubikmeter Krypton gefüllt, so errechnet sich der Druck auf die Flaschenwand zu 22,6 bar. Bettet man die qleiche Krvp_tonmenqe in einen 5 A-Zeolithen ein, so erhält man für das Volumen des beladenen Zeolithen bei einer Beladung von 21,6 Ncm³/g ein gegenüber dem Volumen der Druckgasflasche nur gering erhöhtes Volumen von 66,1 1, bei einer Beladung von 47,1 Ncm³/g ein Volumen von nur etwas mehr als die Hälfte des Druckgasflaschenvolumens, nämlich 30,4 und bei einer Beladung von 77,4 Ncm³/g etwa
  
  des Druckgasflaschenvolumens, nämlich 18,51. Bei der Aufnahme von 3 Normalkubikmeter Krypton ist also das Volumen des beladenen Zeolithen etwa gleich dem Volumen einer Druckgasflasche, die aber in diesem Falle unter einem Druck von 71,4 bar steht. Das 11/2fache Volumen eines mit 77,4 Ncm³/g beladenen Zeolithen gegenüber dem Volumen einer Druckgasflasche entspricht etwa 4 Nm³ Krypton bei einem Druck von 102 bar in der Druckgasflasche.

Comparison between the storage of krypton in compressed gas cylinders and the embedding of krypton in alkaline earth zeolites of type 5 A:

• If 1 normal cubic meter of krypton is filled into a 50 liter pressurized gas bottle, the pressure on the bottle wall is 22.6 bar. Embeds to the qleiche Krvp _t onmenqe in a 5 A-zeolite, a, is obtained at a loading of 21.6 Ncm ³ for the volume of the loaded zeolite / g, a relative to the volume of the compressed gas cylinder only slightly increased volume of 66.1 1 , with a load of 47.1 Ncm ³ / g, a volume of just a little more than half of the compressed gas cylinder volume, namely 30.4 and with a load of 77.4 Ncm ³ / g, for example
  
  of the compressed gas bottle volume, namely 18.51. When 3 normal cubic meters of krypton are taken up, the volume of the loaded zeolite is approximately equal to the volume of a compressed gas bottle, which in this case is under a pressure of 71.4 bar. The 11/2 times the volume of a zeolite loaded with 77.4 Ncm ³ / g compared to the volume of a compressed gas bottle corresponds to approximately 4 Nm ³ krypton at a pressure of 102 bar in the compressed gas bottle.

Claims (2)

1. Method of solidifying radioactive krypton so as to make it ready for ultimate storage with the noble gas pressed into a zeolite matrix at elevated temperature and high presure and kept in the cavities of this matrix while maintaining the pressure and cooling the matrix, with the matrix used being an alkaline earth substituted zeolite 5 A of the general composition

M being a metal of the Mg, Ca, Ba or Sr group, and with the matrix being heated to a temperature above 720 K during the process of pressing in order to assure an irreversible inclusion of krypton.

2. Method according to claim 1 with

a) the zeolites being transferred into a canister and the canister sealed;

b) the sealed canister being evacuated to a pressure below 1 mbar;

c) the noble gas subsequently being introduced into the evacuated canister and fixed in the cavities of the zeolite at a temperature of 720 K to 870 K and a pressure of 200 bar to approx. 2000 bar; and

d) finally, the zeolite loaded with the noble gas being cooled down in a familiar way.