DE2511112B2 - Process for the decontamination of surfaces of metallic materials - Google Patents

Description

The invention relates to a method for decontaminating surfaces of metallic materials, in which the surfaces are brought into contact with molten salt.

HT Fu11am reported on investigations on radioactively contaminated stainless steel samples [report of the Battalc Pacific Northwest Laboratories, Richland, Washington, V. St. A., with the number BNWL-B-277 (July 197.1) J. The stainless steel samples were for this purpose with a three year period old fission product solution treated, so that after the treatment, contamination of the surfaces of the samples by the radio nuclides Sr-WJ, Ru-HKi, Ag-I IDm. Sb-125, Cs-134, Cs-137, Cc-144 and Zr-W could be detected. To remove these radionuclides, a method for salt contamination was investigated, among other things. The contaminated stainless steel samples were placed in an aluminum oxide table, covered with a layer of one or more inorganic salts and heated to the test temperature in an oven. The most effective melts for decontamination have been oxidizing salt melts containing NaNO, or Fc ₂ O. The tests were carried out in a temperature range between 4 (H) ° and 900 ° C, but most of the tests were carried out at K (K) "C. In most cases, the temperature was measured over a period of four hours (contact time) or less constant. For many of the salts used, particularly those containing nitrates, salt decomposition and salt volatilization was a problem at the test temperature.

Elsewhere in this report it says: In order to be able to decontaminate effectively, one must oxidizing molten salt can be used, which corrodes a substantial part of the metallic phase. It was found that the total decontamination factor (DF), which is a number factor for indicating the effectiveness of the procedure in the event of a

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one-time contact of the metal surface _with the molten salt depends to a large extent on the Sate-CT-MetaH weight ratio used. With an original sate-to-metal weight ratio of 3; ' For example, a DF of approx. KMH) was obtained for the first Jn contact. The melting point of NaNO is 307 "C. The thermal decomposition of NaNO begins above the melting point, releasing oxygen and nitrogen oxides At 8 (K) ° C a corrosion rate of approximately 60 mg - cm ² · h '' was found.

The investigations showed that even when the molten salt was repeatedly brought into contact with fresh, contaminated metal surfaces and is optimally recycled, a salt-to-metal weight ratio of 3: 1 or greater is required to achieve a DF of JiK *. II. T. Fullam also emphasizes that this method has a large volume of contaminated SaI /. generated and thus the practical application is restricted from the start.

The disadvantages of the Fullam process are large accumulation of radioactively contaminated waste that has to be solidified for transport and for Final storage, the relatively strong corrosion of the metal surfaces, the high temperature (/ .. B. X00 ° C), the relatively long contact time (at least 1 to 2 hours), as well as the nitrogen oxides that endanger operational safety, including radionuclides can carry with you.

It was also proposed that control rods made of hafnium metal, which were used in a pressurized water core reactor, and were used in this case at their Surface with Snaltprodukl-Nuklidcn, such as / .. B. Co-W), had been contaminated to cut and immersing the Hf pieces in a NaOH melt, removing the radioactive contamination from the Melt is absorbed | K. L. Rohde, R. R. Hammer: “Decontamination of Hafnium Metal Control Rods ", pages 94 to 97, in J. R. Bower: "Chemical and Process Development Branch Annual Report, Fiscal Year 1965", USEAC report Idaho Operations Office, Idaho Falls, V. Sl. A., with No. I DO-14Mi I). Part of an irradiated one was used to establish a decontamination process Hafnium control rods used, animal approx. 9Λ years had been stored long after it was removed from the reactor. Analysis showed a tantalum concentration of less than 0.07% by weight. After the long storage period, the main component of the radioactive fission product nuclides to be removed was Co- (SO, the other nuclides were only present in such small quantities that they were not needed for measurements Role played more. The Dckonlaminalions-efficacy of a NaOH melt could therefore only a nuclide, namely Co-W). In addition, the C'o-60 activities originally present before the contamination treatment were not very high: depending on the sample, in the range between 620 and 45X0 decays / second. The treatments were varied in these investigations so that a temperature V (Mi 450 ", 550" or 570 ° C and a temperature V (Mi 450 ", 550" or 570 ° C) Coordination time between 10 and 90 minutes applied

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became. One attempt (of <j attempts) was accepted. As a result, the treatment is subdivided into three procedural steps compared to the Wsd, from the well-known SaJj ^ hmeten-DekqntaminatiQnsvera) 2 minutes 45O'C, b) 3P minutes 350 ° C and c) drive, the low amounts of radioactive 30 minutes 550 ° C, In another experiment in potentially contaminated salts, two steps arise and solidify: 10 minutes 450 ^o C, 80 minutes 350 o s or must be eliminated dropped below 10%, in two cases even below 1%. Certain particle size ground (between 0.04 - to remove the NaOH residues from the Hf surface and 0.08 ram) and can be applied below 300 ° G, the samples in dilute hydrochloric acid. They meet the requirement that they should not be immersed in the oven, so that the workpiece can pass through the

The disadvantage of this method are, however, that oxyhydrogen flame is not overheated, the flame must

if again the large amount of radioactive waste (here with, for example, nitrogen to a temperature wc-

NaOH), the additionally contaminated hydrochloric acid minus degrees above the melting point of the straight

fail solution and the relatively high temperature to be used salt are cooled. After this

Doors between 450 ° and 570 ° C. The process is spraying the molten salt, the temperature

was also only on Hf Obcrflächcn and only for co ^NOCN ca-15 to about 60 minutes near the

60 contamination tested. Melting point of the salt kept. - With a Gc-

The invention is based on the object of creating a mixture of 24 g NaOH, 27 g KOH and 0.5 g Na ₂ O ₂ , a process that has the disadvantages of being a good immersion bath at 260 ° to 280 ° C DCTs method v «avoids, in particular the seizures 20 contamination activity (residual activity having large amounts of radioactive waste, and with egg 0.3% -, respectively, followed by post-treatment in 2n nem minimum of effort and labor co _HNO3 residual activity 0 , 2%), a maximum of operational safety and Dckon- must be guaranteed for the application of the method according to the invention, the disruptive amination effect. The dckontamid hygroscopic properties of largely urinated workpieces should be reusable and suppressed. This will therefore be practically non-corrosive to the process by adding Na ₂ CO ₃ and increasing the KOH content. The de- sufficient used in the procedure. It is conceivable that further, initially hykontaminatioasmhtel should be able to be made usable for radioactive waste with or without radioactive waste materials with or without radioactive waste materials with normal solidification Can be safely solidified by the type of decontamination agent used in the earth. inventive method and the low labor

The task is done in a simple and easy way of working temperature, which is only a few degrees above the

solved according to the invention in that inorganic, respective melting points are, decomposition

Salt melts forming * Dckontami.v, lionsmittcl in 35 reactions and the development of toxic and / or

A train in a hot oxyhydrogen gas flanime made of Accty-corrosive gases is not to be feared. The crkal-

oils and oxygen or hydrogen and oxygen, decontaminants adhering to the surfaces

must be brought before the introduction of the powder media because of their water solubility

Formed disinfection agent can be easily removed completely with an inert one. The introduction into the bc-

Gas to a temperature a few degrees above -10 known solidification media, such as bitumen, Melting temperature of the respective Dekontamina- glass (Hler Zcmcntlcim, can be used without difficulty

tionmittcls is cooled, in this being melted, as this means the requirement

and in the liquid state at temperatures up to compatibility with the solidification media

/ u of the order of 4 (10 ° C with the help of the gas fill. Due to the low and even corrosion

flame on the surfaces to be deconlaminated 45 sion effect of the according to the invention

be applied in a thin layer, up to large-scale applied decontamination agent, it

left on for one hour and corrosion rates between 0.3 and approx.

then rinsed off with a little water. cnr h found with good contamination effect,

Advantageously, according to the invention, the structure of the metal surfaces remains, such as optical ones

Usable decontamination agent from SaI-511 investigations showed obtained. The work pieces

/ en. Mixtures of salts or mixtures of SaI- can therefore be recycled after decontamination.

zen and / or alkali hydroxides and peroxides are used without further measures,

of the group primary phosphates of potassium and the additional investigations showed that the

Ammonium, mixtures of such primary phosphates are bled after salt contamination

With potassium chloride, mixtures of chlorides of the sodium chloride, allow residual activities to be reduced further, if

trium, potassium and aluminum, mixtures of Na- the rinsing process with water for 5 minutes

trium hydroxide, potassium hydroxide, sodium peroxide and treatment with aqueous, approx. 2N ENT.

Sodium carbonate. In the following, to explain the invention The decontamination agents are listed with a number of execution examples that indicate the

However, do not limit the mean grain size in the range between 0.04 and mi of the invention.
0, OK mm brought into the gas flame.

By the way in which the molten Example I was applied Dckontaminationsmittcls according to the invention on stainless steel samples made of steel 1.4541 were used to

The metallic surface can last for 500 hours in a contamination solution.

Layer thickness i-nwicO, 2 mm or thinner, for example μ solution ("" Co, '"' Sr /" ¹ Y, '"" RuZ ¹¹¹⁶ Rh ₁ ¹¹¹ Cc containing, jc-

0.02 to 0.03 mm when using KH ₂ PO ₄ , cr / .ielt Weils in a concentration of approx. 2 μ Ci / ml) in

be, which after a relatively short time a high pressure autoclave to 300 ° C at 88 atm

len, for example 15 minutes. The contaminations are heated up. These conditions correspond roughly to the

nigen that are present in a Leipzig water reactor. After drying for two hours at 200 ° C., the samples were _{sprayed with potassium dihydrogen phosphate melt (KH 2} PO ,,; melting point 253 "C) for 15 minutes, the melt allowed to cool on the steel samples, then rinsed with water at room temperature and the decrease in activity determined. The process was repeated twice. The measured residual activities were below 1%, based on the originally present contamination. The samples then looked bright. Exceptions occurred when the activity on the samples was in corrosion holes that were already present before the treatment with decontamination agents which is only partially removed by the melt; then the residual activity was around 5% .After treatment, an average of 53 mg / cm ² (= 530 g / m ² ) of salt adhered to the material, corresponding to a layer thickness of 0.2 mm .

Example 2

Stainless steel samples that were contaminated as described in Example 1 were used for decontamination with a melt consisting of 24.0 g NaOH, 30.0 g KOH, 0.5 g Na ₂ O ₂ and 5.5 g NaCO 260 ° C-280 ° C, as described in Example 1, treated. The decontamination steps were carried out as described in Example 1. The residual activities were between 1 and 3% for Co-60 and Ce-144, and 7.5% for Ru-106 / Rh-U) 6. After the salt treatment, the samples looked brsun. After a five-minute treatment with 2N nitric acid, the residual activity fell further down to 60% of this residual activity and the samples became blank.

Example 3

Stainless steel samples that were contaminated as described in Example 1 were in a molten salt,

tu consisting of 11.7 g of NaCl, 14.9 g of KCl and 26.6 g of AlCl ₃ , at a temperature of 130 ° C., as described in Example 1, treated. Residual activities of 0.6 to 6% were achieved. The samples looked blotchy gray after treatment.

In the technique of spraying the molten salts used in the examples - assuming that the coverage is 0.5 mm - about 1.2 kg / m ^{2 of} metallic surface is required. This amount of salt ends up in the liquid when rinsing

2 «Waste. Assuming that a 1% Solution arises, dropping 120 frnr the surface as waste solution at. In contrast, immersion bath tests carried out to compare costs and waste quantities resulted with potassium dihydrogen phosphate that

In immersion baths, 10 to 20 kg of salt per m ^: are required for decontamination (three uses are included in the calculation), which then end up in the radioactive waste completely. About 5% of the amount of salt is obtained in the form of a 1% waste solution.

Claims (1)

Claim: Process stum decontaminating surfaces of metallic materials, in which the Surfaces are brought into contact with molten salts, characterized in that inorganic. Molten salt decontamination agents in one go into a hot one Oxyhydrogen flamrac made of acetylene and oxygen or Hydrogen and oxygen are brought before the introduction of the powdery Qekontaminationsmittcl with an inert gas to a Temperature a few degrees above the melting temperature of the respective decontamination agent is cooled, are melted in this and in the liquid state at temperatures up to the order of 400 ° C with the aid of Apply a thin layer of gas flame to the surfaces to be decontaminated until xu on it for the order of an hour left and then rinsed off with a little water.