Classifications

• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
  • G21F9/00—Treating radioactively contaminated material; Decontamination arrangements therefor
  • G21F9/001—Decontamination of contaminated objects, apparatus, clothes, food; Preventing contamination thereof
  • G21F9/002—Decontamination of the surface of objects with chemical or electrochemical processes
  • G21F9/004—Decontamination of the surface of objects with chemical or electrochemical processes of metallic surfaces

Definitions

• the invention relates to a process for the decontamination of steel surfaces, in particular in reactor cooling circuits, by removing the contaminated surface layer with an acidic aqueous decontamination solution and for preparing the decontamination solution containing the detached radioactive substances for disposal.
• Aqueous solutions of mineral acids have often been used to decontaminate reactor cooling circuits.
• Mineral acids are aggressive substances for the metal of the cooling circuit, and it is therefore extremely difficult to let the acid concentration alone run the decontamination process in such a way that the contaminated surface layer is effectively removed in an acceptable time, but does not corrode the pure metal of the cooling circuit, because Corroded areas in the cooling system could lead to leaks, which may occur due to serious consequences must not arise.
• Aqueous solutions of alkali metal permanganates, nitric acid, sodium persulfate, sodium bromate and preferably hydrogen peroxide are used here to oxidize the contaminated steel surface layer in the first process step.
• aqueous solutions of mixtures of mineral acids, such as sulfuric acid and / or nitric acid, and complex-forming substances, such as oxalic acid, citric acid or formic acid are given, to which corrosion inhibitors, e.g. Iron (III) sulfate, iron (III) nitrate, nitric acid, phenylthiourea or similar can be added.
• corrosion inhibitors e.g. Iron (III) sulfate, iron (III) nitrate, nitric acid, phenylthiourea or similar can be added.
• the use of hydrogen peroxide in the first process stage has the particular advantage that it can easily be broken down into water and oxygen so that the subsequent rinsing with water can be dispense
• the dissolved metallic components together with the radioactive substances are then precipitated from the used decontamination solution of the second process stage.
• the sulfuric and oxalic acid contained in the decontamination solution can be neutralized with calcium hydroxide, so that calcium sulfates and calcium oxalates are formed, which contain a large proportion of the radioactive substances present and are separated from the liquid by filtration.
• potassium permanganate can first be added to the used decontamination solution to decompose the oxalic acid and to obtain manganese dioxide and manganese sulfates, which can then be adjusted by adjusting a pH of about 10 to e.g. Calcium hydroxide can be precipitated.
• the precipitate only precipitates a large, albeit large, proportion of the radioactive substances, so that in both cases the filtrate is still contaminated and must be disposed of in a nuclear manner.
• Such two-stage decontamination processes can be carried out as a continuous process or as a batch process.
• the decontamination of reactor cooling circuits is complex and relatively expensive, in particular if corrosion of the pure metal surfaces is excluded for the desired safety.
• the decontamination solution contains formic acid and / or acetic acid and a reducing agent, preferably formaldehyde and / or acetaldehyde.
• a reducing agent preferably formaldehyde and / or acetaldehyde.
• the liquid is then dyed slightly green, but clear and transparent without turbidity, and their composition can during the treatment of - are relatively easily monitored steel surface. It has been shown that such a decontamination solution removes iron oxides 10 to 50 times faster than the pure base material, and this allows the decontamination process to be carried out without great difficulty in such a way that the pure steel surface is attacked by the pure steel surface and leads to harmful corrosion Decontamination solution is practically excluded. Iron compounds are precipitated from the decontamination solution for disposal. Since the used decontamination solution contains only Fe ions, there are no problems with the precipitation. The precipitates formed have the property of adsorbing the radioactive substances contained in the solution, so that very high precipitation decontamination factors can be achieved by separating the precipitates.
• the separated solid precipitate then contains practically all radioactive substances from the decontamination solution, while the liquid has at most only an insignificant residual activity, which can be below the tolerance limit, and thus regenerates the liquid for reuse or simple chemical disposal by decomposing the dissolved substances in gaseous products and Water, NaOH, possibly Na 2 CO 3 , can be added.
• the chemical composition of the decontamination solution provided according to the invention makes it possible to precipitate the Fe 2 + ions in the form of iron compounds, the density of which corresponds approximately to the density of iron oxide or which can easily be converted into such iron compounds.
• the radioactive waste obtained in a decontamination process is then approximately equal to the material removed from the contaminated surface and is therefore a minimum.
• a reactor cooling circuit made of low-alloy or stainless steel can be decontaminated in a continuous process.
• the size of the inner surface and the capacity of the cooling circuit are known.
• an aqueous solution of formic acid and / or acetic acid and of at least one reducing agent is to be used as the decontamination solution.
• Preferred reducing agents are those which are composed of C, H, 0 and N and contain no harmful foreign elements, such as S.
• Such reducing agents are e.g. Hydrazine, oxalic acid, ascorbic acid, acetic anhydride etc., the decontamination solution according to the invention preferably containing formaldehyde and / or acetaldehyde as a reducing agent.
• radioactive substances are adsorbed in a layer of a mixture of iron oxides, and the thickness and composition of the surface layer to be removed can be determined by prior sampling (CH-PS: Application No. 2184 / 80-7). Based on the data available or determined and the options available, such as in particular the time available for decontamination, heating or cooling devices, etc., the appropriate composition, the required amount and also the process flow for the decontamination solution the broad outline.
• the oxides of the contaminated steel surface are dissolved directly and / or reductively and converted into soluble iron (II) formates and / or iron (II) acetates, which are caused by those in the decontamination solution, especially by them contained reducing agents created stabilizing conditions and in particular oxidation to precipitated iron (III) compounds does not take place.
• Used decontamination solution is therefore slightly green in color, but clearly transparent and without cloudiness and at most contains solid particles of the oxide layer that occur during the solution process, which do not interfere with the decontamination itself or with the treatment of the used decontamination solution for disposal.
• a decontamination solution according to the invention which generally leads to satisfactory results requires e.g. contain only formic acid and formaldehyde, the liter of decontamination solution containing, for example, 7-22 ml formic acid and 12-36 ml formaldehyde.
• decontamination solution according to the invention compared to known decontamination solutions is generally characterized by a low chemical consumption and low costs as well as a high absorption capacity for Iron.
• the used decontamination solution emerging from the cooling circuit is monitored during the detachment process, the Fe 2+ , acid and aldehyde concentrations being continuously checked.
• Such a check is analytically simple and permits reliable control of the entire decontamination process, which reliably precludes inadmissible corrosion of the pure metal surface.
• the iron compounds contained in the decontamination solution emerging from the cooling circuit are precipitated, and the used and thus cleaned decontamination solution is used for reuse, i.e. regenerated for reintroduction into the cooling circuit.
• the iron compounds are preferably precipitated electrolytically by passing the used decontamination solution through an electrolysis stage which contains an iron cathode and a graphite anode.
• Another advantage of the decontamination method according to the invention is that the reactions when the contaminated surface layer is removed are irreversible and therefore no carryover of radioactive substances to surface areas that are not or no longer contaminated is to be expected.
• the decontamination solution After removing the intended layer thickness, the decontamination solution is drained from the cooling circuit. In any case, after draining, any residues will remain in the cooling circuit.
• the decontamination process according to the invention only those residues are present, according to the composition of the decontamination solution, which are thermally converted into iron oxide and into gaseous decomposition products, in particular C0, C0 2 and H 2 0, ie in the cooling circuit, by a simple heat treatment at 175-300 ° C own decomposition products, are decomposed and therefore have no harmful influence on the operation.
• This thermal decomposition of the residues can be carried out by introducing hot air or hot water, but is generally not necessary, since the cooling circuit at Recommissioning warms up to the required temperature in a short time.
• a cooling circuit which still has residual activities after decontamination can be rinsed in a nuclear manner in a conventional manner by ion exchange.
• a rinse will only be necessary in exceptional cases, since residual activities can easily be excluded by means of a corresponding layer thickness to be removed.
• the transporter for the removed radioactive substances is the iron itself which has gone into solution and not some other additional substance, so that if the iron precipitates out of the decontamination solution, practically all the activity will be contained in the precipitate and the separated liquid at best will only have permitted radioactivity.
• the aim is that all radioactive substances contained in the used decontamination solution are adsorbed onto the smallest possible amount of precipitate, that the precipitate is easy to dispose of and that the separated liquid results in as little environmental pollution as possible.
• any substances, such as S-compounds can be used in the disposal precipitation, provided that the precipitation results obtained are satisfactory in an economical manner.
• the iron (III) hydroxide obtained is easier to separate from the liquid than iron (II) hydroxide, for example by filtering, but more precipitant is required for precipitation than with iron (II) hydroxide.
• the radioactive substances contained in the decontamination solution is adsorbed on the precipitated iron hydroxide, and the liquid separated from the precipitate, in the present case an aqueous solution of sodium formate with any residues of formaldehyde, will be very little or not radioactive.
• the sodium formate can now be decomposed oxidatively to NaOH, Na 2 CO 3 , Co 2 and H 2 0.
• An advantage of this precipitation is that the separated precipitate corresponds in weight to the material removed during decontamination, i.e. practically no weight gain has taken place and that the precipitate can be easily disposed of by mixing with cement without further treatment, expediently producing products similar to ferrocement that ensure a particularly low amount of contaminated material to be disposed of.
• the sodium formate obtained can be decomposed.
• the decontamination solution is expediently divided into several batches. After treatment with hydrogen peroxide, the required small amount of precipitant, e.g. NaOH, and after separating the precipitate, the sodium formate obtained is oxidatively, electrolytically or pyrolytically decomposed as indicated above. The liquid product obtained is then used to precipitate the second batch of decontamination solution, etc.
• the precipitation and liquid can be separated by simple filtration.
• the used decontamination solution can contain flocculants, e.g. Polyacrylamide can be added, through which the precipitated particles are combined to form larger particles.
• the precipitate from an earlier precipitation process is used as the preferred flocculant.
• the separated liquid is either prepared for reuse as a decontamination solution or "chemically" disposed of.
• formaldehyde in particular is oxidized to formic acid and the formic acid thus obtained is decomposed together with the existing formic acid by an oxidizing agent in H 2 0 and C0 2 and salts of formic acid are also disposed of.
• Ox y dationsmittel can each be any used, in choosing essentially only on profitability, ie to look for low cost, and that the advantageous chemical disposal is not affected by the oxidizing agent.
• the decontamination method according to the invention can be carried out as a continuously running process with circulated decontamination solution as well as a batch process, the advantages achieved being the same.
• the decontamination process according to the invention can be used to effectively decontaminate surfaces of low-alloy steel and of stainless steel.
• the decontamination process according to the invention reduced the activity to 0.025 ⁇ Ci / cm 2 , which resulted in a material removal of about 10 mg / cm 2 results in a high decontamination factor of 330.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Food Science & Technology (AREA)
• Physics & Mathematics (AREA)
• General Engineering & Computer Science (AREA)
• High Energy & Nuclear Physics (AREA)
• Solid-Sorbent Or Filter-Aiding Compositions (AREA)
• Removal Of Specific Substances (AREA)

Applications Claiming Priority (2)

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• 1981
  • 1981-09-01 CH CH5611/81A patent/CH653466A5/de not_active IP Right Cessation
• 1982
  • 1982-08-09 DE DE8282107178T patent/DE3271935D1/de not_active Expired
  • 1982-08-09 EP EP82107178A patent/EP0073366B2/fr not_active Expired - Lifetime
  • 1982-08-27 US US06/412,375 patent/US4508641A/en not_active Expired - Fee Related
  • 1982-08-30 CA CA000410418A patent/CA1197445A/fr not_active Expired

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