Total decontamination process for radioactive metal material

Abstract: Total decontamination process for radioactive metal material, particularly for the tubes (4) of the nuclear power station tube nests, which process first uses together ultrasounds of controlled frequency (13) and a washing in a first water tank of controlled temperature (5), then only a washing in a second water tank (6) and then together ultrasounds of controlled frequency (20) and a washing in a third tank (7) contanining strong mineral acids under controlled temperature.

Description: The prevent invention relates to a total decontamination process for radioactive metal material, in particular to radioactive metal material presenting big surfaces and recesses, like the tubes for the tube nests in the heat exchangers of a nuclear power station. On life termination of a nuclear power station there is the problem to dismantle the radioactive metal parts and conveniently treat such parts in order to reduce the amount of material to be delivered to the open storages for radioactive material. In general, according to the prior art, radioactive metal surfaces are decontaminated by means of chemical solutions, electrolytic processes, mechanical means, pressurized water jets and application of ultrasounds. In particular, the following documents are known: EP-A-0 106 595 that discloses a decontamination process comprising a step in which a radioactive metal material to be decontaminated is immersed into a water bath under continuous application of ultrasounds; FR-A-2 590 716 that discloses a decontamination process comprising the further immersion of said contaminated material into a water solution of strong mineral acids, the control of the temperature of the bath and the application to said bath of ultrasounds at a defined frequency and power and JP-A-1 233 398, a summary issued from the Patent Office of Japan, that discloses a three step process involving a first immersion in a decontamination liquid bath, a further pre-cleaning liquid bath and a final cleaning step with simultaneous application of ultrasounds. The main drawback common to the prior art processes resides in an unsatisfactory effectiveness when said processes are applied to material presenting big surfaces and recesses, as it happens in said tubes particularly, so that there is always some residual radioactivity preventing from completing the decontamination, i.e. from achieving what is called in this technical field a "total decontamination" and, consequently, from reducing the waste amount to be delivered to said storages. The process according to this present invention obviates said drawback. It comprises the steps: a) a first step of washing said radioactive metal material in a first water bath under continuous first beams of ultrasounds, b) a second step of removing said material from said first water bath and washing said material in fresh water, c) a third step of washing said material in a second bath consisting in a water solution of 3,0% to 4,0% hydrofluoric acid and 5,0% to 6,0% nitric acid at a controlled temperature raging from 60° C to 70°C and simultaneously subjecting said material in said solution to continuous second beams of ultrasounds of controlled frequency and power and d) a fourth step of removing said material from said second bath and washing said material in fresh water. The length of above third step varies according to the strength of the acid bath and decreases as the material dissolves; preferably, a sequence of lifts and immersions is carried out; at the end of each immersion, the material is lifted from the bath in order to carry out a preliminary visual examination and the material presenting metallic gloss on its inner and outer surfaces is removed from the bath and delivered to radiometric checking. The solution is considered exhaust as the iron concentration is higher than 30 g/l (the iron concentration may be determined, for instance, by a spectrophotometer). It will be realized that this invented process is particularly suitable to decontaminate tubes of the tube nests of the heat exchangers in the nuclear power stations and more particularly tube nests of the feeding regenerators in boiling water power stations. The paramount advantage of this invented process resides in that the radioactive material is decontaminated over those standards that are usually accepted for releasing non-inspected or non-conditioned material and in that the waste to be delivered to said storages, consisting in the residuals from the decontaminating solutions, are much reduced in amount. The invention is described in detail with reference to the figure which is a side view of a plant for carrying out the invention. A monorail 1 moves in the direction of arrow 2 a plurality of hung and conveniently spaced cages 3, each containing a set 4 of one hundred tubes each being one meter long and set in the cage vertically; each set of tubes 4 in its cage is dipped sequentially in the tanks 5, 6, 7 and 8 by hoists 9, as detailed below. First tank 5 comprises an inner tank 5A and an outer tank 5B, both covered by a common cover 5C; inner tank 5A contains washing water for the tubes and in outer tank 5B is circulated a diathermic solution to keep at 55° C the temperature of the washing water in tank 5A. Said solution is supplied from a suitable conventional first boiler, not shown in the figure, that gives it the required temperature; a pump, not shown in the figure, supplies the solution to the tank through inlet duct 10 and the solution comes back to said boiler through outlet duct 11. Bottom exhaust means 12 allows to completely empty tank 5B, if necessary. An assembly of ultrasound emitting piezoelectric transducers 13, of 20 kHz frequency, is applied to the outer of side and bottom walls of tank 5B, said transducers being of such specification and number as to maintain in the inner tank 5A an homogeneous power density of 25 Watt per liter (W/1). The tank 5A inner sizes are 50 x 50 x 150 cm (150 cm height). The tube set 4 is left in the washing water during a time period of 2 hours. Second tank 6 recycles a steam of fresh water from an outer source through inlet and outlet ducts, not shown in the figure, to energetically wash the tubes of a set of tubes 4 dipped in said tank together with its cage 3, after removal from tank 5. Tank 6 too comprises bottom exhaust means 14. Third tank 7 comprises an inner tank 7A and an outer tank 7B; the inner tank contains a solution from 3,0% to 4,0% of HF (hydrofluoric acid) and from 5,0% to 6,0% of HNO₃ (nitric acid) (% in volume), in water maintained at the temperature of 70° C by a diathermic solution which is supplied to tank 7B through inlet duct 15 and removed through outlet 16, similarly to the explanation referred to tank 5. Tank 7A is covered by cover 17 provided with an opening connected with an intake tube 18 to remove acid vapours emitted from the solution in the tank. Tank 7B is provided with a bottom exhaust means 21. An assembly of ultrasound emitting piezoelectric transducers 20 is applied to the outer side and bottom walls of tank 7B, similarly to the description referred to tank 5. The ratio between the total inner and outer surface of said tubes and the volume of said hydrofluoric acid and nitric acid solution in this second bath is 3 dm²/l as a maximum. Fourth tank 8 is similar to tank 6 described above. The decontaminated tubes are washed in running water energetically in said fourth tank 8 to release in the water the residual radioactive material particles detached from the tube outer and, in particular, inner surface. This tank too is provided with a bottom exhaust means 22. A cable 23 is associated with a hoist 9 supporting a cage 3 and a clamp 24 at the bottom of said cable holds said cage. It will be realized that tanks and other parts which may be in contact with radioactive materials and with acid are made of suitable material.

Claims: A total decontamination process of radioactive metal material comprising the steps of: a) a first step of washing said material in a first water bath under continuous first beams of ultrasound, b) a second step of removing said material from said first water bath and washing said material in fresh water, c) a third step of washing said material in a second bath consisting in a water solution of 3,0% to 4,0% hydrofluoric acid and 5,0% to 6,0% nitric acid at a controlled temperature ranging from 60°C to 70°C and simultaneously subjecting said material in said solution to continuous second beams of ultrasound of controlled frequency and power and d) a fourth step of removing said material from said second bath and washing said material in fresh water. A process according to claim 1 characterized further in that said step a) is applied during a time period ranging from 50 to 70 minutes, said first water bath is kept at a temperature ranging from 50°C to 60°C and the first and second beams of ultrasound have a frequency ranging from 20 kHz to 22 kHz and a power density of at least 15W/l. A process according to claim 1 characterized in that said step c) is applied during a time period ranging from 75 to 120 minutes. A process according to claim 1 characterized in that the ratio between the total surface of said material and the volume of said hydrofluoric acid and nitric acid solution in said second bath is 3 dm²/l as a maximum.

Inventors: BREGANI FIORENZO, GAROFALO ANTONIO
Applicants: ENTE NAZ ENERGIA ELETTRICA, ENTE NAZIONALE PER L'ENERGIA ELETTRICA - (ENEL)
Application year: 1990
Main classification: G21F9
IPC codes: G21F9