DE3046563A1 - DECONTAMINATING REAGENT AND METHOD FOR DECONTAMINATING A CORE REACTOR OR PARTS THEREOF - Google Patents

Description

description

The invention relates in particular to the decontamination of surfaces contaminated with radioactive materials Cooling surfaces in nuclear reactors, through contact with a circulating decontaminating solution. The invention provides a new decontaminating reagent mixture or reagent solution provided that has improved stability and longer duration of action.

Nuclear reactors with surfaces in which cooling water circulates, on which radioactive precipitates are deposited, were treated with decontaminating reagent solutions to remove such surface contaminants. Usually Treatment causes the generation of radioactive liquid wastes that are difficult and costly too are eliminated.

US Pat. No. 3,272,738 describes the precipitation of radioactive metal corrosion products from EDTA-hydrazine solutions by adding sulfuric acid and ferrocyanide. U.S. Patent 3,873,362 similarly describes the precipitation of radioactive metallic pollution using permanganate, alkaline earth oxides or alkaline earth hydroxides. In U.S. Patent 4,162,229, a cerium (IV) salt is used as a decontamination reagent used.

U.S. Patent No. 3,737,373 describes the use of a 0.1% strength Deuterated oxalic acid in a D-O reactor coolant for Dissolving dirt. The irradiation of this oxalic acid / contaminant system causes a decomposition of oxalates and a precipitation of dissolved contaminants, the then collected by filtration and / or ion exchange. In this process, the deuterated oxalic acid destroyed, so that the addition of further deuterated acid

130036/0679

is necessary to continue decontamination. The reactor has to choose between cooling, sub-critical and hot-critical Conditions are operated in this process *.

The "CAN-DECON" process was developed by the Atomic Energy of Canada Limited developed for the decontamination of shutdown heavy water moderated and cooled reactors, using dilute solutions that cause minimal corrosion and degradation of heavy water cause. Thereby a reduced volume of solid waste material is obtained for disposal and a relative one short decontamination time achieved (see CA-PS 1 062 590). This method consists in using selected acidic Injecting decontamination reagents into a circulating coolant to form a dilute reagent solution, the solution to dissolve deposits and then to contact a cation exchange resin that collects the dissolved cations and the acidic reagents for a new one Use regenerated, circulated and finally the reagents by contact with an anion exchanger to contact to restore the original condition of the coolant (the restoration of the coolant is especially important in the case of heavy water). Suitable acidic reagents are ethylenediaminetetraacetic acid (EDTA), Oxalic acid, citric acid, nitrilotriacetic acid and thioglycolic acid.

The iron / EDTA complex is used as an initial surface treatment agent for steels in energy systems (including nuclear reactors) to create a protective magnetite layer known with good quality. M. Weber et al (Acta Chim. Acad. Sci. Hung. 97 (3), pp. 255-264, 1978) have the Effect of ionizing radiation on the surface treatment system investigated from iron / EDTA complex plus water and found that the radiolytic decomposition this iron (III) complex can be reduced by adding methanol or formic acid.

130036/0679

3Q46563

Some time ago it was found that these are organic acidic decontamination mixtures are subject to radiolysis when used, leading to their decomposition over various Leads over periods of time, causing a loss of decontamination efficiency. An essentially complete one Consumption of EDTA was observed at radiation doses of approximately 0.8 to 1 Mrad. Citric acid used one third was also consumed at this dose, while the oxalic acid concentration remained stable. In performing the CANDU reactor decontamination process, it is believed that the average dose rate is approximately 0.3 Mrad per hour. This leads to the disappearance of EDTA and a significant loss of the Effect within about 3 hours.

The invention is based on the knowledge that from many components existing organic acidic decontamination reagents which may contain oxalic acid and a radiolytic one Subject to decomposition can be improved by adding formic acid to them in amounts which prolong the effect of the mixture.

A preferred decontamination reagent according to the invention contains EDTA, citric acid, oxalic acid and formic acid in an amount that is sufficient for the stability and the To increase the effect of the mixture when exposed to ionizing radiation. The formic acid reduces the decomposition of both EDTA and citric acid and provides oxalic acid as a result of their own radiolysis. Hence causes Formic acid minimal change in reagent formulation. This reagent formulation is particularly advantageous for carrying out decontamination, when it is carried out a longer cycle through the contamination zone takes place and cation exchange resins are used.

The invention also relates to a method for the decontamination of surfaces with radioactive deposits Carrying out a reagent mixture of organic acidic

130036/0679

Decontamination agents including oxalic acid over the contaminated surface is allowed to run, being at the beginning Formic acid is added to the mixture and the presence of the formic acid by at least one more Addition of the same is maintained.

The invention is explained in more detail by means of the accompanying drawings.

Show it:

Fig. 1 is a graph showing the radiolytic Decomposition depending on the radiation dose in the case of EDTA in CAN-DECON solutions a pH of 4.5 in the presence of sodium formate or Represents formic acid;

Fig. 2 is a similar graph showing the consumption of citric acid and the formation of oxalic acid depending on the radiation dose in the case of CAN-DECON solutions at a pH of 4.5, the Contain sodium formate or formic acid?

Figure 3 is a similar graph showing the radiolytic decomposition of EDTA with sodium formate as well as EDTA and citric acid without formate. The radiolytic generation of oxalic acid in the presence formate is also shown (as in Figure 2); '

Fig. 4 is a representation similar to Fig. 3, which graphically shows the decomposition of EDTA and citric acid and shows the generation of oxalic acid in the presence of formic acid;

130036/0679 bathroom original

5 shows in a similar manner the radiolytic decomposition of EDTA in the presence of dissolved iron with and without two concentrations of formic acid. The dashed line shows a comparison test (no iron or formic acid);

6 graphically shows the consumption of EDTA in a similar manner, Citric acid and oxalic acid in the presence of boron and formate;

7 graphically shows the decomposition of EDTA and citric acid in the presence of iron (III) oxalate, iron (II) sulfate and sodium formate with increasing radiation dose.

The presence of formic acid or formations reduces or inhibits the consumption (or decomposition) of the organic Acids to a considerable extent, which prolongs the effect.

The amount of formic acid initially added can be considerable vary, with even small amounts having some effect. Usually the amount of formic acid added is about 1/4 to about two times the amount by weight of the organic acidic decontaminants that undergo radiolytic decomposition. In the case of the preferred system of EDTA, citric acid, oxalic acid and formic acid The respective weight ratios can be 1: (0.6-1.2) :( 0.1-1.2) :( 0.6-3), but these ratios are not critical. Relative proportions of 1: 1: 1: 1 are usually very suitable. In the relatively dilute CAN-DECON type of circulating solution, the concentrations of the same four components are preferably between 0.03 and 0.05 wt%, 0.02 and 0.04 wt%, 0.01 and 0.04 wt%, and 0.03 and 0.15 wt%. It is considered expedient that Formic acid concentration at or above a minimum value of about 0.03% by weight during decontamination

130036/0679

to keep (by further additions, if necessary). Solution concentrations that vary to saturation, are appropriate when decontamination other than CAN-DECON decontamination is carried out in CANDU reactors (for example in reactors cooled with I.easily water, where the coolant can contain boron). boron may also be present in a heavy water coolant or moderator in some cases to control neutron flux.

Does ionizing radiation act on these decontamination reagent mixtures then the radiolytic decomposition of formic acid leads to the formation of some oxalic acid as well as EL · and CO2 · These gases can be in a degassing stage as often used when upgrading the coolant for reuse will. This added oxalic acid is another benefit. The original oxalic acid experiences one itself radiolytic decomposition (to a lesser extent than some of the other acids), but is usually a Net gain in terms of oxalic acid content in the radiolysis of the mixture can be determined. The formic acid does not work significant isotopic deterioration of the coolants in the case of a heavy water coolant or moderator.

The organic acidic decontaminants can consist of oxalic acid and one or more of the components EDTA, citric acid, Nitrilotriacetic acid and thioglycolic acid exist. It was found that mixtures of EDTA, citric acid and oxalic acid are preferred for the decontamination of heavy water reactor systems using the CAN-DECON principle will. The addition of hydrazine is beneficial for some uses. Other mixtures, for example those containing ascorbic acid or acetic acid or other acids mentioned above can be used for other systems be suitable, for example, for light water-cooled reactors.

130036/0679

One possibility for carrying out the invention is preferably to add the organic acid mixture to the coolant of a switched-off reactor and to keep the decontaminating reagents in solution contact with the surfaces to be decontaminated until the radioactive compounds have dissolved, whereupon the coolant solution is passed through a cation exchange resin column (in the H ⁺ or D ⁺ form) and through filters to remove radioactive cations and any suspended solids that may be present and then circulate again. One or more additions of formic acid can be useful in the meantime. After the desired decontamination has been achieved, the solution is sent through both a cation and an anion exchange resin to remove dissolved metals and anions, in particular organic acid anions, whereby the coolant is returned to its normal state. If borate anions are present, some or all of the anion exchangers will become saturated with boric acid, thereby ensuring that a sufficiently high concentration of borate is maintained during each decontamination stage. A degassing stage to remove gaseous analysis products can be useful before the coolant is returned to its operating state.

The ion exchange resins are loaded to capacity as much as possible, usually with the cation exchange resins and mixed ion exchange resins (which are now highly radioactive) disposed of as solid waste material. Such solid waste material is more convenient to dispose of than liquid waste material with fewer environmental problems. If a heavy water coolant or moderator is used for the decontamination, the ion exchange resins are converted into the D ⁺ and OD ~ form in order to avoid a reduction in the deuterium content.

130036/0679

The following examples illustrate the invention without restricting it. For test purposes, a standard decontamination reagent consisting of EDTA, citric acid monohydrate and oxalic acid dihydrate in aqueous solution in concentrations of 0.04 (1.37), 0.03 (1.43) or 0.03 (2.38)% by weight is used ( where the values in brackets are to be understood in itiMol'L). To carry out tests without dissolved metals, the pH is adjusted to 4.5 (normal working range) by adding LiOH. All solutions are irradiated in 50 ml glass syringes using a Gammacell 220 (trademark) Co source at a nominal dose rate of 1.67 χ 10 eV'L «s (16 krad-min). The solution temperatures in the Gammacell device increase with the absorbed dose, but reach a steady state of 42 ° C after a dose of approximately 2 Mrad (2 hours). At a test irradiation at 85 ⁰ C, the syringes are immersed in a thermostatically controlled water bath, wherein the dose rate to 1.3 eV χ 10 ^19-L ^ s ^"1 drops (12.5 kradmin). The acid components of the reagent are separated and analyzed by gas chromatography after esterification with BF-, in methanol.

2+ 3+ trations (measured as Fe and Fe) are spectrophotometric analyzed. Solutions that contain dissolved iron are used in the dark to prevent photolytic decomposition of acids stored.

example 1

The radiolytic decomposition (consumption) of EDTA in a 0.04% by weight concentration in a standard decontamination reagent solution is monitored in the presence of 10 mmol "L sodium formate or 25 mmol" ^L formic acid at a pH of 4.5 and at Gammacell temperatures . At a pH of 4.5, the formic acid is predominantly (85%) in the form of formations. The results are summarized graphically in FIG. In FIG. 1, the dashed line shows, for comparison purposes, the consumption of EDTA in the absence of formic acid. 1 30036/0679

The addition of formic acid / formic acid significantly increases the radiolytic stability of EDTA in these solutions Comparison to the comparison sample (no formate). the The dose required to consume 50% of the EDTA increases from approximately 6 to 15 times in Solutions containing 10 to 25 mmolL formic acid / formate contain. The initial decrease yields correspond to G (-EDTA) = 0.30 and 0.03 molecules / 100 eV, respectively. The dashed Lines in the graphs represent the initial slopes from which G (-EDTA) is calculated will. These values can be compared with G (-EDTA) = 2.3 in the absence of formic acid / formate.

In Fig. 2 the corresponding effects on citric acid and oxalic acid and in the same conditions are shown, the dashed line being the consumption of citric acid in the absence of formate / formic acid. Again, compared to the results obtained in the absence of formic acid / formate, the citric acid is protected by approximately 10-fold with G- (citric acid) = 0.05 ± 0.015, while oxalic acid (right hatred rod) is actually protected during radiolysis with a yield G (oxalic acid) = 5 1.0 + 0.1.

Example 2

The radiolytic decomposition of EDTA and citric acid is monitored in the test reagent solution at 85 ° C. and a pH of 4.5. The results emerge from the graph according to FIG. 3. The oxalic acid concentration is not influenced at doses of <4.7 χ 10 ²² eV-L * " ¹ (10.75 Mrad). In the absence and presence of 10 mmol * L sodium formate, the left and middle curves give up the effect EDTA again The right-hand scale shows the net generation of oxalic acid under the conditions maintained and in the presence of 10 mmol-L sodium formate, this last

130036/0679

citric acid remains relatively unaffected at doses of £ 14 χ 10 ²² eV "L" ¹ (£ 2.2 Mrad).

4 shows the results of a similar experiment in the absence and in the presence of TO mmol / L formic acid. Although the two graphs are not completely the same, both EDTA and Citric Acid, producing oxalic acid in doses exceeding approximately 1 Mrad. In both graphic Illustrations, namely Figures 3 and 4, at 85 ° C EDTA is consumed in an amount approximately twice that Corresponds to the amount observed at έ 42 ° C.

Example 3

The decomposition of EDTA at 85 ° C is followed in the same reagent solutions containing dissolved iron, with and without formic acid. Solutions containing dissolved iron are prepared by heating deaerated reagent solutions to 85 ° C in the presence of pre-oxidized mild steel, namely Inconel 600 (Trade Mark) and stainless steel (Type 410) to a pH of 4.5. The oxidized metals have been prepared by autoclaving at 300 ⁰ C in aqueous solutions whose pH has been adjusted to 10.5 LiOH. This dissolution pretreatment simulates an initial stage of decontamination, but without radiation. In the solutions containing 10 mmol «L of formic acid, the total dissolved iron increases at a pH of 4.5 from approximately 260 to 290 ppm up to 480 + _

2+ 3 +

25 ppm, largely as Fe, the Fe concentration remains unchanged at 80 _ + 10 ppm. If the formic acid concentration is doubled to 20 mmol-L, then the dissolved iron rises to about 725 ppm and the Fe to 235 + 35 ppm. The results come from the Fig. 5 shows. The dashed line represents the EDTA results from Figure 4 without iron or formic acid.

130036/0679

No decomposition of citric acid is observed at radiation doses 4 Mrad.

Before the irradiation, but after the reaction with the pre-oxidized Metal, the initial oxalic acid concentration has dropped by about 75%, apparently due to the Formation of a ferrous oxalate precipitate. A radiolysis at doses of £ 5 Mrad does not further reduce the concentration of oxalic acid, although in some cases it does Increase is observed (the effect is not very reproducible). The initial exposure to the pre-oxidized Metal also has a reduction in EDTA of approximately 10% in the presence of 10 niMol'L of formic acid and um results in about 30% in their absence. A protective effect due to formic acid on the non-radiolytic Degradation of EDTA is achieved during the initial metal run solution, which is an extra benefit. The citric acid concentration remains stable.

These dissolved iron tests indicate that the

2+
Iron (predominantly Fe) also inhibits the radiolytic decomposition of EDTA and citric acid to some extent. This effect is further enhanced by formic acid, but in contrast to the results shown in FIGS. 2 to 4, no oxalic acid is produced in comparable amounts.

The overall effect observed after completing this example with formic acid and dissolved iron is increased Protection for EDTA and citric acid, whereby the initial oxalic acid concentration is somewhat reduced as a result of precipitation will. Formic acid is obviously an aid for iron dissolution.

130036/06 7 9

Example 4

In light water reactors, boron is used to control the neutron flux used for both regular operation and shutdown. The effects of boron on radiolysis of the test reagent mixture containing formic acid, if any such effects occur, are important in relation to the application of the present invention to such reactor systems.

The decomposition of EDTA, citric acid and oxalic acid is monitored in the manner described above, but in the presence of (a) 6000 ppm boron (added as H ₇ BO.,) And

- 1 yy

(b) 2000 ppm boron and 10 mmolL sodium formate.

The results are shown in FIG. 6. The protective effect of the format is comparable to that in its absence of boron (Figs. 1 and 2). Although the initial consumption of oxalic acid (not shown) in the 2000 ppm boron / Formate solutions appear to be increased compared to the 6000 ppm boron solutions, this effect is compensated by a compensating effect Radiolytic production balanced so that ultimately no degradation is observed after approximately 4 Mrad doses.

Example 5

7 shows the decomposition of EDTA and citric acid in the same starting solutions which additionally contain 90 ppm of Fe as iron (III) oxalate, 45 ppm of Fe ⁺ as iron (II) sulfate and 10 mmol of sodium formate. No net consumption or production of oxalic acid is found at doses of £ 4 Mrad. Therefore, the decontamination efficiency of the mixture is still good after 4 Mrad radiolysis.

130036/0679

Example 6

To determine whether the addition of formic acid has any effect on the corrosion of mild steel in the system and on the decontamination of that steel, tests are carried out using a 0.1 wt% standard test solution with and without 0.046 wt% formic acid carried out at 85 ° C. A test coil is used which has an ion exchange cleaning section. Corrosion tests are carried out using test pieces made of mild steel which has been pickled and degreased (using some pieces previously coated with a film as in Example 3). Decontamination tests are carried out using mild steel test pieces that have been degreased and irradiated in the reactor for 8 weeks to make them radioactive. To carry out decontamination tests, a mild steel sheet previously provided with a film is inserted into the spiral in order to generate approximately typical CAN-DECON iron concentrations in the solution. Before each test, the coil was _{flushed with N 2} and the water was deoxygenated.

The corrosion tests show that there was no noticeable difference in the corrosion rate of mild steel due to formic acid occurs. The measured corrosion rates are 0.12 to 0.18 μΐη / hour in the case of the pickled samples and 0.15 to 0.25 μΐη / hour in the case of samples with on the Surface preformed films. Furthermore, there is no decrease in decontamination and cleaning in the presence of formic acid established. These tests are carried out without radiolysis, under radiolytic conditions the would Formic acid, as has already been shown, can prolong the potency and mode of action of a given mixture.

Further tests with EDTA alone with formic acid show one

130036/0879

protective effect and a prolonged effect of the mixture. After irradiation, this mixture contains something Oxalic acid and is suitable for a circulation decontamination in some systems.

Explanation of FIG. 1

Radiolytic decomposition of EDTA at GAMMACELL temperatures in CAN-DECON solutions at a pH of 4.5:

Q- containing 10 mmol-L sodium formate;

X - 25 mmol / L formic acid; the dashed line shows, for comparison purposes, the consumption of EDTA in the absence of formic acid. The dashed line means the initial slopes from which G (-EDTA) is calculated.

Explanation of FIG. 2

Radiolytic decomposition of citric acid and formation of oxalic acid at GAMMACELL temperatures in CAN-DECON solutions at a pH of 4.5:

Δ and V, consumption of citric acid as well

, Formation of oxalic acid (right scale) in solutions containing 10 or 25 mmol-L sodium formate and formic acid contain. The dashed line shows the consumption of citric acid in the absence of formate / formic acid.

Explanation of FIG. 3

Radiolytic decomposition of CAN-DECON solutions at 85 ⁰ C and a pH of 4.5: Q ^un <ä IH 'EDTA in the absence and presence of 10 mmol "L sodium; Q citric acid (no formate); Δ oxalic acid, produced in the presence of formate.

130036/0679

Explanation of the Flg. 4th

Radiolytic decomposition of CAN-DECON lJösungen at 85 ⁰ C and a pH of 4.5: Q, ££, EDTA and O · Φ 'citric acid consumption in the absence and presence of 10 mMol'L formic acid. Δ, generation of oxalic acid in solutions containing formic acid.

Explanation of FIG. 5

Radiolytic decomposition at 85 ° C of EDTA in simulated CAN-DECON solutions, the dissolved iron with and without formic acid contain.

symbol

Fe ²⁺ Fe ³⁺
(ppm) 100 pH Formic acid,
ιπΜόΙ L ~ J- 190 80 4.5 180 90 4.5 195 80 4.5 210 90 4.5-5.2 385 70 4.5 10 390 110 ± 25 4.5. 10 380 ± 20 235 + 35 4.3-4.4 10 490 + 30 4.2 20th

The dashed line shows ^the consumption of EDTA in the absence of dissolved iron or formic acid (from FIG. 4).

130036/0679

Explanation of FIG. 6

Radiolytic decomposition of CAN-DECON solutions in the presence of boron and formations:

openly symbolized solutions that contain 6000 ppm boron □, EDTA; O Ζ ^ Γοηβη33ϋΓβ; Δ, oxalic acid: X and φ, EDTA resp. Citric acid, in solutions containing 2000 ppm boron and 10 mmol «L sodium formate.

Explanation of FIG. 7

Radiolytic decomposition of CAN-DECON solutions at GAMMACELL temperatures in the presence of iron (III) oxalate (90 ppm Fe),

2+ -1

Iron (II) sulfate (45 ppm Fe) and 10 mol-L sodium formate.

Q, EDTA; Q, citric acid. The dashed line shows the decomposition of EDTA in the absence of additives.

1 30036/0679

Claims (10)

MÜ LLKR-IH) HK · DKfJFEl · · g ^ ft & C '"PATEHTANWAtT £ 3946563 DFt. WOLFGANG MÜLLER-BOHt (PATENT ADVOCATE FROM 1927 - 1975) DR. PAUL DEUFEL. DIPL.-CHEM. DR. ALPRED SCHÖN. DIPL.- CHEM.WERNER HERTEL. DIPL.-PHYS. APPROVED REPRESENTATIVES AT THE EUROPEAN PATENT OFFICE REPRESENTATIVES 8EFORE THE EUROPEAN PATENT OFFICE MANDATAtRES AGREES PRES UOFFICE EUROP ^ EN DES BREVET * Ά 2589 i O *) βϊ.! , Ontario, Canada Decontamination reagent and method for decontaminating a nuclear reactor or parts thereof claims 1. Decontamination reagent containing oxalic acid as a component, characterized in that it is organic acid Decontaminants that are radiolytically decomposable and contain formic acid in sufficient quantities to prolong the effectiveness of the decontaminants, the formic acid also being used when the mixture is irradiated provides some oxalic acid with ionizing radiation. 130036/0679 «MUNICH Bt. SIEBERTSTR. 4 POB 860720 CABLE: MUEBOPAT TEL. (0 89) 4740 05 · TELECOPIES XEROX 400 ■ TELEX 5-24.285 ORIGINAL FNSPECTED ' ² "3Q46563 2. Reagent according to claim 1, characterized in that there is at least some ethylenediaminetetraacetic acid (EDTA) contains. 3. Reagent according to claim 2, characterized in that it contains (a) EDTA, (b) citric acid, (c) oxalic acid and (d) formic acid, the approximate relative amounts by weight of (a), (b), (c) and (d) 1: (0.6-1.2) :( 0.1-1.2): (0.6-3). 4. Reagent according to claim 3, characterized in that the relative amounts by weight are approximately 1: 1: 1: 1. 5. Reagent according to claim 3, characterized in that it is in the form of a solution in water or heavy water is present. 6. Reagent according to claim 5, characterized in that the concentration of (a), (b), (c) and (d) in the solution about 0.03-0.05 wt%, 0.02-0.04 wt%, 0.01-0.04 wt%, and 0.03-0.15 wt%. 7. reagent solution according to claim 5, characterized in that the solvent consists of light water or heavy Consists of water and contains boron. 8. A method for decontaminating a nuclear reactor or parts thereof, if carried out an aqueous one Decontamination reagent solution in contact with the to decontaminating surfaces is allowed to circulate in the presence of ionizing radiation, the Solution containing organic acidic decontaminants including oxalic acid, characterized in that that formic acid is initially provided in the reagent solution and the presence of formic acid by at least one additional addition of this acid 130036/0679 3346553 is preserved. 9. The method according to claim 8, characterized in that the formic acid to a minimum concentration in the solution of about 0.03 wt .-% by additives
the circulating reagent solution is maintained. 10. The method according to claim 8, characterized in that the reagent solvent consists of light water or heavy Water and the solution also contains boron. 130036/0679