CA1136398A - Decontaminating reagents for radioactive systems - Google Patents

Abstract

TITLE
DECONTAMINATING REAGENTS
FOR RADIOACTIVE SYSTEMS

INVENTOR

William A. Seddon ABSTRACT OF THE DISCLOSURE
Surface contaminated with radioactive materials can be decontaminated by circulating an aqueous solution of decontaminating reagents which comprises organic acid decontaminating agents usually including oxalic acid, and formic acid, in contact therewith. The reagent preferably comprises both citric acid and EDTA in addition to oxalic and formic acids. It has been found that the efficacy of these organic acid decontaminating reagents preferably comprises both citric acid and EDTA in addition to oxalic and formic acids. It has been found that the efficacy of these organic acid decontaminating reagents can be prolonged under ionizing radiation by the inclusion of formic acid therein.

Description

~3~;3~3 Field of the I _ention This invention is directed to the decontamination of surfaces contaminated with radio-active materials, particularly coolant surfaces in nuclear reactors, by contact with circulating decon-tami-nating solution. ~ novel decontaminating reagent mixture ~
or reagent solution is provided having improved stability ~-and prolonged efficacy.
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Background and Prior Art -10 Nuclear reactors having water coolant clrcuit surfaces on which radioactive deposits occur, have been treated with decontaminating reagent solutions to remove such surface contamination. Usually the treatment results in radioactive li~uid wastes which are difficult and costly to dispose o~.
United States Patent 3,272,738, September 13, 1966, discloses the precipitation of radioactive metal corrosion products from EDTA-hydrazine solutions by addition of sulfuric acid and ferrocyanide. United States Patent 3,873,362, March 25, 1975, similarly precipitates `
radioactive metal contaminants using permanganate, alkaline earth oxides, or alkaline earth hydroxides.
United States Patent 4,162,229, July 24, 1979, utilizes a cerium IV salt as a decontaminating reagent.
Moto~ima et al in United States Patent 3~737,373, June 5, 1973, described the use of 0.1%
deuterated oxalic acid in D2O reactor coolant to dissolve contaminants. Irradiation of this oxalic acid-contaminant system caused decomposition of oxalates and precipltation of dissolved contaminants which were then recovered by 1~3~39~3 filtration and/or ion exchan~e techniques. In this process, the deuterated oxalic acid is destroyed re~uiring additional deuterated acid to eontinue decontamination. The reactor must be eycled between eool, suberitieal eonditions and hot eri-tieal condi-tions in this ~otojima et al process.
The "CAW-DECON" proeess has been devel~ped by ~tomie Energy o~ Canada Limited for decontamination of shutdown heavy water moderated and eooled reactors using ~ 10 dilute solutions minimizing corrosion and downgrading of ':
the heavy water, giving a redueed volume of solid waste for disposal and relatively short decontamination time (see Canadian Patent 1,062,590, September 1~, 1979, S.R. Hateher et al). This method involves injecting seleeted aeidie decontaminating reagents into cireulating eoolant to form a dilute reagent solution, eireulating said solution to dissolve deposits and then to contaet a :
- eation exehange resin which collects the dissolved cations and regenerates the acidic reagents for reeyele, and finally removing the reagents by eontact with an anion exehanger to restore the eoolan-t to its original condition trestoration of the eoolant is partieularly important with heavy water). Suitable aeidie reagents used include ethylenediaminetetraaeetie aeid (EDTA), oxalie aeid, ~itrie acid, nitrilotriacetic aeid and thioglyeolie aeid.
The iron-EDTA complex has been propos~d as an initial surfaee treating agent ~or steels in power plants (including nuelear) to create a good quality proteetive magnetite layer. M. Weber et al ~Aeta Chim. Aead. Sei. Hung.
97(3~, 255-26~, 1978) have studied the effect of ionizing

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radiation on the iron-EDTA comp:Lex -~ water surface txeatin~ system and have Eound that the radiolytic decomposition of this ferric complex can be decreased by additions of methanol or formic acid.
Recently it has been found that these organic acid decontaminating reagent mix-tures are subject to radiolysis in use leadin~ to their decompo-sition over varying times with resulting loss of effectiveness for decontamination. Substantially complete consumption of EDTA was observed at radiation doses of about O.g-l Mrad. Citxic acid used was al50 about 1/3 consumed at this dose, but the oxalic acid concen-tration remained stable. In the CANDU reactor decontamination process, it has been estimated that the average dose rate is about 0.3 Mrad per hour; leading -~ to the disappearance oE EDTA and substantial loss of effectiveness within about 3 hours.

Summarv of the Invention In accordance with the present invention, it has now been found that multicomponent organic acid decontaminating reagents which may includa oxalic acid and which are subject to radiolytic decomposi~ion are improved by incorpora-ting formic acid in sufficient amounts to prolong the efficacy of the mixture.
A preferred decontaminating reagent composition has been developed which comprises EDTA, citric acid, oxalic acid and sufficien~ ormic acid ~o increase the stability and prolong the efficacy of the mixture on exposure to ionizing radiation. The formic acid has been found to decrease the decomposition of .. . ..

~iL3~3~3 both EDTA and citric acid and to yield oxalic acid as a result of its own radiolysis. Thus formic acid causes minimal alteration oE the reagent composi-tion.
This reagent composition is particularly advantageous for decontaminations where prolonged circulation through the contamination ~one and cation exchange ` resins is carried out.
The invention includes, in the process for decontaminating surfaces having radioactive deposits in which a reagent mixture comprising organic acid decontaminating agents including oxalic acid is circulated over the contamina-ted surface, the improvement l of initially incorporated formic acid in the mixture I and maintaining the presence of formic acid by at least `~ one further addition thereof.
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Description of the Drawings Figure 1 is a graph showing radiolytic decomposition vs. radiation dose for EDTA in CAN-DECON :
solutions at pH 4.5 in the presence of sodium formate or formic acid.
Figure 2 is a similar graph showing consumption of citric acid and formation of oxalic acid vs. radiation dose in CAN-DECON solutions at pH 4.5 containing sodium formate or formic acid.
Figure 3 is a similar graph showing radiolytic decomposition of EDTA with sodium formate, and EDTA and citric acid without formate. The radio-lytic production of oxalic acid in the presence of formate is also shown (as in Figure 2~.
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63~8 Fl.gu:re 4 is simi1.ar to Figllre 3 and sho~s graphically -the decomposition of ~.:I)TI~ and cit.ric ac:icl and production of oxalic acid in the presence oE ~o~mic acid.
Figure 5 similarly depicts r~iolyt:ic:
decomposition of EDTA in the presellce of dissolved i.ron with and without two concentrations o formie acid. 'l'he dashed line represents the control (no iron or formic acid).
Figure 6 similarly sho~s c3rap~ically the eonsumption of EDTA, citric acid and oxa]ic acid in the presenee of boron and formate.
Figure 7 graphieally shows th~ decompo~
sition of EDTA and eitric acid in the prese~ee o fer~i.c oxalate, ferrous sulphate and sodi.um formate W.it}
inereasing radiation dose.

The presence o~ ormlc aei~ ~ f`ormate i.on is seen to decrease or retard the eonsumptir~ (or deeomposition) of the organie acids vexy considexabl~ and thus to prolong the effieaeyO

Detailed Deseription The amount of :Eormic cleid ln¢ol~porated initially can var~ eonsiderably wlth even 5~11 ~nC)~mtS
giving some benefit. Usually -t.he amount o~ ~c~rm;.c acid added is from about 1/^~ to about t~ice the weight of the organic acid decontaminating agents subject *o radiolytic decomposition. For the preferred E:DTA, citri.c acid, oxalic acid, formic acid system, the propor~i~ns hy wt.

respeetively :may be 1:(0.6-1.2) ([).1~ 3) h~t these are not critical. The .rel.ative proportions 1~ 1 are normally very sui.ta~le. Xn t:he relative~

~3639~

dilute CAN-DECON type of circulating solution, the concentrations of these same four components preferably would be approximately 0.03-0.05%; 0.02-0.04%; 0.01-0.04~;
and 0.03-0.15% by wt. respecti-~ely. It is considered desirable to maintain the formic acid concentration at or above a minimum value of abou-t 0.03% by wt. during the decon-tamination (by further additions thereof if necessary). Solution concentrations ranging up to saturation would be operative in situations other than CAN-DECON type decontaminations in CANDU reactors (such as in light-water-cooled reactors where the coolant may contain boron). Boron may also be present in heavy water coolant or moderator in some situations to control the neutron flux.
In these decontaminating reagent mixtures, P' on exposing to ionizing radiation, the formic acid radiolytic decomposition has been found to lead to the formation of some oxalic acid as well as H2 and CO2.
These gases can be removed by a degassing step as is fre~uently practised when upgrading the coolant for re-use.
This additional oxalic acid is an added bonus. The original oxalic acid will itself be subject to radiolytic decomposition (to a lesser extent than some of the other acids) but usually there is a net gain in oxalic content - on radiolysis of the mixture. Formic acid would not lead to any significant isotopic downgrading of the coolant in the case of heavy water coolant or moderator.
The organic acid decontaminating agents may comprise oxalic acid and usually one or more of EDTA, citric acid~ nitrilotriacetic acid and thioglycolic acid~
We have found mixtures ~f EDTA, citric acid and oxalic acid ~13~3~3 preferred in decontaminating heavy water reactor systems using CAN-DE~ON principles. The addition of hydrazine has been found advantageous for some applications. Other mixtures such as those including ascorbic acid or acetic acid or other acids mentioned above may be suitable for other systems, for example, light-water-cooled reactors.
In one way of carrying out the inven-tion, we prefer to add the organic acid mixtwre to the coolant of a shutdown reactor and circulate the decontaminating reagents in solution contac-t with the surfaces being decontaminated un-til radioactive compounds are dissolved, the coolant solution subsequently being passed through a cation exchange resin column (in the H+ or D~ form) and through filters to remove radioactive cations an~ any suspended solids~ and recirculated. Further addition(s~
of formic acid may be desirable at an intermediate step.
When the desired decontamination has been achieved, the solution is passed through both cation and anion exchange resins to remove dissolved metals and anions particularly the organic acid anions, thus restoring the coolant to normal.
Where borate anions are present, some or all o~ the anion exchangers would be saturated with boric acld to ensure that sufficiently high borate ion concentration is maintained during each stage of the decontamination. ~
degassing operation to remove gaseous radiolysis products may be desirable before restoring the coolant to service.
The ion exchange resins are loaded as close to capacity as possible and usually the cation and mixed ion exchange resins (which are now highly radioactive) are disposed of as a solid waste material. Such a solid waste ~3639~

is more conveniently disposed of than a liquid waste, and with less environmental problems. Where heavy water coolan-t or moderator is utilized in the decontamination, the ion exchange resins would be converted to the ~ and OD forms in order to avoid downgrading the deuterium conten-t.

The following Examples are intended to be illustrative only. For test purposes, a standard decon-taminating reagent consisted oE EDTA, citric acid monohydrate and oxalic acid dihydrate in aqueous solution at respective concentrations of 0.04 (1.37); 0.03 (1.43);
and 0.03 (2.38) weight ~ (the numbers in parentheses are in mmol-L 1). For tests in the absence of dissolved metals the pH was adjusted to 4.5 (normal working range) by the addition of LioH~ All solutions were irradiated in 50 ml glass syringes using a Gammacell 220 (trademark) Co60 source at a nominal dose rate of 1.67 x 101~ eV-L l s (16 krad-min 1). Solution temperatures in the Gammacell increased with absorbed dose but reached a steady state of 42C after a dose of ~2 Mrad (2 hours~O For test irradiation at 85C the syringes were immersed in a thermostatically-controlled water bath, the dose rate falling to 1.3 x 1019 eV-L l s 1 (12.5 krad.min 1). The reagent acid components were separated and analyzed by gas chromatography after esterification with BF3 in methanol. Dissolved iron concentrations (measured as Fe and Fe3~) were analyzed spectrophotometrically.
Solutions containing dissolved iron were stored in the dark to prevent photolytic decomposition o~ the acids.

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3~8 Exarnple 1 The radiolytic decomposition (consumption) of EDTA at 0.04 wt~ concentration in -the standard decontaminating reagent solution was followed in the presence of 10 mmol-L 1 sodium formate or 25 mmol-L 1 formic acid at pH 4.5 and at Ga~macell tempera-tures. At pH 4.5 the formic acid is present predominantly (85%) as the formate ion. Results are shown graphically in Figure 1. In Figure 1, the dashed line shows for comparison the conswnption of EDTA in the absence of formic acid.
The addition of formic acid/formate clearly enhances the radiolytic stability of EDTA in these solutions compared to the control (no formate). The dose required to consume 50% of the EDTA increases from about 6 to 15 fold in solutions containing 10 to 25 mmol~L
formic acid/formate. Initial disappearance yields correspond to G(-EDTA) = 0.30 and 0.03 molecules/100 eV
respectively. The dotted lines on the graphs represent the initial slopes from which G(-EDTA) was calculated. These values can be compared with G(-EDT~) = 2.3 in the absence of formic acid/formate.
In Figure 2, the corresponding effects on citric acid and oxalic acid are shown under the same conditions, the dashed line being the consumption of citric acid in the absence of formate/formlc acid. Again, relative to results obtained in the absence o formic acid/for~ate,~the citric acid is protected about 10 fold with G(~citric) = 0.05 + 0.015, whereas oxalic acid (xight hand scale) is actually produced durin~ radiolysis - with a yield G(oxalic) = 1.0 ~ 0.1.

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Example 2 -Radiolytic clecomposition of EDTA and citric acid was followed in the -test reagent solution at 85C and pH 4.5. ~esults are shown on the graph in Figure 3. Oxalic acid concentration was unaffected at doses < 4.7 x 1022 eV-L 1 (< 0.75 Mrad). In the ahsence and presence of 10 mmol-L 1 sodium formate, the left hand and center curves respectively show the effect on EDTA. The right hand scale shows a net production of oxalic acid under the conditions and in the presence of 10 mmol-L 1 sodium formate. In this latter solution citric acid remained relatively unaffected at doses < 14 x 1022 eV-L 1 (~ 2.2 Mrad).
Figure 4 shows the results of a similar experiment in the absence and presence oE 10 mmol-L 1 formic acid. Although the two graphs are not in per~ect agreement, both EDTA and citric acid were protected and oxalic acid was produced at doses exceeding ~1 Mrad. In both graphs, Figures 3 and 4, at 85C, EDTA was consumed at a rate about twice that observed at < 4~C.

~xample 3 The decomposition o~ EDTA at 85C ~as followed in the same reagent solutions containiny dissolved iron with and withou-t formic acid. Solutions containing dissolved iron were prepared by heating de-aerated reagent solutions to 85C in the presence of preox;dized carbon steel, XncorleL 600 ~l:rademark~ and type 410 sta:inless steel up to pH 4.5. The oxidized metals had been prepared by autoclaving at 300C in aqueous solutions adjusted to pH 10.5 with LiOH. This dissolution , ~3~ii3~g8 pretreatment would simula-te an initial decontamination stage but in the absence of radiation. In the solutions containing 10 mmol-L 1 formic acid, the total dissolved iron at pH 4.5 increased from about 260~290 ppm up to 480 ~ 25 ppm largely as Fe2 with the Fe concentration remaining unchanged at 80 ~ 10 ppm. Doubling the formic acid concentration to 20 mmol-L 1 further increased the dissolved iron to about 725 ppm and the Fe to 235 ~ 35 ppm. The results are shown in Figure 5. The dashed line represents the EDTA results from Figure 4 without iron or formic acid. No decomposition of citric acid was observed at radiation doses ~ 4 Mrad.
Before irradiation but after reaction with the preoxidi~ed metal, the initial oxalic acid concentra-tion had been decreased by about 75%, apparently due to the formation of a ferrous oxalate precipitate. Radiol~sis with doses < 5 Mrad did not decrease the oxalic acid concentration further and in some cases an increase was observed (the effect was not very reproducible). The initial exposure to preoxidized metal also decreased the EDTA by about 10~ in the presence of 10 mmol-L formic acid, and by about 30% in its absence. A protect:ive effect due to formic acid on the non-radiolytic decomposition of EDTA was achieved during initial metal dissolution, an extra advantage. Citric acid concentration remained stable.
From these tests with dissolved iron, it is indicated that the iron (predominankly Fe~ ) also inhibits to some extent the radiolytic decomposition of EDTA and citric acid. This effect is further enhanced by formic acid but unlike the results presented in Figures 2-4, oxalic acid is not produced in compara~le amounts.

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The overall effect observed on completion of this Example with formic acid and dissolved iron was enhanced protection for EDTA and citric acid with some reduction of the initial oxalic acid concentration due to precipitation. The formic acid was evidently also an aid to iron dissolution.

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In light water reactors boron is added to control the neutron flux for both regular operation and shutdown purposes. The effects of boron, if any, on the radiolysis of the test reagent mixture including formic : acid are important with respect to the application of this ; invention to such reactor systems.
The decomposition of EDTA, citric acid and oxalic acid was followed as before but in -the presence of (a) 6000 ppm boron (added as H3BO3), and . (b) 2000 ppm boron and 10 mmol-L 1 sodium formate.
- The results are show~ in Figure 6. The protective effect of the ~ormate was comparable to that in the absence of : boron (Figures 1 and 2). Although the initial consumption : 20 of oxalic acid (not shown~ in the 2000 ppm boron/formate solutions appeared to be increased over the 6000 ppm boron ;~ solutions, this effect was balanced by a compensating radio- ~.
lytic pxoduction resulting in no net decomposition being detected after about 4 Mrad doses.
Example 5 Figure 7 shows the decomposition of EDTA and citric acid in the same starting solutions containing in .addition 90 ppm Fe3 as ferric oxalate, 45 ppm Fe2~ as ,fexrous sulfate and 10 mmol-L 1 sodium formate.' No net 3Q consumption or production of oxalic acid was detec-ted at doses < 4 Mrad. Thus the decontamination efficacy of the mixture would still be good after 4 Mrad radiolysis.

~L~3~3~l3 Example 6 In order to detexmine whether the addition of foxmic acid had any effect on corrosion of carbon steel in the system and on decontamination of this steel, tests we~e run using the 0.1 wt.% standard test solution with and without 0.046 wt.% formic acid at 85C. A test loop was used which included an ion exchange purification section.
Corrosion runs were carried out on test coupons of carban steel which had been pickled and degreased (and some prefilmed as in Example 3). Decontamination runs were carried out uslng test coupons of carbon steel which had been degreased and exposed to radiation in-reactor for eight weeks to ; render them radioactive. Also for decontamination runs, prefilmed carbon steel foil was added to the loop to cause approximate typical CAN-DECON iron concentrations in the solution. Before each test the loop was purged ~ .
, with N2 and the water used was deoxygenated.
The corrosion tests showed there was no , significant difference in corrosion rate of the carbon steel due,to the formic acid. Corrosion rates measured were from 0.12 to 0.13 ~m/hour for the pickled coupons~
and from 0.15 to 0.25 ~m/h for the prefilmed surEaces.
There was also no decrease in the decontamination and purification achieved with formic acid present. Whlle these tests were conducted in the absence of radiolysis, under radiolytic conditions the formic acid would, as shown earlier, prolong the efficacy and performance of a given mixture.
Further tests with EDTA alone with formic acid showed a protective effect and prolonged efficacy of the mixture. After irradiation this mixture would include some oxalic acid, and would be useful for recirculation de-contamination in some systems.

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~L3~ii398 . .

Figure 1 Caption Radiolytic decomposition of EDTA a-t GAMMACELL temperatures in CAN-DECON solutions at pH

4.5: O - containing 10 mmol~L 1 sodium formate;
X ~ 25 mmol-L 1 formic acid; dashed line shows, for comparison, the consumption of EDTA in the absence o~ formic acid. The dotted lines represent the initial slopes from which G(-EDTA) is calculated.

Figure 2 Caption Radiolytic decomposition of citric acid and formation oE oxalic acid at GAMMACELL temperatures in CAN-DECON solutions at pH 4.5:
and V, consumption of citric acid, and , ~ , formation of oxalic acid (right hand scale) in solutions containing 10 and 25 mmol-L 1 sodium ~ . . ~
formate and formic acid, respectively. The dashed line shows the consumption of citric acid in the absence of formate/formic acid.

Figure 3 Caption Radiolytic decomposition of CAN-DECON
solutions at 85C and pH 4.5: a and ~ , EDT~ in the absence and presence of 10 mmol-L ~ sodium formate, respectively; ~, citric acid (no formate); ~, oxalic acid, production in the presence of formate.

Figure 4 Caption Radiolytic decomposition of CAN-DECON
solutions at 85 C and pH 4.5: ~ , ~ , EDTA and O, ~, citric acid consumption in the absence and presence of 10 mmol-L 1 formic acid, respective~y. ~, production of - oxalic acid in solutions containing the formic acid.
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Figure 5 Caption Radiolytic decomposi-tion at 85 C of EDTA
in simulated CAN-DECON solutions containing dissolved iron with and without formic acid.

SymbolFe2-~ Fe3~ pH formic acid (ppm) mmol.L-l .. _ . _ ,. ,.,.. ,, _ O lgO 100 ~.5 ~__ 180 80 4.5 ---195 go ~.5 ___ 210 80 4.5-5.2 ---~7 385 90 4.5 10 X 390 70 4.5 10 ~ 380*20110+25 4.3-4.4 10 - ~ ~ 490+30235~35 4.2 20 The dashed line represents the consumption of EDTA in -the absence of dissolved iron or formic acid (from Figure 4).
, Figuxe 6 Caption - Radiolytic decomposition of CAN-DECON
solutions in the presence of boron and formate ion:
open symbols solutions containing 6000 ppm boron ~ , EDTA;

O, citric acid; a, oxalic acid: X and ~, EDTA and citric acid, respectively, in solutions containing 2000 ppm boron and 10 mmol-L 1 sodium formate.

- Figure 7 Caption Radiolytic decomposition of CAN-DECON
- solutions at GAMMACELL temperatures in the presence of ferric oxalate (90 ppm Fe ), ferrous sulfate (45 ppm Fe ) and 10 mmol-L sodium formate. ~ , EDTA; O, citric acid. The dashed line represents the decomposition of EDTA in the absence of additives.

Claims (13)

1. A dilute decontaminating reagent aqueous solution comprising: organic acid decontaminating agents subject to radiolytic decomposition including at least one of citric acid and EDTA, and formic acid in sufficient amounts to prolong the efficacy of said decontaminating agents under ionizing radiation, said formic acid concentration being from about 4 to about twice the concentration of said decontamin-ating agents.

2. The dilute solution of claim 1 including at least some ethylenediaminetetraacetic acid (EDTA).

3. The dilute solution of claim 2 comprising:
(a) EDTA;
(b) citric acid;
(c) oxalic acid;
and (d) formic acid;
the approximate relative proportions by wt. of (a), (b), (c) and (d) being 1:(0.6-1.2):(0.1-1.2):(0.6-3) respectively.

4. The dilute reagent solution of claim 3 wherein the relative proportions by wt. are about 1:1:1:1 respectively.

5. The dilute reagent solution of claim 3 wherein the aqueous solvent is heavy water.

6. The dilute reagent solution of claim 3 wherein the concentrations of (a), (b), (c) and (d) in solution are approximately 0.03-0.05%; 0.02-0.04%; 0.01-0.04%; and 0.03-0.15% by wt.

7. The dilute reagent solution of claim 1 including dissolved boron for neutron flux control.

CLAIMS (cont.)

8. The dilute reagent solution of claim 2 including oxalic acid.

9. In a process of decontaminating a nuclear reactor or components thereof in which an aqueous decontami-nating reagent solution is circulated in contact with the surfaces being decontaminated in the presence of ionizing radiation, said solution comprising organic acid decontami-nating agent subject to radiolytic decomposition, the improvement comprising initially providing formic acid in the reagent solution at the start of the decontamination and maintaining the presence of formic acid by at least one further addition thereof.

10. The process of claim 9 wherein the formic acid is maintained at a minimum concentration in solution of about 0.03% by wt. by additions to the circulating reagent solution.

11. The process of claim 10 wherein the reagent solvent is light water and the solution also includes dissolved boron.

12. The process of claim 10 wherein the reagent solvent is heavy water.

13. The process of claim 10 wherein the formic acid concentration is within about 0.03 to about 0.15% by wt.