Although TBP and the hydrocarbon diluent are comparatively stable compounds, they slowly react in Purex systems with formation of degradation products that impair separation performance. The principal deleterious reactions are reaction with radioiodine, hydrolysis, and radiolysis. These will be discussed in turn.

Reaction with radioiodine. Any iodine left in dissolver solutions slowly reacts with TBP and diluent to form iodine compounds that cannot be removed by subsequent alkaline washing. Thus, it is important to remove as much iodine as possible from the dissolver solution before solvent extraction and to use low-inventory contactors in the first, HA, extraction step.

Hydrolysis of TBP. Hydrolysis of TBP occurs stepwise via dibutyl and monobutyl phosphoric acid ([C4H9O] 2P02H and C4H9OPQ3H2) and leads eventually to phosphoric acid. Dibutyl phosphoric acid is the most abundant degradation product. Its rate of formation is influenced by temperature, the nitric acid concentration, the uranium content, and the presence of a diluent, beside the radiation dose. The acidic nature of these hydrolysis products allows in principle cleanup by an alkaline wash.

The effect of TBP degradation products, particularly of dibutyl phosphoric acid, is the formation of strong complexes with uranium(VI), plutonium(IV), zirconium, and niobium. The sequence of complexing strength is Zr > Pu(IV) > U(VI) > Nb.

The uranium and plutonium complexes are strong enough to remain in the organic phase during stripping. In reprocessing LWR fuel, uranium is mainly affected because of its great

Table 10.14 Mutual solubility of water and TBP — dodecane mixtures at 25°C v/o TBP g/liter TBP in H20 Hj 0 in organic 10 0.18 1.2 20 0.24 3.5 30 0.27 7.2 40 0.285 11.5 60 0.31 23.7 100 0.42 64.6

excess. As a consequence, uranium is lost to the waste in the solvent wash. In LMFBR fuel, plutonium is taken up by the degradation products to a significant extent. It can be removed only by an alkaline wash with fluoride addition.

The other detrimental effect of TBP degradation is its complexing of zirconium. This increases the zirconium distribution coefficient and consequently decreases the decontamination coefficient. Moreover, solvent residual radioactivity is increased because of incomplete zirconium reextraction. Another and even more troublesome consequence of zirconium complexing is the formation of precipitates known as crud. This is a severe problem, particularly in mixer-settlers, and has led to a preference for pulsed columns or centrifugal contactors in the first extraction cycle when high-burnup fuel is to be processed.

Table 10.15 shows the effect of temperature and organic-phase nitric acid concentration on the rate of formation of dibutyl phosphate in 30 v/o TBP, as reported by Siddall [SI5].

With the relative volumes of aqueous and organic phases usually present in Purex systems, the amount of DBP formed in the aqueous phase is much smaller than in the organic phase because of the low aqueous solubility of TBP. The practical consequence of these rates is that if the solvent is washed with water to remove HN03 and with aqueous sodium carbonate to remove DBP after less than 15 min contact with process solutions at temperatures under 70°C, the concentration of DBP in process contactors can be held so low that solvent separation performance is not degraded. The DBP concentration after 15 min at 70°C is approximately

Nitration and oxidation. Nitric acid does not react appreciably with TBP at temperatures up to 70°C. At sufficiently high temperatures, however, nitration and oxidation take place. In two instances reaction of TBP-hydrocarbon mixtures with hot, concentrated solutions of nitric acid and uranyl nitrate led to destructive explosions. At Savannah River in 1953 [Cl 1], an evaporator was destroyed while concentrating a solution of nitric acid and uranyl nitrate that contained TBP and a kerosene diluent. At Oak Ridge in 1959 [A8], an explosion occurred in a radiochemical plant evaporator that was concentrating a nitric acid solution of plutonium nitrate possibly contaminated by TBP, diluent, and their radiation degradation products.

Because of these accidents laboratory studies were made at Hanford [Wl] and Savannah River [Cl 1, N5] to determine the conditions under which nitric acid solutions possibly containing TBP could be safely evaporated. Wagner [Wl] reported that a “red oil” formed by extended refluxing of a concentrated aqueous solution of uranyl nitrate, nitric acid, and TBP decomposed autocatalytically when heated to 150°C. Nichols [N5] found that a mixture of 10.5 M HN03 and TBP enters into a runaway reaction when heated rapidly to 130°C, but not at 125°C. Protective measures recommended to prevent future explosive reactions were to (1) minimize the amount of TBP added to the evaporator; (2) permit TBP to steam distill during evaporation; (3) hold the temperature below 130°C until all the TBP has been distilled.

Table 10.15 Rate of formation of DBP in 30 v/o TBP Moles HN03 per liter in solvent Rate of DBP formation, v/o per day, at 25°C 40°C 70°C 0.2 0.0002 0.0010 0.033 0.4 0.0003 0.0017 0.043 0.6 0.0003 0.0015 0.048 0.8 0.0003 0.0015 0.051

Table 10.16 Effect of radiolytic energy density on decontamination in Purex process

Wh/liter Effect

0. 1 Process performance unimpaired

1. Noticeable but not too serious effects

10 Catastrophic loss in decontamination

Radiolysis. Radiation degrades both TBP and hydrocarbon diluent in Purex systems, with formation of molecular fragments, polymers, and nitration products. The main product, however, is the same as from hydrolysis, namely, DBP. The yield of DBP in radiolysis of TBP varies somewhat with the diluent used, water content, type of radiation, and dose rate. Baumgartner and Ochsenfeld [B6] cite production of 20 to 30 mg DBP/liter in 30-min exposure of 30 v/o TBP to 0.2 Wh/liter of radiation in mixer-settlers processing fuel cooled 240 days after 33,000 MWd/MT burnup. Because the density of DBP is 1065 g/liter, the volume percent DBP was

(100) f° °2 ),03>) = 0.0019 to 0.0028 v/o (10.8)

1065 /

Hence the radiation exposure that would produce the same amount of DBP as the 0.0005 v/o produced by hydrolysis for 15 min at 70°C, which has negligible effect on decontamination, is

In addition to DBP, ionizing radiation produces in TBP-hydrocarbon mixtures long-chain acid phosphate esters, nitrohydrocarbons, and nitrate esters that also complex uranium, plutonium, and zirconium, and that cannot be removed by simple alkaline washing. These must eventually be removed either by purging a fraction of the solvent or treating it with strong oxidants [B8].

Siddall [SI 5] summarizes the effects of increasing radiation exposure on decontamination in the first Purex extraction contactor as shown in Table 10.16. The power density, or dose rate, also has an effect on solvent performance. Baumgartner [B5] cited experiments in which

1.2 Wh/liter, delivered to 20 v/o TBP in one pass through the HA and HS contactors, reduced the zirconium decontamination factor from 1000 to 10.

The principal causes of radiolysis in a Purex plant are beta and gamma radiation in the first extracting unit (HA) and alpha radiation from plutonium in the plutonium purification units. Accurate calculation of radiation absorption by solvent is difficult, because it depends on details of the dispersion of aqueous and organic phases and contactor geometry. Blake [Bll] has given equations for estimating solvent radiation absorption when these details are known.

An upper bound for the exposure in the HA unit may be obtained by treating the solvent as uniformly dispersed as small droplets in the aqueous phase, assumed as containing all of the radiation sources. Then the radiation exposure is

R = vD[f + 6(1 ~f)]t (10.10)

where R = exposure of organic phase in watt-hours per liter

t = residence time, hours, of organic phase in contact with aqueous и = volume fraction of aqueous phase D = power density in aqueous phase, watts per liter f = fraction of radiation as beta radiation

1 — / = fraction of radiation as gamma radiation

в = fraction of gamma radiation absorbed in contactor This equation will be applied to the HA contactor of the Barnwell plant. From Tables 10.7 and 10.8, the activity of the aqueous waste HAW stream is

The watts per curie in this stream may be obtained approximately from Table 8.7 as

Hence D = (1251 Ci/liter) (0.00469 W/Ci) = 5.9 W/liter (10.13)

Blake [Bll] gives approximate values for/(0.65) and в (0.4). The volume fraction of aqueous phase v is lower with organic phase continuous than with aqueous. It will be assumed that the HA contactor will be run with organic phase continuous, with и = 0.25. Then

R = (0.25)(5.9 W/liter)[0.65 + (0.4)(0.35)] t = 1.17fh Wh/liter (10.14)

To keep organic exposure below 0.1 (Wh)/liter, the organic residence time should be below

0. 1/1.17 h, or 5 min. Thus, short-residence-time contactors, like the centrifugal contactor specified for Barnwell, are desirable for this primary decontamination service.