Atomic Energy, Vol. 91, No. 2, 2001

INCREASING 129I TRANSMUTATION EFFICIENCY

A. A. Kozar’

UDC 546.718:621.039.7:539.174

The special features of 129I transmutation are discussed. The changes in content and equilibrium concentration of 127I in a target under recycling are calculated, and the conditions for decreasing the required radiochemical capacity in reprocessing of iodine are found. It is shown on the basis of an analysis of the neutron consumption in targets and estimates of the pressure of daughter xenon under the casing that the gaseous products of transmutation must be removed during the irradiation process along a loop channel. To increase the effectiveness and safety of 129I burnup, it is recommended that 129I be included in the volume of the gas permeable porous nuclear-inert targets by permeation with melts of iodine compounds.

About two tons of one of the most environmentally dangerous radionuclides – long-lived 129I with half-life 1.57·107 yr – is produced in the world every year. It is estimated that by 2010 the total accumulation of this radionuclide in off-loaded fuel will be ~60 tons [1, 2]. The 129I yield on fission is approximately 0.9%, and the content in spent fuel from water-moderated water-cooled reactors with the ordinary burnup of 30–40 GW·days/ton is 210–270 g/ton [3]. Long-term reliable isolation of 129I in solidified radioactive wastes is problematic because of the migration characteristics of iodine in the biosphere, which are due to high volatility, complicated chemical behavior, and diverse forms dissolved in water and also low sorption coefficients with respect to the most common minerals and soils [4–6]. For this reason, 129I is currently regarded as one of the main candidates for elimination by transmutation by irradiation with thermal and resonance neutrons. However, it should be noted that in contrast to its main competitor in the order of implementation of transmutation decontamination – fragment 99Tc, which when burned gives the valuable platinide rhutenium [7, 8], there is no such great commercial motivation for eliminating 129I. In addition, the characteristics of the transmutation process for this nuclide have been studied much less than the scientific developments for 99Tc, and the modern experimental achievements in burning 129I do not exceed 6% burnup [9], while 30% burnup has been achieved for 99Tc [10]. Iodine is one of the irreplaceable biogenic elements; its compounds play an important role in metabolic processes, and the content in humans is ~25 mg (4·10–5%) [11], which can serve as an additional argument in favor of transmutation of 129I. 129 I Transmutation Scheme. In contrast to isotopic separation of iodine after its short-lived isotopes have decayed, the target will contain, aside from 129I, the ancillary initial stable nuclide 127I, whose relative content is 12–25% depending on the history of the spent fuel and is mainly 15–20% [12, 13]. For real conditions of irradiation, the transmutation scheme for 129I and 127I (Fig. 1) can be greatly simplified, since for nuclides with competing transition channels, because of the low neutron capture cross sections, β decay occurs with overwhelming probability. The cross sections for radiative capture of thermal neutrons by 129I and 127I are 27 and 6.2 barns, respectively; their resonance systems range from 50 eV up to 4 keV and give resonance integrals of 36 and 147 barns. The burn rate of these isotopes is the same with spectrum hardness γ0 ~ 0.2 (Fig. 2). It should be noted that for large neutron fluence, long-lived 135 Cs (half-life 2.3·106 yr) is produced according to the chain 132Xe(n, γ)133Xe(β–, 5.25 days)133Cs(n, γ)134Cs(n, γ)135Cs. This radionuclide is one of the main candidates for annihilation.

Institute of Physical Chemistry, Russian Academy of Sciences. Translated from Atomnaya Énergiya, Vol. 91, No. 2, pp. 139–146, August, 2001. Original article submitted March 5, 2001. 1063-4258/01/9102-0667$25.00 ©2001 Plenum Publishing Corporation

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Fig. 1. Transmutation scheme for 127I and 129I.

Fig. 2. Transmutation dynamics of 127I (a) and 129I (b) as a function of the thermal-neutron fluence in a spectrum with hardness 0.2: 1) 127I; 2) 128Xe; 3) 128Te; 4) 129Xe; 5) 129I; 6) 130Xe; 7) 131Xe; 8) 132Xe; 9) all other nuclides (133Xe, 133–135Cs and others).

Change of the 127I Concentration in the Target. For neutron-spectrum hardness γ < γ0, 127I gradually accumulates as the irradiation–reprocessing (recycling) of the target is repeated, and the transmutation efficiency for the main starting nuclide decreases. Consequently, the radiative neutron capture cross sections of 129,127I are less their resonance integrals σ129 < I129 and σ127 < I127; in a bulk iodine target the spectrum hardness decreases away from the surface toward the center, and consequently the 127I content can increase even when the transmutation system itself possesses a spectrum with γ > γ0. If the element for irradiation consists of n isotopes with relative concentration c0i (i = 1, ..., n) and burnup qi for a run, then in the absence of transitions between them the content of the mth isotope in the (k + 1)st load (k ≥ 1) after target recycling can be calculated using the recurrence relations n

c1m = c0m ;

∑ i =1

n

c0i =

∑ i =1

n

cki = 1 ; ckm+1 = ckm (1 − qm ) + c1m

∑ cki qi .

(1)

i =1

2 127 127 For transmutation of an iodine target n = 2, c10 = c129 I concentration in 0 , c0 = c0 , q1 = q129, q2 = q127, and the it varies according to the expressions

c1127 = c127 0 ; 668

127 127 127 ck+1 = c127 0 (q129 + ck (q127 – q129)) + ck (1 – q127).

Fig. 3. 127I concentration in iodine targets versus the irradiation cycle with initial iodine 127I content of 15% for a run with 129I and 127I burnup of 50 and 70% (1), 50 and 30% (2), 30 and 40% (3), 30 and 20% (4), respectively.

127 To investigate the behavior it is more convenient to express the function c127 I concentrak in terms of the initial tion in the target, even though this leads to a complicated formula: k −1

127 127 127 c127 k = c0 + c0 (1 − c0 )

i −1 i 127 i −1 i 0 ) ( q127 ) − (c0 ) ( q129 ) ] + ∑ (−1)i Cik [(1 − c127 i =1

k −2

+

i + j +1 i 127 j i j − Cii + j )(1 − c127 0 Ci 0 ) (c0 ) ( q127 ) ( q129 ) , ∑ (−1)i + j Cik+ j (c127

(2)

i, j =1 i + j
where Cik =

1 i!

i

∏ (k − m). m =1

m Setting in Eq. (1) c m k+1 = ck in the limit k → ∞, we can find the equilibrium concentration of the mth isotope in the target from the formula m ceq =

c1m

n

∑ qi ceqi ,

qm (1 − c1m ) i =1

(3)

i≠m

i is the equilibrium concentration of the ith isotope. From Eq. (3) we obtain where ceq 127 127 c127 eq = c0 q129 /( q127 − c0 ( q127 − q129 )).

(4)

As follows from the expression (2), the contribution to ck127 of terms including high burnup q129 and q127, is negligibly small, so that the 127I content actually becomes constant after several irradiation cycles, and all the more rapidly, the larger q129 is (Fig. 3). According to the relation (4), when the sign of the inequality between q129 and q127 remains unchanged, the equilibrium 127I concentration is essentially independent of the 129I burnup, especially for q129 < q127, and in this case its fluctuations accompanying a change in the run parameters can be neglected. Increasing the fraction of resonance neutrons in the spectrum makes it possible to decrease the radiochemical capacity, required for target recovery, by approximately 5–10%. Nonetheless, the striving to increase the spectrum hardness in order to decrease the 127I concentration in the iodine target on account of q129 < q127 (see Fig. 3) is limited at high burnup by accumulation of 131Xe and 129Xe (see Fig. 2), whose 669

resonance integrals are 900 and 250 barns, respectively. Although the capture of neutrons from this part of the spectrum by the indicated nuclides does not greatly influence the 129I transmutation rate because of the overlapping of their resonance peaks is small, the total consumption of neutrons whose value for large transmutation of radioactive wastes increases appreciably increases sharply. Apparently, one system suitable for 129I transmutation is a reactor with a weakly resonant spectrum with hardness γ ~ 0.5, which approximately corresponds to the upper limit of this parameter in VVÉR-400 and -1000 reactors [14]. The possibilities of burning 129I in reshaped fast reactors after formation of a neutron spectrum using various moderators are being examined [13, 15, 16]. Consumption of Neutrons for Transmutation. Xenon poisoning of an iodine target is the limiting factor for achieving high 129I burnup. The choice of maximum burnup can be based on an estimate of the ratio between the neutron consumption for the useful reaction 129I(n, γ)130I and expenditures on the capture reaction of the remaining nuclei in the target only in the thermal part of the spectrum, where the influence of neutron losses on the 129I transmutation rate is determining:

δI =

∑ σi9ci9 + ∑ σi7ci7 + σ127c127 i

i 129 129

σ

c

.

Here σi9 and σi7 are the thermal-neutron capture cross sections of the nuclei of the products of transmutation of 129I and 127I of the ith type, respectively; c129, c127, ci9, and ci7 are, respectively, the relative 129I and 127I concentrations and the concentrations of their daughter nuclides of the type i. According to the terminology adopted, all reactions in the chain of the ancillary starting nuclide are parasitic [14]. However, not only burnup but also production of 129I from 127I occur during irradiation of an iodine target. Since it is impossible to eliminate this harmful process, the neutron capture reactions for daughter 129 I should be regarded as conditionally useful, equivalent to neutron capture by the initial 129I nuclei. Consequently, in the relation presented c129 is the total 129I concentration in the target, including the 129I formed in the 127I chain, and the term in the numerator corresponding to the daughter product 129I is absent. This remark is more of a methodological character, since the production of 129I from 127I is insignificant and has no effect on the computational results. If one proceeds from the requirement that neutrons be economized, their resonance absorption must be taken into account using the formula

δ II =

∑ (σi9 + γ Ii9 )ci9 + ∑ (σi7 + γ Ii7 )ci7 + (σ127 + γ I127 )c127 i

i

(σ129 + γ I 129 )c129

,

where Ii9 and Ii7 are resonance integrals of the terms of the transmutation chains of 129I and 127I, respectively. If only thermal neutrons give the main value and their consumption is the same in accordance with the behavior of the criterion δI in Fig. 4, increasing the spectrum hardness will increase 129I burnup by only a small amount. Thus, burnup of a target with 15% initial 127I content, for which the useful consumption of neutrons is equal to their losses, increases from 46% to only 52% as γ increases from 0.05 to 0.5. Therefore, in terms of the criterion δI the production of transmutation products has no effect on the burnup rate of the starting nuclide or on the economic value of this operation. In reality, the requirement that the total neutron consumption on elimination of 129I, as estimated according to δII, be decreased is better substantiated. In contrast to the preceding case, where the criterion δI is calculated, an increase of spectrum hardness increases the relative neutron loss, though not very appreciably. The different behavior of the functions δI(q) and δII(q) is reflected in the different signs of their partial derivatives: ∂δI /∂γ < 0, while ∂δII /∂γ > 0. Since ∂(δII – δI)/∂γ > 0, as one can see from Fig. 4, the difference between the losses of thermal neutrons and the total losses also increases with γ. Thus, increasing γ to accelerate 129I burnup results in a substantial growth of neutron absorption of resonance neutrons by transmutation products; this can serve as a basis for stopping irradiation in the region of low target burnup: the neutron losses in unavoidable and parasitic reactions are half the useful consumption even with 15–25% burnup and are equal to it with 35–40% burnup. 670

Fig. 4. Fracture of harmful neutron capture reactions in an iodine target with initial isotopic composition 85% 129I + 15% 127I in a spectrum with hardness 0.05 (– – –) and 0.5 (——), calculated for the criteria δI (1) and δII (2), as a function of 129I burnup.

Minimum Admissable 129I Burnup. The lower limit of the boundary of admissable burnup of the nuclide being burned is closely related with the possibility of achieving the transmutation goal itself – decreasing the ecological danger due to the presence of wastes of a given type in the environment by a given factor. There are no long-lived nuclei among the transmutation products of 129I. Consequently, the estimate of the burnup results reduces to determining the decrease in the mass of this nuclide, entering the biosphere as part of low- and medium-level wastes. This greatly distinguishes it from Np, Am, Cm, and others, the α activity of whose burnup products is higher immediately after irradiation than in the starting load, and the required decrease of the toxicity index for wastes entering the human body with drinking water is reached only after long-term storage of the wastes [17–19]. The usual approximate goal for eliminating 129I is to decrease its nonreturnable losses to 5–10% [20, 21] and by 2% for the more optimistic estimates [13]. In other words, even though the overall ecological danger of 129I decreases, the positive effect from burnup will not be obvious because of the existence of an opposite process. For total yearly production P and burnup q in a transmutation run, the required production capacities for recycling targets will be P/q, and the local dose load on workers involved in iodine reprocessing will increase by a factor of q–1 [8]. At the present time the purex process allows extraction of only 95% of the iodine from spent fuel. This parameter also remains in French plans for constructing in 2010–2020 a plant for reprocessing fuel and transmuting EFTRP targets [20] as well as in the industrial program requirements of the Ministry of Atomic Energy of Russia for radiochemical technology, which will be the basis for the closed fuel cycle with transmutation of long-lived waste components [22]. In addition, progress in the development of a technology for catching iodine gives hope for future decrease of iodine losses to 2% [23]. The main transmutation products of 129I are gaseous, with the exception of trace quantities of 128Te formed from ancillary 127I, and cesium isotopes whose accumulation becomes appreciable only at high neutron fluence. Consequently, target recycling is a much easier problem than the removal of iodine from fuel. It should be expected that the relative 129I losses in radiochemical reprocessing of irradiated targets will be lower than that indicated for fuel and will not exceed 1–2% [13]. The total losses of the burned radionuclide into the environment can be determined from the formula [24, 25] WΣ = 1 – q(1 – w0)/[1 – (1 – q)(1 – w)],

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where q is the burnup in one run; w0 are the relative losses accompanying removal from spent fuel; and, w are the relative losses over one target recovery cycle. Hence we obtain the following expression for the lower limit of burnup: qmin = {1 + (WΣ – w0)/[w(1 – WΣ)]}–1, which for w0 = w simplifies to qmin = (WΣ–1 – 1)/(w–1 – 1). With the WΣ < 10% and the realistic (in the near future) situation where w0 ~ 5% and w ~ 2%, the minimum admissable burnup approaches 30%. For stricter requirements WΣ < 5% even taking account of the appreciable progress in the development of iodine catching technology and the decrease in the elementary losses to w0 ≈ 2% and w ~ 1%, burnup should exceed 25% per cycle. It is impossible to achieve such high 129I burnup per run while meeting the requirements for economizing neutrons. Consequently, it becomes necessary to decrease the fraction of parasitic reactions by removing gaseous transmutation products in a loop channel during irradiation [26, 27]. Forms of the Iodine Target. The daughter product xenon is most easily removed using a liquid iodine target, but the scientific–technical substantiation of this scheme is still inadequate. Moreover, a low safety level is characteristic for a process based on a liquid target – as a result of high volatility of iodine and its compounds, a leak in the loop will result in an accident with possible emission of 129I into the atmosphere. Today all simple iodine compounds with nuclear-inert elements that could be viewed as possible chemical forms for a target have now been studied: (K, Na)I, (Bi, Be, Ca, Cu2, Mg, Pb, Sr, V, Zn)I2, (Ce, In, Y)I3, ZrI4, and others; their neutron-physical characteristics differ very little and have no effect on the choice of the most promising target form. Even though in experiments on irradiation of NaI and CeI3 targets for 193 days up to fluence 2·1022 cm–2 with a thermal component 3.2·1021 cm–2 129 I burnup of about 5.13% was achieved for the first compound and 5.87% has been achieved for the second compound [9], the 12% relative difference of burnup can be explained by approximately the same difference in the nuclear density of iodine in the samples. It is possible that compounds which are highly soluble in water should be eliminated from the list of candidates as being the most dangerous for long-term storage before industrial implementation of 129I burnup. KI targets are also doubtful because during irradiation long-lived 40K (with half-life 1.262·109 yr) is produced, though in small quantities. Iodine compounds possess a relatively low melting or decomposition temperature – mainly, 400–900°C – and low strength. Consequently, to increase transmutation safety they must be incorporated in a strong diluent. The simplest solution is probably permeating simple nuclear-inert matrices, such as ZrC, ZrO2, MgO, Al2O3, and others, with melts of iodine compounds. For ~1 cm fuel pellets with ~50 µm pores, the free volume in the pellets can be easily filled up to 80% even after a single permeation [27]. Such targets possess a rigid spatially-continuous framework, which limits their destruction in chemical changes of the iodine component during irradiation, and their high gas permeability makes it possible to remove transmutation products [26–28]. Similar targets can also be obtained by compression under conditions where the diluent fraction is somewhat smaller than the fraction of iodine compound particles, which creates the target framework. However, even though xenon diffuses easily, it is difficult to remove xenon from compressed targets because the gas permeability is too low. When one iodine atom is removed from molecules containing several iodine atoms, the compounds will partially decompose and free iodine will be released and the compounds will be transformed into chemically active radicals and ions. In contrast to targets where iodine-containing molecules are homogeneously distributed throughout the volume, in heterogeneous targets the influence of the second process on the target structure is not so great because of the low surface contact between the diluent and the inclusions of iodine compounds. Internal Xenon Pressure. It also makes sense to remove gaseous products of 127,129I transmutation in order to decrease the pressure on the target casing, because if leaks are present, iodine vapor can escape together with xenon, creating an emergency situation. The xenon partial pressure can be most easily estimated for compounds containing a single iodine atom, i.e., compounds of the form MeI, where Me is a metal. It can be assumed (with an error of less than 1%) that the mass of the xenon formed is equal to the mass of the iodine burned. If the initial relative free volume – the volume not occupied by matrix materials and iodine compounds – is ε0 and the volume of the iodine compound is ε, then, introducing the notation η ≡ ε0 /ε show672

Fig. 5. Xenon pressure under the casing of a NaI (——) and KI (– – –) target versus iodine burnup at 400°C and η = 0.4 (1), 1.33 (2), and 6 (3).

ing the fraction of the volume of the iodine component that consists of voids in the target, we obtain in the absence of chemical reactions the following relation between the xenon pressure under the casing and the total burnup q of iodine isotopes: P = RT [q–1ηVMeI – (VMeI – VMe)]–1, where R is the universal gas constant, T is the temperature (in K), VMeI and VMe are, respectively, the molar volume of MeI and Me. For Me ≡ K, Na, because of the low melting temperature, the molar volume of the liquid metals must be used. If there is no diluent ε0 = 1 – ε, and consequently η = ε–1 – 1 without any change in the physical meaning of the parameter. The porosity of materials that show promise for burning 129I can reach 70%. If the initial free volume of such a matrix is filled with an iodine compound in the range 15–70%, then voids in the target will occupy 20–60%, the iodine-component volume in the target will be 10–50%, whence η = 0.4–6. For undiluted iodine target, the range of values of η indicated corresponds to a change in the relative volume of the iodine compound in the range 14–71%. Even with a small 30% iodine content in NaI or KI targets and the same volume of the matrix framework (ε = 0.3, ε0 = 0.4, η = 1.33), already with a low burnup from 20% the internal xenon pressure at 400°C will be tens of MPa (curve 2 in Fig. 5), so that increasing the 129I concentration to increase the transmutation efficiency becomes problematic. The substantial pressure under the target casing makes it unnecessary to maintain a high safety level for the 129I burnup scheme, which is another argument in favor of removing daughter gases during irradiation. Conclusions. The expected decrease of the ecologically dangerous radionuclide 129I by transmutation is much lower than for actinides and 99Tc [19, 24, 25]. Consequently, it is not the first candidate for nuclear destruction. As a result of the low 129I burnup rate, effective utilization of 129I requires specialized high-flux apparatus with thermal and intermediate neutron spectrum, and such equipment requires developing systems for continuous removal of gaseous transmutation products. However, the special biological role of iodine in living systems and the difficulty of reliably localizing iodine require that its radioactive isotope 129I be removed in the near future.

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