Thermodynamic Study of the Chlorine Content of Stainless Steel

Abstract

No thermodynamic data for the solution of Cl in ferrous alloys were found in the literature. This is in accord with recent Accelerator Mass Spectroscopy (AMS) analyses which showed that Cl contents in stainless steel (SS) are in the order of a few ppb. However, based on older chemical analyses of Cl in the order of 100 ppm, SS that has been irradiated with thermal neutrons in nuclear reactors is considered a major source of the long-lived ³⁶Cl isotope in nuclear waste. In this study, the potential Cl contamination of SS originating from production and refinement processes is investigated. Unlike ferrous alloys, blast-furnace and steelmaking slags can dissolve significant amounts of Cl. The equilibrium distribution of Cl species between slags and gas phase was calculated for various steelmaking processes using the FactSage 7.2 software and databases. The results showed that despite the high volatility of metal chlorides at high temperatures, significant fractions of Cl can be retained in the slag phase even at 1600 °C. Chloride may also be incorporated in non-metallic inclusions originating from secondary refining. Based on these results and on several further assumptions, various scenarios for explaining, and also avoiding, Cl contamination of steel are discussed.

Appendix A: Potential Sources, Analyses, and Environmental Impact of 36Cl Reported in the Literature

Chlorine has 25 isotopes in total. These include two stable isotopes, ³⁵Cl and ³⁷Cl, with natural abundances of 75.76 pct and 24.24 pct, respectively, and ³⁶Cl, which is the only long-lived radioactive chlorine isotope, with a half-life of (3.013 ± 0.015)·10⁵ years. All the other radioactive isotopes of chlorine are short-lived (half-lives of minutes or less).[77]

Much attention has been paid to ³⁶Cl in the environment due to its long half-life and its natural and anthropogenic origins.[1] Trace amounts of radioactive ³⁶Cl exist in the environment, in a ratio of about (7 to 10)·10⁻¹³ to 1 with stable chlorine isotopes.[2,3] This corresponds to a concentration of approximately 1 Bq/(kg Cl).[4]

³⁶Cl is produced in the atmosphere by spallation of ⁴⁰Ar by interactions with cosmic ray protons and by the nuclear reaction ³⁶Ar (n, p) ³⁶Cl with cosmic ray neutrons. In the top meter of the lithosphere, ³⁶Cl is generated primarily by thermal neutron activation of ³⁵Cl and spallation of ³⁹K and ⁴⁰Ca.[3,78]

In a nuclear reactor, ³⁵Cl that is present as an impurity in the cooling water, nuclear fuel, metallic materials, or in concrete is activated by thermal neutron capture to ³⁶Cl. The reaction ³⁵Cl + ¹n → ³⁶Cl + γ is characterized by a high cross section (43.63 barn); thus, even trace amounts of ³⁵Cl in nuclear fuel and structural reactor materials are sufficient to produce significant amounts of ³⁶Cl, in the presence of a high flux of thermal neutrons.

³⁶Cl decays in one step to stable isotopes, mainly by beta minus decay (β⁻, 98.1 pct) to ³⁶Ar, and a small amount by electron capture/beta plus decay (ec/β⁺, 1.9 pct) to ³⁶S. Hence, the decay products are mainly the noble gas argon and some sulfur.

Once scattered through the environment, chlorine quickly acquires the chloride anionic form Cl^–, the thermodynamically stable species of chlorine in water. Natural redox processes virtually never involve this form. Dissolved chlorine Cl₂(aq) and most oxidized forms of chlorine such as hypochlorite ClO^–, chlorite ClO₂^–, and chlorate ClO₃^– are highly reactive and are quickly reduced to chloride in the environment.

The only fairly stable form of oxidized chlorine in aqueous solutions is perchlorate, ClO₄^–, due to the kinetic stability of its tetrahedral structure. Due to its relative stability, perchlorate can also occur as a contaminant in drinking water and groundwater; its isotopic composition could be used to identify the sources of contamination.[77]

The original chemical form of ³⁶Cl in radioactive waste forms is unknown. However, in the long run and especially under reducing conditions, which are expected in most of the planned deep underground repositories for radioactive waste, all the oxidized forms of chlorine, including perchlorate, if present at all, will be reduced to chloride.

Chloride forms highly soluble salts with all major and minor components of ground and surface waters: all alkali and alkaline earth elements, iron, manganese, nickel, copper, aluminum. These salts are only important if rock salt is considered as host rock of a geological repository. The only sparingly soluble phases, also known as naturally occurring minerals, are chlorargyrite (AgCl) and calomel (Hg₂Cl₂). Other pure phases formed with “soft” metal cations like HgCl₂ or PbCl₂ are soluble in the gram per liter range.

In the case of a spent fuel repository, the anion Cl⁻ will not sorb on bentonite minerals due to electrostatic repulsion from the negatively charged clay mineral surfaces, and it will not be incorporated in any solid phase of the bentonite backfill or the host rock.

In fact, chloride is used as “conservative tracer” in diffusion experiments with clay or granite containing solids, as it is not retarded by any sorption or co-precipitation effects.

Considering the chemical behavior of the chloride anion in clay or granitic ground waters, acting as a “conservative tracer,” the large isotopic dilution of ³⁶Cl by stable ³⁵Cl and ³⁷Cl in the near and far field of a deep geological repository, and the long half-life of ³⁶Cl, no significant retardation effects are expected for ³⁶Cl.

Hence, ³⁶Cl is a dose-relevant nuclide in geological repositories for radioactive waste relying on the retention of radionuclides as a safety principle. Mitigating the dose of ³⁶Cl by improving sorption in the engineered barriers does not appear to be feasible.

Therefore, an accurate estimate of the source term will be crucial for the post-closure safety assessment of radioactive waste repositories. However, the inventory of ³⁶Cl is almost exclusively derived from calculations based on measured or merely assumed trace concentrations of its stable isotopes, which are impurities in nuclear fuel or structural reactor materials.

The only measurements of stable chlorine and ³⁶Cl in nuclear fuel have been published by Tait et al.[79] They found that the total average Cl impurity level of four un-irradiated CANDU UO₂ fuel samples was (2.3 ± 1.1) ppm, which is less than the 5 ppm initial Cl impurity concentration in spent fuel, assumed in the first safety assessment calculation for ³⁶Cl.[80] The latter value is cited also in recent publications, e.g., Pipon et al.[81]: “Pristine chlorine … is present as an impurity in the nuclear fuel (< 5 ppm).”

No ³⁶Cl is left in vitrified high-level waste from reprocessing. The volatile chlorine has evaporated during the reprocessing steps of the dissolved spent fuel; it is found in the surroundings of reprocessing plants.[82] Thus, even if fuel dissolution was involved during the production of the historical waste, no ³⁶Cl from the fuel inventory is expected to remain in the waste.

The cladding also contains traces of stable chlorine. The only measurements published are for Zr-2.5Nb pressure tubes for CANDU reactors: Aitchison and Davies[83] report Cl impurities between 1 and 5 ppm. They state that the chloride in Zr-2.5Nb tubes originates from the Kroll process for producing Zr from crude Zr sponge. Zirconium is reacted with Cl₂ to produce ZrCl₄, which is purified by sublimation and then reacted with molten Mg to regenerate Zr and leave MgCl₂.[83] By using Auger spectroscopy and EDX analysis, they detected an approximately 10 nm layer of complex zirconium-carbide-chloride precipitate in strips of low-energy fractures on Zr-2.5Nb tubes. Stringers containing Cl have also been observed in Zircaloy-2 and Zircaloy-4,[84] where stringer density in forged and rolled products depended on vacuum re-melting parameters.

Stainless steel is used extensively in nuclear reactors as a construction material and may be exposed to high neutron fluxes. Even if the ³⁶Cl content in stainless steel is assumed relatively low, e.g., set to 1 ppm as the detection limit of the analysis method used, it has quite an impact on the results of the analysis because of the extremely high mobility of the nuclide. However, Robertson et al.[5] report that SS 304 used in reactor internal hardware has Cl concentrations ranging from less than 50 to 130 ppm Cl. Hou et al.[85] state: “No report on the analysis of steel for ³⁶Cl is available.”

Nothing is known about the chemical form of ³⁶Cl in any of these waste forms.

The report of Robertson et al.[5] is often cited as literature source for the content of <50 to 130 ppm chlorine in SS 304 (e.g., Hummel[86]). A closer look at that report[5] shows that these values are mentioned only in Table 2.3 of the report; at the bottom of the table also the sources are given where the results collected in the table were taken from (without specifying which reference is for which value). Their own measurements, reported in Table 5.1 for SS 304, result in chlorine values below detection limit for all 3 samples of SS 304 analyzed, and in the text (p. 49) they state that “Cl-36 was detectable in only the spent ion exchange resin samples…”.[5] Inspection of the data[5] revealed that the values <50 to 130 ppm chlorine content in SS 304 originate from Evans et al.,[6] which is listed at the bottom of Table 2.3 of the Robertson et al. report.[5] It is interesting to note that Evans et al.[6] analyzed un-irradiated steels by neutron activation, while Robertson et al.[5] analyzed irradiated samples using radiochemical methods.

In the Evans et al. report,[6] it is stated that they obtained SS samples from various reactors under construction and analyzed ³⁶Cl by neutron activation. They calibrated the neutron activation method with standards, using NBS steel standards and the results are given in Table 4.3, p. 39 of their report. For ³⁶Cl analysis, there are no values reported in the table for a total of 11 (5+6) NBS SS samples, while, e.g., for fly ash in the same table they report 44 ppm and 42 ppm for the fly ash standards (or previous measurements). From this we assume that (a) there is no detectable ³⁶Cl in the 11 SS samples from NBS and (b) this is probably a double check, because NBS also does not report any chlorine in the samples, based on previous analysis.

The analysis of the steel samples from the various reactor sites is reported in Table 4.7 and the average composition of steel in Table 4.8 of the Evans et al. report.[6] In Table 4.7, one reads that the ³⁶Cl data for the various steel samples were North Anna <50 ppm, Susquehanna <60 ppm, Enrico Fermi <60 ppm and <70 ppm, and Belle-Fonte 130 ppm. In Table 4.8, the average ³⁶Cl in SS 304L is reported as 70 ppm, with range <50 to 130 ppm for the 5 samples analyzed. Thus, it appears that the only reason why they report above-detection-limit Cl content in SS 304 is the result of the analysis of the Belle-Fonte sample (130 ppm). In the report it is stated that 2 samples of SS 304 from Belle-Fonte were obtained according to Table 3.2, one from the reactor vessel and one from vessel cladding. We could not find which of these two (or both) gave 130 ppm. On the other hand, they have analyzed 11 NBS standard samples and 4 samples from other reactors which all resulted below their detection limit. No mention of these samples is made in the Robertson report,[5] which has 3 additional own steel samples analyzed resulting in chlorine contents below detection limit.

A much more intensive ³⁶Cl analysis campaign in various ferrous materials was undertaken by Nirex, UK in the late 1990s. The results were summarized in a report.[87] The analytical method used was radiochemical neutron activation analysis (RNAA). According to this method, the fast dissolution of about 0.7 g of neutron activated ferrous material in the form of millings was followed by separation of chlorine and counting of ³⁸Cl. In total, 176 SS and other ferrous material samples were analyzed. The measured chlorine concentrations in 170 of 176 samples were below detection limits, which were all low, in the range 0.06 ppm to 0.97 ppm, except for Cr/Mo/V steel for which the detection limits were between 0.26 ppm and 3.3 ppm. Other impurities in ferrous materials such as ³⁴S and ³⁹K are not significant sources of ³⁶Cl.

In their conclusions, the authors of Reference 87 state, “These measurements confirm the view that the PNNL results, which were the only measurements available before this program of work, are suspect. Chlorine concentrations in excess of 100 ppm are considered implausible for a number of reasons: during the manufacture of steel blast-furnace temperatures are sufficiently high to cause volatilization of iron chlorides; it is important to produce steels with low chlorine concentrations because steels are known to corrode when exposed to chlorine. All the evidence suggests that the measurements of chlorine concentrations above the limits of detection by Pacific Northwest Laboratory were doubtful, and the limits of detection were so high that the measurements below these limits are of little value.”

The analytical method used in the Nirex report,[87] together with the results of the analysis of 30 steel samples for 6 different steel types, has been published in the open literature[63] and values below detection limit were reported for all samples.

Similar values below detection limit were reported by Hou et al.[85] for irradiated SS (below 0.2 ppm) from a Danish reactor.

The use of AMS (Accelerator Mass Spectroscopy) allows ³⁶Cl measurements in steel with sensitivity better than 10⁻¹⁴ relative to stable chlorine or detection limits of the order of 1 ppb.[7] The measurements showed much higher chlorine levels in the first and second leaching of the ‘surface layer’ as compared to the levels of chlorine measured after the final, total dissolution of the sample (see discussion in Section III–F). The paper[7] also mentions the analysis of drill cores from 4 steel samples (cylinders of 1 mm diameter and 4 mm length) which are expected to give better reproducibility. The results of these analysis were delivered by O. Forstner, one of the co-authors of the paper,[7] to SKB and are reported here. The analysis of 6 doublet samples from 3 types of steel were below the detection limit of 2 ppb, while only one type of steel (58316) had 5.8 ppb and 6.0 ppb in the doublet samples, with an error of 0.5 ppb. The drill core samples were apparently taken from different steel types than the drill shaving samples of Winkler et al.[7]

It is evident that with more sensitive analytical techniques such as AMS, one can certainly exclude chlorine concentrations of the order of hundreds of ppm. With the hypothesis of chlorine contained in non-metallic inclusions or in slag inclusions and assuming a maximal content of 0.1 pct Cl in them, the steel of the Belle-Fonte sample discussed above should have contained 13 pct non-metallic or slag inclusions. This high percentage could have resulted more likely from large, undetected, exogenous inclusions rather than from the finely dispersed endogenous, non-metallic inclusions that usually originate e.g., from deoxidation processes in secondary refining; however, sample contamination or analytical errors might also have been important factors.

In summary, finding chlorine contents below 2 ppb in 75 pct of the analyzed samples gives a good ground to the claim that very low, if at all, levels of chlorine have to be expected in steel.