Some Interesting Work On Silicotitanate Ion Exchange for the Removal of Cesium from Hanford Tanks.
The paper to which I'll briefly refer is this one: Effect of Na Concentration on Cs Distribution with Crystalline Silicotitanate in Tank Waste Simulants Emily L. Campbell, Amy M. Westesen, Ashley N. Williams, and Reid A. Peterson Industrial & Engineering Chemistry Research 2023 62 (19), 7650-7656
The Hanford Reservation outside of Richland, Washington (the site of the Pacific Northwest National Laboratory) is where much of the plutonium for US nuclear weapons was isolated, using various processes, most recently the Purex process, a solvent extraction process.
The wastes from these processes was stored in very large tanks, rather in an ad hoc fashion, during the Cold War arms race with the former Soviet Union. Some of these tanks are leaking.
I have been fascinated with the chemistry of these tanks for quite sometime, since they are very useful for understanding the geochemistry of fission products far beyond the Oklo fossil nuclear reactors in Gabon, Africa.
I once encountered a overly defensive dumb shit here who carried on about a collapsed tunnel at Hanford and was inspired (by the amusing absurdity) to study some properties of fission products there and in the Nevada Nuclear Weapons Test Site. It was fun and instructive to write: 828 Underground Nuclear Tests, Plutonium Migration in Nevada, Dunning, Kruger, Strawmen, and Tunnels .
While rote antinukism kills people, some of antinuke stupidity can be inspiring, albeit in a negative way.
In the period between 1950 and 1980, and for some time beyond, plutonium processing for weapons grade plutonium - which is necessarily dilute in solid solution for physics reasons - was done with little concern for minimizing the effort to permanently handle the wastes. In particular, acid neutralizations were frequently carried out using sodium hydroxide meaning that the valuable fission product radiocesium ended up in the presence of ions from which it is difficult to separate, as the group 1 elements, lithium, sodium, potassium, rubidium and cesium have very similar chemistry.
Cesium is a prominent fission product, in the fission of the common actinides, the mass number 137 is near or at the maximum yield. The relatively long lived isotope Cs-137, half-life of about 30.08 years, a potassium mimetic is a prominent isotope in used nuclear fuel and the raffinates for fuel recycling via solvent extraction procedures like the Purex process used at Hanford.
One pathway for separation of sodium and cesium - the use of sodium can and should be eliminated in future reprocessing that will be necessary if we wish to save the world (perhaps we don't so wish) - is via Ion Exchange.
This paper is about an inorganic ion exchange material, a silicotitanate.
Under irradiation, generally in the UV range, but also certainly in the gamma and X-ray range, titanium oxides have remarkable properties for the destruction of organic pollutants. Thus a cesium silicotitanate might prove very useful for the destruction of some very recalcitrant pollutants.
This paper is about characterizing the behavior of this ion exchange material which is nonelutable: It is difficult to remove the cesium from the ion exchange material; this is good for isolation but bad for recovery of the cesium for other uses. (I love to think about uses for radiocesium; many are already in practice but some unique other uses suggest themselves.
Anyway, from the paper itself:
The renewed interest in CST as the ion exchange media for cesium removal has led to a re-evaluation of the Zheng Anthony Miller (ZAM) isotherm model used to predict Cs exchange in tank waste. (8−10) Manufacturing changes in CST production and conversion to the engineered form of the ion exchange media have resulted in deviation from the original isotherm model. More recently, the Campbell Westesen Peterson (CWP) isotherm model was developed from experimentally determined Cs capacity and simplified ZAM equilibria expressions to include only binary substitution of monovalent ions. The CWP model significantly improved the ability to predict Cs loading from complex matrices... (10)
...Hanford tank waste supernate can exceed 8.0 M Na and there is interest in understanding the impact of Na concentration on Cs removal. As it stands, tank waste is diluted to nominally 5.6 M Na prior to processing through TSCR. There is value in understanding the impact of Na concentration on Cs removal from a processing standpoint, as well as a modeling standpoint, to calculate column breakthrough and economics of waste volume treatment, compared to column changeouts due to Cs breakthrough. At nominally one million dollars per CST column, it would be advantageous to optimize the feed conditions to minimize secondary waste and subsequent processing, streamline processing time, and still achieve the 99.9% Cs-137 removal goal. For example, can greater CST utilization be achieved by operating at either higher or lower sodium molarity?
A series of batch contact experiments were conducted to investigate the impact of Na concentration on Cs removal. Batch contact experiments were performed in two simulants: one with only Na+, OH–, and NO3– as the simple simulant, and the other with added carbonate and nitrite. The simulants were prepared at 8 M Na and then subsequently diluted to achieve a Na concentration range of 3.5–8.0 M. Batch contacts utilizing waste from tank AP-107 that had been previously decontaminated as the matrix were also performed where an aliquot of the AP-107 was concentrated by volume (evaporated) to 7 and 8 M Na, and the remaining Na concentrations were prepared via dilution of the 6.2 M Na...
The work was done with simulants of tank waste, as well as actual tank waste from tank AP-107. AP-107 is a double shelled tank that began taking waste in 1986. About 57% of the cesium-137 present in 1986 in the tank has decayed to stable barium-137, but around 43% remains. (The number of deaths associated with the storage of fission products and residual actinides in tank AP-107 is zero.)
Here's what the authors found out:
A plot of the Cs loading, corrected for Na dilution, thereby maintaining the Cs/Na ratio, indicated a linear change in the Cs and Na activity coefficients. In the event that the ratio of activity coefficients was changing proportional to dilutions, the dilution corrected Cs loading would remain constant across the suite of Na concentrations. A deeper dive into the activity coefficients according to Bromley and HSC reveal that the estimated activity coefficients from both methods do not align well with the experimental data. The Bromley method is linear, whereas the HSC prediction has more of a curved shape, which more closely resembles the data. In future isotherm model development, the discrepancy in activity coefficient concentrations should be addressed.
The batch contact results and associated isotherms generated in this work can serve as a starting point for cost-benefit analysis discussions when processing tank waste with different Na concentrations. Cs distribution was highest at low Na concentrations and the associated matrix also contributed to Cs distribution values, where solutions with CO32– and lower nitrate resulted in a higher distribution constant. Without running a full ion exchange column test, which is costly and time-consuming, the batch contact work herein should suffice for estimation of load behavior in a process setting. (18)
This is OK, I guess, for a dump mentality, although again, radiocesium loaded titanates can prove very valuable materials for the treatment of chemical wastes.
Since cesium, in particularly radiocesium, is an extremely interesting material with many uses, I expect that more careful schemes for separations of fission products from fresh nuclear fuels will recover cesium in a different way, but in terms of separating it from historical matrices, this isn't a bad idea. I rather like it.
Have a nice day tomorrow.