Nuclear-driven production of renewable fuel additives from waste organics

The study investigates whether ionizing radiation from nuclear facilities can be used to valorize waste glycerol from biodiesel production into valuable chemicals, specifically acetol and solketal, thereby coupling nuclear energy with biorefining. The context is the need for low-carbon, non-intermittent energy sources and improved economics for nuclear plants, alongside the oversupply and low value of glycerol. The hypothesis is that γ radiation (and mixed neutron+γ fields) can initiate acid-catalyzed pathways via radiolysis that enhance yields of acetol and solketal from glycerol-containing mixtures, providing a potentially scalable, economically attractive co-production route with nuclear systems.

Prior research established extensive effects of ionizing radiation (radiolysis) in organic media, with typical γ-radiolysis G-values around 0.1–1 µmol J−1 and limited industrial applications (e.g., Dow’s ethyl bromide process, 1963). Nuclear co-production concepts have been explored for hydrogen and desalination but with profitability similar to electricity-only generation. Glycerol, a biodiesel by-product, is oversupplied and low-value; catalytic valorization to acetol and solketal is known but hampered by catalyst deactivation, harsh conditions, separation challenges, and long reaction times. Chemoselective advances exist for acetol and solketal, yet radiolysis-based glycerol valorization had not been explored. Literature on aqueous glycerol radiolysis (e.g., Baugh et al.) reports acetol formation and dose-rate effects. Radiolysis theory (Samuel-Magee) describes spur chemistry, LET effects, and roles of reactive species (H3O+, radicals, solvated electrons), informing expected dependencies of product yields on radiation type and dose rate.

Experimental: Mixtures of glycerol (neat and aqueous) and ternary glycerol-water-acetone compositions were prepared gravimetrically. Samples in polypropylene cryovials (5 mL) were irradiated in the TRIGA Mark II reactor (JSI). Two irradiation modes: (i) γ-only (delayed γ during reactor shutdown; dose rates 15.8–40.5 Gy min−1), and (ii) mixed neutron+γ (reactor critical; dose rates 1630–6540 Gy min−1; for dose-rate study 520–8170 Gy min−1). Absorbed doses: typically 50 kGy for comparison; dose series at 20–100 kGy. Dose determinations used calibrated instrumentation and validated MCNP reactor models. Post-irradiation, samples were frozen, transported, and analyzed within 30 days. Analytical chemistry: GC-MS (Shimadzu TQ8040) with internal standard calibration (butan-2-ol) quantified acetol, solketal, acetic acid, among others. Radiation-chemical yields (G-values) were computed from analyte moles and absorbed energy; uncertainties combined calibration RSD, dilution, and dose errors (±11–15%). Mechanistic analysis: Interpreted results via spur/track chemistry, considering formation of stabilized protonated glycerol cations and H3O+ as acid catalysts, radicals (HO•, H•), solvated electrons, LET and dose-rate effects, viscosity and temperature dependencies (Stokes–Einstein). Scale-up modeling: MCNP 6.1.1 simulations estimated γ dose rates in two geometries: (i) 2 GWth PWR with glycerol-filled pipes in the reactor cavity (mixed field), and (ii) spent fuel pool with arrays of pipes irradiated by γ from spent fuel (γ-only). Models were expanded to accommodate multiple pipes/assemblies (up to 50 pipes in PWR cavity; up to 560 pipes and 1780 fuel elements in spent fuel pool). Empirical G-values from the most productive mixtures were combined with simulated dose rates and assumed 50 kGy cycles to estimate annual production capacities, including an EU-wide extrapolation to 180 NPPs.

- Product identification: Acetol and solketal are major radiolysis products from glycerol-containing mixtures; acetic acid also detected in ternary mixtures due to acetone radiolysis. - G-values (γ-only, unless noted): - Solketal: Up to 1.53 ± 0.2 µmol J−1 in ternary glycerol–acetone–water mixtures; headline solketal G-value reported as 1.5 ± 0.2 µmol J−1. - Acetol: Up to 1.8 ± 0.3 µmol J−1 in aqueous glycerol (69 mol% water); headline acetol G-value 1.8 ± 0.2 µmol J−1. - Mixture effects: - Adding 69 mol% water to glycerol increases acetol yield ~3× vs neat (from 0.6 ± 0.07 to 1.8 ± 0.3 µmol J−1). Further dilution reduces acetol yield, consistent with literature (e.g., 0.22 µmol J−1 at 0.93 mol% glycerol). - Adding acetone to aqueous glycerol increases solketal yield ~34× under γ irradiation (from 0.045 ± 0.005 to 1.53 ± 0.2 µmol J−1). Mixed-field yields show smaller relative gains. - Dose and dose-rate dependencies: - For acetol, γ-ray G-values tend to decrease with increasing absorbed dose, attributed to competing reductions by solvated electrons; mixed-field acetol yields increase with dose, implying different balance of spur processes. - Increasing dose rate reduces yields, observed for solketal (500–8200 Gy min−1) and consistent with expectations for acetol. - Radiation field effects (LET): γ-only (low LET) generally yields higher G-values than mixed neutron+γ (higher LET) due to less spur overlap and higher yields of short-lived acidic species (e.g., H3O+, protonated glycerol cation) that catalyze acetol and solketal formation. - Mechanism: Evidence supports acid-catalyzed pathways driven by protonated glycerol cation (from direct action on glycerol) and/or H3O+ (from water radiolysis). Radical-initiated α-hydrogen abstraction on glycerol forms α-hydroxy radicals leading to acetol via acid-catalyzed rearrangement and chain propagation; solketal forms via chemical ketalization with acetone catalyzed by acidic species. - Scale-up projections: - Single PWR spent fuel pool (γ-only): Solketal 57 ± 6 t year−1; acetol 13 ± 2 t year−1; consumes ~57 ± 8 t year−1 glycerol and ~25 ± 2 t year−1 acetone. - EU-wide extrapolation to 180 NPPs: Solketal ≈ (1.0 ± 0.1) × 10^4 t year−1; equivalent solketal volume (9.6 ± 1) × 10^6 L year−1, enabling ~1.9 × 10^8 L year−1 of E5-like petrol blend at 5 vol% solketal. - Integration advantages: γ-only spent fuel pool scenario avoids neutron activation risks and leverages existing nuclear infrastructure with potentially negligible radiation processing costs.

The findings demonstrate that ionizing radiation can drive the conversion of waste glycerol into acetol and solketal with competitive radiation-chemical yields for organics, validating the concept of coupling nuclear facilities with biorefinery processes. The higher yields under γ-only irradiation indicate that low-LET fields favor the formation of transient acidic catalytic species (protonated glycerol cations and H3O+), which are central to the acid-catalyzed mechanisms. Dose-rate sensitivity and LET effects align with spur/track chemistry predictions, where higher LET or dose rates increase spur overlap and recombination, lowering effective concentrations of catalytic and radical intermediates. Mixture composition strongly modulates outcomes: water enhances radical mobility and catalytic species formation up to an optimum; acetone availability is essential for solketal formation. Scale-up modeling suggests meaningful industrial potential, especially in γ-only spent fuel pool configurations, offering co-generation of value-added chemicals and enhancing the nuclear value proposition. The results are relevant to renewable fuel strategies since solketal can serve as a fuel additive, potentially contributing to increased renewable content in petrol blends while valorizing biodiesel waste.

This work reports the first radiation-induced process for solketal synthesis and enhanced acetol production from glycerol, achieving solketal G-values of 1.5 ± 0.2 µmol J−1 and acetol G-values of 1.8 ± 0.2 µmol J−1 under γ irradiation in optimized mixtures. Mechanistic analysis implicates transient acidic species (protonated glycerol cation and H3O+) and α-hydroxy radicals in acid-catalyzed pathways. MCNP-based scale-up indicates the γ-only spent fuel pool scenario offers superior annual production capacities, projecting up to ~1 × 10^4 t year−1 solketal across European facilities. Although current yields may not yet surpass chemoselective routes, leveraging waste ionizing sources could enable low processing costs. Future work should target yield and selectivity improvements (e.g., scavenging solvated electrons, oxygen removal, process optimization), alongside engineering assessments (process modeling, corrosion, safety) to advance nuclear-chemical co-production.

- Experimental irradiations used high dose rates and discrete doses (e.g., 50 kGy), which may differ from practical continuous processing conditions. - Mixed neutron+γ fields reduced yields and raise activation concerns; the study emphasizes γ-only scenarios to mitigate activation but detailed activation/corrosion assessments are pending. - Product selectivity and overall conversion/yield require improvement to match best chemocatalytic processes; competing reactions (e− solv, O2 scavenging) limit yields. - Economic analysis is indicative; solketal price data are scarce and full cost models (including separation, purification, and plant integration) are not provided. - Scale-up projections assume representative facility geometries and availability; variability across plants and regulatory constraints may affect feasible throughput. - Additional safety, materials compatibility, and process control studies are needed for industrial deployment.