Generation of ultrafast, transient, highly acidic pH spikes in the radiolysis of water at very high dose rates: relevance for FLASH radiotherapy

Monte Carlo multi-track chemistry simulations were carried out to study the effects of high dose rates on the transient yields of hydronium ions (H3O+) formed during low linear energy transfer (LET) radiolysis of both pure, deaerated and aerated liquid water at 25 °C, in the interval ∼1 ps – 10 μs. Our simulation model consisted of randomly irradiating water with N interactive tracks of 300-MeV incident protons (LET ∼ 0.3 keV/µm), which simultaneously impact perpendicularly on the water within a circular surface. The effect of the dose rate was studied by varying N. Our calculations showed that the radiolytic formation of H3O+ causes the entire irradiated volume to temporarily become very acidic. The magnitude and duration of this abrupt “acid-spike” response depend on the value of N. It is most intense at times less than ∼10–100 ns, equal to ∼3.4 and 2.8 for N = 500 and 2000 (i.e., for dose rates of ∼1.9 × 109 and 8.7 × 109 Gy/s, respectively). At longer times, the pH gradually increases for all N values and eventually returns to the neutral value of seven, which corresponds to the non-radiolytic, pre-irradiation concentration of H3O+. It is worth noting that these early acidic pH responses are very little dependent on the presence or absence of oxygen. Finally, given the importance of pH for many cellular functions, this study suggests that these acidic pH spikes may contribute to the normal tissue-sparing effect of FLASH radiotherapy.

Nuclear-driven production of renewable fuel additives from waste organics

Non-intermittent, low-carbon energy from nuclear or biofuels is integral to many strategies to achieve Carbon Budget Reduction targets. However, nuclear plants have high, upfront costs and biodiesel manufacture produces waste glycerol with few secondary uses. Combining these technologies, to precipitate valuable feedstocks from waste glycerol using ionizing radiation, could diversify nuclear energy use whilst valorizing biodiesel waste. Here, we demonstrate solketal (2,2-dimethyl-1,3-dioxolane-4-yl) and acetol (1-hydroxypropan-2-one) production is enhanced in selected aqueous glycerol-acetone mixtures with γ radiation with yields of 1.5 ± 0.2 µmol J^-1 and 1.8 ± 0.2 µmol J^-1, respectively. This is consistent with the generation of either the stabilized, protonated glycerol cation (CH₂OH-CHOH-CH₂OH₂⁺) from the direct action of glycerol, or the hydronium species, H₃O⁺, via water radiolysis, and their role in the subsequent acid-catalyzed mechanisms for acetol and solketal production. Scaled to a hypothetically compatible range of nuclear facilities in Europe (i.e., contemporary Pressurised Water Reactor designs or spent nuclear fuel stores), we estimate annual solketal production at approximately (1.0 ± 0.1) × 10⁴ t year^-1. Given a forecast increase of 5% to 20% v/v% in the renewable proportion of commercial petroleum blends by 2030, nuclear-driven, biomass-derived solketal could contribute towards net-zero emissions targets, combining low-carbon co-generation and co-production.

Single-strand DNA-nanorod conjugates - tunable anisotropic colloids for on-demand self-assembly

Directed self-assembly uses different stimuli to initiate and control the interaction between nanocrystals. Protonation at reduced pH represents a convenient stimulus for initiating self-assembly. Prior work has focused on protonation-induced hydrogen bonding between peptide or amino acid functionalized nanocrystals for reversible cycling between dispersed and aggregated states. Here, we discuss a fundamentally different approach, in which changes in pH modify the nonspecific interparticle interaction between Au nanorods conjugated with single-stranded (ss) DNA. While electrostatic repulsion stabilizes dispersed suspensions at neutral pH, protonation in acidic solution modifies the DNA corona, turning the interaction between the rods attractive and triggering their self-assembly. Analysis of in-situ electron microscopy of ssDNA-Au nanorods in solution is consistent with a van der Waals attraction of charge-neutral monomers at acidic pH. The results demonstrate ssDNA-conjugated anisotropic nanostructures as versatile building blocks with stimuli-programmable interactions for on-demand self-assembly.

Ultra-High Dose-Rate, Pulsed (FLASH) Radiotherapy with Carbon Ions: Generation of Early, Transient, Highly Oxygenated Conditions in the Tumor Environment

It is well known that molecular oxygen is a product of the radiolysis of water with high-linear energy transfer (LET) radiation, which is distinct from low-LET radiation wherein O2 radiolytic yield is negligible. Since O2 is a powerful radiosensitizer, this fact is of practical relevance in cancer therapy with energetic heavy ions, such as carbon ions. It has recently been discovered that large doses of ionizing radiation delivered to tumors at very high dose rates (i.e., in a few milliseconds) have remarkable benefits in sparing healthy tissue while preserving anti-tumor activity compared to radiotherapy delivered at conventional, lower dose rates. This new method is called "FLASH radiotherapy" and has been tested using low-LET radiation (i.e., electrons and photons) in various pre-clinical studies and recently in a human patient. Although the exact mechanism(s) underlying FLASH are still unclear, it has been suggested that radiation delivered at high dose rates spares normal tissue via oxygen depletion. In addition, heavy-ion radiation achieves tumor control with reduced normal tissue toxicity due to its favorable physical depth-dose profile and increased radiobiological effectiveness in the Bragg peak region. To date, however, biological research with energetic heavy ions delivered at ultra-high dose rates has not been performed and it is not known whether heavy ions are suitable for FLASH radiotherapy. Here we present the additive or even synergistic advantages of integrating the FLASH dose rates into carbon-ion therapy. These benefits result from the ability of heavy ions at high LET to generate an oxygenated microenvironment around their track due to the occurrence of multiple (mainly double) ionization of water. This oxygen is abundant immediately in the tumor region where the LET of the carbon ions is very high, near the end of the carbon-ion path (i.e., in the Bragg peak region). In contrast, in the "plateau" region of the depth-dose distribution of ions (i.e., in the normal tissue region), in which the LET is significantly lower, this generation of molecular oxygen is insignificant. Under FLASH irradiation, it is shown that this early generation of O2 extends evenly over the entire irradiated tumor volume, with concentrations estimated to be several orders of magnitude higher than the oxygen levels present in hypoxic tumor cells. Theoretically, these results indicate that FLASH radiotherapy using carbon ions would have a markedly improved therapeutic ratio with greater toxicity in the tumor due to the generation of oxygen at the spread-out Bragg peak.

Formation of Local, Transient “Acid Spikes” in the Fast Neutron Radiolysis of Supercritical Water at 400 °C: A Potential Source of Corrosion in Supercritical Water-Cooled Reactors?

The use of supercritical water (SCW) in GEN IV reactors is a logical approach to the ongoing development of nuclear energy. A proper understanding of the radiation chemistry and reactivities of transients in a reactor core under SCW conditions is required to achieve optimal water chemistry control and safety. A Monte Carlo simulation study of the radiolysis of SCW at 400 °C by incident 2 MeV monoenergetic neutrons (taken as representative of a fast neutron flux in a reactor) was carried out as a function of water density between ∼150 and 600 kg/m3. The in situ formation of H3O+ by the generated recoil protons was shown to render the “native” track regions temporarily very acidic (pH ∼ 1). This acidity, though local and transitory (“acid spikes”), raises the question whether it may promote a corrosive environment under proposed SCW-cooled reactor operating conditions that would lead to progressive degradation of reactor components.

Ultra-high dose-rate (FLASH) radiotherapy: Generation of early, transient, strongly acidic spikes in the irradiated tumor environment

Monte Carlo simulations of γ/fast electron-radiolysis of water show that the in situ formation of H₃O⁺ temporarily renders each "native" isolated spur/track region very acidic. For pulsed (FLASH) irradiation with high dose rate, this early time, transient "acid-spike" response is shown to extend evenly across the entire irradiated volume. Since pH controls many cellular processes, this study highlights the need to consider these spikes of acidity in understanding the fundamental mechanisms underlying FLASH radiotherapy.

Radiolysis and radionuclide production in a flowing-water target during fast 40Ca20+ irradiation

A flowing-water target was irradiated with a 140 MeV/u, 8 nA 40Ca20+ beam to test the feasibility of isotope harvesting at the upcoming Facility for Rare Isotope Beams. Among other radionuclides, 2.6(2)E-6 48Cr and 5.6(5)E-6 28 Mg nuclei were formed for every impingent 40Ca and were collected through ion exchange. Radiolysis-induced molecular hydrogen evolved from the target at an initial rate of 0.91(9) H2 molecules per 100 eV of beam energy deposited. No radiation-accelerated corrosion of the target material was observed.

“Acid spike” formation in the fast neutron radiolysis of supercritical water at 400 °C studied by Monte Carlo track chemistry simulations

A reliable understanding of radiolysis processes in supercritical water (SCW) cooled reactors is required to ensure optimal water chemistry control. In this perspective, Monte Carlo track chemistry simulations of the radiolysis of pure, deaerated SCW at 400 °C by 2 MeV mono-energetic neutrons were carried out as a function of water density between 0.15 and 0.6 g/cm3. The yields of hydronium ions (H3O+) formed at early time were obtained based on the G values calculated for the first three generated recoil protons. Combining our calculated G(H3O+) values with a cylindrical track model allowed us to estimate the concentrations of H3O+ and the corresponding pH values. An abrupt, transient, and highly acidic pH response (“acid spikes”) was observed at early times around the “native” fast neutron and recoil proton trajectories. This intra-track acidity was found to be strongest at times of less than a few tens to a hundred of picoseconds, depending on the value of the density considered (pH ∼ 1). At longer times, the pH gradually increased for all densities, finally reaching a constant value corresponding to the non-radiolytic, pre-irradiation concentration of H3O+, due to the autoprotolysis of water. Interestingly, the lower the density of the water, the longer the time required to reach this constant value. Because many in-core processes in nuclear reactors critically depend on the pH, the present work raises the question whether such highly acidic pH fluctuations, though local and transitory, could promote or contribute to corrosion and degradation of materials under proposed SCW-cooled reactor operating conditions.

Low linear energy transfer radiolysis of supercritical water at 400 °C: in situ generation of ultrafast, transient, density-dependent "acid spikes"

There is growing interest in the radiation chemistry of supercritical water (SCW), as its use as a coolant in a nuclear reactor (Generation IV) is the logical evolution of the current (Generation III or less) water-cooled reactors. However, current knowledge about the potential effects of water radiolysis in a Gen-IV supercritical water-cooled reactor (SCWR) is incomplete. In this work, Monte Carlo track chemistry simulations of the low linear energy transfer (LET) radiolysis of SCW (H2O) at 400 °C are used in combination with a spherical "spur" model to study the effect of water density on the in situ radiolytic formation of H3O+ ions and the corresponding abrupt, transient, highly acidic pH response ("acid spikes") that is observed immediately after irradiation. The magnitude and duration of this acidic pH effect depend on the water density in the considered range of 0.15-0.6 g cm-3. It is strongest at times less than a few tens of picoseconds with the pH remaining nearly constant at ∼1.6 and 1.9 for the highest ("liquid-like") and lowest ("gas-like") density, respectively. At longer times, the pH gradually increases for all densities and finally reaches a constant value corresponding to the non-radiolytic, pre-irradiation concentration of H3O+, due to the autoprotolysis of water. Our results show that the lower the density of the water, the longer the time required to reach this constant value, ranging from ∼50 ns at 0.6 g cm-3 (pH ∼ 5.6) to ∼1 μs at 0.15 g cm-3 (pH ∼ 8.5). The generation of these highly acidic pH fluctuations around the "native" radiation tracks, though local and transient, raises questions about the potential implications of this effect in proposed Gen-IV SCW-cooled reactors regarding corrosion and degradation of materials.

In situ electron microscopy of the self-assembly of single-stranded DNA-functionalized Au nanoparticles in aqueous solution

Solution-phase self-assembly of DNA-functionalized nanoparticles into mesoscale structures is a promising strategy for creating functional materials from nanocrystal building blocks. The predominant approach has been the use of Watson-Crick base pairing between complementary bases in designated 'sticky ends' to trigger programmable self-assembly into ordered superlattices. Here we demonstrate the ordered self-assembly of Au nanoparticles conjugated with single-stranded (ss) DNA in acidic solutions. Au nanoparticles functionalized with thiolated ssDNA are protected against coalescence and the DNA conformation undergoes significant modifications at low pH, which can be associated with the protonation of adenine bases and the formation of a parallel poly-adenine duplex, which govern the interaction between ssDNA-Au nanoparticle conjugates. In situ liquid cell electron microscopy enables real-time imaging of the self-assembly process and the identification of key characteristics, such as the preferred structural motifs and interparticle separations in the native solution environment. Our results highlight alternatives to conventional base-pairing interactions for building DNA-directed nanoparticle superlattices.

Prediction of rate constants of important chemical reactions in water radiation chemistry in sub and supercritical water – non-equilibrium reactions

The rate constants for reactions involved in the radiolysis of water under relevant thermodynamic conditions in supercritical water-cooled reactors are estimated for inputs in simulations of the radiation chemistry in Generation IV nuclear reactors. We have discussed the mechanism of each chemical reaction with a focus on non-equilibrium reactions. We found most of the reactions are activation controlled above the critical point and that the rate constants are not significantly pressure dependent below 300 °C. This work will aid industry with developing chemical control strategies to suppress the concentration of eroding species.

Self-radiolysis of tritiated water. 4. The scavenging effect of azide ions (N3−) on the molecular hydrogen yield in the radiolysis of water by 60Co γ-rays and tritium β-particles at room temperature

The effect of the azide ion N₃⁻ on the yield of molecular hydrogen in water irradiated with ⁶⁰Co γ-rays (∼1 MeV Compton electrons) and tritium β-electrons (mean electron energy of ∼7.8 keV) at 25 °C is investigated using Monte Carlo track chemistry simulations in conjunction with available experimental data. N₃⁻ is shown to interfere with the formation of H₂ through its high reactivity towards hydrogen atoms and, but to a lesser extent, hydrated electrons, the two major radiolytic precursors of the H₂ yield in the diffusing radiation tracks. Chemical changes are observed in the H₂ scavengeability depending on the particular type of radiation considered. These changes can readily be explained on the basis of differences in the initial spatial distribution of primary radiolytic species (i.e., the structure of the electron tracks). In the “short-track” geometry of the higher “linear energy transfer” (LET) tritium β-electrons (mean LET ∼5.9 eV nm⁻¹), radicals are formed locally in much higher initial concentration than in the isolated “spurs” of the energetic Compton electrons (LET ∼0.3 eV nm⁻¹) generated by the cobalt-60 γ-rays. As a result, the short-track geometry favors radical–radical reactions involving hydrated electrons and hydrogen atoms, leading to a clear increase in the yield of H₂ for tritium β-electrons compared to ⁶⁰Co γ-rays. These changes in the scavengeability of H₂ in passing from tritium β-radiolysis to γ-radiolysis are in good agreement with experimental data, lending strong support to the picture of tritium β-radiolysis mainly driven by the chemical action of short tracks of high local LET. At high N₃⁻ concentrations (>1 M), our H₂ yield results for ⁶⁰Co γ-radiolysis are also consistent with previous Monte Carlo simulations that suggested the necessity of including the capture of the precursors to the hydrated electrons (i.e., the short-lived “dry” electrons prior to hydration) by N₃⁻. These processes tend to reduce significantly the yields of H₂, as is observed experimentally. However, this dry electron scavenging at high azide concentrations is not seen in the higher-LET ³H β-radiolysis, leading us to conclude that the increased amount of intra-track chemistry intervening at early time under these conditions favors the recombination of these electrons with their parent water cations at the expense of their scavenging by N₃⁻.

The effect of the azide ion on the yield of molecular hydrogen in water irradiated with ⁶⁰Co γ-rays and tritium β-electrons at 25 °C is investigated using Monte Carlo track chemistry simulations.