Response Of Isotopes Research Articles

Modeling the response of Norway spruce tree-ring carbon and oxygen isotopes to selection harvest on a drained peatland forest.

Continuous cover forestry (CCF) has gained interest as an alternative to even-aged management particularly on drained peatland forests. However, relatively little is known about the physiological response of suppressed trees when larger trees are removed as a part of CCF practices. Consequently, studies concentrating on process-level modeling of the response of trees to selection harvesting are also rare. Here, we compared, modeled and measured harvest response of previously suppressed Norway spruce (Picea abies) trees to a selection harvest. We quantified the harvest response by collecting Norway spruce tree-ring samples in a drained peatland forest site and measuring the change in stable carbon and oxygen isotopic ratios of wood formed during 2010-20, including five post-harvest years. The measured isotopic ratios were compared with ecosystem-level process model predictions for ${\kern0em }^{13}$C discrimination and ${\kern0em }^{18}$O leaf water enrichment. We found that the model predicted similar but lower harvest response than the measurements. Furthermore, accounting for mesophyll conductance was important for capturing the variation in ${\kern0em }^{13}$C discrimination. In addition, we performed sensitivity analysis on the model, which suggests that the modeled ${\kern0em }^{13}$C discrimination is sensitive to parameters related to CO2 transport through stomata to the mesophyll.

Response of Water Isotopes in Precipitation to a Collapse of the West Antarctic Ice Sheet in High-Resolution Simulations with the Weather Research and Forecasting Model

Abstract The West Antarctic Ice Sheet (WAIS) may have collapsed during the last interglacial period, between 132 000 and 116 000 years ago. The changes in topography resulting from WAIS collapse would be accompanied by significant changes in Antarctic surface climate, atmospheric circulation, and ocean conditions. Evidence of these changes may be recorded in water-isotope ratios in precipitation archived in the ice. We conduct high-resolution simulations with an isotope-enabled version of the Weather Research and Forecasting Model over Antarctica, with boundary conditions provided by climate model simulations with both present-day and lowered WAIS topography. The results show that while there is significant spatial variability, WAIS collapse would cause detectable isotopic changes at several locations where ice-core records have been obtained or could be obtained in the future. The most robust signals include elevated δ18O at SkyTrain Ice Rise in West Antarctica and elevated deuterium excess and δ18O at Hercules Dome in East Antarctica. A combination of records from multiple sites would provide constraints on the timing, rate, and magnitude of past WAIS collapse.

Response of stable isotopes (δ2H, δ13C, δ15N, δ18O) of lake water, dissolved organic matter, seston, and zooplankton to an extreme precipitation event

Lake ecosystems process and cycle organic substrates, thus serving as important bioreactors in the global carbon cycle. Climate change is predicted to increase extreme weather and precipitation events that can flush nutrients and organic matter from soils to streams and lakes. Here we report changes in stable isotopes (δ2H, δ13C, δ15N, or δ18O) of water, dissolved organic matter (DOM), seston, and zooplankton in a subalpine lake at short time resolution following an extreme precipitation event between early July to mid-August 2021. Water from excess precipitation and runoff remained in the lake epilimnion and coincided with increasing δ13C values of seston (−30 ‰ to −20 ‰), due to the input of carbonates and terrestrial organic matter. Particles settled into deeper lake layers after two days and contributed to the uncoupling of C and N cycling as the lake responded to this extreme precipitation event. Following the event, there was an increase in bulk δ13C values of zooplankton (from −35 ‰ to −32 ‰). Throughout this study, δ13C values of DOM remained stable throughout the water column (−29 ‰ to −28 ‰), while large isotopic fluctuations in DOM δ2H (−140 ‰ to −115 ‰) and δ18O (+9 ‰ to +15 ‰) values suggested DOM relocation and turnover. Integrating isotope hydrology, ecosystem ecology, and organic geochemistry offers an element-specific, detailed approach to investigating the impact of extreme precipitation events on freshwater ecosystems and particularly aquatic food webs.

Responses of Picea meyeri to Climatic Factors Revealed by Tree Ring Isotopes and Water Use Efficiency on Luya Mountain of North-Central China

Along with tree ring width, carbon isotopes are also good proxies for climate change. Water use efficiency (WUE) can be calculated more quickly and accurately based on carbon isotopes. In this study, according to the principle of dendroclimatology, the sequence of δ13C and WUE of tree rings of Picea meyeri are built. Pearson correlation analysis and multiple regression analysis are used to explore the response of carbon stable isotopes of Picea meyeri to climate change, which revealed the relationship between δ13C of Picea meyeri and climatic factors. Based on δ13C, we calculated the WUE of Picea meyeri and analyzed its response to climate change. The main conclusions are as follows: (1) The δ13C of Picea meyeri decreases year-by-year from 1957 to 2020, in the range from −23.89‰~−21.67‰, and the average value is −22.67‰. The water use efficiency of Picea meyeri increases in the range from 17.26~61.31, with an average of 39.45. (2) The δ13C of Picea meyeri is negatively correlated with temperature, which has the highest correlation with the temperature of the growing season (c5–c9), and its coefficient is higher than that of the mean temperature of each month. (3) There is a significant positive correlation between WUE sequence and temperature. Meanwhile, due to the effect of precipitation and temperature, the Picea meyeri is subject to drought stress to some extent. Above all, temperature is the main climatic factor affecting the δ13C and WUE of Picea meyeri on Luya mountain.

Carbon and oxygen isotopes of the Lianglitage Formation in the Tazhong area, Tarim Basin: Implications for sea‐level changes and palaeomarine conditions

Carbon isotopes can be used to interpret sea‐level changes during deposition, but the accuracy of sea‐level changes indicated by shallow‐water carbonates remains unclear. We carried out sedimentary microfacies and stable isotope analysis of carbonate rocks in Upper Ordovician Lianglitage Formation in Tazhong area, Tarim Basin, to examine the response of isotopes to high‐frequency cycles of shallow‐water carbonate rocks. The Lianglitage limestones can be divided into four types of microfacies that were deposited in reef and shoal environments on a platform margin. A total of sixty‐one (61) selected limestone samples have the δ13C value varying from 0.5993‰ to 1.6228‰ (average 1.1364‰). The extremely low correlation coefficient of carbon and oxygen isotopes indicate that the samples can represent the deposition seawater, thus the estimated Z value and temperature show that the Lianglitage Formation was deposited in the normal marine environment in a tropical to subtropical zone. The Lianglitage Formation shows a clear deposition trend from tidal flat to reef shoal and then to an open platform. The sedimentary environment controls the difference in carbon and oxygen isotopes of shallow‐water carbonates through the rate of burial of organic carbon, and thus the δ13C tend to increase when sea level rise. Hence the carbon isotopic composition of shallow‐water carbonate rocks can reflect sea‐level changes. The δ13C of Lianglitage Formation in well TZ72 shows four sedimentary cycles (20–40‐meters‐thick) controlled by sea‐level changes, which were also recorded in this Formation from other parts of the Tarim Basin.

Response of nitrogen isotopes to paleo-environment and organic carbon accumulation in a Late Paleozoic alkaline lake, Junggar Basin

The geochemistry of nitrogen (N) in ancient lacustrine systems, especially in alkaline conditions, is poorly understood. To fill the knowledge gap, we investigated alkaline lacustrine sediments of the Upper Paleozoic Fengcheng Formation (ca. 300 Ma) in the Mahu Sag, Junggar Basin, northwestern China. Based on the general characteristics and stratigraphic variations in N isotopes, their relationship to the paleo-environmental setting and type/accumulation of organic matter were evaluated. The lacustrine sediments can be divided into alkaline (δ15N > 10–12‰) and circum-neutral (δ15N < 10–12‰) groups in general. The geochemical N cycle was mainly controlled by the pH of the lake water, and ammonia volatilization triggered N isotopic fractionation. In contrast, N isotopes are not significantly affected by the salinity and redox state of the water mass. The δ15N, particularly for the alkaline group, exhibit a correlation with the type of organism present (i.e., the algae/bacteria ratio), and also the hydrocarbon-generation potential and type of organic matter. This reflects the response of lake water pH to the types of organisms present and accumulation of organic matter. The N isotopic compositions of lacustrine sediments, and the factors that control this, are more complex than in the marine system, and are dependent on the biomass and paleo-environmental setting.

Metasomatic modification of Sr isotopes in apatite as a function of fluid chemistry

Apatite is a ubiquitous accessory mineral in crustal rocks. The Sr-isotope record of apatite has a wide range of applications in earth science studies. However, apatite has been documented to be easily altered by certain fluids. Currently, the impact of metasomatic alteration on Sr-isotopic abundances in apatite is not well known. In order to better understand this issue, well-characterized fluorapatite grains have been metasomatized experimentally at a temperature of 800 or 600 °C and a pressure of 200 MPa. Fluids used included a CO2-H2O mixture and NaF-, CaCl2-, and HCl-bearing solutions, all of which were doped with a standard solution with a known 87Sr/86Sr ratio. In the fluorapatite + CO2-H2O experiments, the fluorapatite grains were not altered by the fluids; thus, their Sr isotope compositions were generally kept unchanged. However, the other fluids induced partial to complete alteration of fluorapatite. In experiments involving the NaF- or HCl-bearing solutions, the Sr content remained constant or was increased in altered areas of the fluorapatite, and the Sr isotopes underwent changes with partial isotopic signature of the reacting solutions. In experiments involving CaCl2, the Sr content was decreased in the altered fluorapatite because high Ca activity in the solution caused Ca to replace Sr on the Ca site. Notably, the Sr isotopic ratios are still changed, although to a relatively small extent. The efficient Sr isotopic exchange between fluid and apatite is attributed to the rapid transport rate of Sr between the reaction-interface fluid and bulk fluid surrounding the apatite. This experimental study demonstrates that the response of apatite Sr isotopes to metasomatic alteration is mainly controlled by the chemistry of fluids. Overall, Sr isotopes become susceptible to hydrothermal alteration once the apatite is chemically reactive with the fluids. Therefore, it is important to evaluate the fluid-rock history, especially the conjectured fluid composition before using Sr isotopes from apatite as a geochemical tracer.

Reconstruct hydrological history of terrestrial saline lakes using Mg isotopes in halite: A case study of the Quaternary Dalangtan playa in Qaidam Basin, NW China

Saline lakes are sensitive to climatic changes, however, it is challenging to reconstruct paleoclimate based on terrestrial evaporite records using conventional elemental and isotopic proxies. Magnesium is a major element in saline lakes, and the Mg isotope composition of brine is responsive to climate-driven processes such as carbonate precipitation and freshwater input. However, little has been explored on the application of Mg isotopes to studies of saline lakes. In this study, the Middle Pleistocene halite deposit from the Xiaoliangshan (XLS) evaporite section in the Qaidam Basin, Northwest China, was selected as a case to evaluate the response of Mg isotopes in the saline lake to environmental events. The Mg isotope data of the halite are complemented by geochemical analyses major elements and Sr-Cl isotopes of the halite component, and C-O isotopes of the associated carbonates. The 87Sr/86Sr ratios of the halite remained homogeneous (0.7111 to 0.7112) throughout the section, suggesting that the material source did not change significantly during the precipitation of the halite beds. By contrast, element ratios (Mg/Na and K/Na) and stable isotope ratios of C, O, Cl, and Mg show remarkable fluctuations along with the sediment profile. Based on the C-O-Cl isotope data, we identified events of freshwater recharging and desalination in an overall dry climatic background that were recorded in the halite. Notably, halite δ26Mg values vary by up to 2‰ in the section (from −1.63‰ to 0.46‰), and the low δ26Mg signature of halite was interpreted to reflect the input of light Mg isotopes into the saline lake in a freshwater recharging event, while the high δ26Mg values were produced by the precipitation of carbonate minerals under arid climatic conditions. Collectively, we suggest that Mg isotopes in terrestrial halite could be a sensitive tracer of basin hydrology. This study for the first time demonstrates the potential of Mg isotopes in terrestrial evaporites to unravel paleoclimatic events.

Quantifying the effect of shade on cuticle morphology and carbon isotopes of sycamores: present and past.

PremiseReconstructing the light environment and architecture of the plant canopy from the fossil record requires the use of proxies, such as those derived from cell wall undulation, cell size, and carbon isotopes. All approaches assume that plant taxa will respond predictably to changes in light environments. However, most species‐level studies looking at cell wall undulation only consider “sun” or “shade” leaves; therefore, we need a fully quantitative taxon‐specific method.MethodsWe quantified the response of cell wall undulation, cell size, and carbon isotopes of Platanus occidentalis using two experimental setups: (1) two growth chambers at low and high light and (2) a series of outdoor growth experiments using green and black shade cloth at different densities. We then developed and applied a proxy for daily light integral (DLI) to fossil Platanites leaves from two early Paleocene floras from the San Juan Basin in New Mexico.ResultsAll traits responded to light environment. Cell wall undulation was the most useful trait for reconstructing DLI in the geological record. Median reconstructed DLI from early Paleocene leaves was ~44 mol m−2 d−1, with values from 28 to 54 mol m−2 d−1.ConclusionsCell wall undulation of P. occidentalis is a robust, quantifiable measurement of light environment that can be used to reconstruct the paleo‐light environment from fossil leaves. The distribution of high DLI values from fossil leaves may provide information on canopy architecture; indicating that either (1) most of the canopy mass is within the upper portion of the crown or (2) leaves exposed to more sunlight are preferentially preserved.

Multipole Response of Ni and Sn Isotopes and its Momentum Dependence

We have used a self-consistent Skyrme-HF plus Continuum RPA model to study the low-multipole response of stable and neutron/proton-rich Ni and Sn isotopes using suitable Skyrme parametrizations. Experimental studies of these nuclei are being planned at RIKEN. We focus on the momentum dependence of the strength distribution, as it may provide information on the structure of excited nuclear states.

Response of Mg isotopes to dolomitization during fluctuations in sea level: Constraints on the hydrological conditions of massive dolomitization systems

Marine dolomitization processes are characterized by complex variations in hydrological conditions and pore-fluid chemistry. Deposition of massive dolomitized carbonates on the Comanche Platform of south Texas, USA, during the Albian coincided with multiple fluctuations in sea level, thereby providing an ideal setting to study the response of magnesium isotopes to dolomitization during eustatic sea-level change. In this study, we conducted Mg, C, and O isotope analyses and complimentary mineralogical and petrographic investigations of dolomitized massive carbonates of the Albian Edwards Group in the Comanche Platform. Petrographic observations indicate that the carbonate rocks in the studied units were not altered by deep burial diagenesis or hydrothermal fluids. Based on our petrographic observations and the trace element and C–O isotope data, we infer the dolomites were formed via syn-depositional dolomitization during a period of low sea level. The δ26Mg values of the dolomites increase rapidly from −2.5‰ to −1.8‰ in the basal part of the unit, reflecting a change in fluid chemistry caused by dolomitization in a restricted marine environment. Subsequently, δ26Mgdolomite values gradually decrease back to their initial value of approximately −2.5‰ due to seawater replenishment during transgression. The observed variability in dolomite δ26Mg values reflects changes in the connectivity of the platform with the open ocean during marine transgressions. However, the δ26Mgdolomite values do not vary with the high-frequency eustatic sea-level change recorded in the lithological variations, indicating uniform hydrologic conditions of the massive dolomitization system despite the hydrodynamic variations in the sedimentary environment. Therefore, we propose that massive dolomitization systems are mostly fluid-buffered and, as a result, Mg isotopes of dolomites can be used to trace changes in the paleo-marine environment, such as basin connectivity.

High-precision equation of state benchmark for cryogenic liquid deuterium at ultrahigh pressure

We provide a principal Hugoniot of cryogenic liquid deuterium at a pressure of $27lPl240$ GPa, with reflected-shock data of up to $\ensuremath{\sim}830$ GPa. The maximum density reaches $\ensuremath{\sim}1.49$ $\mathrm{g}/{\mathrm{cm}}^{3}$, about 8.7 times the initial density. Our independent principal Hugoniot experimental data broadly support the wide-regime equation of state (WEOS) model, which well matches Fernandez-Pa\~nella et al. [Phys. Rev. Lett. 122, 255702 (2019)] and Knudson et al. [Phys. Rev. Lett. 118, 035501 (2017)] experimental data over the observed pressure range up to 550 GPa, and most of three sets of reflected shock data are in accordance with our theory up to 1 TPa. Our high-precision experimental results establish an important benchmark equation of state of deuterium and conform to the WEOS model quite well. Our work is useful for the development of the high-pressure response of hydrogen's isotopes, directly related to inertial confinement fusion, planetary science, and metallization.

Temporal variations and evaporation control effect of the stable isotope composition of precipitation in the subtropical monsoon climate region, Southwest China

Stable hydrogen and oxygen isotopes in precipitation, which are sensitive to climate change and the water cycle, are widely used as natural tracers to investigate hydrological processes and paleoclimate reconstruction. High resolution event-based precipitation sampling was conducted in Guilin, a subtropical monsoon climate region in Southwest China, from 2010 to 2016 to identify the temporal variability of stable isotopes of precipitation and its dominant meteorological factors. The results showed that the stable isotopes of precipitation exhibited significant seasonal variability, with lower values in the wet season and higher values in the dry season, reflecting the changes in the character of precipitation in response to seasonal variability of the moisture sources and atmospheric dynamics. δ18O values were significantly negatively correlated with temperature and potential evapotranspiration (ET0) on the daily and monthly timescales; however, there was no significant correlation between monthly δ18O and the amount of rainfall. Multivariate regression analysis showed that when temperature, precipitation amount and ET0 were considered, 39% of the variability of the monthly oxygen isotope composition of precipitation was explained. Monthly mean temperature had a higher covariation with precipitation δ18O, indicating that the variations of precipitation isotope were more dependent on temperature, but not substantially dependent on the amount effect of rainfall. Howerver, the main influence of meteorological factors on the isotopic variations of precipitation varied seasonally. Temperature strongly influenced the isotopic composition of precipitation in summer, while the amount effect was prominent due to the enhanced convective activity characterized by more depleted δ18O values. Although evapotranspiration is a minor contributor of precipitation isotope variability, the impact of evapotranspiration on the enrichment of precipitation isotopes in autumn is a substantial control and should be considered, which makes a major contribution to compensate for the depletion in heavy isotopes of rainfall. The non-linear relationship between d-excess and ET0 was attributed to the variable contribution of large-scale convective moisture and local evaporation, as well as water vapor transfer processes. The response of precipitation stable isotopes to convective events was characterized by distinct stage patterns, associated with the water vapor sources and physical processes during rainout. The significant depletion of precipitation δ18O values during convective events was not due to the ‘amount effect’, but due to large-scale convection activity. These findings provide further insight into intraevent-scale isotopic variations associated with organized convection and their key influencing factors, which ultimately improves our interpretation of the paleoclimate records in subtropical monsoon regions.

Supernova neutrino scattering off Gadolinium odd isotopes in water Cherenkov detectors

In this work the supernova neutrino (SN) charged-current interactions with Gd odd isotopes (A=155 and 157) are studied. We use measured spectra and the quasiparticle-phonon model (MQPM) to calculate the charged current response of odd Gd isotopes to supernova neutrinos. Flux-averaged cross sections are obtained considering quasi-thermal neutrino spectra.

Nuclear response in a finite-temperature relativistic framework

A thermal extension of the relativistic nuclear field theory is formulated for the nuclear response. The Bethe-Salpeter equation (BSE) with the time-dependent kernel for the particle-hole response is treated within the Matsubara Green's function formalism. We show that, with the help of a temperature-dependent projection operator on the subspace of the imaginary time (time blocking), it is possible to reduce the BSE for the nuclear response function to a single frequency variable equation also at finite temperature. The approach is implemented self-consistently in the framework of quantum hadrodynamics based on the meson-nucleon Lagrangian. The method is applied to the monopole, dipole and quadrupole response of $^{48}$Ca and to the dipole response of the tin isotopes $^{100,120,132}$Sn, in particular, to a study of the evolution of nuclear collective oscillations with temperature. The article is dedicated to the memory of Pier Francesco Bortignon and devoted to the developments related to his pioneering ideas.

The Response of Magnesium, Silicon, and Calcium Isotopes to Rapidly Uplifting and Weathering Terrains: South Island, New Zealand

Silicate weathering is a dominant control on the natural carbon cycle. The supply of rock (e.g. via mountain uplift) has been proposed as a key weathering control, and suggested as the primary cause of Cenozoic cooling. However, this is ambiguous because of a lack of definitive weathering tracers. We use the isotopes of the major cations directly involved in the silicate weathering cycle: magnesium, silicon and calcium. Here we examine these isotope systems in rivers draining catchments with variable uplift rates (used as a proxy for exposure rates) from South Island, New Zealand. Overall, there is no trend between these isotope systems and uplift rates, which is in contrast to those of trace elements like lithium or uranium. Li and Si isotopes co-vary, but only in rapidly uplifting mountainous terrains with little vegetation. In floodplains, in contrast, vegetation further fractionates Si isotopes, decoupling the two tracers. In contrast, Mg and Ca isotopes (which are significantly affected by the weathering of both carbonates and silicates) exhibit no co-variation with each other, or any other weathering proxy. This suggests that lithology, secondary mineral formation and vegetation growth are causing variable fractionation, and decoupling the tracers from each other. Hence, in this context, the isotope ratios of the major cations are significantly less useful as weathering tracers than those of trace elements, which tend to have fewer fractionating processes.

New γ-ray detector CATANA for in-beam γ-ray spectroscopy with fast RI beams

The γ-ray detector CATANA (CAesium iodide array for γ-ray Transitions in Atomic Nuclei at high isospin Asymmetry) has been constructed to detect γ-rays from nuclear reaction with radioactive ion beams at RIKEN RIBF. CATANA consists of 100 square frustum-shaped CsI(Na) crystals coupled with photomultiplier tubes. The crystal shapes and positions are arranged to minimize the distance between the crystals so as to have better calorimetric properties and Doppler-shift correction capability. The CATANA array was employed to measure the dipole response of the neutron-rich calcium isotopes 50,52Ca, and the γ-rays from several known transitions were observed.

The response of Li and Mg isotopes to rain events in a highly-weathered catchment

Storms are responsible for up to ~50% of total annual rainfall on tropical islands and result in rapid increases in discharge from rivers. Storm events are, however, notoriously under-sampled and their effects on weathering rates and processes are poorly constrained. To address this, we have undertaken high-frequency sampling of Quiock Creek catchment, a Critical Zone Observatory located in Guadeloupe, over a period of 21 days, encompassing several storm events. Chemical and isotopic (Li and Mg) analyses of different critical zone reservoirs (throughfall, soil pore water, groundwater and river water) were used to assess the interactions between rock, water and secondary minerals. The Li concentrations and δ7Li values of these different reservoirs range from 14 to 95 nmol/kg and 1.8 to 16.8‰, respectively. After several rain events, the average δ7Li value (13.3‰) of soil solutions from the lower part of the soil profile (>~150 cm below the surface) was unchanged, whereas in the upper part of the profile δ7Li values increased by ~2–4‰ due to increased contribution from throughfall. By contrast, the δ26Mg value of soil waters in the upper part of the soil profile were not significantly affected by the rain events with an average value of −0.90‰. The δ26Mg values of the different fluid reservoirs were generally close to the value of throughfall (~−0.90‰), but higher δ26Mg values (up to −0.58‰) were measured in the deeper parts of the soil profile, whereas groundwaters that have a long residence time had lower δ26Mg values (down to −1.48‰). These higher and lower values are attributed to, respectively, adsorption/desorption of light Mg isotopes on/from the surface of clay minerals. The δ7Li value of the river waters was ~9.3‰, with a Li concentration of 60 μmol/kg, but during a storm these values decreased to, respectively, 7.8‰ and 40 μmol/kg. This change in δ7Li is consistent with an increased contribution of Li from the soil solution. Thus, even in highly weathered catchments, changes in hydrological conditions can have a significant impact on weathering processes and therefore the composition of river waters delivered to the ocean.

Supernova neutrino scattering off Gadolinium even isotopes in water Cherenkov detectors

Neutrinos in water can be detected thanks to several reactions. The most important one is the inverse beta decay ν̄e + p → n + e+. The detection of 2.2 MeV γ from neutron capture on free protons is very difficult. The feasibility of Gadolinium (Gd) doping in water Cherenkov detectors essentially reduces background signals and enhances the sensitivity to neutrino detection. In this work the supernova neutrino charged-current interactions with the most abundant Gd even isotopes (A=156,158 and 160) are studied. We use measured spectra and the quasiparticle random phase approximation to calculate the charged current response of Gd isotopes to supernova neutrinos. Flux-averaged cross sections are obtained considering quasi-thermal neutrino spectra.

Isotopic and spectral effects of Pu quality in Th-Pu fueled PWRs

UK plutonium (Pu) management is expected to focus on the use of uranium-plutonium (U-Pu) mixed oxide (MOX) fuel. However, research has shown that thorium-plutonium (Th-Pu) may be a viable alternative, offering favourable performance characteristics. A scoping study was carried out to determine the effect of isotopic composition and spectral hardening in standard and reduced moderation Pressurised Water Reactors (PWRs and RMPWRs). Lattice calculations were performed using WIMS to investigate safety parameters (Doppler Coefficient (DC), Moderator Temperature Coefficient (MTC), Void Coefficient (VC) – in this case Fully Voided Reactivity (FVR) – and Boron Worth (BW)), maximum theoretically achievable discharge burnup, Pu consumption and transuranic (TRU) composition of spent nuclear fuel (SNF) for the two reactor types. Standard grades of Pu were compared to a predicted UK Pu vector.MTC and FVR were found to be strongly influenced by the isotopic composition of the fuel. MTC was determined to be particularly sensitive to positive ‘peak’ contributions from fissile isotopes in the energy range 0.1–1 eV which diminish as the Pu content increases. The more extreme nature of the perturbation in FVR cases results in key differences in the contributions from fissile isotopes in the thermal energy range when compared with MTC, with no positive contributions from any isotope <500 eV.Where the requirement for MTC to remain negative was the limiting factor, a higher maximum fissile loading, discharge burnup and Pu consumption rate were possible in the PWR than the RMPWR, although the two reactors types typically produced similar levels of U233. However, for the majority of Pu grades the total minor actinide (MA) content in SNF was shown to be significantly lower in the RMPWR. Where FVR is the limiting factor, the maximum fissile loading and discharge burnup are similar in both reactor types, while increased Pu consumption rates were possible in the PWR. In this case, lower concentrations of U233 and MAs were found to be present in the PWR. These results are for a single pass of fuel through a reactor and, while the response of fissile isotopes at given energies to temperature perturbations will not vary significantly, the maximum achievable discharge burnup, Pu consumption rate and TRU build-up would be very different in a multi-recycle scenario.