Laurent Cantrel

Review of literature on ruthenium behavior in nuclear power plant severe accidents

During a hypothetical severe accident in a pressurized water reactor (PWR), fission products (FPs) are released from the nuclear fuel and may reach the reactor containment building. Among the FPs, ruthenium is of particular interest due to its ability to form volatile oxide compounds in highly oxidizing conditions. In addition, ruthenium is a very hazardous compound because it is chemically toxic and also because of its radiotoxicity. The topic of ruthenium is examined in terms of nuclear safety issues. A review of the literature regarding ruthenium oxides properties, gaseous and aqueous chemistry is compiled. The study focuses on ruthenium tetroxide (RuO4), which is highly reactive and volatile and is the most likely gaseous chemical form under the conditions prevailing in the containment. The interactions between ruthenium oxides and containment surfaces, which could be most important in overall ruthenium behavior, are also discussed. Finally, an evaluation of the possible revolatilization phenomena of ruthenium adsorbed on PWR containment surfaces or dissolved in the sump under superoxidizing conditions (radiolysis) is also presented. In this case, ruthenium dioxide (RuO2) must also be considered. Knowledge of all these phenomena is required to accurately predict ruthenium behavior and to make a best-estimate assessment of the potential ruthenium source term.

Reaction kinetics of a fission-product mixture in a steam-hydrogen carrier gas in the phebus primary circuit

Radioiodine entering the containment from the postaccident primary circuit in vapor or gaseous form, as observed in the Phebus FPT0 and FPT1 tests, has a direct impact on the source term evaluation. State-of-the-art fission-product transport codes based on the assumption of thermochemical equilibrium failed to predict this phenomenon. In this work the standard approach of assuming the instantaneous establishment of thermochemical equilibrium is questioned and it will be argued that kinetic limitations may have existed under the severe-accident boundary conditions of the FPT0 and FPT1 tests. To this end a simple monodimensional transport model was developed in an attempt at introducing kinetic aspects within the primary circuit. A number of homogeneous gas-phase reactions between selected fission products and structural materials, complemented by condensation reactions, underlies the kinetic model. In the absence of experimental data, the kinetic constants were estimated using the transition-state theory or semi-empirical methods. The kinetic model was then applied to the analysis of Phebus FPT0 and FPT1 yielding a satisfactory agreement between experimental data and model predictions.

Theoretical Study of the Gas-Phase Reactions of Iodine Atoms (2P3/2) with H2, H2O, HI, and OH

The rate constants of the reactions of iodine atoms with H(2), H(2)O, HI, and OH have been estimated using 39, 21, 13, and 39 different levels of theory, respectively, and have been compared to the available literature values over the temperature range of 250-2500 K. The aim of this methodological work is to demonstrate that standard theoretical methods are adequate to obtain quantitative rate constants for the reactions involving iodine-containing species. Geometry optimizations and vibrational frequency calculations are performed using three methods (MP2, MPW1K, and BHandHLYP) combined with three basis sets (cc-pVTZ, cc-pVQZ, and 6-311G(d,p)). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the cc-pVnZ (n = T, Q, and 5), aug-cc-pVnZ (n = T, Q, and 5), 6-311G(d,p), 6-311+G(3df,2p), and 6-311++G(3df,3pd) basis sets. Canonical transition state theory with a simple Wigner tunneling correction is used to predict the rate constants as a function of temperature. CCSD(T)/cc-pVnZ//MP2/cc-pVTZ (n = T and Q), CCSD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p), and CCSD(T)/6-311++G(3df,3pd)//MP2/6-311G(d,p) levels of theory provide accurate kinetic rate constants when compared to available literature data. The use of the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ and CCSD(T)/6-311++G(3df,3pd) levels of theory allows one to obtain a better agreement with the literature data for all reactions with the exception of the I + H(2) reaction R(1) . This computational procedure has been also used to predict rate constants for some reactions where no available experimental data exist. The use of quantum chemistry tools could be therefore extended to other elements and next applied to develop kinetic networks involving various fission products, steam, and hydrogen in the absence of literature data. The final objective is to implement the kinetics of gaseous reactions in the ASTEC (Accident Source Term Evaluation Code) code to improve speciation of fission transport, which can be transported along the Reactor Coolant System (RCS) of a Pressurized Water Reactor (PWR) in case of a severe accident.

Study of RuO4 decomposition in dry and moist air

During a hypothetical severe nuclear accident on a pressurized water reactor (PWR), it is of primary importance to assess potential radionuclide release into the environment, and thus to better understand the volatile ruthenium tetroxide stability, in the containment building, due to its high radiotoxicity. The stability of RuO4(g) in dry and moist air, under conditions representative of a PWR containment building, is investigated. RuO4 decomposition occurs in bulk gas phase, without any specific affinity with surfaces. The kinetic rate law of RuO4 reduction is found to be dependent on the presence of steam. The humidity seems to play a catalytic role, as well as the presence of ruthenium dioxide deposits. The temperature is also a key parameter. In the presence of steam, the half-life times of RuO[4] are found to be respectively of 5 h and 9 h at 90 °C and 40 °C. A chemical reaction scheme consistent with the experimental observations is proposed.

Radiolytic Oxidation of Ruthenium Oxide Deposits

Abstract In the case of a hypothetical severe accident in a nuclear pressurized water reactor, the formation of radiotoxic RuO4(g) may occur in the reactor containment building, resulting from the interactions of ruthenium oxide deposits with the oxidizing medium induced by air radiolysis. Consequently, this gaseous ruthenium tetroxide may be dispersed into the environment; therefore, the determination of the ruthenium deposits behavior is of primary impo rtance for nuclear safety studies. An experimental study, performed by the French Institut de Radioprotection et de Sûrete Nucleaire (IRSN), using a gam ma irradiator cell (EPICUR facility at IRSN/Cadarache) has been carried out in order to obtain experimental data on these interactions. The results showed that radiolytic oxidation of ruthenium oxide deposits leads to the formation of gaseous ruthenium tetroxide to a significant extent. A comparison between the revolatilized Ru fractions obtained experimentally and those obtained by calculations based on the rate laws modeling ozone irradiation effect, established in previous studies, is presented. The disagreement observed is discussed. It appears that the oxidation resulting from air/steam radiolysis products is enhanced in comparison with pure ozone effect.

A theoretical study of the H-abstraction reactions from HOI by moist air radiolytic products (H, OH, and O (3P)) and iodine atoms ( 2P3/2)

The rate constants of the reactions of HOI molecules with H, OH, O ((3)P), and I ((2)P(3/2)) atoms have been estimated over the temperature range 300-2500 K using four different levels of theory. Geometry optimizations and vibrational frequency calculations are performed using MP2 methods combined with two basis sets (cc-pVTZ and 6-311G(d,p)). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the cc-pVTZ, cc-pVQZ, 6-311+G(3df,2p), and 6-311++G(3df,3pd) basis sets. Reaction enthalpies at 0 K were calculated at the CCSD(T)/cc-pVnZ//MP2/cc-pVTZ (n = T and Q), CCSD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p), and CCSD(T)/6-311++G(3df,3pd)//MP2/6-311G(d,p) levels of theory and compared to the experimental values taken from the literature. Canonical transition-state theory with an Eckart tunneling correction is used to predict the rate constants as a function of temperature. The computational procedure has been used to predict rate constants for H-abstraction elementary reactions because there are actually no literature data to which the calculated rate constants can be directly compared. The final objective is to implement kinetics of gaseous reactions in the ASTEC (accident source term evaluation code) program to improve speciation of fission products, which can be transported along the reactor coolant system (RCS) of a pressurized water reactor (PWR) in the case of a severe accident.

Theoretical study of plutonium(IV) complexes formed within the PUREX process: a proposal of a plutonium surrogate in fire conditions.

We present a relativistic quantum chemical study to determine the best surrogate for plutonium(IV) to be used in experimental investigations of the behavior of plutonium-nitrate-TBP in fire conditions that might occur in the nuclear fuel refining process known as PUREX. In this study geometries and stabilities of Pu(NO3)6(2-) and Pu(NO3)4(TBP)2 complexes were compared to that of equivalent complexes of selected elements from the lanthanide and actinide series (Ce, Th, U) chosen on the basis of similar ionic radii and stability as tetravalent species. PBE and PBE0 DFT functionals have proven to be sufficient and affordable for qualitative studies, performing as good as the wave function based correlated method MP2. On the basis of our results, cerium(IV) appears to be a good surrogate for plutonium(IV).

Impact of the Si/Al ratio on the selective capture of iodine compounds in silver-mordenite: a periodic DFT study

Silver modified zeolites with a mordenite structure can capture volatile iodine compounds (I2 and ICH3) which can be released during a severe nuclear accident. However under these particular conditions, molecules such as CO and H2O present in the containment atmosphere are expected to inhibit the adsorption of iodine compounds. In the present work, periodic density functional theory calculations have been carried out to investigate the interaction of I2, ICH3, H2O and CO molecules in silver-exchanged mordenite with various Si/Al ratios with the aim of finding values that favor a selective adsorption of I2 and ICH3. Computational results show that the interaction energies of CO and H2O remain of the same order of magnitude (from -120 to -140 kJ mol-1 for CO and from -90 to -120 kJ mol-1 for H2O) for all the investigated Si/Al ratios. In contrast, ICH3 is increasingly strongly adsorbed as the Si/Al ratio decreases, from around -145 kJ mol-1 when Si/Al = 47 to -190 kJ mol-1 for Si/Al = 5. The same trend is observed for I2 with a larger amplitude: from -135 kJ mol-1 for Si/Al = 47 to -300 kJ mol-1 for Si/Al = 5. Therefore, the use of silver-exchanged mordenite with Si/Al ratios of 5 or 11 would drastically limit the inhibiting effect of contaminants on the adsorption of volatile iodine species. Also for the same ratios, a spontaneous dissociation of I2 during its adsorption is observed, leading to the formation of AgI complexes which are prerequisite for the immobilization of iodine in the long term.

A Density Functional Theory and ab Initio Investigation of the Oxidation Reaction of CO by IO Radicals.

To get an insight into the possible reactivity between iodine oxides and CO, a first step was to study the thermochemical properties and kinetic parameters of the reaction between IO and CO using theoretical chemistry tools. All stationary points involved were optimized using the Beckes three-parameter hybrid exchange functional coupled with the Lee-Yang-Parr nonlocal correlation functional (B3LYP) and the Møller-Plesset second-order perturbation theory (MP2). Single-point energy calculations were performed using the coupled cluster theory with the iterative inclusion of singles and doubles and the perturbative estimation for triple excitations (CCSD(T)) and the aug-cc-pVnZ (n = T, Q, and 5) basis sets on geometries previously optimized at the aug-cc-pVTZ level. The energetics was then recalculated using the one-component DK-CCSD(T) approach with the relativistic ANO basis sets. The spin-orbit coupling for the iodine containing species was calculated a posteriori using the restricted active space state interaction method in conjunction with the multiconfigurational perturbation theory (CASPT2/RASSI) employing the complete active space (CASSCF) wave function as the reference. The CCSD(T) energies were also corrected for BSSE for molecular complexes and refined with the extrapolation to CBS limit while the DK-CCSD(T) values were refined with the extrapolation to FCI. The exploration of the potential energy surface revealed a two-steps mechanism with a trans and a cis pathway. The rate constants for the direct and complex mechanism were computed as a function of temperature (250-2500 K) using the canonical transition state theory. The three-parameter Arrhenius expressions obtained for the direct and indirect mechanism at the DK-CCSD(T)-cf level of theory is 1.49 × 10(-17) × T(1.77) exp(-47.4 (kJ mol(-1))/RT).

Thermochemistry of small iodine species

We present a systematic study of the thermochemistry for a set of iodine species relevant to atmospheric chemistry. The reactions include H, O and I atoms and H2, OH, HI, I2, iodine monoxide, hypoiodous acid (HOI) and H2O species. The calculations presented were performed using completely renormalized coupled cluster theory including single, double and non-iterative triple substitutions in conjunction with the ANO-RCC basis sets developed for scalar relativistic calculations. The second-order spin-free Douglas–Kroll–Hess Hamiltonian was used to account for the scalar relativistic effects. The calculations also included spin–orbit corrections and semi-core correlation contributions. The resulting reaction enthalpies and Gibbs energies at 298 K have been compared with the experimental data. On the basis of a set of selected reactions we suggest an updated value for Δ f H298K° of HOI based on the set of isogyric reactions: −69.0 ± 3.7 kJ mol−1.