Natural nuclear fission reactor

A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions occur. The conditions under which a natural nuclear reactor could exist had been predicted in 1956 by Paul Kuroda. The remnants of an extinct or fossil nuclear fission reactor, where self-sustaining nuclear reactions have occurred in the past, can be verified by analysis of isotope ratios of uranium and of the fission products (and the stable daughter nuclides of those fission products). An example of this phenomenon was discovered in 1972 in Oklo, Gabon by Francis Perrin under conditions very similar to Kuroda's predictions. Oklo is the only location where this phenomenon is known to have occurred, and consists of 16 sites with patches of centimeter-sized ore layers. There, self-sustaining nuclear fission reactions are thought to have taken place approximately 1.7 billion years ago, during the Statherian period of the Paleoproterozoic, and continued for a few hundred thousand years,

Modelling of fuel fragmentation, relocation and dispersal during Loss-of-Coolant Accident in Light Water Reactor

Vladimir Vladimirov Brankov

Recent LOCA tests with high burnup fuel at the OECD Halden Reactor Project and at Studsvik demonstrated the susceptibility of the fuel to fragment to small pieces, to relocate and possibly cause a hot-spot effect and to be dispersed in the event of cladding rupture. However, the LOCA safety criteria defined by the US NRC are still based on fuel tests with fresh and low burnup fuel and therefore require revision for high burnup fuel. In this context a PhD project with the goal of developing new models for high burnup fuel fragmentation, relocation and dispersal during Loss of Coolant Accident in Light Water Reactors was launched at Paul Scherrer Institute in June 2013 with a financial support from swissnuclear. The work continued with base irradiation simulation with a closer look at the fission gas release measurements of high burnup BWR fuel rods.The data showed significant scatter for fuel rods at the same average burnup, originating from the same fuel assembly and having almost the same enrichment. The scatter was explained with a BWR-specific mechanism for fission gas trapping. It was motivated with Scanning Electron Microscopy images at the pellet-cladding interface that showed very strong fuel-cladding bonding layer on one side of the fuel rod. Base irradiation with the EPRI's FALCON code coupled with an in-house advanced fission gas release and gaseous swelling model GRSW-A was done for selected high burnup BWR fuel rods. The calculated fission gas release was overestimated in all cases. This was expected, because at the time the modelling did not account for fission gas trapping. From the available modelling and experimental data, a model for fission gas trapping was proposed. After calibration, the agreement between calculated and measured fission gas release was significantly improved. Fuel fragmentation modelling was focused on fuel pulverization. This is a high burnup fuel-specific phenomenon, in which the fuel pellet periphery may fragment to sizes less than 100 ÎŒm. Such fragments are very mobile and can be easily relocated and dispersed in the event of cladding rupture regardless of the rupture opening size. Therefore, the fuel fragmentation model addresses the potentially most important mode of fragmentation - fuel pulverization. Fuel relocation inside fuel rod is simulated during cladding ballooning and until cladding failure. The model takes as input the time-dependent cladding deformation supplied by FALCON and the fragment size distribution either provided directly by experimental data or by the fuel fragmentation model. Fuel relocation was modelled under the specific assumption that outermost fragments (e.g. pulverized fuel) relocate first, resulting in a large packing factor and local cladding temperature increase at the balloon (i.e. hot-spot effect) and, as a consequence, in enhanced cladding oxidation at the balloon. Fuel dispersal is modelled by solving the mass, energy and momentum conservation equations for two phases - the gas inside the fuel rod and the fraction of fuel which is considered "moveable". Although the model uses simplified geometrical representation of the fuel rod and some other simplifying assumptions, the underlying reason for fuel dispersal (namely the interfacial friction between the gas outflow and the solid) is explicitly simulated. The model for fuel dispersal is calibrated using Halden and Studsvik LOCA tests.

EPFL2017

Fission Rates Measured Using High-Energy Gamma-Rays from Short Half-Life Fission Products in Fresh and Spent Nuclear Fuel

Hanna Kröhnert

In recent years, higher discharge burn-ups and initial fuel enrichments have led to more and more heterogeneous core configurations in light water reactors (LWRs), especially at the beginning of cycle when fresh fuel assemblies are loaded next to highly burnt ones. As this trend is expected to continue in the future, the Paul Scherrer Institute has, in collaboration with the Swiss Association of Nuclear Utilities, swissnuclear, launched the experimental programme LIFE@PROTEUS. The LIFE@PROTEUS programme aims to better characterise interfaces between burnt and fresh UO2 fuel assemblies in modern LWRs. Thereby, a novel experimental database is to be made available for enabling the validation of neutronics calculations of strongly heterogeneous LWR core configurations. During the programme, mixed fresh and highly burnt UO2 fuel lattices will be investigated in the zero-power research reactor PROTEUS. One of the main types of investigations will be to irradiate the fuel in PROTEUS and measure the resulting fission rate distributions across the interface between fresh and burnt fuel zones. The measurement of fission rates in burnt fuel re-irradiated in a zero-power reactor requires, however, the development of new experimental techniques which are able to discriminate against the high intrinsic activity of the fuel. The principal goal of the present research work has been to develop such a new measurement technique. The selected approach is based on the detection of high-energy gamma-ray lines above the intrinsic background (i.e. above 2200 keV), which are emitted by short-lived fission products freshly created in the fuel. The fission products 88Kr, 142La, 138Cs, 84Br, 89Rb, 95Y, 90mRb and 90Rb, with half-lives between 2.6 min and 2.8 h, have been identified as potential candidates. During the present research work, the gamma-ray activity of short-lived fission products has, for the first time, been measured and quantitatively evaluated for re-irradiated burnt UO2 fuel samples with burn-ups of about 36 and 46GWd/t. Based on experiments carried out with these fuel samples in a reference test lattice of the PROTEUS reactor, fresh-to-burnt-fuel fission rate ratios could be determined. The 1σ uncertainties on the derived fission rate ratios are 1.7 to 3.4% and are mainly due to the statistical uncertainties. Calculated values of the fission rate ratios, as obtained using the Monte Carlo code MCNPX, have been shown to agree with the experimental results within these uncertainties. In deriving fresh-to-burnt-fuel fission rate ratios,142La and 138Cs have emerged as the preferred fission products. Their fission yields for the main fissile isotope (235U, 239Pu and 241Pu) are similar, which makes them relatively insensitive to the exact composition of the burnt fuel. Finally, a measurement station for the future LIFE@PROTEUS experiments has been proposed and evaluated, along with a detailed formulation of recommendations for optimised irradiation and measurement strategies. The estimated accuracy for the foreseen measurements of fission rate ratios between fresh and highly burnt fuel pins is 1 to 2%. The contribution of nuclear-data related uncertainties have been pointed out as possibly representing the main constraint on the achievable accuracy in future experiments. In brief, the present research work has established a novel experimental technique for measuring and comparing fission rates in fresh and highly burnt fuels in a zero-power research reactor such as PROTEUS. Moreover, possibilities have been presented for the further optimisation needed for a future, routine application of the technique.

EPFL2011

X-ray micro-diffraction studies of heterogeneous interfaces between cementitious materials and geological formations

Philip Pattison

In the present study the challenge of analyzing complex micro X-ray diffraction (microXRD) patterns from cement clay interfaces has been addressed. In order to extract the maximum information concerning both the spatial distribution and the crystal structure type associated with each of the many diffracting grains in heterogeneous, polycrystalline samples, an approach has been developed in which microXRD was applied to thin sections which were rotated in the X-ray beam. The data analysis, performed on microXRD patterns collected from a filled vein of a cement clay interface from the natural analogue in Maqarin (Jordan), and a sample from a two-year-old altered interface between cement and argillaceous rock, demonstrate the potential of this method. (C) 2013 Elsevier Ltd. All rights reserved.