[en] The quantitative aspects of premixing and rate of fragmentation in steam explosions are addressed. For premixing the experiments are focused on the water-depletion phenomenon predicted to occur within the two-dimensional, three-phase, transient mixing zone of a high temperature melt poured into a pool of coolant. These experiments are scaled to yield similar water-depletion regimes as expected in the lower plenum of the reactor vessel. The first, preliminary, results are consistent with numerical predictions. For fragmentation the experiments are focused on observing single exploding melt drops in a steady, elevated pressure field, prototypic of an escalated explosion. The first, preliminary, data demonstrate the interplay between the thermal and hydrodynamic components of the fragmentation-driving mechanism(s), and provide the promise that on such a basis appropriate constitutive laws can be made available for the numerical computation of the escalation and propagation of steam explosions

[en] The primary purpose of this paper is to present recent results on certain key experimental aspects and mechanisms of steam explosions. In addition, sample results of integral calculations, incorporating the recent experimental results, are presented. The scope of this work is focused to the premixing and propagation phases of energetic explosions. On premixing, the authors present experimental results in scaled (1/8-scale) geometries of the lower plenum of a PWR. The computer code PM-ALPHA was used as the scaling tool to ensure the experiment is run under similar water depletion regimes as that predicted for the reactor. This water depletion (from the mixing zone) is the key physical mechanism that limits the energetics of such large scale explosions, and to a large extent these experiments are focused on it. Thus, instead of a molten material, hot solid particles are used in these experiments. Local, instantaneous liquid fraction measurements are made, within the three-phase mixing zone, by means of a new instrument developed specifically for this purpose. For calculating escalation and propagation, the key ingredient is the fuel fragmentation kinetics. These kinetics are explored experimentally in this work, for the first time, at conditions that simulate a propagating explosion. Finally, illustrative integral calculations are presented for large scale pours in the lower plenum

[en] The authors are investigating the possible adverse effects of (1) backstreaming ion emission from the Bremsstrahlung converter target and (2) the interaction of the resultant plasma with the electron beam during subsequent pulses for multi-pulse radiography facilities. These effects would primarily manifest themselves in a static focusing system as a rapidly varying x-ray spot. To study these effects, they are conducting beam-target interaction experiments on the ETA-II accelerator (a 6.0 MeV, 2.5 kA, 70 ns FWHM pulsed, electron accelerator). They are measuring spot dynamics and characterizing the resultant plasma for various configurations. Thus far, their experiments show that the first effect is not strongly present when the beam initially interacts with the target. Electron beam pulses delivered to the target after formation of a plasma are strongly affected. They have also performed initial experiments to determine the effect of the beam propagating through the plasma. This data shows that the head of the beam is relatively robust, but that backstreaming ions from the plasma can still manifest itself as a dynamic focus toward the tail of the beam. They report on the details of the experimental work to suppress these effects

[en] As part of the Dual Axis Radiography Hydrotest Facility, Phase II (DARHT II) multipulse Bremsstrahlung target, we have been performing an investigation of (1) the possible adverse effects of backstreaming ion emission from the Bremsstrahlung converter target and (2) maintaining sufficient target density to ensure dose in latter pulses. Theory predictions show that the first effect would primarily be manifested in the static focusing system as a rapidly varying x-ray spot. From experiments performed on ETA-II, we have shown that the first effect is not strongly present when the beam initially interacts with the target. Electron beam pulses delivered to the target after formation of a plasma are strongly affected, however. Secondly, we have performed studies of the effect of the time varying target density on dose and seek to demonstrate various techniques for maintaining that density. Measurements are presented of the target density as a function of time and are compared with our hydrodynamic models

[en] We have fielded a neutron imaging system at the National Ignition Facility to collect images of fusion neutrons produced in the implosion of inertial confinement fusion experiments and scattered neutrons from (n, n') reactions of the source neutrons in the surrounding dense material. A description of the neutron imaging system is presented, including the pinhole array aperture, the line-of-sight collimation, the scintillator-based detection system and the alignment systems and methods. Discussion of the alignment and resolution of the system is presented. We also discuss future improvements to the system hardware. (authors)

[en] Inertial Confinement Fusion experiments at the National Ignition Facility (NIF) are designed to understand and test the basic principles of self-sustaining fusion reactions by laser driven compression of deuterium-tritium (DT) filled cryogenic plastic (CH) capsules. The experimental campaign is ongoing to tune the implosions and characterize the burning plasma conditions. Nuclear diagnostics play an important role in measuring the characteristics of these burning plasmas, providing feedback to improve the implosion dynamics. The Neutron Imaging (NI) diagnostic provides information on the distribution of the central fusion reaction region and the surrounding DT fuel by collecting images at two different energy bands for primary (13-15 MeV) and downscattered (10-12 MeV) neutrons. From these distributions, the final shape and size of the compressed capsule can be estimated and the symmetry of the compression can be inferred. The first downscattered neutron images from imploding ICF capsules are shown in this paper. (authors)

[en] Time gating a neutron detector 28m from a NIF implosion can produce images at different energies. The brighter image near 14 MeV reflects the size and symmetry of the capsule 'hot spot'. Scattered neutrons, ∼9.5-13 MeV, reflect the size and symmetry of colder, denser fuel, but with only ∼1-7% of the neutrons. The gated detector records both the scattered neutron image, and, to a good approximation, an attenuated copy of the primary image left by scintillator decay. By modeling the imaging system the energy band for the scattered neutron image (10-12 MeV) can be chosen, trading off the decayed primary image and the decrease of scattered image brightness with energy. Modeling light decay from EJ399, BC422, BCF99-55, Xylene, DPAC-30, and Liquid A leads to a preference from BCF99-55 for the first NIF detector, but DPAC 30 and Liquid A would be preferred if incorporated into a system. Measurement of the delayed light from the NIF scintillator using implosions at the Omega laser shows BCF99-55 to be a good choice for down-scattered imaging at 28m.

[en] A summary of data and results from the first neutron images produced by the National Ignition Facility (NIF), Lawrence Livermore National Laboratory, Livermore, CA, USA are presented. An overview of the neutron imaging technique is presented, as well as a synopsis of data and measurements made to date. Data from directly driven, DT filled microballoons, as well as indirectly driven, cryogenically layered ignition experiments are presented. The data show that the primary cores from directly driven implosions are approximately twice as large, 64 ± 3 μm, as indirectly driven cores, 25 ± 4 and 29 ± 4 μm and more asymmetric, P2/P0 = 47% vs. ∼ 14% and 7%. Further, comparison with the size and shape of X-ray image data on the same implosions show good agreement, indicating X-ray emission is dominated by the hot regions of the implosion. (authors)

[en] Directly laser driven and X-radiation driven DT filled capsules differ in the relationship between neutron and X-ray images. Shot N110217, a directly driven DT-filled glass micro-balloon provided the first neutron images at the National Ignition Facility. As seen in implosions on the Omega laser, the neutron image can be enclosed inside time integrated X-ray images. HYDRA simulations show the X-ray image is dominated by emission from the hot glass shell while the neutron image arises from the DT fuel it encloses. In the absence of mix or jetting, X-ray images of a cryogenically layered THD fuel capsule should be dominated by emission from the hydrogen rather than the cooler plastic shell that is separated from the hot core by cold DT fuel. This cool, dense DT, invisible in X-ray emission, shows itself by scattering hot core neutrons. Germanium X-ray emission spectra and Ross pair filtered X-ray energy resolved images suggest that germanium doped plastic emits in the torus shaped hot spot, probably reducing the neutron yield. (authors)