Compact Fission Research Articles

Shielding factors for a fission-powered mars exploration rover

Background: With the advances in compact fission power systems and Stirling converters, the efficiency and portability of electrical energy production systems has increased substantially thanks to NASA’s Kilopower project. These 1-10 kWe-class reactors are compact enough to be transportable by pressurized rovers, allowing an extended human reach across the surface of Mars to produce scientific data at a rate that greatly surpasses the rate of robotic rover data collection. If rovers were to harness fission power while mobile, it is possible for near-limitless range. This capability could allow one crew on Mars for under a “Conjunction” class mission as outlined in NASA’s Mars Design Reference Architecture (DRA) 5.0 to visit many geological sites of interest instead of requiring additional landings to explore distant features of the Martian surface. This study explored the parameters that affected the mass of the shielding required to protect the crew against a fission reactor embedded in the chassis of a pressurized rover. Methods: An analytical approach was devised to estimate the required mass of a hypothetical solid tungsten shield under various conditions. Results: Power levels below 3.4 kWe were found to be impractical for rover applications. Between 3.4 kWe and 10 kWe would be able to recharge the rover without returning to base camp. The increase in shielding mass from a 3.4 kWe reactor and a 10 kWe reactor was 6%. The reduction between a 10 kWe reactor at 3 m and 6 m was 7%. Varying the shield thickness in accordance with astronaut activities resulted in a 19% reduction. Powering the reactor off before exiting the vehicle resulted in an additional 65% reduction. Conclusions: Knowledge of the crew activities and locations had the greatest impact on the required shielding mass and the mission activities should be well-understood before finalizing a shield design.

Three-dimensional potential energy surface for fission of 236U within covariant density functional theory* *Supported by the National Natural Science Foundation of China (11875225, 11790325, 11790320), the Special Fund from the China Nuclear Data Center, the Fundamental Research Funds for the Central Universities, and the Fok Ying-Tong Education Foundation

We calculate the three-dimensional potential energy surface (PES) for the fission of the compound nucleus U using covariant density functional theory with constraints on the axial quadrupole and octupole deformations as well as the nucleon number in the neck . By considering the additional degree of freedom , the coexistence of the elongated and compact fission modes is predicted for . Remarkably, the PES becomes shallow across a large range of quadrupole and octupole deformations for small , and consequently, the scission line in the plane extends to a shallow band, leading to fluctuations of several to ten MeV in the estimated total kinetic energies and of several to approximately ten nucleons in the fragment masses.

Scission configuration in self-consistent calculations with neck constraints

The calculations of the potential energy surface are essential in the theoretical description of the fission process. In the constrained self-consistent approach, the smooth evolution of nuclear shape is described from the ground state until a very elongated one with a narrow neck. In all microscopic calculations, the rupture of the neck at scission is associated with a substantial change of nuclear matter density distribution and rapid energy decrease. In this paper, we show that there is no discontinuity of the potential energy surface at scission when multi-constrained calculations are applied with the neck constraint. An early rupture of the neck at lower quadrupole and octupole moments is discussed as competitive with the conventional fission path. We discuss the neck properties in the scission configuration. We find that the neck radius in the asymmetric fission mode cannot decrease below 2 fm, and the nuclear matter density cannot decrease below the saturation density. In the compact fission mode, nuclear density may go down to half of the saturation density before the rupture of the neck.

Experimental investigation on hydrodynamics and mass transfer of a novel fission gas removal equipment for molten salt reactor

The separation of fission gases like 135-Xe and 85-Ke from the liquid nuclear fuel is a focused topic in the molten salt reactor (MSR) community. In this paper, a compact fission gas removal equipment with a serpentine tube as the gas-liquid contactor is introduced. The novel design is free of any moving or rotating components, bringing great convenience in the real-time fuel treatment. As crucial factors influencing the gas removal performance, the two-phase hydrodynamics and interfacial mass transfer characteristics in the vertical serpentine tube are investigated under a water-nitrogen-oxygen system. Both highspeed photographing and conductivity probe are employed to capture the evolution of bubbly flow inside the tubular contactor. The oxygen concentration differences between the inlet and outlet of the device are measured under various operating conditions, and the overall volumetric mass transfer coefficient kla is derived to evaluate the gas removal efficiency. Experiment results imply a periodic transition of volume fraction profile between the off-center distribution in the downward flow and the near-wall distribution in the upward flow due to the centripetal effect in the U-bends. Bimodal distributions of bubble equivalent diameter are observed because of the intensive bubble coalescence inside the elbow. The overall kla is found to be linearly proportional to the liquid Reynolds number and gas volume fraction. The dimensionless mass transfer correlation proposed in this work predicts well the overall volumetric mass transfer coefficient.

Pre-Scission Model Calculation of Fission Fragment Mass and Total Kinetic Energy Distributions for Even-Even Fm, No and Rf Isotopes

The main properties of the fission fragments in spontaneous fission of even-even isotopes of Fm (Z=100), No (Z=102) and Rf (Z=104) are estimated using a pre-scission point model. The underlying potential energy surfaces are calculated with Strutinsky’s shell correction procedure. The parametrization of the nuclear shapes is based on Cassini ovals generalized by the inclusion of three additional shape parameters: α1, α4 and α6. It represents a natural way to describe scission configurations. The corresponding fragment-mass distributions are estimated supposing they are due to thermal fluctuations in the mass asymmetry degree of freedom. A detailed comparison with all existing data for Fm, No and Rf isotopes is presented. For these three series of isotopes the experimentally observed transition from asymmetric to symmetric fission, that happens with increasing mass number A, is well reproduced. In lighter isotopes (e.g. 254Fm and 254Rf) two mass-asymmetric fission modes are predicted to occur with comparable yields: one having relatively compact and the other relatively elongated scission configurations. On the other hand, in heavier isotopes (e.g. 264Fm and 264Rf) the fragment-mass distributions are predicted to be narrow single-peaked around A/2 corresponding to essentially one compact fission mode. We call this type of fission ”super-symmetric”. The corresponding distributions of the total kinetic energy of the fragments are also calculated (in the point-charge approximation) and compared with measurements. Despite the fact that the dynamical effects were neglected, we have obtained a quantitative agreement with the experimental data.

A compact multi-plate fission chamber for the simultaneous measurement of 233U capture and fission cross-sections

233 U plays the essential role of fissile nucleus in the Th-U fuel cycle. A particularity of 233 U is its small neutron capture cross-section which is about one order of magnitude lower than the fission cross-section on average. Therefore, the accuracy in the measurement of the 233 U capture cross-section essentially relies on efficient capture-fission discrimination thus a combined setup of fission and γ-detectors is needed. At CERN n_TOF the Total Absorption Calorimeter (TAC) coupled with compact fission detectors is used. Previously used MicroMegas (MGAS) detectors showed significant γ-background issues above 100 eV coming from the copper mesh. A new measurement campaign of the 233 U capture cross-section and alpha ratio is planned at the CERN n_TOF facility. For this measurement, a novel cylindrical multi ionization cell chamber was developed in order to provide a compact solution for 14 active targets read out by 8 anodes. Due to the high specific activity of 233 U fast timing properties are required and achieved with the use of customized electronics and the very fast ionizing gas CF4 together with a high electric field strength. This paper describes the new fission chamber and the results of the first tests with neutrons at GELINA proving that it is suitable for the 233 U measurement.

Hyperdeformation and Clustering in the Actinide Region

The excitation energy spectrum of fission resonances has been measured with high energy resolution using the 235U(d,pf) reaction in order to study hyperdeformed (HD) rotational bands and HD nuclear shapes. The moments of inertia of the rotational bands and the energy of the ground state in the third minimum were determined. Another signature of these highly deformed states, their enhanced α decay, was also observed. By studying cold or compact fission in the 232Th(n,f) reaction around HD resonances, we obtained data for heavy clustering.

Cold deformed fission in232U(n, f) and239Pu(n, f)

Masses, charges and kinetic energies of light fission fragments from the reactions232U(n, f) and239Pu(n, f) induced by thermal neutrons have been measured on the Cosi fan tutte spectrometer of the Institut Laue-Langevin in Grenoble. Both at very high and very low kinetic energies marked fine structures in the mass yields and odd-even staggerings in the charge yields are observed. In the framework of a scission point model the results are shown to point to compact and deformed scission configurations, respectively, where at scission the fragments carry no intrinsic excitation energy. The two limiting processes may, therefore, be called cold compact fission (usually known as cold fission) and cold deformed fission. The latter process as a general phenomenon of low energy fission has come into focus only recently.