Lifetime changes in the characteristics of thermionic converters

The substantiation of the lifetime stability of the emission properties of electrodes and electric characteristics of the electricity-generating channel (EGC) and element (EGE) of a space thermionic nuclear power system for an operating period of 5–10 or more years is examined. The basic reasons for the changes in the emission properties of EGE/EGC electrodes and a Topaz type reactor-converter are analyzed on the basis of test data and data from experimental simulation of the working processes in the interelectrode medium. A scheme is proposed for the working process occurring in the interelectrode medium of the EGE; the scheme is based on the Topaz materials and technology. The stability of the emission characteristics of electrodes with respect to possible changes during prolonged operation is analyzed for the most advanced designs of space nuclear power systems. The changes in time and the corresponding changes in the output electrical characteristics are estimated. 36 references. State Science Center of the Russian Federation—A. I. Leipunskii Physics and Power-Engineering Institute. Translated from Atomnaya énergiya, Vol. 89, No. 1, pp. 57–67, July, 2000.     

Databank for the current-voltage characteristics of thermionic converters

Investigations of thermionic converters, supporting design investigations and developments in space nuclear power systems, are being conducted in the Laboratory of Studies of Fundamental Problems of Conversion of Various Forms of Energy at the State Science Center of the Russian Federation—Physics and Power-Engineering Institute. As a result, a great deal of experimental data has been accumulated in the form of the current-voltage characteristics of arc operating regimes with different electrode materials. The databank stores the experimental data in electronic form. At present it contains the results of 17 experiments. For data handling convernience, a program for viewing the entire digital information has been developed. The results of a search can be printed out. The data control apparatus makes it possible to select the data required by the user. A current version of the databank is discussed as an example. 6 figures. 1 table, 13 references. State Science Center of the Russian Federation—A. I. Leipunskii Physics and Power-Engineering Institute. Translated from Atomnaya énergiya, Vol. 89, No. 1, pp. 71–74, July, 2000.     

High-output thermionic converter based on a metal-oxygen system on the collector

The results of comprehensive experimental investigations of thermionic converters with collectors containing oxygen in an adsorbed near-surface layer and capable of decreasing the barrier index below 2 eV are presented. A vanadium-doped alloy based on chromium, which was investigated in a converter in a pair with different emitters consisting of platinum and single crystal tungsten oriented along the (110) face, was examined as an example of an efficient metal-oxygen system on the collector. The emission-adsorption properties of electrodes and the output electrical characteristics, obtained under working conditions of the converter, as well as surface characteristics of the collector, such as the elemental composition temperature, and others, are described. 6 figures, 21 references. State Science Center of the Russian Federation—A. I. Leipunskii Physics and Power-Engineering Institute. Translated from Atomnaya énergiya, Vol. 89, No. 1, pp. 39–48, July, 2000.     

Nonmonotonic potential barrier for electrons inside an adsorbed layer on the collector of a thermionic converter

A self-consistent computational-theoretical model of electron emission is developed. The model is based on the variational principle using a multielectron approximation taking account of the interelectronic interaction. such as echange and correlation effects. The simplifying assumption in which the ion cores are replaced by a semiinfinite positively charged uniform background (jelly model) is used. The electron density profile in the semiinfinite jellium model is assumed to be uniform in the transverse directions. The calculations showed that an additional potential barrier, which retards electrons, arises on the effective potential inside the layer adsorbed on the electrode. Semiempirical relations describing the height and width of the barrier are obtained, 4 figures, 7 references. State Science Center of the Russian Federation—A. I. Leipunskii Physics and Power-Engineering Institute. Translated from Atomnaya énergiya, Vol. 89, No. 1, pp. 67–71, July, 2000.     

Investigation of the reflector temperatures of a space craft thermionic reactor

In a fast incore thermionic spacecraft reactor for nuclear propulsion, the temperature rise due to the neutron heating in the reflector control drums is investigated. The reactor is fuelled with (U-Ta)C, consisting of 80UC-20TaC with a sinter density of 80% and controlled with the help of rotating drums imbedded into the beryllium reflector. The control drums contain natural B₄C strips (with 20% ¹⁰B and 80% ¹¹B) and produce nuclear heat via neutron absorption in ¹⁰B. The neutronic analysis has been conducted in S₁₆-P₃ and S₈-P₃ approximation with the help of one- and two-dimensional neutron transport codes ANISN and DORT, respectively. Calculations are conducted for a reactor with a core radius of 22 cm and core height of 35 cm leading to 50 kW_el in power phase. Reflector drums with 100% natural B₄C in form of strips (drum diameter=13.5 cm, strip width=5 mm) at the outer periphery of the radial reflector of 16 cm thickness would make possible reactivity changes of Δk_eff,max=10.7% without a significant distortion of the fission power profile during all phases of the space mission. A reduction of the B₄C in the strips to 20 and 10% would still allow a reactivity change of Δk_eff,max=8.4 and 7.7%, respectively, amply sufficient for an effective control of a fast reactor during all phases of the space mission. By a nuclear thermal thrust around F=5000 N and a specific impulse of 670 s⁻¹ at an hydrogen exit temperature around 1900 K, the maximum temperature in the drums rises to 1023 K, with 100% natural B₄C content in the strips, far below the melting point of beryllium. The maximum drum temperature is depressed to 663 and 519 K, with 20 and 10% natural B₄C content in the strips, respectively.     

Adsorption and emission features of collectors of thermionic energy converters in the case of mass transfer in the interelectrode gap

Translated from Atomnaya Énergiya, Vol. 69, No. 3, pp. 196–197, September, 1990.     

Investigation on spent fuel characteristics of reduced-moderation water reactor (RMWR)

The spent fuel characteristics of the reduced-moderation water reactor (RMWR) have been investigated using the SWAT and ORIGEN codes. RMWR is an advanced LWR concept for plutonium recycling by using the MOX fuel. In the code calculation, the ORIGEN libraries such as one-group cross-section data prepared for RMWR were necessary. Since there were no open libraries for RMWR, they were produced in this study by using the SWAT code. New libraries based on the heterogeneous core modeling in the axial direction and with the variable actinide cross-section (VXSEC) option were produced and selected as the representative ORIGEN libraries for RMWR. In order to investigate the characteristics of the RMWR spent fuel, the decay heat, the radioactivity and the content of each nuclide were evaluated with ORIGEN using these libraries. In this study, the spent fuel characteristics of other types of reactors, such as PWR, BWR, high burn-up PWR, full-MOX-PWR, full-MOX-BWR and FBR, were also evaluated with ORIGEN.

It has been found that about a half of the decay heat of the RMWR spent fuel comes from the actinides nuclides. It is the same with the radioactivity. The decay heat and the radioactivity of the RMWR spent fuel are lower than those of full-MOX-LWRs and FBR, and are the same level as those of the high burn-up PWR. The decay heat and the radioactivity from the fission products (FPs) in the spent fuel mainly depend on the burn-up and the burn-up time rather than the reactor type. Therefore, the decay heat and the radioactivity from FPs in the RMWR spent fuel are smaller, reflecting its relatively long burn-up time resulted from its core characteristics with the high conversion ratio. The radioactivity from the actinides in the spent fuel mainly depends on the ²⁴¹Pu content in the initial fuel, and the decay heat mainly depends on ²³⁸Pu and ²⁴⁴Cm. The contribution of ²⁴⁴Cm is much smaller in RMWR than in MOX-LWRs because of the difference in the spectrum. In addition, from the waste disposal point of view, the characteristics of the heat generation FP elements, the platinum group metals, Mo and the long-lived FPs (LLFPs) were also investigated.     

Neutralization effect for Ne+ scattering from a Cu(110) surface

Intensity and structure of the energy spectra of Na⁺ and Ne⁺ ions scattered from a Cu(110) surface are governed by multiple scattering and neutralization effects. These were studied for ion energies between 600 eV and 1 keV and in the temperature range from 100 to 600 K in the experiment and by computer simulation. Na⁺ scattering directly reflects the crystallographic structure of the (110) surface. The temperature effects can be used to analyze thermal motions of surface atoms in terms of a surface Debye temperature for specific vibrational directions. The contributions of single and multiple scattering events to the energy spectra are analyzed and for Ne⁺ a strong trajectory-dependent neutralization is found. The comparison of the neutralization of Ne⁺ and Na⁺ leads to a Ne⁺ ion survival probability of a few percent for single scattering, less than 1% for double scattering, and a value of less than 10⁻³ for scattering from atoms below the top atomic layer. A simple neutralization model is developed to explain the observed survival probabilities.