Fast spectrum

The plutonium-uranium fuel cycle has particular advantages in fast spectrum... 

From a comparison of the fast spectrum to the delayed one it becomes obvious that a series of vibrational satellites grows in with time, such as the satellites at 229,261,416, 528, and 710 cm The corresponding vibrational modes couple only to the slowly decaying sublevel I (bold-face numbers in the first column of Table 3). 

The fast and non-delayed emission spectrum (Fig. 22 a) shows nearly the same structure as the time-integrated spectrum measured at T = 20 K. (Fig. 14) However, the satellites that result from state III (9 cm higher lying peaks of the doublet structure in the 20 K emission spectrum) do not occur in the fast spectrum. Obviously, at T = 1.3 K, an emission of state III is not observed. This is due to the very fast sir processes to the lower lying triplet substates II and I. (Compare also Sect. 4.2.9.) Thus, the fast spectrum represents the non-thermalized emission spectrum of state II. The Boltzmann distribution does not apply immediately after the excitation pulse, since the thermal equilibration is relatively slow. (For details see Refs. [22,24].) This state II emission spectrum is assigned... 

Fast spectrum-acquisition capabilities Unlike quadrapole or ion-trap instruments, the acquisition of a mass spectrum is not based on scanning. Typical pulse rates of TOF instrument are 20-50 kHz. Spectra from different ion-introduction events are accumulated, resulting in improved S/N due to the averaging of random noise. The resulting mass spectra are stored at 4-10 spectra/s. The TOF is an integrating rather than a scanning instrument. 

The fast-spectrum reactors with full recycle of actinides would be designed with on-site spent fuel reprocessing and fuel fabrication to minimize the on-site inventory of long-lived radioactive waste. Modern robotic equipment can be used for reactor refueling and for fuel reprocessing. The spent fuel reprocessing and fuel fabrication facilities must be developed to close the nuclear fuel cycle and use all the energy available in natural uranium. 

The Westcott formulation for the effective cross sections a and a is useful only for well-moderated thermal reactors, where the approximations of the neutron spectra ate more reasonable. Even in such reactors, more detailed calculations of actual neutron spectra and effective cross sections are necessary for precise reactor design. The Westcott cross sections are not applicable to fast-spectrum reactors, where neutron moderation and thermalization are suppressed. 

Fast-breeder oxide fuel is also assumed to contain 25 ppm of residual nitrogen [Kl]. Typical average fast-spectrum cross sections are 0.135 mb for 0(w. 7) and 14 mb for N(n, p)... 

Transmutation. Recycling actinides to the LWRs will decrease the average material neutron multiplication factor by only 0.8 percent, provided that they are of high purity [C2], Recycling to LMFBRs, however, will be preferred. There will be less neutron capture in impurities, such as lanthanides, and the average fission-to-capture ratio of the actinides should be higher in a fast spectrum than in a thermal one. 

One of its attractive features of rhenium is that it is a spectral shift absorber (SSA), which means that it has a low relative absorption cross section for fast neutrons while in the thermal spectrum its absorption cross section increases dramatically. This has safety applications for the reactor design in accident scenarios. Rhenium has an absorption cross section of in the fast spectrum, however the magnitude of the difference between the absorption cross section and the fast fission cross section of is low compared to the difference at a thermal spectrum. It also provides a barrier that protects Niobium 1% Zirconium from nitrogen attack and damage caused by other fission products that outgas from the fuel. Most of the other SSA materials have a relatively low melting point, making them less attractive. 

In this scenario the reactor is immersed in water, and the gas flow region is flooded. It is assumed that the radial reflectors are all removed by any splashdown into water. This scenario results in the moderation of the fast neutrons and normally the increase in cross section with decreased neutron energy would result in an increase in reactivity. The inclusion of rhenium in the core between the fuel and the NblZr cladding negates this effect, as it is a Spectral Shift Absorber. SSA s are materials that are relatively transparent to neutrons in the fast spectrum but a massive absorber at the lower end of the energy spectrum [King, 2005]. 

King, J.C., El-Genk, M. S., Spectral Shift Absorbers for Fast Spectrum Space Nuclear Reactors., in proceedings of Space Technology and Applications International Forum (STAIF 2005), edited by M. El-Genk, AIP Conference Proceedings 746, New York,... 

Because lead is chemically inert with the water required to generate steam for power production, and because the vapor pressure of lead is extremely low, lead-cooled reactors do not require the massive containment structures of LWRs and thus have the potential to be less expensive than current LWR technology, particularly if refueling occurs at greater than 10-year intervals. In whatever form, improvements in fast-spectrum reactor technology and the ability to pro-... 

Although helium gas was used in the previous illustration, similar arguments can be made for CO2 as a coolant. Carbon dioxide has been the preferred coolant for the magnox and AGR reactors helium is required for high-temperature reactors using graphite as a moderator and either helium or CO2 might be used in a fast-spectrum reactor. The choice of a gas coolant for a particular reactor can depend on heat-transfer considerations, chemical and radiation stabUity, and interaction effects, or perhaps other factors. 

Studies on the gas-cooled, fast-spectrum reactor have shown that a potential advantage of carbide fuel over oxide is derived from its increased conductivity, increased heavy metal density, and decreased moderating effect. The improved conductivity of the carbide over the oxide may allow the maximum heat removal per foot of fuel element to be raised from 20 to about 40 kW/ft. Increased heavy metal density and decreased moderating effect of the carbide allow the possibility of a harder neutron spectrum and therefore an increase of about 10% in the breeding ratio. 

The last row in Table X shows the uranium requirements assuming a complete economy of gas-cooled, fast-spectrum reactors. The conditions shown are typical of those calculated for such a reactor (/4). Under these conditions, the doubling time for breeding would be about eight years, whereas that for reactor installation is assumed to be six years, so that a net consumption of uranium (for this particular assumed buildup rate) would be required, even for this reactor. 

The supercritical-water-cooled reactor (SCWR) ( Fig. 58.21) system features two fuel cycle options the first is an open cycle with a thermal neutron spectrum reactor the second is a closed cycle with a fast-neutron spectmm reactor and full actinide recycle. Both options use a high-temperature, high-pressure, water-cooled reactor that operates above the thermodynamic critical point of water (22.1 MPa, 374°C) to achieve a thermal efficiency approaching 44%. The fuel cycle for the thermal option is a once-through uranium cycle. The fast-spectrum option uses central fuel cycle facilities based on advanced aqueous processing for actinide recycle. The fast-spectrum option depends upon the materials R D success to support a fast-spectrum reactor. 

The options for burning TRU include both thermal and fast critical reactors and thermal and fast-spectrum subcritical accelerator-driven systems (ADSs). O Figure 61.5 is a schematic of one of many possible transmutation schemes involving both aqueous partitioning and nonaqueous partitioning. 

Fast group constants were obtained from the MUFT-4 code which performs the cross section averaging over the fast spectrum but assumes a homogeneous cell. 

A fast Spectrum refractory-metal critical experiment with a moderating reflector has been operated in Idaho to determine the basic nuclear characteristics of this type of reactor. This paper describes the measurements that have been performed with this critical experiment. 

D. J. McGOFF, FORM, a Fourier Transport Fast Spectrum Code for the IBM-709, NAA-Sr-Memo 5766 National American Aviation (Se. 1960). 

Fast Spectrum Measurements On a Sub-critical Section of the Thermionic Critical Experiment, S. C. Cohen, R. A. Mdpre, J. L. Russell, Jr. (GGA) ... 

A new criticality parameter, surface multiplication (Ms), was reported on in 1961 by Woodcock/ This parameter is defined as the number of neutrons that leave a core boundary as a result of a single entering neutron. Although no measured values of surface multiplication have been previously reported, Monte Carlo calculations have shown It to be a useful and valid index Of criticality. Monte Carlo calculations have shown that at least for fast-spectrum systems, lAfs varies in ah approximately linear manner with linear dimenslm from zero size through the point of criticality. If verified, this property could allow the prediction of critical size frmn measurements on systems far subcrttlcal. 

Neutron Spectrum Measurernents in a Compact Fast-Spectrum Critical Assembly,... 

Differential neutron spectra were measured In center of a fast-spectrum molybdenum-reflected critical assembly using spherical proton-recoil detectors. The critical assembly was buiU to investigate geometrical variables and reactivity effects of-materials for a small, fast-spectrum conceptual reactor (lithium-cooled, U nitride-fueled). Measurements were made at Atomics Intemational where the critical asseimbly was designed, built, and operated. Calculations were made, at the NASA Lewis Research Center. 

JOHN L. ANDERSON and WENDELL MAYO, Effect of Adding Lithium Nitelde, Hafitlum, Tantalum, and Tungsten to a Fast-Spectrum, Molybdenum-Reflected Critical Assembly," NASA TM X-S2787, National iteronautics and Space Administration (May 1970). 

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