Epithermal-neutron region

The search for a liquid for use at high temperatures and low pressures in a fluid-fueled reactor led to the choice of either fluorides or chlorides because of the requirements of radiation stability and solubility of appreciable quantities of uranium and thorium. The chlorides (based on the isotope) are most suitable for fast reactor use, but the low thermal-neutron absorption cross section of fluorine makes the fluorides a uniquely desirable choice for a high-temperature fluid-fueled reactor in the thermal or epithermal neutron region. 

The SDP is an average value over the epithermal neutron energy region. A good neutron moderator should divert few neutrons from the fission process, i.e. the neutron absorption cross section must be small. In this respect both heavy water (i.e. D2O) and carbon are... 

This simple method for calculating reaction rates in a mixed thermal-epithermal spectrum is called the Hogdahl convention (Hogdahl, 1965). It assumes that in the thermal neutron region the activation cross section varies with neutron energy as the 1/v law. This is... 

The assumption that the activation cross section varies with neutron energy as 1/v in the thermal neutron region is valid for most (n,y) reactions. The two reactions that deviate the most from the 1/v assumption are Lu(n,y) Lu (typically +0.4%/K) and Eu(n,y) Eu (typically —0.1%/K). The reaction rates for these two reactions, relative to a monitor reaction like Au(n,y) Au, will depend on the thermal neutron temperature in the irradiation channel used. A new set of equations, the Westcott formalism (Westcott 1955), was developed to account for these cases and used the Westcott g T ) factor, which is a measure of the variation of the effective thermal neutron activation cross-section relative to that of a 1/v reaction. In the modified Westcott formalism, the following differences are also included the Qo(a) value of the Hogdahl formalism is replaced by the So(< ) value, and the thermal to epithermal flux ratio, f, is replaced by the modified spectral index, r a) TJTo). To use this formalism with the kg method (De Corte et al. 1994), it is necessary to measure the neutron temperature, r , for each irradiation and a Lu temperature monitor should be irradiated. The Westcott formalism needs to be implemented only when analyzing for Lu and Eu. There are several other non-1/v nuclides Rh, In, Dy, Ir, and Ir, but for these the error... 

The MSR produces power by circulating a molten salt and fuel mixture through graphite core flow channels. The slowing down of neutrons by the graphite moderator in the core region provides the epithermal neutrons necessary to produce the fission power for sustained operation of the reactor. 

Another approach to the determination of Li in RPV steel was by neutron activation analysis (NAA) followed by radiochemical analysis for H. This was far more complex than the ICP-MS analysis as it involved the irradiation of the inactive steel specimens in the CONSORT reactor at Ascot and transport of the activated steel to the Springfields Laboratory for radiochemical analysis. Six inactive specimens of RPV steel from TRA were irradiated for 63 hours in a neutron flux comprising flux values of 960 x 10 n cm s 44 X 10 n cm s and 300 x 10 n cm s for thermal, epithermal and fast regions of the energy spectrum respectively. 

Vibrational spectra obtained over the whole range requires relatively high neutron energies in the epithermal region and, as will be seen from later chapters in this book, spallation sources are pre-eminent in this field. However, where a limited energy range is acceptable reactor sources can be very powerful. 

Figure 2, The general features of a reactor neutron spectrum. Key A, thermal region B, epithermal region and C, fast region.

The theoretical methods have been previously reported and involve the computer codes ZUT and TUZ (resonance capture rates)/ THERMOS (thermal-neutron reaction rates), and MUFT (epithermal smooth reaction rates). Homogenization of the lattice was by flux-volume weighting in the fast and thermal-energy regions, and by volume weighting In the intermediate energy region. 

A one-dimensional transport theory code ANISN with the Hansen-Roach cross-section set and a buckling approximation of the form OB p for the finite longitudinal cylinder dimension, were used for the analysis of the experimental data. The largest difference between computed and experimental values of k ff was 0.016, as shown in the table. The code also permits calculation of the neutron current densities at the fuel-reflector interface and, therefore, provides an indication of the number and energy of the neutrons that are returned to the fuel region. The computations thus indicate that the changes In reactivity of the solution cylinders induced by the steel reflector are primarily the result of the competition between the effective decrease in the reflection of the thermal component and the effective increase in the reflection of the epithermal component of the neutron flux at the fuel-reflector interface with increasing thicknesses of steel. 

Figure 1. Typical excitation function for neutron-induced reaction, illustrating l/ii dependency and resonance peaks in epithermal region...

The life cycle of a neutron in a thermal reactor, moderator, or reflector can be considered to be divided on an energy scale into thermal and epithermal regions. The Argonaut experiment "Measurement of Thermal Neutron Diffusion Length in H2O" concerns itself with the fate of the neutron in the thermal portion of such a cycle this experiment completes the picture by considering the neutron behavior during its epithermal career. 

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