Neutron multipliers

The idea is to use a particle accelerator producing neutrons by spallation to feed a fuel/moderator assembly where the neutrons multiply by fission chain reactions. 

The most important parameter of the detection system is its response function. We have studied this extensively in Monte Carlo and other calculations. The calculated time-spectrum response to monoenergetic neutrons is composed of a Gaussian timing curve (2.97-ns FWHM), a trapezoidal contribution from detector thickness and non-axial paths, and an exponential tail, calculated by Monte Carlo, from multiple scattering in the neutron scintillator. (Spectrum distortion due to neutrons multiply scattered by structural and other parts of the apparatus and arriving at the neutron... 

Neutron multiplying parameters for large SP placement spacing had been studied using RITM reactor code, in which the neutron transfer equation is solved by the method of successive collisions. The composition of the canister (R=ll cm) fuel zone is represented by the uranium-zirconium SNF with enrichment of 56.7% by... 

The Li nucleus can absorb a fast (above 3 MeV) neutron to produce a tritium nucleus, an alpha particle, and a slower neutron. A moderated neutron can be absorbed by a Li nucleus to produce a tritium and an alpha. Neutronic calculations indicate that a thick sphere of natural lithium could breed about 1.8 tritium atoms for each tritium atom burned in a fusion reaction (1 ). Structure and portions of the volume left open for fueling or driver beams reduce the 1.8 tritium breeding ratio. If the ratio falls below 1.0, it may be increased by addition of a neutron multiplier such as Be or Pb, and by isotopically enriching the Li in °Li. 

Neutron multipliers have also come into use. These usually consist of a tank containing highly enriched The uranium releases 100 n for each trigger neutron sent in the... 

The assumptions of transport theory. As you well know, the theory which permits one to calculate the neutron densities or fluxes is an essentially statistical theory and is called transport theory. This theory goes back to the last century and Boltzmann s book on the kinetic theory of gases [1] can still be read to advantage. The fimdamental concept is the so-called neutron flux x,EySl t), This quantity gives for the time t the number of neutrons, multiplied with their speed, which satisfy the following conditions ... 

In these, r = InE/Eo is the logarithm of the energy of the neutron in units of the thermal energy Eq. The q r) is the number of neutrons per imit volmne in unit r range, multiplied with their velocity no is the density of thermal neutrons, multiplied with their velocity. D and Dq are the diffusion constants (divided by the velocity) for fast and thermal neutrons the former is a function of r. 5 is the slowing down per imit volmne. So is the value of S for thermal energies, i.e., r = 0,... 

In these Ut is the density of thermal neutrons multiplied by their velocity, q t) is proportional to the slowing down density t is the age of neutrons, to the age of thermal neutrons, k the multiplication constant, Kf the reciprocal diffusion length of thermal neutrons in the mixture. The above equations were derived by Fermi on the assumption that the number of collisions which a fission neutron suffers before it becomes thermal is the same for all neutrons and that the distance between two collisions is also a well-defined quantity depending only on the energy. 

Criticality Studies of a Neutron Multiplier, R. C. Lloyd, S. R. Bierman, E. D. Clayton. B. M. Durst (BNWL)... 

There has been considerable need for neutron irradiation units with high neutron fluxrfor use in research, industry, and education. Sources of various types have been used lor this purpose for maiiy years, but have lacked the intensity and volume, with level flux, for larger sample irradiation. While the research reactor can produce the desirable neutron flux, construction and operation is rather costly. A neutron multiplier has been described. .using a subcritical lattice of enriched uranium rods,with a central cylindrical flux trap. A Cf. source is used to provide the initial neutrons to drive the unit. This lattice unit was loaded at the Critical Mass Laboratory to study aspects of criticality including keff and effects of temperature, voids, and added material. 

S. H. LEVINE, "Fundamentals of Neutron Multipliers, Californium-252 Utilization Meeting, San Diego, California (Nov. 4-6, 1975). 

So vhat has all this got to do with a reactor Well, the power level is directly proportional to the number of neutrons, and neutrons multiply, so if the multiplication is greater than 1.00, ve are raising power. Just as we were making more money - so the reactor is supercritical. On the other hand, if the multiplication is less than 1.00, as was the case when we were losing money, the reactor would be subcritical, and the power level would go down. When the multiplication is exactly 1.00, the power level does not change, so the reactor is exactly critical. 

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