Leakage, neutron

Severe restrictions on the mass of the bomb can be circumvented by using pieces of a shape more difficult to shoot. For example, a flat plate of actual material tamped on only one side, has a minimum thickness below which it can no longer support a chain reaction, no matter how large its area, because of neutron leakage across the untamped surface. If two such plates were slid together, untamped surfaces in contact, the resulting arrangement could be well over the critical thickness for a plate tamped on both sides, and the mass would depend only on the area of the plates. 

Neutron leakage during moderation from fission to U resonance energy r)25e(l -Pi) 0.0228... 

Neutron leakage during moderation from U resonance to thermal energy ri2sePiP(I -Pt) 0.0031... 

A is the fraction of neutrons which are not lost through leakage to the surroundings the non-leakage factor). In order to minimize the neutron leakage, the reactor core is surrounded with a neutron reflector which for thermal neutrons in LWRs is water (graphite or beryllium are sometimes used in other reactor designs) for fast neutron reflection iron is frequently used. 

It is seen that in (19.17) increases with decreasing buckling, and, because B oc 1/r (19.21a), with increasing size of the reactor. This is a result of the neutron production being a voliune effect (proportional to r for a sphere), whereas leakage is a surface effect (proportional to r ). For each reactor there is a minimum critical size k g - 1) below which the surface to volume ratio is so large that neutron leakage is sufficient to prevent the fission chain reaction. 

The primary goal of this study was to ensure that there was sufficient excess reactivity in the neutron multiplication factor to keep the reactor critical for the 10 year lifespan while ensuring that the reactor would be subcritical during major accident scenarios. The position of the reflector can be used to set the multiplication factor of the reactor. Burnup in the reactor causes a proliferation of additional materials to absorb neutrons and reduces the density of fissile materials, lowering the neutron multiplication of the reactor. This can be offset by closing the reflector, which decreases the neutron leakage of the system. This is shown in Figure 5-1. 

The stracture of the reflector is shown in Fig. 4. The upper part of reflector must be made of a material with a lower reflection effect than the coolant itself in order to increase its ability to control neutron leakage. The reflector therefore has a gas cavity which can increase 3% Ak of the reflector reactivity compared to a reflector without a cavity. 

A full scale critical experiment is important to evaluate the calculated results such as reactivity coefficients and critical conditions. As neutron leakage is enhanced in the 4S core, the conventional calculation method is not sufficient to accurately predict the core characteristics. A critical experiment is thus the most urgent R D item. 

In.addition, there was a special experiment designed to establish empirically the total neutron leakage through 8 in. of steel. In this experiment steel bricks were placed in a stack 8 in. thick and 4 ft square. This stack was erected next to the outside surface of the graphite. The steel in turn was backed by a 21-in.-thick stack of paraffin also 4 ft square in the transverse dimensions. Bare and cadmium-covered indium foils placed in the paraffin were activated by tbe neutrons which penetrated the steel. Coaq>lete details of thie experiments and the interpretation of the reduced data have been reported elsewhere. ... 

Thus the formula used for estimating the neutron leakage current out of the thermal shield is... 

In removing the fissioning suspension from the reaction zone for heat exchange, replacement or other purposes, it is preferred to conduct the removal so that the chain reaction is discontinued while the suspension is out of the reaction zone. In accordance with this invention, this may be done by changing the shape of the liquid suspension so that the external surface per unit volume thereof is increased when the liquid is removed whereby neutron leakage from the exterior thereof is increased. Alternatively, the liquid suspension may be withdrawn from a reactor provided with a neutron reflector into a container which has no reflector or which is capable of losing a... 

Consider, for example, the condition which would obtain in the chain reaction shown in FIG. 18 if the volume of slurry were suddenly increased to such an extent that a portion of the neutrons normally lost by fast and thermal neutron leakage were saved by such increase in... 

In high concentrations of substantially pure form, plutonium can also be used, when properly combined with a neutron moderator, to sustain a slow neutron 35 chain reaction in a neutronic reactor of relatively small size wherein the neutron leakage is high. In other words, it can be used as an efficient source of large quantities of neutrons, and the neutrons thus produced can be used to produce another fissionable isotope such as U a,... 

The use in a reaction zone of a substantially pure fissionable isotope to provide a high K factor thereby leading to a reactor of small size and high neutron leakage. 

U233 and 9433 may be dispersed in water as soluble 6 uranyl and plutonyl (PuOa ) salts, such as uranyl and plutonyl nitrate, sulphate or fluoride, or as an alkali metal uranyl or plutonyl carbonate. Such a solution with the proper isotope concentration in water of either type and in a container of the proper size will sustain a 10 chain reaction with a low neutron loss except for exterior neutron leakage, which will be very high. 

As liquid type reactors are small when incorporating a fissionable isotope in high concentration, exterior neutron leakage is high and in consequence such reactors are ideal neutron sources for use in isotope converters. Such converters are more fully described and claimed in the copending application of Wigner et al., filed November 13, 1945, Serial No. 628,322, now Patent No. 2,813,321. 

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