Neutron reflector

Criticality Precautions. The presence of a critical mass of Pu ia a container can result ia a fission chain reaction. Lethal amounts of gamma and neutron radiation are emitted, and a large amount of heat is produced. The assembly can simmer near critical or can make repeated critical excursions. The generation of heat results eventually ia an explosion which destroys the assembly. The quantity of Pu required for a critical mass depends on several factors the form and concentration of the Pu, the geometry of the system, the presence of moderators (water, hydrogen-rich compounds such as polyethylene, cadmium, etc), the proximity of neutron reflectors, the presence of nuclear poisons, and the potential iateraction with neighboring fissile systems (188). As Httle as 509 g of Pu(N02)4 solution at a concentration Pu of 33 g/L ia a spherical container, reflected by an infinite amount of water, is a critical mass (189,190). Evaluation of criticaUty controls is available (32,190). 

In the 1941 paper with Yu. B. Khariton [40], the problem of the critical size of a sample of 235 U in the fission of nuclei by fast neutrons was considered. The calculations showed that, in order to sustain a chain fission reaction by fast neutrons in a sample of 235 U surrounded by a heavy neutron reflector, it is sufficient to have only ten kilograms of pure 235U isotope. Here also a theory is given which allows calculation of the critical mass of... 

HYLIFE Lithium Flow. The lithium jets are injected into the 8-m-high HYLIFE chamber with 9.5 m/s initial velocity (72.2 m /s flow rate). Additional coolant in the neutron reflector increases the flow to 86.2 m /s. The mixed-mean temperature rise in the lithium is 18 K, and the peak lithium temperature is 500°C. Eleven recirculation pumps, each with 7.8 m /sec (124,000 gpm) capacity, return the lithium to the top of the vessel. About 9.8 m /s is diverted from the flow loop to four Li-Na intermediate heat exchangers which in turn drive twelve steam generators. The plant gross electric power is 1236 MWg 135 MW is used to drive the 4.5 MJ - 5 % efficient laser, 95 MW is used elsewhere in the plant, and 1006 MW is the net power. 

The critical mass of, for example, a sphere of pure plutonium-239 metal in its densest form (alpha-phase, density 19.8 g/cm) is about 10 kg. The radius of the sphere is about 5 cm, about the size of a small grapefruit. If the plutonium sphere were surrounded by a natural uranium neutron reflector, about 4.4 kg, the radius of the sphere would be about 3.6 cm, about the size of an orange. A 32 cm thick beryllium reflector reduces the critical mass to about 2.5 kg, a sphere with a radius of 3.1 cm, about the size of a tennis ball. 

Beryllium is used in nuclear reactors as a neutron reflector and as an alloy with the fuel elements. 

In nuclear reactor technology hafnium-free zirconium(IV) oxide is used as a neutron reflector, due to its low neutron capture cross-section. 

Xo is the radially averaged flux at the centre-line, z = 0, of the reactor (neutrons/mVs), e is the elongation factor. Close to unity, e makes allowance for the effective increase in reactor height produced by the neutron reflector, c = 1.0 for a reactor with no reflector, while e 1.2 for most commercial reactors. 

Operations with fissile materials may be performed safely by complying with any one of the subcritical limits given in Sec. 8.2 provided the conditions under which it applies are maintained. A limit shall be applied only when the effects of neutron reflectors and of other nearby fissionable materials are no greater than reflection by an unlimited thickness of water, t The limits shall not be applied to mixtures of U, and and Pu. 

USE Source of neutrons when bombarded with alpha particles according to the equation jBe + JHe J C + jn This yields about 30 neutrons per million alpha particles. Also as neutron reflector and neutron moderator in nuclear reactors. In beryllium copper and beryllium aluminum alloys (by direct reduction of beryllium oxide with carbon in the presence of Cu nr Al). In radio tube parts. In aerospace structures. In inertial guidance systems. 

Metal 10% of total Aircraft disk brakes, X-ray transmission windows, space-vehicle optics and instru-usage ments, aircraft/satellite structures, missile parts, nuclear-reactor neutron reflectors,... 

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. 

The smallest critical sizes are obtained for homogeneous systems of pure fissile nuclides with maximum neutron reflection. For neutrons with the fission energy spectrum, the critical mass of a metallic sphere of pure is 22.8 kg, that of is 7.5 kg, and that of Pu is 5.6 kg, assuming a 20 cm uranium metal neutron reflector. For fission by thermal neutrons the smallest critical size of a spherical homogeneous aqueous solution of 1102804 without reflector requires 0.82 kg of in 6.3 1 of solution. The corresponding figures for are 0.59 kg in 3.3 1, and of Pu, 0.51 kg in 4.5 1. 

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... 

The variation in critical mass which is required to sustain a neutron chain reaction depends to a very substantial degree upon the nature and thickness of the neutron reflector. 

It is known that a self-sustaining chain reaction can be obtained in devices known as neutronic reactors utilizing natural uranium, as a result of slow neutron fission of the content of the natural uranium. In such reactors, discrete bodies of natural uranium of high purity are disposed, usually in the form of a lattice arrangement of spheres or rods, in a neutron moderator such as graphite, baryllium or heavy water of high purity, surrounded by a neutron reflector. Neutron absorption in the U s content of the natural uranium during the reaction leads to the production of the transuranic isotope 94, known as plutonium (symbol Pu), which is fissionable in much the same manner as 94 3 or Pu 3 is formed in... 

According to the present invention the novel breeder system comprises a neutronic reactor wherein and 25 heavy water (D2O) neutron moderator are combined in a chain reacting composition surrounded by a neutron reflector of heavy water containing a fertile isotope or isotopes in solunon or in suspension. The fertile material absorbs neutrons emanating from the chain reacting 30 composition and is thus converted to thermally fissionable material. 

The tank 10 is surrounded by a tank 15 formed of a material having a relatively small neutron capture cross section such as those mentioned above. The tank 15 contains a neutron reflector of heavy water and fertile material such as, for example, a slurry of approximately 1 gram of Th02 per cubic centimeter of heavy water. The slurry within tiie tank 15 is indicated at 14 (FIG. 1) and is circulated by means of inlet and outlet conduits 41 and 42. 

The tank 86 is, in turn, surrounded by another tank 92 which contains an outer neutron reflector 94 in the form 25 of beryllium or graphite. The tank 92 is enclosed within and supported by a concrete vault 96 affording a biological shield around the system. 

A general object of the present invention is to design an effective fast neutron reflector disposed around a fast neutron reactor to diminish neutron losses from the periphery thereof, thereby making possible a reactor of... 

A more specific object of the invention is to provide a novel composite neutron reflector around a fast neutron... 

Precaution Flamm. solid powd. mod. fire hazard as powd. or dust or when exposed to flame si. explosion hazard as dust incompat. with halocarbons reacts incandescently with fluorine and chlorine reacts with Li, P Hazardous Decomp. Prods. Heated to decomp, in air, emits very toxic fumes of BeO NFPA Health 3, Flammability 1, Reactivity 0 Uses Structural material in space technology moderator In nuclear reactors source of neutrons windows for x-ray tubes in gyroscopes, computer parts, inertial guidance systems additive in solid-propellant rocket fuels beryllium-copper alloys aircraft brakes neutron reflectors... 

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