Fission Reactor Applications of Carbon

A good moderator should possess the following properties  

Moderator Slowing-down power, cm 1 Moderating ratio 

In summary, it is evident that the only moderators of merit are based on elements of low atomic weight. Practically, this limits the choice to elements of atomic number less than sixteen. Gases are of little use as moderators because of their low density, but can be used effectively in chemical compounds such as H20 and D20. The choice of potential moderators of practical use thus rapidly reduces to the four materials shown in Table 3. Comparing the candidate moderators in Table 3 with our requirements listed above we may note  

Grade Source Forming Method11 Bulk Density g/cm3 Elastic Modulus 1 GPa Strengthb MPa Thermal CTEb lO K 1 

CP-1 was assembled in an approximately spherical shape with the purest graphite in the center. About 6 tons of uranium metal fuel was used, in addition to approximately 40.5 tons of uranium oxide fuel. The lowest point of the reactor rested on the floor and the periphery was supported on a wooden structure. The whole pile was surrounded by a tent of rubberized balloon fabric so that neutron absorbing air could be evacuated. About 75 layers of 10.48-cm (4.125-in.) graphite bricks would have been required to complete the 790-cm diameter sphere. However, criticality was achieved at layer 56 without the need to evacuate the air, and assembly was discontinued at layer 57. The core then had an ellipsoidal cross section, with a polar radius of 209 cm and an equatorial radius of309 cm [20], CP-1 was operated at low power (0.5 W) for several days. Fortuitously, it was found that the nuclear chain reaction could be controlled with cadmium strips which were inserted into the reactor to absorb neutrons and hence reduce the value of k to considerably less than 1. The pile was then disassembled and rebuilt at what is now the site of Argonne National Laboratory, U.S.A, with a concrete biological shield. Designated CP-2, the pile eventually reached a power level of 100 kW [22], 

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Nuclear applications of nanocapsules are related to the emitting physical properties of the encapsulated material. Emitted radiation can be electromagnetic of high energy (y), electrons or positrons (/3), alpha particles (" He nucleus), or fission products [67]. These emitters can be in themselves radioactive or can be activated by a nuclear reaction, usually a neutron capture. The particular advantage of carbon nanocapsules in nuclear applications is related to the protective characteristics that the carbon capsule confers to the interior product. Experiments on irradiation of fullerenes have shown that knocked carbon atoms from one cage are foimd in another fuUerene and even form dimers and trimers by a recoil-implantation mechanism [68]. The observed major damage of capsules in nanoencapsulated molybdenum irradiated in a nuclear reactor was produced by... 

In Japan, the studies were facilitated by the access to the fusion (JT-60) and fission neutron reactor (JMTR, HTTR), for quite large samples, and by the know-how of Japanese industry in the field of carbon fibers and composites. PAN-based C fibers, instead of pitch-based ones (for 2D composites, with an ex-pitch C matrix), were chosen for irradiation tests (for fission application), since a larger fracture load and deformation could be obtained [34]. By the end of the 1990s, the main material studied in Japan was a 2D PAN-based C fiber/mix (ex-phenolic - - ex-pitch) C matrix material... 

Fluidized-bed CVD was developed in the late 1950s for a specific application the coating of nuclear-fuel particles for high temperature gas-cooled reactors. PI The particles are uranium-thorium carbide coated with pyrolytic carbon and silicon carbide for the purpose of containing the products of nuclear fission. The carbon is obtained from the decomposition of propane (C3H8) or propylene... 

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