Spherical nuclear fuel

Sectioning Step Not applicable Abrasive Grain size Mounting Working surface Cold mounting in epoxy resin Lubricant Load RPM Time and coolant N min  

Lapping Diamond 9 pm Metall/Kunstsoff-Scheibe gerillt Suspension 18 120 10 

Polishing Diamond and colloidal Si02 3 pm 0.05 pm Perforated synthetic fiber cloth Suspension water-based 18 120 8 

Coating Nickel applied by electroless method, or TiN applied by PVD method Sectioning Diamond wheel Mounting In a cold mounting medium (0.6 mm), low speed  

Step Abrasive Grain size Working surface Lubricant and coolant Load N RPM Time min 

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Example 7.5 Cooling nuclear pellets Spherical nuclear fuel pellets generate heat at a rate per unit volume, q, and being cooled at the boundary by convection heat transfer. For a single pellet on start up, we have... 

Table 52. Recommended preparation of spherical nuclear fuels (coated particles). Nuclear fuel UO2 - pyrocarbon - silicon carbide SiC...

A spherical nuclear fuel element is surrounded by a spherical shell of metal that shields the system. The heat generated within the fuel element is a function of position. This behavior can be approximated by the expression... 

The sol-gel process has been developed for the production of spherical oxide fuels with a particle size of up to 1 mm for use in nuclear reactors [B.56]. The following operations are involved in converting the initially aqueous sol of colloidal particles into calcined microspheres ... 

In the nuclear industry one of the main applications of colloid technology is in the handling and reprocessing of radioactive waste, where the problems are mainly concerned with the flocculation and separation of particulate radioactive materials. A second application is that of the sol-gel process to the preparation of nuclear fuel in the form of spherical particles of uniform size. 

The HTR-10 test reactor uses spherical fuel elements which are made completely of ceramic materials. Uranium dioxide as nuclear fuel is in the form of coated particles which are dispersed in the graphite matrix of the fiiel elements. Graphite serves as neutron moderator... 

CP-1 was assembled in an approximately spherical shape with the purest graphite in the center. About 6 tons of luanium 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 mbberized 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]. 

Fig. 14. HTGR fuel elements (a) prismatic core HTGR fuel element (b) cross section of a spherical fuel element for the pebble bed HTGR. Reprinted from [88], 1977 American Nuclear Society, La Grange Park, Illinois.

Solid fuels other than carbon-based materials can also be agglomerated to obtain specific properties. For example, in recent developments spherical agglomerates are produced from enriched uranium powder as a fuel for specific nuclear reactors (Section 6.10.3). Tab. 6.10-1 lists some of those solid fuels that have been or are being processed most commonly with agglomeration technologies to improve their properties for various applications. 

A 10 MW(th) HTGR test module, HTR-10 [89], is under construction since 1995 by the Institute of Nuclear Energy Technology (INET) of Tsinghua University in Beijing. It is a pebble bed reactor with 27,000 spherical fuel elements and a coolant outlet temperature of 700 °C (later 950 °C). Its operation is expected to start end of 1999. 

Two concepts originating from the Russian Research Centre Kurchatov Institute — the fast spectrum gas cooled BGR-300 with advanced porous fuel and the thermal spectrum MARS with TRISO based pebble bed of spherical fuel elements and molten salt coolant — employ the technologies for which a partial experience database already exists in the Russian Federation from previous nuclear R D programmes. 

The Fixed Bed Nuclear Reactor (FBNR) concept assumes the use pressurized water reactor (PWR) technology, but incorporates hi temperature gas cooled reactor (HTGR) type fuel and the concept of a suspended fixed bed core. Spherical fuel elements are fixed in the suspended core by the flow of water coolant. Any accident signal will cut off the power to the coolant pump causing a stop in the flow. This would make the fuel elements fall out of the reactor core, driven by gravity, and enter a passively cooled fuel chamber where they would reside in a subcritical condition. The Fixed Bed Nuclear Reactor (FBNR) is a simplified version of the fluidized bed nuclear reactor concept [XII-1 to XII-9]. In the FBNR, spherical fuel elements are in a fixed position in the core therefore, there is no concern about the consequences of multiple collisions between them, an issue that may be raised about the fluidized bed concept. Relatively little work has been done for the fixed bed nuclear reactor so far, but the experiences gained from the development of a fluidized bed reactor can facilitate the development of the FBNR. 

Similar to other nuclear installations, the defence-in-depth concept incorporated in the MARS provides for multiple barriers to radioactivity release from the fuel and for measures to maintain the integrity of these barriers. Such a barrier structure largely leans upon the known properties of the fuel (spherical fuel elements with coated particles), i.e., the retention of a large amount of radionuclides in a ceramic fuel kernel and the prevention of radionuclide release to the coolant by the fuel particle coatings. The graphite matrix of fuel elements that has an ability to absorb certain radionuclides facilitates a reduction of radioactivity release to the coolant. A two-circuit plant scheme provides an additional barrier to radioactivity release to the environment. 

The function of the main heat transport system is to remove nuclear heat generated in the spherical fuel elements using natural convection of the molten salt coolant in all operation modes, i.e., under normal operation and in accidents. Fig. XXVIII-6 shows a schematic diagram of heat removal paths for the MARS plant. 

Compute k for a reflected spherical reactor which has an active core radius R 37.2 cm and a spherical shell reflector of thickness 20 cm. For this calculation use (1) the material composition described for the reactor in Prob. 4.8, in the case Nm/Nf = 15,500 (2) nuclear cross sections for fuel and moderator given in Prob. 4.8 (3) the two-group method. 

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