Type Fuel Elements

Dispersion-type elements are those in which discrete particles of fissile material are dispersed in a metallic or ceramic matrix. The matrix effectively 

The structure of a typical coated particle is shown in Fig. 5.4. The fuel kernel is a sphere of uranium carbide with a diameter in the range of 100-400 pm. The innermost coatihg consists of a layer of pyrolytic carbon laid down by deposition from hydrocarbon gases in a high-temperature fluidized bed. This inner layer absorbs fission fragment recoils and is made relatively porous to provide voidage for the accommodation of fission gases. The next layer is made up of silicon carbide, which is particularly effective in the 

The oxidation of graphite in air has led to the use of carbon dioxide or helium as coolant in power reactors. The use of carbon dioxide is limited by the onset of the reaction 

During the baking process in the production of high-grade graphite, the evolution of gas from the pitch binder gives the material a porous structure. 

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The Arbeitsgemeinschaft Versuchsreaktor (AVR) and Thorium High-Temperature Reactor (THTR-300) were both helium-cooled reactors of the pebble-bed design [29,42,43]. The major design parameters of the AVR and THTR are shown in Table 10. Construction started on the AVR in 1961 and full power operation at 15MW(e) commenced in May 1967. The core of the AVR consisted of approximately 100,000 spherical pebble type fuel elements (see Section 5). The pebble bed was surrounded by a cylindrical graphite reflector and structural carbon... 

The core has heterogeneous arrangement and uses dispersion-type nuclear fuel. Core consists of a set of FA and sets of reactivity control and safety rods. FAs incorporate burnable poison (gadolinium) rods to compensate the core excessive reactivity. The core uses smooth-pin type fuel element with a clad ng made of zirconium alloy. 

In order to have a definite chemical system to study, we have chosen a reactor which is probably not very realistic as a power producer, but one which has the advantage that we know both the design details of the fuel element and the cost and design data for the chemical processing plant in which this fuel element is presently handled. We have adopted an MTR type fuel element which is composed of enriched uranium aluminium alloy, but we have assumed that the is replaced by Since any thermal reactor power economy... 

In summary, we can say that we are deeply convinced that it is possible to reprocess enriched fuel elements for about 1 per gram by being realistic in the plant design. We do not think that oiu choice of the MTR type fuel element... 

Reference design Block-type fuel elements 3 x 360 MW He-turbine, integrated arrangement of the three turbosets in the prestressed concrete pressure vessel. 

Decision to modify the HHT reference design by providing one single He-turboset with 1240 MW and block-type fuel elements in 1975... 

Decision to cancel the HHT project in 1981 and to concentrate instead on a shorter term feasible HTR with the steam cycle Thermal power 3000 MJ/s with block-type fuel elements and later with pebble bed core. 

On the other hand, the USA adopts a block type fuel element in HTGRs. In this fuel element, coated particle fuel and graphite powder are mixed and sintered, then shaped into thin rods, and loaded and sealed into holes in a hexagonal column-shaped graphite block. The experimental... 

A research program was initiated in 1976, at ttie Battelle-Pacific Northwest Laboratories Critical Mass Laboratory, to provide experimental criticality data that can be used in criticality assessments of LWB-type fuel element shipping casks and storage facilities. The intent of this paper is to provide an overview of this experiment program at Battelle, both in surveying the experiments performed to date and describing future work., , ... 

Criticality safety evaluations for handling mixed Pu-U oxide-type fuel elements depend heavily on computational analysis with experimentally validated computer codes and cross-section data. A series of critical experiments has been performed with fast test reactor fuel pins in water at the Battelle-Pacific Northwest Critical Mass Laboratory in support of the Advanced Fuel Recycle... 

As a result of these experiments, we now have experiment data for FTR-type fuel elements interspersed with solid neutron absorbers. These data can be used to benchmark calculation schemes used in the criticality analysis of these types of fuels. 

The fuel bum-up in the reactor is very low. The current reactor core configuration is number 17, which started in December 1991, and it is expected to last until the end of 2006, when some standard fuel elements will be replaced with FLIP type fuel elements (70% enrichment). 

Flip Type Fuel element (70%) Graphite reflectors... 

The RP-0 reactor is located at IPEN headquarters in the San Boija district, close to downtown Lima. It is a critical facility that reached criticality for the first time in July 1978 using extruded rod type fuel elements made with 20.09% enriched UO2 mixed with graphite and aluminium cladding. In 1991 the core was converted to 19.75% enriched fuel. In June of that year the reactor reached criticality using MTR type fuel elements supplied by the Argentinian National Commission of Atomic Energy (Comision Nacional de Energia Atomica — CNEA). 

The PFPWR50 is a small PWR with proposed mild variant of the use of HTGR type TRISO fuel within graphite columns packed in Zr-alloy claddings of conventional LWR type fuel elements. It uses a hexagonal lattice that is tighter than the square lattice of conventional... 

The fabrication cost of a core based on pebble bed of micro fuel elements is 40% lower than that of a traditional core based on rod-type fuel elements [X-4]. The reason behind this is that the fabrication of a pebble bed core eliminates costly welding and air-tightness control operations, as well as mechanical treatment of uranium dioxide kernels. 

The direct cooling of micro fuel elements by light water coolant to a certain extent excludes their failure under a positive reactivity insertion. The reason behind this is that the average time of heat transfer from a micro fuel element to the coolant is about 0.03 seconds (for a micro fuel element of 1.8 mm diameter). Therefore, any positive reactivity inserted over more than 0.03 s will be effectively compensated by the evaporation of a water coolant-moderator. Different from that, in a PWR or a BWR with standard rod-type fuel elements the characteristic time of heat transfer from fuel to the coolant is 3-5 s, depending on fuel rod diameter. If positive reactivity is inserted faster, this will result in fuel melting, while the coolant will not evaporate before the cladding fails. 

It can be seen that the character of this scenario is also different for the two core types. For a standard WER-1000 with rod-type fuel elements, the decrease of power takes place very slowly because of the positive Doppler reactivity being inserted when fuel, which is at 1000°C in normal operation, gets cooled. Fission reaction is stopped after 1000 s, when nearly all primary-circuit water is released through the safety valves. In this, the temperature of zirconium claddings exceeds 1000°C after about 20 s after the accident start. 

Such an accident is never considered for the reactors with cores based on rod-type fuel elements, as core cooling with an acceptable temperature of zirconium claddings cannot be provided in this case. For VKR-MT, the rupture of reactor vessel bottom is considered as a beyond design basis accident. 

LIU, L.L., et al.. Behaviour of EBR-II Mk-V-type fuel elements in simulated loss-of-flow tests. Journal of Nuclear Materials, 204, pp. 194-202 (1993). 

Further evolution of the MARS concept provides for an increase of coolant temperature at the core outlet as appropriate structural materials are developed. The resulting concept in which HTGR type fuel elements are cooled by molten salt would be attractive for very high temperature non-electric applications, such as hydrogen production [XXVni-7]. 

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