Void reactivity

Interaction of void/reactivity coupling with flow dynamics and heat transfer Interaction among a small number of parallel channels Interaction of direct contact condensation interface with pool convection A flow excurision initiates a dynamic interaction between a channel and a compressible volume... 

Void reactivity effect under fuel leakage of coolant % -0.6 -1.8... 

Enhancing reactor safety due to elimination of coolant boiling (coolant boiling point is 1670 C, boiling point of sodium is 870 C) in the most heat stressed fuel subassemblies (FSA) even in the case of the severest accidents. It makes the realization of coolant positive void reactivity effect practically impossible. Besides that, the use of chemically inert LBC eliminates occurrence of explosions and fires if there is coolant contacting with air and water which is possible in emergency situations ... 

The reflector is installed inside the reactor vessel and the heat generated in the reflector is cooled by sodium. The equivalent core diameter is 0.8m which satisfy negative void reactivity requirements. The reflector length is 1.5m and the reflector gradually moves up to control the reactivity leading to bum-up. The axial power distribution changes as shown in Fig. 3 according to the reflector position. 

VOID REACTIVITY -2.5 (Diffusion Model) -1.0 (Transport Model)... 

One of the attractive features of the fast reactor is its hard neutron spectrum. To expand this feature, a metallic fuel core is employed in the 4S. However, it is more difficult to reduce void reactivity for a core with a harder spectrum. It is very important to design the void reactivity to be negative in order to prevent a severe nuclear accident in the event of sudden loss of coolant, sudden loss of coolant flow or a large gas bubble entrainment in the core. 

For the selected core, the void reactivity of the total core is -1 at the end of life based on the transport calculation. Other temperature feedback coefficients are all negative as shown in Table 4. 

Positive void reactivity coefficient of the reactor, made tolerable by the high thermal inertia of the sodium coolant in the amounts generally used and by the consequent difficulty for the reactor to reach boiling conditions. 

As a conclusion, JUPITER experiment and analysis was found to possess sufficient consistency on the whole, especially for the prediction of criticality, space-dependency of C/Es in core region, and sodium void reactivity, which were persistent problems in the past JUPITER evaluations. It was also recognized that there is, however, some room for further improvements about the C28/F49 ratio, reaction rate distribution in blanket region, and Doppler reactivity. Efforts are now being conducted from various viewpoints such as re-evaluation of experimental and analytical errors, application of new most-detailed analytical tools, comparisons with other experimental cores, and refrnement of statistical tests for physical consistency. 

Highly enriched mixed oxide (MOX) fuels and Pu fuels without uranium were considered for Pu burning enhancement. It was found that Pu consumption rates essentially depend on Pu enrichment. Both bumup reactivity loss and Doppler coefficient are important criteria for highly enriched MOX fuel cores. Cores without uranium were found to consume the Pu at a very large bumup rate close to the theoretically maximum value of 110-120 kg/TWhe. The introduction of UO2 in an internal blanket is effective in enhancing the Doppler coefficient, it causes a minor increase in the sodium void reactivity in non-uranium cores. 

Feasibility studies have been performed to investigate the basic characteristics (transmutation rate, bumup reactivity, Doppler coefficient, sodium void reactivity, maximum linear heat rate, etc.) of a fast reactor core with MA transmutation, the following items were considered ... 

In the first half of 1996, tests were completed on substantiation of the neutronics of the BN-800 reactor core with the sodium plenum at the top, allowing to minimize positive component of the sodium void reactivity effect in case of completely dried mixed uranium-plutonium fuel core. 

The final stage was devoted to the measurements of the control rod worth and sodium void reactivity effect in the core sector including all three different enrichment zones (BFS-58-4 critical assembly was simulating the reactor at the beginning of the run after refueling). 

Preliminary analysis made in 1996, showed, that the whole q>erimental program carried out, gave the possibility to determine the sodium void reactivity effect with sufficient accuracy (- 0.2 0.3% Ak/k). 

The preliminary investigation showed that introduction of minor actinides in fuel results in great increasing of sodium void reactivity effect (SVRE) and therefore the using of traditional fast reactors is impossible because it isn t possible to assure zero value of SVRE which is dictated by Russia Safety... 

As a result of a common interest for developing severe accidents analysis codes, a first common benchmark exercise about BN-8(X) in its non-zero void reactivity version has been proposed jointly by EU and IPPE at the December 1992 meeting of the WAC Group. Li December 1994, a second common follow-up benchmark exercise about BN-800 in its nearly-zero void reactivity version has been proposed jointly by EU and lAEA/IWGFR with the aim of including also India and Japan besides IPPE/Russia. 

Unprotected Loss Of Coolant (ULOF) comparative calculations of the Russian BN-800 core with nearly-zero void reactivity. 

As far as the WAC group is concerned, the benchmark exerdse about BN-8(X) in its nearly-zero void reactivity version focused on severe transient accident conditions of the ULOF type. At their yearly review meeting of May 94, the IWGFR of IAEA had agreed to support this new BN-800 calculational exercise proposed by IPPE. Participation includes Germany, France, UK, Italy and Russia as the traditional partners of the WAC comparative exercises, as well as USA, Japan and India as additional "IAEA" parmers. 

The main features of the reactor under consideration are chosen close to the BN-800 reactor with nearly-zero sodium void reactivity core design. In particular ... 

Reactor power is chosen in the range of 1500-2KX) MW (thermal). Sodium void reactivity effect is (0 to -0.1) x l0 The number of the zones with different enrichment is 2 or 3. A multi-batch loading scheme is assumed. The following reactivity effects are taken into account sodium thermal expansion and void reactivity Doppler effect axial core expansion radial core expansion control rod drive expansion. IPPE provides reactivity worth tables and specifies correlations for calculations of structural feedback effects of reactivity (with reference data). The duration of the LOF scenario to be computed is taken as about KXX) s. 

The coolant void reactivity coefficient was first analyzed for a core with a 10% coolant fraction, a 10% enrichment, and fuel fractions ranging from 10% to 50%. The results (Fig. 3.2) show that for fuel fractions less than 30%, complete voiding of the Flibe coolant from the core could result in a positive reactivity addition. As the fuel concentration is increased to provide more realistic excess reactivity values and longer core bumup times, the relative importance of the absorption and moderation in the Flibe is reversed, and the overall void coefficient is then negative as the uranium-to-carbon atom ratio exceeds approximately 0.05. For an NaZrFs salt, the void coefficient is positive for fuel fractions less than 60%. 

The void coeffieient results for the updated reference AHTR model are shown in Fig. 3.8 and are similar to the initial SNL results. For the coolant fraction of 7.6% and an enrichment of 10%, a negative void reactivity effect can be attained for a fuel fraction greater than -0.25 for pure Li Flibe and greater than -0.5 for Flibe with 0.01% Li content. Inereasing the fuel enrichment allows for slightly lower void reactivity effects, as also shown in Fig. 3.8. 

Arranging the configuration in an annular geometry with the same coolant fraction and fuel fraction was not found to be helpful in decreasing the void reactivity effect. In fact, slightly more positive effects are found, although the results were within two standard deviations of each other. It is unclear as to why the effects could be slightly more positive. 

Void reactivity effect calculations were performed for the cores near the end of the bumup cycle to determine the effect of the lower content and larger fission product inventory. The results showed that the void reactivity was about the same value as for the fresh core configurations. 

A maximum hypothetical accident (MHA) has been studied which presumes an unidentified 2.00/see ramp reaetivity insertion of indefinite size and duration, coupled with a failure to scram. This then triggers the maximum sodium voiding reactivity effect of 1.60 which is assumed to take place in 0.1 sec. 

A parametric study carried out in order to establish the design orientations of burner cores taught us first that a considerable reduction of the fuel inventory (or dilution ) is always necessary to be able to operate a large core with a high plutonium content and therefore with an attractive plutonium burning performance. This dilution results in a decrease in in-pile fuel residence time as well as in a reduction (favourable) of the sodium void reactivity, whereas a decrease of the uranium content of the fuel brings about a reduction of the Doppler effect, a decrease of the conversion ratio which causes a daily reactivity loss ttiat makes it difficult to achieve long irradiation cycles, as well as a reduction of tiie delayed neutron fraction. 

In all cases the core performance was determined in terms of safety parameters such as the Doppler coefficient and the sodium void reactivity, the fuel cycle length (taking account of the rapid loss of reactivity with bumup), the absorber worth requirement, the fuel residence time, etc. It was possible to demonstrate the feasibility of the core, including the variants mentioned above, in terms of overall safety and economic performance. 

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