Fuel burnup

Reactivity Control. The movable boron-carbide control rods are sufficient to provide reactivity control from the cold shutdown condition to the full-load condition. Supplementary reactivity control in the form of solid burnable poison is used only to provide reactivity compensation for fuel burnup or depletion effects. The movable control rod system is capable of bringing the reactor to the subcritieal when the reactor is an ambient temperature (cold), zero power, zero xenon, and with the strongest control rod fully withdrawn from the core. In order to provide greater assurance that this condition can be met in the operating reactor, the core is designed to obtain a reactivity of less than 0.99, or a 1% margin on the stuck rod condition. See Fig. 7. 

Distribution ratios and transport were carried out on real HAW arising from dissolution of a mixed oxide of uranium and plutonium (MOX) fuel (burnup 34,650 MW d/tU), where uranium and plutonium have been previously extracted by TBP.86 The experiments were performed in the CARMEN hot cell of CEA Fontenay aux Roses with two dialkoxy-calix[4]arene-crown-6 derivatives (diisopropoxy and dini-trophenyl-octyloxy). High cesium distribution ratios were obtained (higher than 50) by contacting the HAW solution with diisopropoxy calix[4]arene-crown-6 (0.1 M in NPHE). Moreover, the high selectivity observed with the simulated waste was confirmed for most of the elements and radionuclides (actinides or fission products Eu, Sb, Ce, Mo, Zr, and Nd). The residual concentration or activity of elements, other than cesium, was less than 1% in the stripping solution, except for iron (2%) and ruthenium (8%) the extraction of these two cations, probably under a complexed... 

High-level waste thus includes the concentrated wastes that arise from reprocessing of commercial or defense nuclear fuel that contain virtually all the fission products and transuranium radionuclides (except plutonium) in spent fuel. However, the definition does not mention the constituents of the waste, and it is only qualitative because concentrated is not quantified and the minimum fuel burnup that would yield high-level waste is not specified. Although the definition given above referred only to liquid (aqueous) waste, it is clear from further discussions in 10 CFR Part 50, Appendix F (AEC, 1970), that AEC intended that high-level waste also would include concentrated solid waste derived from liquid high-level waste that was suitable for permanent disposal. 

The fuel elements are held in position by grid plates in the reactor core. The fuel burnup to which a reactor may be operated is expressed as megawatt-days per kilogram (MWd/kg), where MWd is the thermal output and kg is the total uranium (sum of U-235 and U-238). In light-water power reactors the core may be operated to about 35 MWd/kg (about 3.5% burnup) before fuel elements have to be replaced. In liquid metal fast breeder reactors (LMFBRs) and high temperature helium gas-cooled reactors (HTGRs), the burnups may exceed 100 MWd/kg ( 10% burnup of the heavy metal atoms). 

The performance requirements for ceramic nuclear fuel elements include the following dimensional stability to high fuel burnups, fission product retention, corrosion resistance, high thermal performance, fabricability, economic advantage, inspectability, and chemical reprocessing and recycling. 

X 10 MWd/MT. This corresponds roughly to a heat of fission of 200 MeV for U. Fuel burnup B, in megawatt-days per metric ton, is thus related to the weight fraction w of fuel fissioned by... 

Despite the drawback of large bumup, rhodium SPNDs are used extensively in nuclear power plants, especially in PWRs for the determination of power distribution, fuel burnup, and other information related to the performance of the core. The detectors are inserted into a certain number of instrumented fuel assemblies through guide tubes. Every instrumented assembly has seven equally spaced SPNDs (a background detector and a thermocouple are also included in the package see Fig. 14.29) for the measurement of the flux at seven axial locations. The outputs of the detectors, corrected for background, are transmitted to the plant computer, where after appropriate corrections are applied, the power, fuel burnup, plutonium production, etc., are calculated. Every PWR has a least 50 instrumented assemblies, which means that the flux is monitored at more than 350 locations. 

Boron may also be used as a burnable poison to compensate for the change in reactivity with lifetime. In this scheme, a small amount of boron is incorporated into the fuel or special burnable poison rods to reduce the beginning-of-life reactivity. Bumup of the poison causes a reactivity increase that partially compensates for the decrease in reactivity due to fuel burnup and accumulation of fission products. Difficulties have generally been encountered when boron is incorporated directly with the fuel, and most applications have used separate burnable poison rods. 

High fuel burnup rate can cause the reactor to be refueled earlier than designed. Swelling can cause excessive pressure on the cladding, which could lead to fuel element cladding failure. 

The accuracy of decay heat calculations depends on the individual heat generation rate from fission product decay nuclides and actinides, and the burnup calculation for its production and transmutation. To obtain experimental data and to improve the accuracy of related calculations, the decay heat of MK-II spent fuel subassemblies was measured at the JOYO spent fuel storage pond [7], The fuel burnup was approximately 66 GWd/t and the cooling time was between 40 and 385 days. The measured decay heat is shown in Fig. 9. 

A Run-to-Cladding-Breach (RTCB) test is planned in JOYO. The RTCB test is expected to improve the FBR fuel performance. The results will increase the fuel burnup and extend the cladding life-time. As part of the preparation work, the FFD system has been upgraded to improve its accuracy and reliability and FP traps have been installed. A series of simulated fuel failure tests has been conducted [10]. 

Table 6.3 illustrates the magnitudes of various reactivity feedback coefficients and their variation with fuel burnup. 

It has been said that as fuel burnup progresses, moderator and water void coefficients tend to become positive. Furthermore, a step that is proposed to be taken in the RBMKs since Chernobyl is to force a... 

The fuel reliability should be considered in the context with the purpose to increase fuel burnup. At the beginning of 1999 a total quantity of the FA discharged during all time of operation of 11 reactors was 5819 (110 fuel cycles). 194 of them were identified as leaking. 

Summarizing the presented data we can state that the increase of Ukrainian WWER-1000 fuel burnup has not worsened fuel reliability. 

Ukrainian NPPs have been operating advanced fuel assemblies with Zr alloy-110 and Zr alloy-635 guide tubes (GT) and Zr alloy-110 spacer grids (SG) since 1995. According to the calculations the substitution of the steel by Zr alloy in materials of guide tubes and spacer grids increases the fuel utilization efficiency by 8.2%. Advanced fuel implementation allows to increase fuel burnup by 5-7%. 

The maximum allowable fuel burnup in natural uranium appears to be limited to about 4000 to 5000 MW-d/tonne by swelling of the... 

This type of fuel element has the advantage that the structural parts of the element are unaffected by fission recoil damage, and, hence, the fuel burnup is not limited by impairment of the structural integrity of the fuel element. Furthermore, since these fuel elements consist only of graphite and the fuel materials themselves, both of which can withstand very high temperatures, a large improvement in the maximum allowable fuel surface temperature becomes possible—1000°C (1800°F) or higher. 

In-pile tests of coated fuel particles and fuel element sections indicate excellent behavior even after very large burnup exposures. The first Peach Bottom core is designed for an average fuel burnup of about... 

Table X indicates the combined uranium requirements for new inventory and fuel burnup per megawatt(e)-year assuming a growth of nuclear power such that the doubling time is six years. This growth rate is approximately that expected in the United States (and probably other countries) in the period from about 1975 to 2000 A.D. (54,55). The reactor conditions shown in the table were chosen to reflect advances in technology expected for each of the reactor concepts.

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