Bumup limits

The causes and conditions of general achievements and setbacks in reactor and liquid metal coolant technologies are presented in the FRDB, e.g. those determining breeding characteristics (core geometry, fuel enrichment and fissile isotope content, volume fractions, intrinsic limits and smeared density of fuel and blanket pellet, etc), and the fuel bumup limits (chemical composition, fuel fabrication technology, neutron flux, dimensions, cladding and wrapper material, etc.)... 

In light of recent incomplete control rod insertion events, the incomplete understanding of the root causes, and the rate at which such problems appear, the ability to insert the control rods fully must be demonstrated at appropriate intervals in order to meet the current licensing basis. A proposed Bulletin Supplement is under consideration by the USNRC which would specify bumup limits as one way to resolve the problem. Should licensees choose to limit bumups to less than those stated, the problem... 

Recent experimental data suggest that high bumup fuel may be more prone to failure during design-basis transients and reactivity insertion accidents than previously thought. Tests on the relationship between fuel failure enthalpy and bumup for pressurized water reactor fuel rods indicate lower failure initiation enthalpy thresholds (measured in differential calories/gram) than was crmsidered in the evaluation of currently approved fuel bumup limits. 

Core configuration and design limitations (reactivity coefficients, bumup limits, inspection, etc.) ... 

Because reactor unwatering considerably worsens decay heat pick-up from Spent Fuel Assemblies (SFAs), it should be performed after some period of fuel hold up in non-unwatered reactor. Many calculations were performed taking into account both factual and possible-limiting state of cores depending on fuel bumup. Two options were examined ... 

To find the re activity-limited bumup of fuel in n-zone scatter refueling, B , note that at the end of life, the freshest nth fraction of fuel will have had bumup of approximately fi /n, the next older nth fraction 2fi /n, etc., and the oldest nth fraction, ready for discharge, will have reached B bumup. The reactivity of this mixture of fuel is... 

Reactivity-limited Bumup in PWR with Modified Scatter Fueling... 

Watt [W2] has used the computer codes CELL and CORE to evaluate the reactivity-limited bumup of a 1060-MWe PWR operated with modified scatter refueling as a function of the... 

The bumup increases roughly linearly with enrichment. The dashed line shows that the variation of reactivity-limited bumup of 3.2 w/o enriched fuel with the number of fuel zones predicted by the simple Eq. (3.6) is a fair representation of the more accurate computer result. 

To illustrate use of Fig. 3.14, the example of the line L =0.9 will be discussed. Suppose that this 1060-MWe reactor is expected to operate at an availability-based capacity factor L = 0.9 with a 1-year interval between refuelings. The minimum fuel-cycle cost of 41 million will occur at a batch fraction /= 5 and a feed enrichment of 3.75 w/o U. This will require fuel to sustain an average burnup B of slightly over 40,000 MWd/MT. If average bumup should be limited for mechanical reasons to slightly over 30,000 MWd/MT, the minimum fuel cycle cost of 42 million will occur at / = 5 and a feed enrichment of 3.2 w/o, the combination suggested by the manufacturer for this reactor. 

Despite the inability of these equations to represent accurately the concentration of hi er plutonium isotopes, the reactivity-limited bumup attainable from fuel initially containing 3.2... 

Each of the reactor criticals contains some fraction of burned fuel, ranging from about 50% to 100% of all assemblies. Because of the neutron absorption in U followed by P decay, u is present in all burned fuel assemblies the fractional content will depend on the initial enrichment and assembly bumup. An energy-dependent bias in is known to exist in the 27BURNUPLIB cross-section library for systems containing plutonium.This bias is discussed further in the following section. The net effect of this bias will be an increased value of k jf for lower-energy (more thermalized) systems. Note that this plutonium bias is not limited to the 27BURNUPLIB instead, it appears to be inherent in current plutonium cross-section data. " ... 

Significant cost savings in security and development can be had if the reactor uses a Category-Ill level of SNM rather than Category I. Security for facilities with highly enriched fuel on site can cost roughly 30 M per year and the slow down in experimental operations caused by this security can cause an additional cost. The first option requires a well-moderated reactor the second requires a somewhat moderated reactor. The first option may end up being very limited in power and lifetime because of fuel bumup the second option has more latitude for power and bumup. 

A proven fuel cycle is very essential for PFBR. Though mixed carbide fuel has been used for FBTR due to non availability of enriched uranium, risks associated with carbide fuel fabrication, higher cost coupled with limited bumup potential limited experience on reprocessing of the fuel have led to adoption of mixed oxide (MOX) fuel. This fuel has shown excellent performance with respect to bumup, has well proven reprocessing technology and has also been used in most of the large sized FBR. 

Reactor plutonium recovered from low enriched uranium spent fuel (less than 5% U-235) constitutes a typical example of a mixture of radionuclides with known identity and quantity for each constituent. Calculations according to para. 404 of the Regulations result in activity limits independent of the abundance of the plutonium radionuclides and the bumup within the range 10 000-40 000 MW d/t. The following values for reactor plutonium can be used within the above range of bumup, the Am-241 buildup taken into account, up to five years after recovery ... 

VII.25-VII.27], and efforts have been made to validate computational methods using data selected from these compendiums [VII.27-VII.29]. The measured isotopic data that are available for validation are limited. Of farther concern is the fact that the database of fission product measurements is a small subset of the actinide measnrements. In addition, the cross-section data for fission product nuclides have had much less scrutiny over broad energy ranges than most actinides of importance in INF. Fission prodncts can provide 20-30% of the negative reactivity from bumup, yet the uncertainties in their cross-section data and isotopic predictions reduce their effectiveness in safety assessments with bnmnp credit. 

VII.73. An example of variability in measurement technique is provided by France, which currently specifies the use of a simple gamma detector measurement to verify bumup credit allowances for less than 5600 MW d/MTU but more direct measurement of fuel bumup for allowance of higher irradiation [VII.39]. For this second measurement, France rehes on two instruments that verify the reactor bumup records based on active and passive neutron measurements. In the USA a measurement device similar to one used in France has been demonstrated by Ewing [VII.40, VII.41] to be a practical method for determining if an assembly is within the acceptable fuel region of Fig. VII.2. If the axial bumup profile is identified as an important characteristic of the spent nuclear fuel that is relied upon in the safety analysis, then similar measurement devices could also potentially be used to ascertain that the profile is within defined limits. 

The results of the post-irradiation examination of the ANTIMAG experiment irradiated in PHENIX (250 10 cjq>ture/cm ) showed the efficiency of the shroud around B4C pellets B4C fragments restrained, limitation of clad carburation and showed also saturation of dedensification at high bumup. 

The reactor core consists of 69 typical (17 X 17) PWR fuel assemblies with a reduced active length (2,92 m) to limit the pressure losses and with a low power density (70 KW/1) for increased design margins. Soluble boron and burnable poisons are used for shutdown and fuel bumup reactivity control. The use of burnable poisons for partial reactivity control results in a lower soluble boron concentration and assures a non positive moderator temperature coefBcient at any operating condition. 

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