Fuel Bumup

A PWR can operate steadily for periods of a year or two without refueling. Uranium-235 is consumed through neutron irradiation uranium-238 is converted into plutonium-239 and higher mass isotopes. The usual measure of fuel bumup is the specific thermal energy release. A typical figure for PWR fuel is 33,000 MWd/t. Spent fuel contains a variety of radionucHdes (50) ... 

The low-power-density, low enrichment reactor core uses soluble boron and burnable poisons for shutdown and fuel bumup reactivity control. Low worth grey rods provide load following. A heavy uranium flywheel extends the pump coastdown to allow for emergency action during loss-of-flow transients. 

However, spent fuel is not a waste until it is so declared. As in the definitions of high-level waste discussed previously, the constituents of spent fuel and the minimum fuel bumup or concentrations of radionuclides produced by irradiation are not specified. High-level waste then is defined in two parts as ... 

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 ... 

Two cores had 6-% fuel enrichment, the remaining cores 21 % enrichment. Mean fuel load therein was equal to 46.8 kg. According to specifications, maximal fuel bumup made up 20 % of the initial fuel load on average. 

Reactor pressure vessel Core inlet temperature Core outlet temperature Coolant inlet pressure Coolant flow Core power density Average fuel bumup Refuelii interval Gas turbine cycle type... 

High fuel bumup based upon the HTTR type fuel. 

Design fuel bumup is between 60,000 and 100,000 MWd/MT, instead of a few hundred in the irradiated thorium processed at Savannah River and Hanford. This causes uranium and fission-product concentrations and fission-product radiation levels to be much higher than in the inadiated thorium processed at Savannah River and Hanford. 

The fuel utilization is referred to as bumup. The bumup may be expressed as the percentage of fuel used before it must be replaced. For example, 1 % bumup means that for each ton of fuel 10 kg of the fissile plus fertile atoms have been consumed (in fission and capture). However, usually the fuel bumup is given in amount of energy obtained per ton of initially present fuel atoms (in case of mixed U - Pu fuels per ton of initial heavy metal. 

FIG. 21.6. Typical variation of the ratio between the Cs and Cs radioactivities with fuel bumup. 

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. 

Totally (as of 1/11/97) 36 reactor core reloads have been carried out at peak discharge bumup of 10% h.a. and experimental fuel bumup of 11.8% h.a. 

During 1986 and 1987 the reactor was changed over to the modified core (01M) with the peak bumup as high as 8.3% h.a. to improve fuel performance and to increase BN600 reactor fuel bumup. The new peak bumup values were 6.5% h.a. for the lowly enriched fuel, 6.9% h.a. for the intermediately enriched fuel and 8.3% h.a. for the highly enriched fuel. The principal difference between the first load type core and the modified core was an increase in the height of a fuel fissile section from 750 to 1000 mm and utilization of three uranium 235 fuel enrichments, i.e. 17%, 21% and 26% instead of 21% and 33%. 

By present time the 11.3% h.a. peak fuel bumup BN600 reactor core design has been elaborated and is at the stage of approval and finalization. The activities on development of the 12% h.a. peak bumup core have been started. 

The capital cost of FBR is about 1.5 to 2.5 times that of thermal reactors and significant cost reduction is essential for its successful deployment. Cost reduction measures adopted in LMFBR include elimination of ex-vessel sodium storage, decrease in number size of components of heat transport system, compact layouts, increasing operating temperature, increasing plant life and increasing fuel bumup. 

Nuclear instrumentation and control systems present a greater challenge. Even though it may be possible to develop a system that is inherently controlled by temperature for all normal operations, it appears that it will always be necessary to include control devices and instrumentation in the nuclear system to adequately control and monitor startup from cold shutdown, total shutdown, and possibly to compensate for fuel bumup. Design configurations that reduce the number and complexity of these instrumentation and control systems and components need to be developed. 

During the MK-II operation, extensive data were accumulated from start-up and core characteristics tests. These core management and core characteristics data were compiled into a database [2]. The core management data includes core specifications and configurations, atomic number densities before and after irradiation, neutron and gamma flux, neutron fluence, fuel bumup, and temperature and power distributions. The core characteristics data include excess reactivities, control rod worths, and reactivity coefficients, e.g., temperature, power and bumup. These core characteristics data were recorded on CD-ROM for user convenience. 

The second modification of the reactor core made in 1993 facilitated reaching design fuel bumup of 10% h.a. 

However presented statistics is not correct for determination of fuel bumup influence on fuel pin failure rate, because the average share of tested FA after the first and second year of operation is 30-50%, after the third year of operation -60-80% and after the fourth year of operation -90-100%. ( The rejection criterion of leaky FA is the value of iodine-131 activity =1-10 Ci/L in water of testing system.). 

During inspections of FAs with Zr GTs and SGs which have completed 3 cycles the displacement of SGs was revealed. In some FA the displacement was revealed after 2 fuel cycles. Structurally SGs were not reliably attached to GTs. Displacement of SGs happened in case where the fuel bumup exceeded 25-27 MWd/kgU. 

Currently it is proved, that the increase of fuel bumup under compensatory technical measures implementation does not worsen FA bow situation. Analysing the WWER-1000 and PWR operational experience we may state that, at present we possess a more detailed and complete information on evolution of a FA bow and measurement of RCCA insertion for WWER-1000 than for PWR. 

The experience of WWER-1000 FA operation and principal results of the irradiated FAs examination allow to accept the possibility of further fuel bumup increase. Fuel reliability is satisfactory. 

The further improvement of FAs is necessary. That will allow to reduce the front- end fuel cycle cost (specific natural uranium expenditure), to reduce amount of spent fuel and, consequently, the fuel cycle back- end costs, and to increase the fuel bumup. 

BOWDEN, R.L., THORNE, P.R., STRAFFORD, P.I., The methodology adopted by British Nuclear Fuels pic in claiming credit for reactor fuel bumup in criticahty safety assessments , ibid., pp. lb.3-10. 

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. 

Over the totality of the year, the plant was coupled to tfie grid for 246 days (nearly 70 % of the time) and supplied 3.4 billion kwh. The availability of the reactor, apart from scheduled outage, was 95 %, fuel bumup reaching 318.8 equivalent nominal power days by the end of the year. 

Maximum fuel bumup of 30,390 MWd/t for Mark I (70% PuC-30% UC) core without any fuel failure was achieved. The central fuel subassembly with a maximum bumup of 25,030 MWd/t was discharged for post Irradiation Examination (PIE). PIE is mostly completed (see para 4.5.3). 

In last 40 years, the fuel bumup in light-water reactors (LWRs) has increased from -20,000 to -60,000 MWd/ton. This increased bumup has reduced refueling operations and was a major factor in reducing refueling times. The same economic drivers exist for the LS-VHTR and may result in significant reductions in refueling time as the technology is developed. 

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