Fuel failure rates

The new regulation would increase the cost of electricity generation since it tightens the exposure limit. To minimize its impact, we paid extra attention to defining the hot zone and clean zone. The workers are not allowed to enter the clean zone from the restricted area (hot zone) except through the access building. In the process, radiation work and the worker s movement were carefully reviewed. Also, to reduce the impact of stringent requirement, we have taken the credit in the fuel failure rate based on the improved fuel performance(l% fuel failure rate to 0.25% fuel failure rate). 

The fuel failure rate at the SE-EBO NPP (2008) is comparable with fuel failures at other NPPs (see Table 4.2). 

Table 4.2 Fuel failure rates at various NPPs during the 1980s and 1990s ...

Example 4. A particular microprocessor (MPU) is assigned for a fuel-injection system. The failure rate must be estimated, and 100 MPUs are tested. The test is terrninated when the fifth failure occurs. Failed items are not replaced. This type of testing, where n is the number placed on test and ris the number of failures specified, is termed a Type II censored life test. 

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 nominal fuel failure and fission product release rates are estimated using the segment average values of the as-manufactured fuel attributes given in Tables 4.2-4 and 4.2-16, nominal core operating conditions (with the exception of thermal power which is taken as 102 percent of full power), nominal fuel failure models, and nominal estimates of the fission product transport properties. 

The fuel failure and fission product release rates at the upper 95 percent bound are determined by a full-core propagation-of-errors analysis comparable to that described in Reference 11. The uncertainties in the as-manufactured fuel attributes (e.g., defect fractions as defined in Table 4.2-4), the core operating conditions, the fuel failure models, and the fission product transport properties are combined by conventional statistical techniques to determine the total variances in the fuel failure and fission product release rates from which the upper 95 percent bound values can be calculated. 

Two basic types of control systems commonly employed are shown in Figures 9.45 and 9.46 [23]. Control system A is used mostly with wheel atomizer processes and consists of two circuits. The first circuit controls outlet air temperature by feed rate regulation. The inlet air temperature, controlled by the second circuit, is corrected by the fuel combustion rate. The control system is provided with a safety system that prevents any damage in case of failure in the feed system followed by rapid increase in the outlet air temperature. 

Two different types of solenoid valves are used to block fuel flow to a burner in a SIS. The valves are piped in series. Both valves should close when a dangerous condition is detected. Both valves have one failure mode, fail-danger, with a failure rate of 0.0008 failures per year. Both valves are tested once every year. Based on the differences between the valves, a common cause beta factor of 0.01 is assigned. What is the PFD of the valve subsystem including common cause ... 

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.). 

Fig. 3 show the time evolution of the average fuel pin failure rate (FPFR). The average FPFR is defined as follows ... 

FIG. 3. Fuel pin failure rate for fuel of Ukrainian WWER-1000. 

The failure monitoring was conducted within the period of 41 hours and 27 minutes. That led to estab-hshment of the failure rate (6.95 failures per day) and the mean repair time (1.81 minutes). The failure rate is high, yet this high number is also due to the frequent initiation of the fuel cell system, frequent load with the nominal power output, occasional overloads and fluctuating voltage within the distribution network at the inverter connection point. The values of failure rate and the mean repair time shall be specified using... 

Under accident conditions of transport, irradiated fuel pins may fail, with subsequent radioactive release into the package containment system. Data on the fuel fission product inventory, possible failure rate of pin cladding and the mechanism of activity transfer from the failed pin into the containment system are therefore required to enable the package leaktightness to be assessed. 

Simplified thermal-hydraulic analyses, which were benchmaiked against GT-MHR data, indicated that for a fixed core outlet temperature of 1000°C for the coolant, the peak fuel temperature in the AHTR during normal operation will be 110-130°C cooler than for the prismatic helium-cooled NGNP design, and the average fuel temperature at the core outlet will be 30-50°C cooler. This is a direct result of the superior heat transfer properties of the molten salt relative to helium. This is significant because the failure rate of the coated particle fuel increases with increasing temperature. 

Coated-particle graphite-matrix fuel developed for high-temperature gas-cooled reactors (HTGR) in the United States and Germany starting in the 1960s. These fuels have demonstrated low failure rates for temperatures up to about 1600°C and moderate bumup. 

Efforts are being made to develop a new relational data base system of LMFBR component reliability data on an engineering work station. The system is based on CREDO (Centralized Reliability Data Organization), a cooperative project between PNC and the USDOE, which ended in 1992. As part of the data analysis reliability parameters were quantitatively estimated for sodium mechanical pumps, i.e., failure rates, probability of common cause failures and lepairability. Additionally, risk-related data on energy production systems such as solar photo voltaic energy system and LMFBR nuclear fuel cycle have been collected. 

The low power density of the core, increased design margins, self-regulation of the flow rate through the fuel assemblies and the slow dynamics of tranments ensure that there will be no fuel failures in normal or emergency conditions. 

To summarize, the suitability of UO2 as a reactor fuel for both thermal and fast reactors has been confirmed by a wealth of experience, and the failure rate of fuel pins can be made acceptably low (down to about 1 in 10" ) by careful quality control in manufacturing. Particular problems which have arisen, such as fuel densification and damage due to pellet-clad interaction, have been solved by adjustment of the initial fuel density and pin gas pressure, and by limiting the rate at which large power increases are allowed to occur. Alternatives to the oxide fuel, such as uranium carbide and uranium nitride, have been investigated less extensively, but hold promise of superior performance in fast reactor systems. 

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