Cladding temperature, peak

Figures 4-12-4-17 depict the important phenomena of the transient and show the critical parameters. The difficulty of keeping a high mixture level in the core is evident. The presence of a second clad temperature peak is a consequence of this fact. See Table 3-1 in Chapter 3 for a list of typical external releases in this type of accident.

Such kind of changes also will impact the peak clad temperature, and consequently thermal limits will be modified, if such is the case. 

Analysis has shown that the peak clad temperature following a large break (LOCA) is about 800 °C which is substantially lower than a PWR plait of current design and well below NRC limit of 1204°C. For small LOCA (s 8 inches) analysis show that core uncovery does not occur. 

The NRC has amended 10 CFR 50.46 Appendix K to permit an alternative ECCS analysis method in addition to the conservative approach to ECCS analysis. This alternative consists of a realistic ECCS analysis plus an accounting for the uncertainty of the calculation in the adverse direction. This method should produce a reduced calculated peak clad temperature and would, therefore be beneficial with respect to plant operation and lifetime. The actual degree of benefit would, however, vary from vendor to vendor due to design differences. 

Peak fuel cladding temperature shall not exceed 1204°C (2200°F). 

MASLWR implements safety systems that do not rely on actively powered external systems such as pumps. Analyses indicate that the passive safety systems are able to keep the peak cladding temperature below the design limit of 1200°C (2200 °F) for design basis accidents. The following passive systems have been incorporated into the MASLWR design ... 

FIG. XXII-10. Peak cladding temperature versus fuel pin outer diameter. 

FIG. XXII-11. Peak cladding temperature versus core inlet temperature. 

FIG. XXIII-6. Peak cladding temperature, core outlet temperature, and maximum S-CO2... 

Figure XXIII-7 shows the dependency of the S-CO2 Brayton cycle efficiency upon the core inlet temperature. In the calculations, the heat exchanger tube height is chosen to satisfy the peak cladding temperature constraint of 650°C. It is confirmed that a fuel rod outer diameter of 1.30 cm and an inlet temperature of 438°C maximize the Bra)don cycle efficiency.

FIG. XXIII-7. Supercritical carbon dioxide Brayton cycle efficiency versus Pb core inlet temperature subject to peak cladding temperature constraint of650°C. 

CORE INLET, CORE OUTLET, AND PEAK CLADDING TEMPERATURES... 

Because of the relatively low volumetric decay heat for GT-MHR waste packages, the peak fuel/graphite temperatures are significantly lower than the corresponding fuel/cladding temperatures within LWR waste packages. The GT-MHR peak fuel temperatures are 220°C versus 350 C LWR fuel. 

The analyses performed demonstrate that the LOCA acceptance criteria are met in the case of the small-break LOCA. The 254 mm cold leg break exhibits the limiting minimum inventory condition that occurs during the initial blowdown period and is terminated by accumulator injection. The APIOOO design is such that the minimiun inventory occurs just prior to IRWST injection for all breaks except the 254 mm cold leg break. All breaks simulated in the break spectrum produce results that demonstrate significant margin to peak cladding temperature acceptance criteria limits. 

A large amount of experience has been accumulated on the so called HT9 alloy considered as reference material for metallic driver fuel of EBR-II and FFTF (Refs. 7.6, 7.7, 7.17). The highest exposure doses were reached with FFTF oxide fuels at limited peak cladding temperature (600X), with a record level of about 200 dpa, without cladding failure. Furthermore, some of the lead tests were performed at cladding temperatures in the range of 640 C-660 C. Post irradiation results confirmed the inherent characteristics of this type of material ... 

The figures XXIII-2 and XXIII-3 show peak values of fuel and cladding temperatures for each fuel assembly during the fuel cycle (without accounting for uncertainty factors). The peak fuel temperature is reached in a fresh fuel assembly and makes 1375 K (Fig. XXIII-2) the peak cladding temperature is 888 K (Fig. XXIII-3). 

FIG. XXIII-3. Peak cladding temperatures of RBEC-M core during irradiation, K. 

The estimated peak fuel and cladding temperatures in accidents without scram are given in Table XXIII-6. 

TABLE XXIII-6. PEAK FUEL AND CLADDING TEMPERATURES IN ACCIDENTS WITHOUT SCRAM... 

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