Cladding design limitations

If the coolant undergoes a phase change (boiling), a different approach must be used since the design limit is clad heat flux, not clad temperature. The CHF is used as the limit to prevent film boiling and fuel rod failure. Figure 22.29 shows the heat balance. 

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 ... 

Design limits Temperature of fuel element cladding with account 650... 

Design limit on mechanical Existence of fuel-clad gap - absence of pellet-clad mechanical interaction ... 

A. 1624. The transient analysis may show that the fuel design limits have been exceeded, resulting in some fuel cladding damage. An estimate of the type of damage, the quantity of fuel affected and other factors (such as fuel and cladding temperatures, coolant characteristics and chemical interactions) should be provided. 

Design limit for preventing excess heat up of the fuel rod cladding. 

Eabrication techniques must take into account the metallurgical properties of the metals to be joined and the possibiUty of undesirable diffusion at the interface during hot forming, heat treating, and welding. Compatible alloys, ie, those that do not form intermetaUic compounds upon alloying, eg, nickel and nickel alloys (qv), copper and copper alloys (qv), and stainless steel alloys clad to steel, may be treated by the traditional techniques developed for clads produced by other processes. On the other hand, incompatible combinations, eg, titanium, zirconium, or aluminum to steel, require special techniques designed to limit the production at the interface of undesirable intermetaUics which would jeopardize bond ductihty. 

Except for components designed in accordance with provisions of (c)(1) above, fluid service requirements for materials stated in this Code shall not restrict their use as cladding or lining in pipe or other components. Fluid service requirements for the outer material (including those for components and joints) shall govern, except that temperature limitations of both inner and... 

As anticipated, there have been occasional equipment failures involving reactors, but the safety systems have been sufficiently redundant so that one or more have always worked. Even in the Three Mile Island accident in 1979, the safety systems worked as designed. Much of the damage resulted from operator actions to override the safety systems. As concluded in the Reactor Safety Report, the limitations of the operator created and then seriously aggravated the Three Mile Island incident. Nevertheless, the features of the containment system prevented significant exposure to the plant personnel or any off-site individual, this despite failure of the barriers provided by the fuel cladding and the primary coolant system. 

However, it is recommended to set a temperature captor inside the autoclave wall near the autoclave bottom where the lowest temperatures may occur, so that the temperature limit fixed by the autoclave design be not reached otherwise, the autoclave must be replaced. Another hazard might also appear on these stainless steel cladded autoclave if this cladding is perforated or cracked and CO2 corrodes the carbon steel (in presence of water) so a strict inspection must be made frequently to ensure that no such cladding perforation occurs. 

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