Safety Margins in PWRs: 10 CFR 50.46 & Beyond

[kollier]

As it is stated in 10 CFR 50.46 maximum clad temperature should not exceed 2200 F(1204 C)
but I have read in "Nuclear Safety" by G. Petrangelli that fissuring of the cladding starts at about 800-900 C.
I want to know the safety limit or margin for cladding in NPPs.
I could't find any regulation or standard other than those mentioend in 10 CFR ,but if fissuring starts at 800 C why the safety margin is not about 800 C?

 

[Astronuc]

If by fissuring of the cladding, one means breach, i.e. cracking, and generally axial or longitudinal cracking (as opposed to circumferential fractures), one should understand that the breach, while releasing fission products, does not affect the coolability of the fuel rods. In other words, the breach occurs at low strain levels, which means that fuel rods mostly maintains it's geometry and coolability.

I'd be interested to see the discussion by G. Petrangeli, because PCI (pellet-cladding interaction) can occur under normal operating conditions, and that is why the utilities operate nuclear fuel with restrictions on power ascension rates. On what page is the cladding temperature limit mentioned?

In addition, the French REPNa tests show that cladding can survive large strains, but that is dependent on local burnup and the quantity and distribution of hydrides in the Zr-alloy cladding.

The 1204 C test has to do with cladding oxidation and embrittlement in the reflood stage of a LOCA. The concern there is ballooning of the cladding as opposed to breach, and the potential of breakaway oxidation.

I found the book here.
http://books.google.com/books?id=5X...a=X&oi=book_result&ct=result&resnum=4#PPR6,M1

There is a useful reference from OECD.
http://books.google.com/books?id=DC...=X&oi=book_result&ct=result&resnum=4#PPA27,M1

 

[kollier]

It was my mistake, Petrangeli doesn't say that fissuring "starts" at 800-900C.
It is written in page 21 of the book:
"The first consequences of uncontrolled overheating
of the core are the fissuring of the fuel
claddings (at about 1073–1173K (800–900C)), while
their normal operating temperature is about 623K
(350C)) and their subsequent oxidation reaction
with water or with steam (above 1473K (1200C))
which generates heat and hydrogen."

Moreover Bengt Pershagen in his book, Light Water Reactor Safety, on page 51 (3.4.6 Metal-water reaction) is written:
"At temperatures 880-900 C, clad oxidation begins to increase[ not starts, as Astronuc mentioned] leading to the formation of hydrogen and the release of heat ...
The reaction rate depends strongly on the temperature and on the thickness of the oxide deposite. At 1200C the heat release is about as large as the average nuclear power in the fuel during normal operation.
Within 15 minutes, about 15% of the cladding is oxidized..."


By the way I know that the situation is critical at 1200C but isn't it important not to pass 800C?