NUCLEAR REACTOR CORE PROBLEMS
DOE-HDBK-1017/2-93
Plant Materials
Nuclear Reactor Core Problems Summary (Cont.)
Fuel Densification
Densification, which is the reverse of swelling, is a result of irradiation.
Such behavior can cause the fuel material to contract and lead to
irregularities in the thermal power generation.
Three principle effects:
An increase in the linear heat generation rate by an amount directly
proportional to the decrease in pellet length
An increased local neutron flux and a local power spike in the axial
gaps in the fuel column
A decrease in the clearance gap heat conductance between the pellets
and the cladding. This decrease in heat transmission capability will
increase the energy stored in the fuel pellet and will cause an
increased fuel temperature.
To minimize these effects on power plant operation, limits are established on
the power level rate of change and the maximum cladding temperature
(1200C) allowable during a loss of coolant accident.
Fuel Cladding Embrittlement
Embrittlement is caused by hydrogen diffusing into the metal. Cladding
embrittlement can lead to cladding failure.
Zircaloy-4 and different fabrication processes are used to minimize the effect
of hydrogen embrittlement.
Fuel Burnup and Fission Product Swelling
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.
Operational maximum and minimum coolant flow limitations help prevent
extensive fuel element damage.
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