22.31411.5612.084113.14Fall 2006
Problem Set VI
Due 10126106
his
problem i l l u s t r a t e s a mechanism by which c l a d stresses can be
reduced during up-power ramps by approaching f u l l power a t a slower
rate.
1.
Geometry (20°C)
The c l a d outside diameter i s 11.2 nun; and t h e c l a d thickness i s 0.7 mm.
2.
Clad Material Properties (assumed constant)
Young's Modulus
Poisson's Ratio
C o e f f i c i e n t o f Thermal
Expansion
;9 = c l a d creep r a t e (sl.) .
where a has the u n i t s MPa.
9
3.
Operating Conditions
.
= 76 GPa;
= 0.25;
= 6.7 m1m.K; and
= (1 x 10-9)ag;
-
A t h o t zero power, the c l a d i s a t 280°C. t h e coolant pressure i s 15.5 MPa,
t h e gas pressure i n s i d e the rod i s 5.4 MPa, and the clad i s touching
t h e f u e l w i t h zero contact pressure.
A t h o t f u l l power the coolant and gas pressures are unchanged, the
average c l a d temperature i s 375'C, and t h e r a d i a l displacement o f t h e
o u t e r surface o f the f u e l has increased by 35 run.
Consider two cases:
- An up-power
- An up-power
4.
ramp from zero t o f u l l power i n 30 minutes;
r i i p from zero t o f u l l power i n 30 hours.
Calculational Basis
- Use a s i n g l e r i n g t o
represent t h e c l a d
- Consider
t h a t t h e r a d i a l d e f l e c t i o n a t the f u e l outer surface
and the c l a d average temperature vary l i n e a r l y w i t h time
during each up-power ramp.
- Use only
e l a s t i c , thermal, and creep s t r a i n s .
s t r a i n s equal zero a t h o t zero power.
- Use zero a x i a l f o r c e from f u e l - c l a d
contact.
L e t the creep
5.
Questions
Obtain t h e following quantities upon reaching f u l l power i n each uppower ramp:
.,
- c l a d stress;
- clad strain;
- radial
and
displacement a t the c l a d o u t e r surface.