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4. Core with zirconium hydride in fuel elements<br />

For enhancement of plutonium burn-up rate a moderated zone containing zirconium hydride in<br />

the fuel element may be used. Zirconium hydride is a good moderator and has good inert properties.<br />

Application of zirconium hydride moderated zones in fast reactors were considered in papers [8-9].<br />

We have considered the reactor core with fuel elements containing zirconium hydride layer<br />

located in the gap between the fuel pellet and steel cladding or around the steel cladding. All<br />

dimensions of the fuel pin were taken as in the reference core. Due to the shifting of the neutron<br />

spectrum to the thermal energy region more than 70% of the plutonium burn-up events occurs in the<br />

thermal energy region. The use of the fuel elements with zirconium hydride and plutonium<br />

enrichment of 20%-23% permits to burn up about B Pu<br />

= 25 kg/FPY (at the P r = 80 MW) of plutonium<br />

at the average power density in the fuel of q f<br />

= 227 W/cc. There are two possible arrangements of<br />

zirconium hydride layer in fuel element: (1) HZr arranged abutting to the outer surface of the steel<br />

cladding, and (2) HZr layer may be arranged in the gap between the fuel pellet and the steel cladding<br />

of the fuel element.<br />

In Table 2 results of computations of the reactor core with different arrangement of HZr layer in fuel<br />

element are presented. Burn-up rate of plutonium in fuel elements with HZr layer<br />

(∆d 2<br />

(HZr) ≈ 0.045 ÷ 0.072 cm) arranged abutting to the outer surface of fuel pin is equal to<br />

B Pu<br />

= 24.8 kg/FPY, while B Pu<br />

= 21.6 kg/FPY in the core with HZr layer (∆d 1<br />

(HZr) ≈ 0.035 cm) arranged<br />

inside the fuel element. The average power density is equal to q f<br />

= 228 W/cc (21.8 W/g) in fuel elements<br />

with HZr layer arranged abutting to the outer diameter of fuel cladding, and q f<br />

= 258W/cc (24.7 W/g) in<br />

fuel elements containing HZr layer inside fuel element.<br />

Table 2. Fuel elements containing H 2<br />

Zr cladding arranged in different positions:<br />

(1) Abutting to the outer diameter of the steel cladding.<br />

(2) In the gap between the fuel pellet and the steel cladding.<br />

Parameter 23.225%<br />

Reference core<br />

Plutonium enrichment, %<br />

20.0% 23.2% 55%<br />

Position of HZr – (1) (2) (1) (2)<br />

r fuel<br />

, cm 0.09 0.09 0.08 0.09<br />

R fuel<br />

, cm 0.357 0.357 0.33 0.301<br />

r clad<br />

, cm 0.3685 0.3685 0.3685 0.3685<br />

R clad<br />

, cm 0.425 0.425 0.425 0.425<br />

k eff<br />

0.9839(5) 0.9831(5) 0.9811(6) 0.9828(6)<br />

k s<br />

0.980 0.982 0.981 0.984<br />

I p<br />

(mA) 1.67 1.51 1.61 1.41<br />

q f<br />

(W/g) 21.98 21.82 24.73 31.00<br />

k v<br />

= q max<br />

/q f<br />

1.35 1.37 1.32 –<br />

B Pu<br />

, (kg/FPY) 13.7 24.8 21.6 33.2<br />

Fuel element with plutonium enrichment 55% and two moderated zones of zirconium hydride is<br />

also considered. The first HZr zone is arranged abutting to the outer diameter of fuel element (the<br />

thickness of the layer is equal to 0.07 cm) and the second zone is arranged in the gap between the fuel<br />

812

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