After these very simple examples, we are able to ‘build’ a more realistic reactor. As a starting point, we want to study a light water reactor (critical) with a lead reflector, loaded with UO2 fuel. Suppose that the water volume is about 4 times more than the fuel volume. The core is a cylinder with a diameter equal to its height. The core radius is 1 m, the reflector thickness is 50 cm and the iron tank containing the two is 5 cm thick.

Homogeneous core

In a first step, suppose that we consider a homogeneous core of (H2O + UO2) with 1.35% (compared to 238U) 235U enrichment.

The condition VH2O/ VUO2 = 4 implies a fuel density of pfuel = 2.8 g/cm3 (taking UO2 « 10 g/cm3) and the atomic composition of the fuel is 6 moles of H2O for 1 mole of UO2. Now, we are able to write the MCNP input file:

Homogeneous core c

c Exterior c

1 0 1:-2:3 imp:n=0 c

c Iron tank c

2 1 -7.87 -1 2 -3 (4:-5:6) imp:n=1 c

c Lead reflector c

3 2 -10.34 -4 5 -6 (10:-11:12) imp:n=1 c

c Core c

4 3 -2.8 -10 11 -12 imp:n=1

c tank/reflector surfaces

1 cz 155

2 pz -155

3 pz 155

4 cz 150

5 pz -150

6 pz 150

c reflector/core surfaces

10 cz 100

11 pz -100

12 pz 100

c Material

ml 26000.55c 1 $ Iron of the tank

m2 82000.50c 1 $ Lead of the

reflector

m3 92235.60c 0.0135 92238.60c 0.987 & $ 235U (1.35%) + 238U 1001.60c 12. 8016.60c 8. $ 6 H2O + 1 O2 (of the

fuel)

sdef pos 000 erg 2.5 kcode 1000 1 10 150 totnu

The keff of the reactor with that geometry is keff = 1.001 ± 0.001.