Phase diagram of water

Safety Features in a PWR
• The control rods can be lowered fully in the case of an emergency
• Should the pressure drop in the primary loop and the water start to
boil, the creation of bubbles (voids) decreases the moderation and also
the absorption. The effect on the moderation is the more significant
and the chain reaction stops and the reactor is no longer critical
• The moderation is also decreased if the core temperature rises, as
this increases the Doppler broadening of the 238 U resonances, which
decreases the resonance escape probability p
• A loss-of-coolant accident (LOCA) in which the water in the
primary loop is lost requires additional emergency cooling to be
available. The outer containment vessel provides a final barrier and
worked successfully in the Three Mile Island accident
Power Output of Nuclear Reactor
Reaction rate R = (Neutron Flux)×(Cross-section)×(Number of Nuclei)
Flux φ = Neutrons m −2 s −1 Number of Nuclei = N
Cross-section σ = effective area, unit is barn = 10 −28 m 2
Example: Reactor core contains 10 4 kg of uranium enriched to 2%
in 235 U. Cross-section for neutron induced fission of 235 U = 579 barns.
Flux φ = 10 18 m −2 s −1 . Calculate the power output.
Number of 235 U nuclei = 10 4 (1000/238)(6 ×10 23 )(0.02) = 5.0×10 26
R = φσN = 10 18 ×579×10 −28 ×5.0×10 26 = 2.9×10 19 s −1
Energy per fission = 200 MeV = 200×10 6 ×1.6×10 −19 = 3.2×10 −11 J
So power output = 3.2×10 −11 ×2.9×10 19 = 0.93 GW th .
10 4 kg U(2%) 5.0×10 26 × 3.2 ×10 −11 J = 1.6 × 10 16 J ≡ 0.5 GW th y
Fast Breeder Reactors (FBR)
Predicted fossil reserves
~ 8.10 22 J
Fission reactors (thermal neutron) ~ 4.10 21 J
Fast breeder reactors (fast neutrons) ~ 2.10 23 J
fast breeder reactors are possible long-term solution to world’s
energy needs (~10 3 years) - ~50 times fission reactor energy reserve
Fission reactors consume 235 U so < 1% uranium utilised
Fast breeder reactors have small core of highly enriched fissile fuel
with no moderator. Emitted fast neutrons convert surrounding 238 U
to fissile 239 Pu quicker than fuel consumed by fast neutron induced
fission in core.
Basic reactions
β − β − α
n + 238 U 239 U 239 Np 239 Pu 235 U
t 1/2
=23.5m t 1/2
=2.35d t 1/2
=24,000y
‘fertile’ ie
spawns Pu
‘fissile’ ie
chain reaction