Page 77 - Dynamics and Control of Nuclear Reactors

6.3 Samarium-149 poisoning 69

6.2.12 Xenon in molten salt reactors
Fission product gases can be removed continuously in molten salt reactors by simple
sparging. Consequently, these reactors do not have a major issue with Xe-135.

6.3 Samarium-149 poisoning
Samarium-149, also has a very large absorption cross section for thermal neutrons
4
(σ a 4.2 10 b). Sm-149 results from decay of Promethium-149. Pm-149 is a
daughter product of Neodymium-149. Nd-149 has a fission yield of 0.014 and it
decays into Pm-149 with a half-life of 1.7h. Since the half-life of Pm-149 is much
longer (47h), it is acceptable to treat Pm-149 as the direct fission product.
The equations for Pm-149 and Sm-149 variations are (modified from [2])
dP 0
¼ γ σ f Φ λ P P 0 (6.23)
dt P

dS 0
¼ λ P P S σ aS Φ (6.24)
0
0
dt
where
0
P ¼P/N f
0
S ¼S/N f
P¼Promethium-149 concentration
S¼Samarium-149 concentration
N f ¼concentration of fissile material
γ P ¼fission yield of Promethium-149 (0.014)
s )
λ P ¼decay constant of Promethium-149 (4.1 10 6 1
4
σ aS ¼Samarium-149 absorption cross section (5.3 10 b at 2200m/s and
4
3.36 10 b effective for moderator at 300°C)
As in the treatment of Xe-135, the steady state poisoning due to Sm-149 is given by
0
Poisoning¼S (σ aS /σ af )
The steady state value of S is given by
0
S ss ¼0.014 (σ f /σ aS )
0
Therefore
0
Samarium poisoning, S ss ¼0.014 (σ f /σ af )¼0.012
Poisoning of 0.012 reactivity units is equal to 1.8 dollars. Note that the steady
state Sm-149 poisoning is independent of the flux level.
Sm-149 buildup after startup is very slow because of the long half-life of Pm-149.
However, once Sm-149 reaches equilibrium, it remains unchanged thereafter.
Fig. 6.12 shows poisoning due to Sm-149 after startup of the reference reactor.