Effects of process-generated hydrogen on RPV walls

4.4. Radiolysis of the primary water

4.4.2 Chemical reactions between radiolytic species Besides the production of radiolytic species, the reactions between the species has a big influence on the chemistry in the primary water. As such it also influences the H-concentration in the water and therefore also the partial pressure of H 2 in the RPV. Similar to the production rate of species by radiolysis, an expression can is available for the production rate of species by chemical reactions. Reaction rate theory has shown the production rate is:

N X j =1

M X m,s =1

N X j =1

M X s =1

i =

(4.12)

R c

k j c s c m − c i

k j c s

where R c i is the chemical generation rate of species i, mol/m 3 s, k constant, in m 3 /mol · s for the chemical reaction j. c i , c s and c m are the concentrations of species i, s and m, in mol/m 3 , respectively, and N is the total number of different reactions considered in the model, while M is the total number of different species. [54] The rate constant, k , is a temperature dependent property of the reaction. This temperature dependency is expressed by the Arrhenius law: j is the reaction rate

R

1 T

1 T 0 −

E a

k = k 0 exp

(4.13)

where k 0 is the reaction rate constant at a temperature, T 0 , E a is the activation energy corresponding to that reaction in Jmol -1 , R is the universal gas constant, 8.3145 Jmol -1 K -1 and T is the temperature in Kelvin. This so called “activation energy” can be interpreted as the minimum energy required for the colliding species for the chemical reaction to take place. [48] The combination of both the radiolytic and chemical production rate of each specie gives the resulting concentration. The total production rate of each specie in the primary water is then given by: k si C s # (4.14) The concentration of H and H 2 will be of major interest as they influence the final concentration of hydrogen in the RPV wall steel. R i = G γ i Γ γ 100 + G n i Γ n 100 + G α i Γ α 100 ! ˜ Fρ N v + " N X s =1 N X m =1 k sm C s C m − C i N X s =1

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