Kinetics With Delayed Neutrons

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Kinetics With Delayed Neutrons. B. Rouben McMaster University EP 4P03/6P03 2008 Jan-Apr. Point-Kinetics Equations.

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Kinetics With Delayed Neutrons

B. Rouben

McMaster University

EP 4P03/6P03

2008 Jan-Apr

Point-Kinetics Equations
• In a previous presentation, we derived the point-kinetics equations, which govern the time evolution of the neutron density n and that of the delayed-neutron-precursor concentrations, Cg:
Case without Delayed Neutrons
•  is a very short time:
•   0.9 ms in CANDU
•   0.03 ms in LWR
• It is easy to see that if there were no delayed neutrons at all, the point-kinetics equations would reduce to
• Thus, without delayed neutrons, the neutron density would grow (or drop) exponentially as
Case with Delayed Neutrons
• Delayed neutrons change this significantly.
• To solve the point-kinetics equations, we can try exponential solutions of the form
The Inhour Equation
• We can divide by n to get an equation for .
• Eq. (6) is usually recast into another form (the Inhour equation) by substituting
Inhour Equation
• The Inhour equation is a complicated equation to solve in general, as the left-hand side is a discontinuous function which goes to  at several points.
• e.g., for G = 6 [from Duderstadt & Hamilton]
General Solution
• There are (G+2) branches in the graph of the Inhour equation, and (G+1) roots for  (intersections with line ).
• If  < 0 all roots will be negative, but
• If  > 0 one root will be positive (1), and all other roots will be negative
• The general solution for n and Cg is then a sum of (G+1) exponentials:
General Solution (cont.)
• By convention, we denote 1 the algebraically largest root (i.e., the rightmost one on the graph)
• 1 has the sign of .
• Since all other  values are negative (and more negative than 1 if 1 <0), the exponential in 1 will survive longer than all the others.
• Therefore, the eventual (asymptotic) form for n and Cg is exp(1t), i.e., the power will eventually grow or drop with a stable (or asymptotic) period .
General Solution (cont.)
• In summary, for the asymptotic time dependence:
• For  not too large and positive (i.e., except for positive reactivity insertions at prompt criticality or above):
• , i.e., things evolve much more slowly than without delayed neutrons
Solution for 1 Delayed-Neutron-Precursor Group
• If we assume only 1 delayed-neutron-precursor group, the Inhour equation becomes a bit simpler:
Solution for 1 Delayed-Neutron-Precursor Group (cont.)
• Now there are 3 branches and 2 roots for :
• When  < 0, both  values are negative
• When  > 0, one  value is positive, and the other is negative.
• When  > 0, one  value is 0, the other is negative.
• Again, we label the algebraically larger one 1.
• With 1 precursor group, the equation for  can also be written as a quadratic equation:
• which can be solved exactly:
Solution for 1 Delayed-Neutron-Precursor Group (cont.)
• If we substitute the form into the point-kinetics equations, we get

The general solution is

Solution for 1 Delayed-Neutron-Precursor Group (cont.)
• Using the values of 1 and 2 from Eqs. (14) & (13)
• The 2nd term decays away very quickly (typically in 1 s), therefore the neutron density (or flux/power) experiences a prompt jump or drop by a factor /(-) [this is good as long as  is not too large with respect to ]
Prompt Jump or Drop
• Illustration of prompt jump – prompt drop is similar:

(Lamarsh Fig. 7.4)

Relationships at Steady State
• The point-kinetics equations apply even in steady state, with =0.
• The relationship between the precursor concentrations and the neutron density can be obtained by setting the time derivatives to 0 in the point-kinetics equations. For G precursor groups at steady state (subscript ss):
• From Eq. (21) we get
• [Note: This relationship holds also at all points in the reactor.]
• Summing Eq. (22) over all g yields back Eq. (21), since
Will the Precursor Have a Prompt Jump?
• Eq. (18) gave us the general solution for the precursor:
• The 2nd term will decay away very quickly. Let’s evaluate the first term, using n1 and 1 from Eq. (19)
• Thus the precursor concentration has a smooth exponential behaviour, no prompt jump/drop.
More on the Prompt Jump/Drop
• The prompt jump or drop holds even if the reactor was not initially critical.
• Thus, each time there is a sudden insertion of reactivity, there is a step change in reactivity, the neutron density (or flux/power) will change by a factor /(-).
• Following the prompt jump/drop, the time evolution of the neutron density (or flux or power) will be according to the stable period 1/1.
Prompt Criticality
• The condition  =  corresponds to:
• This means that in this case, even if we ignore the delayed neutrons ( ), keff will be = or >1, i.e., the reactor is critical on prompt neutrons alone. This isprompt criticality.
• The delayed neutrons then no longer play a crucial role, and when  increases beyond (prompt supercriticality),very very short reactor periods (< 1 s, or even much smaller, depending on the magnitude of ) develop.
• Thus, it is advisable to avoid prompt criticality.