Page 29 - Dynamics and Control of Nuclear Reactors

20 CHAPTER 3 The point reactor kinetics equations

There are differences between the delayed neutron data for thermal fissions and fast
fissions in U-235, U-233 and Pu-239, but they are small.
The reader should be aware that different references report slightly different
values for delayed neutron data. These differences have little influence on practical
reactor simulations. Also, note that Pu-239 and U-233 have much smaller delayed
neutron fractions than U-235. This has an impact on dynamic behavior.
A reactor fueled with low-enrichment U-235 produces Pu-239 by neutron capture
in U-238. Consequently, the ratio of Pu-239 to U-235 increases as the reactor oper-
ates and the “effective” delayed neutron fraction decreases.
A single delayed neutron group is often used for approximate calculations. In this
case, approximate average values for the effective delayed neutron fraction and
effective delayed neutron decay constant are required. Typical values for a U-235
fueled reactor are 0.0067 for the effective delayed neutron fraction and 0.08s 1
for the effective decay constant (see Section 4.6 for calculating the effective decay
constant). The main utility of the model with a single delay group is that it provides a
simple, easy-to-implement tool for illustrating the features of reactor transients. We
will use the model with a single delayed neutron precursor group for illustrating some
features of a reactor transient, but the full six-group model will be used in simulations
to illustrate reactor maneuvers.

3.2.2 Photoneutrons from nuclei excited by gamma rays
Several isotopes can produce photoneutrons by interaction with high energy gamma
rays produced in fission reactions. The materials that can produce neutrons by inter-
acting with fission gamma rays are deuterium, lithium, beryllium, and carbon. These
materials have a small enough binding energy (all have binding energies of 7.25MeV
or less) to enable photoneutron-producing reactions with gamma rays having ener-
gies that occur during fission reactions. Fission gamma rays are not energetic enough
to cause (γ, n) reactions in other materials.
In photoneutron production, the high energy gamma ray causes the target nucleus
to transition to an excited state. This excited state persists until the nucleus emits a
neutron. The process of neutron emission follows the law of radioactive decay as
characterized by a half-life. The different photoneutron producing nuclei have
half-lives ranging from around 2.5s to 12.5days.
The photoneutron yield is much smaller than the delayed neutron yield from
fission products, but the half-lives of some of the photoneutron precursors are
much longer than the precursor half-lives of delayed neutrons from fission
products. Consequently, photoneutron production can become larger than delayed
neutron production from fission products after time passage following shutdown
of the reactor. This occurs after several minutes after shutdown in CANDU
reactors.
Photoneutrons are produced in reactors that use Deuterium by the (γ, n) reaction
between Deuterium and 2.225MeV gamma rays. The coolant and moderator in
CANDU reactors are almost pure D 2 O. Even light water reactors have a small