Title:

Maximisation of the doppler effect in thermal reactors


Author(s):



Published by:

Publication date:

ECN
NUCLEAIR

1998


ECN report number:

Document type:

ECNR97007

Other


Number of pages:

Full text:

57

Download PDF

Abstract:
Increase of the fuel temperature in a nuclear reactor leads, or can lead,to (1) A Doppler broadening of the resonances of the nuclides in the fuel;
(2) An expansion of the fuel; and (3) A shift of the Maxwellian part of the
spectrum to higher energies. These processes together introduce a certain
amount of reactivity, which can be expressed in the socalled fuel
temperature reactivity coefficient. The reactivity effect of the third
process is very small, because the Maxwell spectrum is to a major extent
determined by the moderator temperature. Moreover, the reactivity effect due
to an expansion of the fuel is small too, for most thermal systems. When the
second and third processes can be neglected, the fuel temperature reactivity
effect is fully determined by the Doppler effect. The fuel temperature
reactivity coefficient is then called the Doppler coefficient of reactivity.
The Doppler broadening of the resonances causes an increase of resonance
absorption, due to a decrease of selfshielding. The competition between
resonance fission at the one hand and resonance capture at the other hand
determines the sign and magnitude of the reactivity induced by an increase of
the fuel temperature. In welldesigned nuclear reactors the Doppler effect
due to resonance capture by fertile nuclides exceeds the Doppler effect due
to resonance fission, which implies that an increase of the fuel temperature
causes a negative reactivity effect and a correspondingly negative Doppler
coefficient. Since the Doppler effect is a prompt effect, occurring
simultaneously with the dissipation of kinetic energy of the fission products
into temperature, it is very important in the study of rapid power
transients. In this report, the Doppler coefficient of reactivity is defined
in chapter 2. Chapter 3 discusses the geometry of the unitcell for which the
calculations are performed and describes the fuel types that have been
investigated. In chapter 4 the 'Doppler efficiency' is introduced and three
methods by which it can be calculated are presented. Chapter 5 discusses the
results of the calculations of the Doppler efficiency, based on both the
NR(IM)theory and the Nordheim Integral Method. Chapter 6 presents the
results of the calculations of the Doppler coefficient of reactivity under
the constraint of constant k#infinity# In this calculation, the Doppler
coefficient of reactivity is calculated for different configurations of the
unitcell, but all yielding the same k#infinity#. This is done for both
realistic fuels and artificial fuels. For the latter the fissile resonance
absorbers are replaced by artificial 1/v fissile nuclides in order to isolate
the resonance absorption effects caused by the dominant resonance absorbers.
The conclusions with respect to the maximisation of the absolute value of the
Doppler coefficient are presented in chapter 7. The appendices are auxiliary
to chapter 4. 21 refs.
Back to List