Fission Product Yield Data for the Transmutation of Minor Actinide ...

the fissioning nucleus and the number of
neutrons emitted by fragments at symmetry;
(h) The multiplicity distribution of neutrons
emitted from fragments is crucial for the
correct calculation of pre- and post-neutron
emission mass distributions — this quantity
should be carefully investigated and
evaluated, and more measurements are
needed to derive more reliable systematics;
(i) Assumptions made about neutron distributions
in calculations could be checked against
measured pre- and post-neutron emission
mass distributions and adjusted accordingly.
If the effects of multi-chance fission are part of
the calculations (purely theoretical approaches), the
following parameters need to be chosen with care:
(1) Parameters and penetrability of the fission
barrier;
(2) Contributions of multi-chance fission
components as functions of incident energies;
(3) Energies at which the asymmetric fission
modes disappear;
(4) Preferred fission modes of different fissioning
nuclides as a function of excitation energy;
(5) The number of neutrons emitted before and
after scission, but also between saddle and
scission;
(6) Neutron multiplicity distributions from
fragments.
5.2.3.5. Remarks on systematics
and theoretical models
Our present knowledge of the expected
impacts on the observed mass distribution is insufficient
to predict the real shapes. There are neither
sufficiently reliable experimental data nor confident
predictions by systematics and theoretical models.
Hence, we can only say that the assumptions
associated with shape symmetry and the calculation
of the point of mass symmetry are incorrect, but we
cannot tell whether and at what energies the error is
significant.
At lower energies up to 30 MeV, the
deviations from Gaussian and shape symmetry are
probably so small that the uncertainty is negligible
compared with all other uncertainties. Discrepancies
up to 2.5% absolute yield around the peaks
and in the slopes of the valley are mainly due to
differences in the positions of peak maxima, the
peak shapes and the widths of valleys, while the
peak heights are less discrepant. At still lower
energies up to 10 MeV, structure in the peaks should
be visible due to the two fission modes ST-1 and ST-
2, but in several models this effect cannot be
detected because the peak functions are too broad.
Furthermore, the experimental results are not
conclusive because of differences in the observed
structure. A detailed analysis should clarify the
shape and position of the structure and the energy
at which ST-1 starts to become insignificant.
The survey plots of the mass distribution
parameters between 13 and 20 MeV have revealed
deviations from the smooth energy dependences for
some calculations. The nature of these ‘irregularities’
could not really be explained, although they
may arise from the onset of a new fission chance.
Further investigations are strongly recommended.
At energies above 30 MeV, the predicted
shapes are discrepant. At the present time there is
little indication of what the correct shapes should be
and which model gives the best predictions. All that
can be noted is that the calculations of Wahl,
Katakura and the theoretical model predictions
seem to be far from reality, even considering the
probable necessary adjustments to the experimental
data. The following questions need to be clarified in
a detailed analysis, particularly for this high energy
region:
(a) How significant is the expected broadening of
the peaks?
(b) At what energy does the symmetric peak start
to become significant and dominate?
(c) Are three peaks visible at a certain energy, or
does the broadening of the peaks due to
multi-chance fission result in a broad and flat
plateau?
5.3. CONCLUSIONS AND
RECOMMENDATIONS
Accuracy requirements for fission yields in
waste transmutation studies are ill defined. We
assume that only fission yields ≥2% are important,
and should be known to about 25% relative
accuracy, amounting to roughly 1.5% absolute yield
uncertainty for peak fission yields of 6–7%. At low
energies for 238 U, where agreement is better, the
discrepancies among calculations are 2.5% in many
cases. Clearly for those nuclides included in part B
of the benchmark exercise the agreement is worse,
even without detailed analyses. Therefore, the
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