Nuclear Science and Engineering Research

8-1

Method of Accurately Designing New Types of Reactors－ Advanced Method Using Experiments (Extended Cross Section Adjustment) －(a) Cross-section adjustment(conventional method)(b) Extended cross-section
adjustment

True
design value

True
design value

Designvalue

Design value calculatedby the true
Designcross-section setvalue○

Design value calculatedby the true
cross-section set
Consideration of
the target corecharacteristics○○○

Design value
without correction○○
Experimental data○
Design value
without correction

Table 8-1 Comparison of prediction accuracy
The “prediction” is the ratio of the design value to that without
correction. The “uncertainty” represents the sum of the design
value uncertainty components induced by the experiment,analysis method, and nuclear data. The extended cross section
adjustment improves the prediction accuracy more efficiently
than the conventional method.

Design target
coreparameters(a) Cross-section
adjustment(conventional method)
Prediction○(b) Extended crosssection adjustment

Uncertainty
Uncertainty
Prediction(%)(%)

Experimental data

Fig.8-4 Features of the new adjustment method
The conventional method (a) aims to produce a design value
close to the calculation value by using the directly unknowable
true cross section set. In contrast, the present method(b) makes the design value closer to the true design value by
considering the target core characteristics.

To design a new-type of reactor, it is necessary to accurately
predict the nuclear characteristics of the reactor coreparameters, which are determined by the reactions between
neutrons and nuclei. These core parameters can be predictedby nuclear data and computer codes. The former describe thefeatures of the nuclear reactions, whereas the latter analyze thebehavior of neutrons. The accuracy of the predicted value canbe validated with a full-scale, well-simulated experiment, aso-called mock-up experiment. However, such an experiment
would be very difficult because it incurs huge costs andrequires full-scale nuclear fuels.
To solve these problems, a design method called crosssection (set) adjustment has been used in fast reactor core
design study. The cross section set consists of nuclear data
converted to be applicable to the computer codes. The crosssection adjustment method based on the Bayes theorem
enables us to improve the prediction accuracy of the coreparameters by adjusting the cross section set to existing
experimental data. In this method, the target core parameters
calculated by the adjusted cross section set are used directly as
the design values. This method, therefore, has the advantagethat all the design values are physically consistent with eachother.
On the other hand, another design method called theCriticality

0.99770.30

0.99820.21

Powerdistribution

1.000

1.4

0.996

0.9

Control rod
worth

0.969

1.4

0.968

1.1

Improved prediction accuracy

extended bias factor method was developed in a previous
study by the Japan Atomic Energy Agency. This method has
the advantage that it can give the most accurate design values.
However, the extended bias factor method does not attempt tomaintain consistency among the design values.
In the present study, a new design method called extended
cross section adjustment is developed. The new method
theoretically integrates the two existing design methods
mentioned above. The conventional adjustment method altersthe cross section set in order to maximize its correctness byoptimizing it to the existing experimental data. In contrast,
the present method adjusts the cross section set to minimize
the uncertainty of the design value by taking into account thetarget core characteristics. Therefore, it is possible to improve
the prediction accuracy further (Fig.8-4). By applying thenew method to a typical large fast reactor design analysis, it isdemonstrated that the prediction accuracy is improved more
efficiently than in the conventional method (Table 8-1).
The new method not only clarifies the theoretical relation
between the conventional cross section adjustment method
and the extended bias factor method, but also successfully
improves the prediction accuracy. In the future, the new
method will be used for a reactor design study as a design
method that efficiently improves the prediction accuracy.

Reference
Yokoyama, K. et al., Extended Cross-Section Adjustment Method to Improve the Prediction Accuracy of Core Parameters, Journal of Nuclear Science andTechnology, vol.49, no.12, 2012, p.1165-1174.

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JAEA R&D Review 2013