Revision of the JENDL FP Fission Yield Data

Some fission yields data of JENDL FP Fission Yields Data File 2011 (JENDL/FPY-2011) revealed inadequacies when applied to delayed neutron related subjects. The sensitivity analyses of decay heat summation calculations also showed some problems. From these results the fission yields of JENDL/FPY-2011 have been revised. The present report describes the revision of the yield data by emphasizing the sensitivity analyses.


Introduction
The JENDL/FPY-2011 file [1] was released in 2011.After the release of the file, several feedbacks from the users of the file were reported.One of them is from delayed neutron application [2] where it was shown that the yield of 86 Ge played significantly.The sensitivity analyses of decay heat summation calculation also showed inadequacies of some fission yield data.
The sensitivity analysis method of decay heat summation calculation has been developed [3] for the estimation of the uncertainties emerging from nuclear data used.In the course of the sensitivity analyses of fast neutron fission, the comparison of the uncertainties between fast and thermal neutron fissions was performed and it was found that the uncertainties of the calculated decay heat for thermal neutron fission were unexpectedly larger than those for fast neutron fission at short cooling times after a fission burst when the JENDL/FPY-2011 file was employed.During the search for the reason, it was found that the fission yields of the nuclides contributing to the large uncertainties of the decay heat calculations seemed to be too large.The yield data of the nuclides which showed unexpectedly large values were revised.
In the present paper, the comparison of the uncertainties between the thermal and the fast neutron fissions and the revision of the JENDL yield data are described.

Comparison of decay heat uncertainties for thermal and fast neutron fission
The uncertainties of decay heat summation calculations were derived for thermal and fast neutron fissions using JENDL FP Decay Data File 2011 (JENDL/FPD-2011) and JENDL/FPY-2011.fission, the uncertainties of the fast neutron fission and the thermal neutron fission are not so different from each other.However, in the case of 235 U fission, the uncertainties of the thermal neutron fission are much larger than those of the fast neutron fission at shorter cooling times than 0.1 s.The nuclides contributing to the uncertainties at the short cooling times are shown in Figure 2 as a time depending sensitivity.On the left-hand side, the nuclides with large sensitivity for 235 U thermal neutron fission are indicated.On the right-hand side, those for 235 U fast neutron fission are indicated.As seen in the figures, the nuclide of 86 Ge has large contribution to the thermal fission but no significant contribution to the fast neutron fission.The contributions of 88 As and 100 Rb are not significant but sizable contribution to the uncertainties of the 235 U thermal neutron fission and no contribution to incident neutron energy, it is true that the yields are different from each other for thermal and fast fission.However, the differences shown in Table 1 seem to be too large.The charge distribution of the same masses as those nuclides, that is, A = 86, 88 and 100 are shown in Figure 3. From the yields corresponding to 86 Ge ( Z = 32 for A = 86), 88 As ( Z = 33 for A = 88) and 100 Rb ( Z = 37 for A = 100) look like abnormally large comparing with other yields.

Fission yields of JENDL/FPY
The fission yield data of JENEL/FPY-2011 are based on the evaluation of ENDF/B-VI [4].Although some modifications were carried out for the compilation of the JENDL file because of the differences of decay chains and isomers adopted in the JENDL and ENDF files, basically the independent yields of the JENDL file were taken from those of ENDF/B-VI.As for the 86 Ge case, the problem of the ENDF yield data was claimed previously [5].And recent analysis of delayed neutron also pointed out that the yield data of 86 Ge of JENDL/FPY-2011 severely affect the reactivity analysis of a reactor [2].
The independent fission yields of JENDL and ENDF are listed in Table 2.As pointed out above, the yield value of the JENDL file is similar to that of ENDF.As the ENDF/B-VI yield data are based on the reported ones in the report of LA-UR-94-3106 [4] which contains detailed data sources used for the evaluation work.In the appendix A of the report, the measured cumulative yield of 86 Ge by 235 U thermal neutron fission is listed to be 0.87 ± 0.12 % as listed in Figure 4.The red line shows the yield of 86 Ge with the reference of 90rud1.The reference designated as 90rud1 is Radiochimica Acta 49, 155-191 (1990) [6].However, the value of 0.87 ± 0.12 for 86 Ge is not found in the original paper.There is no yield data of 86 Ge in the paper.The isotope WONDER-2015  whose measured yield is 0.87 ± 0.12 % is 86 Se, not 86 Ge.So the value of the appendix A of LA-UR-94-3106 seems to be incorrectly cited.As the cumulative yield of 86 Se seems to be mistaken to be that of 86 Ge, the independent yield of 86 Ge would be also adjusted to give the mistaken cumulative yield.
As for other nuclide yields, similar mistakes were considered to have happened.Then the independent yields of the nuclides mentioned above were discarded and newly calculated to give new yield data of JENDL.

Calculation of independent yield
The independent fission yields were calculated following the equations in the report of JENDL/FPD-2011 [3].The independent yield y i (A, Z) of a nuclide with charge Z and mass A is expressed by: where Y(A) is the mass yield, f (A, Z) the fractional independent yield, F x (A, Z) the pairing and oddeven correction factor and r i (A, Z) the isomeric yield ratio.The fractional independent yield f (A, Z) was calculated by: where Z p (A) is the most probable charge of mass A, σ A the Gaussian width of mass A and N(A) the normalization factor.The normalization factor N(A) is given as the summation of all elements of each mass becomes 1.0.
The value of Z p (A) and σ A were taken from LA-UR-94-3106.
The new charge distributions of A=86, 88 and 100 are shown in Figure 5. Comparing with Figure 3, the abnormal behaviors of 86 Ge, 88 As and 100 Rb seem to be disappeared.The new values were compared with those of the JEFF evaluated values [7], too.The new values and those of JEFF evaluation are listed in Table 3.It is found out that the new JENDL values are same order as those of JEFF.

Figure 5 .
Figure 5. Charge distribution of new fission yields for A = 86, 88 and 100.
total uncertainties of the calculated decay heat values at short cooling times after a fission burst are shown in Figure1.In this figure, the results of 235 U and 239 Pu fissions are shown.In the case of 239 Pu The a e-mail: j_katakura@vos.nagaokaut.ac.jpDOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 201

Table 1 .
those of the fast neutron fission.The independent fission yields of those nuclides for the thermal neutron fission of 235 U in JENDL/FPY-2011 are listed in Table1.As the fission yields depend on Comparison of independent fission yields of JENDL/FPY-2011.
Figure 2. Important sensitivity coefficients at short cooling times after fission.EPJ Web of Conferences 08004-p.2