Transmutation Calculation

Transmutation performances of the hydride MA target and related core features in FBRs have been evaluated with the method shown in Table 16.2. A three­dimensional continuous energy Monte Carlo Code MVP [9] and MVP-BURN [10] are used as burn-up calculations for evaluating the transmutation of MAs. The cross-section library applied in the calculations is JENDL-4.0, which is processed to be adjusted to the MVP code. In the burn-up calculation, the prediction-correction method is employed to improve accuracy with millions of neutron histories for the criticality calculation, where the accuracy of Eigen value is about 0.04 %.

Table 16.2 Calculation method for MA transmutation

Items

Methods

Notes

Computation

method

Three-dimensional continuation energy Monte Carlo analysis code; MVP (burn-up routine is MVP-BURN)

1,200,000 neutron histories with 120 batches. Initial 20 batches are run to establish the initial neutron source distribution

Nuclear data

JENDL-4.0 library

Calculation

model

Pin heterogeneous model

Table 16.3 Comparison of reduction ratio of MAs

Target

Loading mass (kg)

Reduction mass (kg/year)

Reduction ratio after 1 year

Effective half life (year)

Case1:

MA-hydride

335

91.1 (33.0)a

0.272

2.19

Case2:

MA-metal

335

27.6 (9.4)a

0.082

8.07

Ratio: Case1/ Case2

1.00

3.30

3.30

0.27

aValues in parentheses are reduction masses by fissions

Calculations have been done for two kinds of transmutation target. In case 1, the transmutation target was the MA hydride of (MA01, Zr09)H16. Calculation with metal MA0.1Zr0.9 target without H was done in case 2. The results of calculations are summarized in Table 16.3, where effective half-life is defined as the time such that the residual amount of MA is decreased to half of the MA loaded during the burn-up. The effective half-life is calculated to be 2.19 years in case 1 and

8.7 years in case 2, mainly because of the softened spectrum effect induced by the MA-hydride. The transmutation rate of the MA-hydride target is about three times higher than that of the MA-metal target. Figure 16.4 shows the change of total MA and each element of MA in the MA-hydride target with increase of time. Major elements in MA, that is, Np and Am are decreased simultaneously during the burn — up. The contribution of long-lived Cm (245Cm and 246Cm) is much smaller than that of Np and Am. The change of total MA in the MA-metal target is also shown in Fig. 16.4 for comparison.

The major mode of the transmutation in the present method is not fission but neutron capture (see Table 16.3). As shown in Fig. 16.5, Am and Np are mainly transmuted to Pu because of neutron capture, beta decay, and alpha decay. Recycled Pu is used as a driver fuel in this reactor.

image113

Fig. 16.4 Change of each element in MA assemblies

 

242Cm *

 

* 244Cm *

 

243Cm —n;—

 

245Cm

 

246Cm

 

242mAm

 

26msec

 

242Am 0

 

241Am

 

243Am

— ГГ7

 

244mAm

 

-Л4.4у

 

image224

* 240Pu 0

 

238Pu

 

239Pu

 

^236d

 

image225

237Np

 

238Np

 

239Np

 

image226
image227

235U

 

236U

 

238U

 

Fig. 16.5 Chain transmutations for actinide nuclides

 

image114

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