20.08.2015   Nuclear fuel cycle science and engineering

Transmutation of long-lived fission products (LLFPs)

With respect to the transmutation of FPs, much less work has been performed in the last decades. In part this is because, compared to the transuranics, their radiotoxicity is lower and shorter lived so that, after about 250 years, most of them have decayed. Nevertheless, some FPs are very long lived and may be mobile in the environment so that, often, they are major contributors to the very long-term radiological impact of deep geological disposal. The long-lived FPs (LLFPs) that deserve most attention in this respect are 2 29I, 135Cs, 79Se, 99Tc and 126Sn (see Table 17.5).

Подпись: Woodhead Publishing Limited, 2012

Reactor type

PWR

FR

ADS

‘ч Fuel type Parameter

MOX (Pu only, reference)

Homog

TRU

recycle

Pu only

Homog. TRU recycle, CR = 1 and MA/Pu ~0.1

Homog. TRU recycle, CR = 0.5 and MA/Pu ~0.1

Homog. TRU recycle, CR = 0.5 and МА/ Pu ~1

MA targets

(Heterog.

recycle)

U-free fuel CR = 0 and МА/ Pu ~1

Decay heat

1

x3

x0.5

x2.5

x12

x38

x40

x100

Neutron source

1

x8000

~1

x150

x1000

x4000

x5000

x20000

 

512 Nuclear fuel cycle science and engineering Table 17.5 Properties of the main long-lived FPs (LLFPs)

Isotope Half-life Type of Thermal Dose Fraction in

(y) decay power (ingestion) irradiated fuel

(W/Bq) (Sv/Bq) (g/t)*

14C

5.7

X

103

в

1.6

X

10-14

5.7

X

10-10

1.3

X

10-1

36Cl

О

CO

X

105

в~, в+

4.4

X

10-14

см

об

X

10-10

1.6

X

100

79Se

6.5

X

104

в

6.5

X

10-15

со

см

X

10-9

4.7

X

100

93Zr

1.5

X

106

в

2.6

X

10-15

см

‘sT

X

10-10

со

00

X

101

99Tc

2.1

X

105

в

1.4

X

10-14

сб

X

10-10

см

об

X

102

107Pd

6.5

X

106

в

1.4

X

10-15

Г—

сб

X

10-11

X

см

102

126Sn

1 X

105

в

СМ

‘si1

X

10-14

5.1

X

10-9

2.0

X

101

126Sb

‘si;

CO

X

10-2

в

сл

о

X

10-13

6.9

X

10-6

129I

1.6

X

107

в

1.3

X

10-14

7.4

X

10-8

1.7

X

102

135Cs

CO

cm

X

106

в

9 X

10-15

1.9

X

10-9

1.3

X

103

151Sm

CO

о

X

101

см

со

X

10-15

9.1

X

10-11

1.6

X

101

Note:

*PWR-UOX (3.5% U-235 enrichment, BU = 33 GWd/t)

Unlike transuranics, FP transmutation does not produce supplementary neutrons as fission products are neutron consumers. LLFP transmutation therefore requires a large neutron surplus to be available. If this is expressed in terms of the fraction of a reactor fleet that would be needed to perform the LLFP transmutation, the value comes out at between 8 and 15%,23 i. e., a very large (and, very probably, unrealistic) figure.

Finally, it should be emphasized that the short-term heat production is essentially related to 90Sr and 1 37Cs, which are not candidates for transmutation because their relatively short half-life (~30 years) is such that no transmutation process can provide a comparable ‘transmutation half-life’, (evaluated as the product of the microscopic capture cross section of the isotope considered times the neutron flux). One way to handle these two isotopes could be to recover them with an appropriate chemical process, then store them and let them decay.

In summary, the transmutation of the so-called LLFPs is no longer envisioned by any major international program.