atw 2018-05v6

inforum

atw Vol. 63 (2018) | Issue 5 ı May

(a) Parameter variation: t cool

(b) Parameter variation: k

(c) Parameter variation: D 1

50 u(t,P) [°C]

40

30

20

10

0

0 1 10 10 2 10 3 10 4 10 5

50 u(t,P) [°C]

40

30

20

10

0

55 y

100 y

200 y

300 y

450 y

(d) Parameter variation: D 2

0 1 10 10 2 10 3 10 4 10 5

50 u(t,P) [°C]

40

30

20

10

0

7.6 m

9 m

11 m

15 m

asymp.

(g) Parameter variation: n, D 1 , D 2

27, 40 m, 7.6 m

9, 65 m, 12 m

9, 65 m, 15 m

1, −, 15 m

asymp.

0 1 10 10 2 10 3 10 4 10 5

| | Fig. 3.

Time [y]

Temperature evolution at P = P 2 for the

baseline configuration C a (black) and

alternative configurations (blue).

A-F: One parameter changed (parameter in

title); G: Three parameters changed.

Host rock composition as in Figure 2.

peak for D 1 = 40 m, this is reduced to

just 32 y to peak for D 1 = 60 m. The

envelope area required for siting the

repository increases proportionally

to drift spacing. This option requires

to increase the length of the access

tunnel to the drifts, while the total

length of the disposal drifts remains

unchanged. An increase from 40 m to

60 m increases space requirements by

50 % in envelope area. The cost and

side effects have to be weighed against

the benefit of a significantly lower and

earlier temperature peak. Also, with

more space available between drifts,

retrieval could be facilitated.

Increasing batch spacing D 2 from

7.6 m to 15 m leads to a remarkable

reduction of peak temperature at P 1 ,

cutting the amplitude by over 40 %

(Figure 2d, Table 3a). Peak time

remains roughly constant for small

D 2 and decreases abruptly between

11 m and 15 m. This option requires

doubling the total length of the disposal

drifts, while the number of

batches remains unchanged. The area

for disposal increases proportionally

50 u(t,P) [°C]

40

30

20

10

0

0 1 10 10 2 10 3 10 4 10 5

50 u(t,P) [°C]

40

30

20

10

0

k = 4

k = 3

k = 2

k = 1

(e) Parameter variation: n

0 1 10 10 2 10 3 10 4 10 5

Time [y]

n = 27

n = 7

n = 3

n = 1

to batch spacing. This cost has to be

weighed against a significant reduction

of peak temperature. Retrieval is

handicapped on the one side by longer

drifts and favoured on the other by

substantially decreased temperatures.

Passing from 27 to 3 disposal drifts

leads to a limited reduction of peak

temperature, while passing from 3

to 1 drift leads to a more significant

drop (Figure 2e). Accordingly, the

reduction in peak temperature per

row is slight for large n and important

for small n (Table 3a). Similar to the

effect of drift spacing, the temperature

peak shortens significantly. For

n = 3, the temperature peaks 40 y

after waste disposal (compared with

316 y in base line configuration).

Decreasing the numbers of drifts

significantly influences the geometry

of the repository; the area required

remains constant, but its envelope

becomes long and narrow. For low

values of n, the difficulty of “fitting”

the repository into a host formation

may increase. In panel e, the number

of drifts is reduced in steps from 27 to

1. Accordingly, their length increases

from 616 m (27 drifts) to 2.4 km

(7 drifts) to 5.6 km (3 drifts) and to

16.7 km for a single drift. The technical

effort consists in ensuring longer

operating times for the individual

drifts.

Finally, the repository can be

divided into clusters (in some cases, it

might have to, due to the presence of

determining geological features). The

clusters are assumed to be sited in

sufficient distance to remain thermally

unaffected from each other. The

effect is shown in Figure 2f. Thermal

benefits on peak temperature are

limited unless the clusters are made

50 u(t,P) [°C]

40

30

20

10

0

0 1 10 10 2 10 3 10 4 10 5

50 u(t,P) [°C]

40

30

20

10

0

(f) Parameter variation: s

Time [y]

40 m

50 m

60 m

80 m

asymp.

27 x 81

15 x 45

9 x 27

3 x 9

1 x 1

0 1 10 10 2 10 3 10 4 10 5

very small (i.e. holding well below

10 % of the total number of batches).

The required area per cluster decreases

proportionally to the number

of batches it holds, which can be an

advantage for “fitting” the repository

in perturbed sites. In panel f, the

disposal area is fractioned into clusters

of various size (indicated by n×m,

with s = nm| option / nm| baseline ). As

for panel c, this option requires to

increase the length of the access

tunnels.

Panel g presents results for

alter native configurations with n

decreased and D 1 , D 2 increased, other

parameters unchanged. The asymptotic

case is represented by the configuration

D 1 , D 2 → ∞. The hatched area

represents the scope for thermal

dimensioning below the temperature

curve belonging to the baseline configuration.

A large fraction of the

scope for thermal dimensioning is

available if triple the amount of space

with respect to the baseline configuration

is reserved, allowing for efficient

action on D 1 , D 2 and n.

Another point of interest is the

thermal benefit at the time horizon

10 4 y, when steel waste canisters are

expected to fail. At this point, the

ambient conditions in the canister core

change from dry to wet, thus enabling

liquid-phase reactions that were previously

excluded. For thermally activated

diffusion-reaction processes, the

rates depend on the ambient temperature.

While all engineering parameters

can be used to substantially reduce

the temperatures in first 10 3 y, this is

no longer the case in the following 10 4

y. Except for the asymptotic cases,

there is little response in temperatures

at the horizon 10 4 y for changes in t cool ,

DECOMMISSIONING AND WASTE MANAGEMENT 323

Decommissioning and Waste Management

Scope for Thermal Dimensioning of Disposal Facilities for High-level Radioactive Waste and Spent Fuel ı Joachim Heierli, Helmut Hirsch, Bruno Baltes

More magazines by this user
Similar magazines