Signos motivados

In Section 10.4.2 of TR 11-01 summarizes SKB‘s treatment of the periglacial and glacial cycles. As the

region enters the periglacial time domain SKB assumes Forsmark is shows biosphere characteristics

similar to those of the later parts of the initial temperate period, i.e. the landscape will consist of terrestrial

ecosystems, mainly forests and mires, with few or no lakes and no sea. Parts of the area, especially those

with fine-grained sediments in central Öregrundsgrepen, can potentially be used for long-term agriculture.

Small areas with mainly organic soils are assumed to be cultivated for limited periods. SKB also assumes

the pattern for discharge of deep groundwater, as well as the conditions determining transport and

accumulation of radionuclides in the landscape will be similar to those prevailing during the late part of the

initial temperate period.

IRT finds all these assumptions reasonable and consistent with assumptions made for the initial temperate

period. The general assumption of the repeatability of future glacial cycles is common to performance

assessments.

The periglacial time periods are assumed to involve freezing of almost all the land surface with only

limited, unfrozen areas called taliks. Taliks often occur under lakes or rivers in the permafrost region, and

are the only spots in the periglacial landscape where radionuclides released from the repository can be

transported up to the biosphere. Given that lakes and streams often are locations for human settlement and

land use, SKB posits that taliks are potentially the locations where humans could be exposed to

radionuclides upwelling from the geosphere. SKB reasonably assumes the generally low productivity in

the permafrost region would require use of larger areas to supply the resources needed by even a small

community. Therefore, IRT understands that SKB assumes humans living near taliks would derive some of

their food from contaminated groundwater with higher concentrations of radionuclides than might be found

during interglacial periods, but that not all their food would come from this area. Unlike the interglacial

time period for which SKB seems to assume 100% of the critical group food sources come from

contaminated areas

, SKB assumes some of the periglacial climate critical group‘s food supply may be

more contaminated than that for a critical group living in a temperate climate, while other parts of the

periglacial climate critical group food supply would be completely uncontaminated. Hence, the average

dose to the periglacial climate critical group might be similar to that of the critical group living in the

interglacial periods.

This seems reasonable for bioaccumulation in food, but does not address the potentially larger contribution

to dose from drinking water assuming future humans drink water exclusively from the taliks, which seems

like it would be the only place where unfrozen water would appear at the surface. Hence, IRT

recommends SKB provide:

 A rationale why the periglacial critical group would also be able to derive part of their drinking

water supply from areas other than the talik; or

 Consider potentially a higher drinking water dose to the critical group for the periglacial periods; or

 Reconsider the appropriateness of assuming the periglacial climate critical group engages in the

same combination of agricultural practices and hunting/gathering assumed for the interglacial

critical group.

For glacial conditions, the IRT agrees with SKB that the only situation under when humans or other biota

may be exposed to high concentrations of radionuclides from the repository is when the retreating ice-front

is situated near the Forsmark area and the area is submerged. SKB also notes that no long-term

radionuclide accumulation is likely during glacial conditions due to the rapid turnover rate of groundwater

in the retreating ice front reservoir. It was nevertheless useful that SKB considered the potential existence

of radionuclide pathways to humans during the glacial periods.

IRT is assuming this about SKB‘s assumptions about the temperature period from the words SKB uses to describe the critical group behavior for the periglacial time periods, although an explicit statement by SKB to this effect was not found by the IRT.

SKB also assumes the main uncertainties in the landscape development during the remaining part of the

reference glacial cycle are essentially the same as those dominating during the initial temperate period, i.e.

1) the configuration of the landscape, 2) the timing of different events, and 3) the composition and

properties of species and communities inhabiting the future landscape. The IRT agrees that even though it

is impossible to describe in detail the landscape development during a complete glacial cycle, the

systematic landscape analysis and the approach for estimating doses encompasses most of the potential

future landscape configurations for the reference glacial cycle.

For the global warming climate, SKB states that: ―the prerequisites for transport and accumulation of

radionuclides in the biosphere during temperate periods of the global warming variant are assumed to be

similar to those in the initial temperate period of the reference evolution.‖ This also seems a reasonable

assumption, if true. The IRT evaluated whether the differences in biosphere LDFs for both the current

interglacial and the hypothesized global warming biosphere were significant in terms of the calculated total

dose rate versus time. Global warming is considered by SKB to have an effect on temperature (increase of

the annual air temperature of 3.5°C and increase in mean annual precipitation by 20 mm). This effect has

been examined by SKB through specific LDF‘s (landscape dose conversion factors). Figures 13-7 and 13-

8 in TR-11-01 show that the global warming biosphere LDF‘s can be higher than during the interglacial

period (i.e., agriculture in temperate conditions) for a few radionuclides, but are very close to the

interglacial LDF‘s, i.e., not more than a factor of 10 for two elements (Cs-135 and U-238). SKB considers

that a tenfold increase of these nuclides would not affect the final risk. The IRT evaluated how important

the dose contributions from U-238 and Cs-135 are to the overall dose assessment. For example, Figures

13-15 and 13-16 in TR-11-01 provide the contributions to the total estimated dose rate for those

radionuclides contributing the most to the total calculated dose rate. In the text for these figures, SKB

notes: ―The legends are sorted according to descending peak annual effective dose over one million

years‖. Since neither of these two radionuclides appears in these figures, neither radionuclide contributes

more than a small amount to the total dose estimates. As a result, dose/risk assessments are performed for

a single biosphere representative of the interglacial climate. While the justification for this could be better

documented, via, for example, a calculation case using the global warming LDF‘s, the IRT agrees that

assuming the LDFs for the interglacial climate is justified.