Bioshield concrete

Due to geometric (shielding) differences the specific activity at the inner surface of the bioshield of a Loviisa unit will be about two orders of magnitude greater than that of a TVO reactor. To some extent the nuclide-wise distributions are different, too. In the Loviisa bioshields H-3, Fe-55, Co-60 and Ni-63 are the most important radionuclides from the point of view of a prompt dismantling, which at present is the preferred option for the Loviisa reactors. In the long-term the activity will be determined by Ca-41 and to a smaller extent C-14, Cl-36, Ar-39 (if it remains in the concrete) and Ni-59. In the bioshield concrete of the... 

Calcium-41 Ca-41 is produced by neutron activation of natural Ca-40. It has been found to exceed the GQ by a factor of 2 in both graphite fuel struts and desiccant from HNA (3 streams). The reported desiccant value is an upper limit probably based on trace contamination by graphite dust. In decommissioning wastes, activation of the concrete bioshield would also be expected to produce Ca-41 but these wastes streams are regarded as low level wastes in the NIREX inventory and hence GQ values do not apply. Measurements of Ca-41 can be obtained after chemical separation of Ca, which is done routinely for Ca-45 measurements. After any Ca-45 (t 14 =163 days) has decayed away, it can be measured by liquid scintillation counting. Procurement of direct standards from NPL would be required. In fresh samples, if the Ca-45 has been measured, then the Ca-41 could be estimated by comparison of activation... 

The most obvious application of this method is in waste characterisation associated with nuclear decommissioning, where activated concrete bioshields are a potential source of activity. A core from a bioshield has been analysed using gamma spectrometry, analysis and determination. The core used was too old for any " Ca to be detected, but... 

Figure 4. Distribution of activity in activated concrete bioshield core.

At this point in time only a limited comparison between the DOT 3.5 and MCBEND predictions has been possible. A limited comparison of these results for the bottom com concrete bioshield region of the reactor vault is given in section 4 which shows good agreement. 

The Monte-Carlo calculations have only lecMitly been completed and there has been a limited amount of time for collaboration between Fuji and NNC to compare results. Consequently at this point in time the results available for publication are limited to a comparison of the thermal neutron fluxes extending radially through the bulk concrete bioshield at the level of the bottom of the core (along the line x - x in Figure 12). 

When compared with other studies, e.g. [7], it has been concluded the concentrations of the most important elements in the bioshield materials of the Finnish nuclear reactors are now known from the point of the activation inventory calculations. The main uncertainty concerns the actual water content of the concretes, because the samples were not from the irradiated parts of bioshields. 

Figure 1. Inner surface activity in the bioshields (serpentine concrete) of the Loviisa reactors 151. 

Figure 2. Inner surface region activity in the bioshields of the Olkiluoto reactors A) Concrete B) Reinforcing bars /5/.

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