Dose estimation for reuse of material contaminated by Fukushima Daiichi NPP accident

Seiji Takeda (JAEA) presented.

As a result of the Fukushima accident, caesium was dispersed over a 250 km from the nuclear power plant (Figure 17). A large challenge has therefore been how to handle the large volume of contaminated material generated as a result of clean-up operations. One strategy for addressing this issue would be the reuse of material.

Figure 17. Cs-134 and Cs-137 deposition following the Fukushima accident.

(http://savechild.net/wp-content/uploads/2011/06/zentaidojyo.gif)

In June 2011, the Nuclear Safety Commission issued a policy to ensure safety of workers and members of the public from the treatment and disposal of contaminated wastes

(http://warp.da.ndl.go.jp/info:ndljp/pid/9483636/www.nsr.go.jp/archive/nsc/info/20110603_2.pdf [in Japanese]). The policy stipulated that worker doses should not exceed 1 mSv/y and doses to residents in the vicinity of treatment facilities should also remain below 1 mSv/y. Recycled materials have to be monitored to ensure activity that any doses would be below 10 µSv/y. These dose criteria were used, along with the results of dose estimation, to calculate the radioactive caesium concentration corresponding to the dose criteria.

Wastes arising from decontamination activities are treated in the same way, irrespective of the location from which they arise. The treatment is determined according to the level of

contamination (Figure 18). Wastes with contamination levels exceeding 100,000 Bq/kg are sent for interim storage. The Ministry of Environment planned to reuse some contaminated materials to reduce the volume of wastes sent to landfill

(http://www.env.go.jp/jishin/attach/memo20120111_shori.pdf [in Japanese] and

http://josen.env.go.jp/en/documents/pdf/documents_04.pdf?130111 ), but there were no criteria

in place by which reuse can be evaluated. A dose assessment was therefore carried out to establish the required criteria for the reuse of contaminated materials.

Two sets of criteria were required for the reuse of contaminated materials, one for unlimited use, the second for limited and specified reuse. The New Act established an unlimited reuse criterion of 100 Bq/kg (Cs-134 and Cs-137), based on existing clearance levels for concrete and metals.

However, the clearance level had not been evaluated for the reuse of wooden and other wastes and a dose estimate was required to confirm the validity of the clearance level for such materials.

A key factor was the balance between the large volume of contaminated material being generated and the need for materials for use in reconstructing infrastructure damaged by the earthquake and tsunami. A dose assessment to establish new criteria for the reuse of wastes, primarily concrete, in infrastructure was focussed on the demand for construction materials and the restoration plan for the area in and around Fukushima.

Figure 18. Flow chart for the treatment of contaminated wastes according to the level of contamination.

For contaminated wood, the focus was on the use of wood chips within five reuse scenarios:

particle board, paper, compost, biomass power generation and mulch (Figure 19). Several scenarios were considered for workers and members of the public exposed as a result of the manufacture and reuse of wood-based products, taking into account source volume, level of radiation contamination, exposure time and type of exposure (external, ingestion, inhalation).

Figure 19. Working process and final products considered for the reuse of contaminate wood chips An assessment code, PASCLR2, was developed to derive clearance levels on the basis of a dose estimation assuming a unit radioactive caesium concentration in wood chips. Dose conversion factors for external exposure pathways were based on conservative source geometry assumptions relative to working conditions for transportation, temporary storage and processing of wood chip products. Other parameters were derived from previous clearance estimates undertaken in Japan.

The code was used to derive the minimum radioactive caesium concentration (Cs-134 + Cs-137) that was practicable for the reuse of wood chip under each scenario, relative to the effective dose criteria. All concentrations corresponding to the dose criteria were greater than 100 Bq/kg, with the lowest concentration value relating to the reuse of wood chips in particle boards (120 Bq/kg)(Figure 20).

Figure 20. Minimum radioactive Cs concentration (134Cs + 137Cs) which is practicable for the reuse based on the results of dose estimation

In the dose estimation for limited reuse purposes, doses to workers and members of the public at all stages between production of recycling material and final reuse were calculated, based on realistic information on the treatment and reuse of material in and around Fukushima. Pathways considered including seepage of rainwater to groundwater following reuse of materials for the construction of pavements or coastal defences and subsequent use of well water. The transfer of caesium from soil to plants was also evaluated for protection forests and migration in groundwater and uptake in marine products. Again, caesium isotope concentrations relating to criteria ensuring

safety were derived for the different scenarios (production of recycling material and reuse in pavements or roads or to develop coastal protection forests). The dose assessment took into account the type of material used, the thickness of shielding materials and where reused materials were located e.g. roads or pavements. The highest activity concentration that could be reused in inland infrastructure projects equated to 2,700 Bq/kg when material was used as a 30 cm sub-base course material for road construction with a 30 cm cover layer consisting of base course material and asphalt. Concrete wastes with contamination of up to 3,000 Bq/kg could be reused for coastal protection forest development where shielding material of 1 m was used.

Two types of criteria were therefore required in the initial stage of response to the Fukushima accident to allow general recycling of materials with free release to the market and to achieve volume reduction of contaminated materials and progress infrastructure reconstruction. The development of these criteria during the initial stages of the accident response was an important aspect for progressive recovery from the accident. Further work is ongoing to determine guidelines and criteria for the reuse of soil removed as part of the immediate remediation activities and for the reuse of materials such as concrete and metals with low levels of contamination that will be generated during decommissioning activities at the nuclear power plant.

4.4 Emergency limitation of radionuclide concentrations in foodstuffs: from