Research and Development Relating to the Accident at the TEPCO’s Fukushima Daiichi NPS

1-11 For the Safe Handling of Dehydrated Sludge Contaminated by Radioactive Cesium－ Dose Estimate for Workers and Public with Incineration and Disposal of Dehydrated Sludge －Temporary Storage Scenario
Public near temporary storage
place(direct and skyshine radiation)

Unloading,transportationand landfill

Reuse of disposal site
after the closure

Disposal Scenario
Use of park
Residence

Temporary storage
and disposal

Dehydratedsludge

Disposal ofincinerationashes

Migration ofradionuclide ingroundwaterIncineration

Ingestion of crops : adult

Park

External : children

Prohibition of reusing dehydrated sludge
and its ashes as cement material and so on

Minimum Value: 8900 Bq/kg

Ingestion of water :children

External : adult

Disposalsite use
Incineration
Scenario

Inhalation : worker
External : worker

Use of well water

Residence near
the incinerator

Direct ingestion : worker

Groundwater
migration(well)

Construction ofhouse,
agriculture

Maintenance of incinerator,transportation

Incineration ofdehydrated sludge

Landfillwork

Agriculture

Residence

Ingestion of crops : adult
External in agriculture :worker

It is necessary to forbid reusing
the disposal site for residence
and agriculture.

External : children

Necessity of restriction and management
for the disposal site reuse

External : adult
1E+1

1E+2

1E+3

1E+4

1E+5

1E+6

1E+7

1E+8

Radioactive cesium concentration in contaminatedmaterial corresponding to the effective dose criteria(Bq/kg)

Fig.1-22 Scenarios for incineration and disposal of dehydrated
sludge contaminated by radioactive cesium
External and internal exposure pathways in the transportation,incineration, and temporary storage of the sludge are specified
on the basis of information on actual work conditions andtreatment record for the sludge. Specified scenarios andpathways are guaranteed to be exhaustive through a comparison
with the pathways used in previous clearance level estimate.

Fig.1-23 Estimated results of limiting radioactive cesium
concentration to ensure safe incineration and disposal
Minimum radioactive cesium concentration in dehydrated
sludge and incineration ashes, corresponding to the effective
dose criterion (1 mSv/y for engaging in treatment of materials
contaminated by the severe accident, and 10 μSv/y for postclosure in disposal), is estimated to be 8900 Bq/kg under thecondition of limited reuse of the site.

The ground surface environment in Fukushima Prefecture
and the surrounding area has been contaminated byradionuclides released from the TEPCO’s Fukushima Daiichi
NPS. Soil particles that adsorbed radioactive cesium flow
into sewage and water resources for tap water. Dehydrated
sludge containing high concentrations of radioactive cesium
was generated by sewage and water supply processing. The
contaminated sludge should be incinerated and disposed ofwhile ensuring the radiation safety of the workers and thepublic. However, there was no criterion for treatment of thesludge.
The dose to the workers and the public during incinerationand disposal was estimated to provide the technicalinformation for determining the criterion. We calculated theminimum radioactive cesium concentration from the doseresults, corresponding to the effective dose criterion indicatedby Nuclear Safety Commission of Japan (Near-term policy
to ensure the safety for treating and disposing contaminatedwaste around the site of Fukushima Dai-ichi Nuclear Power
Plants, NSC, June 3, 2011).

We focused on the similarity between the actual situationof general incineration and disposal and a series of scenarios
in previous clearance level estimation. By incorporating
additional exposure pathways peculiar to the sludge into thescenarios based on the clearance level estimation, we could
set up comprehensive and reliable scenarios and pathways
over a short period of time, as shown in Fig.1-22. According
to the information on the actual work conditions and treatment
record of the sludge, suitable parameters in each scenario wereset for a realistic estimation.
The estimated doses for the main exposure pathways areshown in Fig.1-23. Under the condition of a radioactive
cesium concentration of less than 8000 Bq/kg, generalincineration and disposal of the sludge is justifiable under therestriction that disposal site reuse be limited to residential andagricultural use.
This result was used as technical information for
determining the criterion for sludge treatment by the Ministryof Land, Infrastructure, Transport and Tourism.

Reference
Takeda, S. et al., Dose Estimate for Incineration and Disposal of Dehydrated Sludge Contaminated by Radioactive Cesium of High Activity Concentration,
Hoken Butsuri, vol.47, no.4, 2012, p.247-259 (in Japanese).

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JAEA R&D Review 2013