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A Plan and Its Safety Assessment of VLLW Disposal Site in order to Dispose of Waste Materials Generated from Decommissioning of Tokai

Nuclear Power Plant

K.Tanaka1,2, H.Noguchi2, S.Nomura2, H.Tanabe2, K.Morii2, K.Fujimura2

1The Institute of Applied Energy (IAE) Tokyo, Japan2The Japan Atomic Power Company (JAPC) Tokyo, Japan

E-mail contact of main author: k-tanaka@iae.or.jp

Abstract. The Japan Atomic Power Company (JAPC) has planned to dispose of both metal wastes and concrete wastes generated from decommissioning of Tokai nuclear power plant (TK1). TK1 is the first commercial nuclear power plant in Japan that is a gas-cooling reactor and is now under decommissioning. According to a law of Japan, the disposal site is categorized as a trench type of near surface disposal without artificial constructions. We call such a disposal site L3 site in Japan. The site will be located in the northern part of TK1 site. JAPC will construct two trenches named A-trench and B-trench. JAPC will dispose of metal wastes in A-trench and concrete wastes in B-trench. We have performed safety assessments to ensure necessary measures for maintaining the site safely. Scenarios relevant to public exposure in an operation phase and in a post-closure phase have been considered to develop conceptual and mathematical models. In order to identify scenarios, characteristics of meteorology, geography, hydrology, geohydrology and social environment around the site were investigated. The assessments provide that the public exposure from the site is low enough for limits of the requirements.

Key Words: VLLW, near surface disposal, safety assessment, decommissioning

1. Introduction

Tokai nuclear power plant (TK1), which is a graphite moderate and CO2 gas-cooling reactor (GCR), is the first commercial nuclear power plan in Japan and is under decommissioning [1]. The Japan Atomic Power Company (JAPC) who is an operator of TK1 has planed to dispose of very low-level waste (VLLW) generated from decommissioning in TK1 site. According to a law of Japan, this disposal site is categorized as a trench type of near surface disposal without artificial constructions, which we call L3 in Japan [2].

JAPC has already submitted an application of the disposal site, which is called TK-L3, for regulatory body. In tasks for preparing the application, we have assessed public exposure the public exposure around TK-L3 through various pathways of various scenarios. The assessments provided that the public exposure would be low enough both during the operation and after a controlled period. It has met a legal limit of Japan.

2. A specification of TK-L3

TK-L3 will be located in the northern part of TK1 site as shown in FIG. 1. An area of the site is 6.0×10+6 m2 and total amounts of waste are 1.6×10+4 tons. JAPC will construct two trenches named A-trench and B-trench. JAPC will dispose of metal wastes in A-trench and concrete wastes in B-trench. Metal wastes will be mainly generated from dismantling of Steam Rising Units (SRU) and from piping which connects a reactor and SRU. Surface of

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SRU internals and inner surface of the piping are contaminated by radionuclides, which arise from activations of corrosion products and from dispersion of fission products. Concrete wastes are generated from dismantling of the primary Biological Shielding Wall (BSW) and some parts of the secondary BSW. The concrete wastes are activated by neutron irradiation. Radiological specification of waste materials to be disposed of is shown in TABLE 1.

In order to perform observations about a structure of groundwater-flow and the stratum in TK1 site, JAPC has dug 35 observation-wells for boring investigations as seen in FIG. 2. The observation showed that the groundwater always flows toward the sea and

that the upper limit of the groundwater is around 5.6m in depth.

TABLE 1: RADIOLOGICAL SPECIFICATION OF WASTE MATERIALS

RadionuclideUpper limit of

radioactivity con-centration (Bq/ton)

Total radioactivity

(Bq)Radionuclide

Upper limit of radioactivity con-

centration (Bq/ton)

Total radioactivity

(Bq)3H 3.0x109 1.4x1012 90Sr 1.0x107 1.7x109

14C 5.0x107 1.2x1010 137Cs 7.0x106 9.1x108

36Cl 1.0x108 4.6x1010 152Eu 3.0x108 5.6x1010

41Ca 2.0x107 3.4x109 154Eu 9.0x106 2.5x109

60Co 8.0x109 1.3x1011 α-nuclide 4.0x106 1.4x108

63Ni 3.0x109 6.6x1010

FIG.3 shows a cross-section view of a trench of TK-L3. There will be three layers of waste materials in the trenches of TK-L3 of which the depth is 4m. Since the depth will be enough to prevent that waste materials soak in the groundwater, radionuclides will migrate to the groundwater by only rain infiltrating into the waste material. Cover soil of thickness 2.5 m

FIG.3 CROSS-SECTION VIEW OF TK-L3

Distance of 400m from the shoreline

: Site boundary line

Tokai NPP

Location of TK-L3

: Location of a well for observation of the flow of the groundwater

FIG.2 LOCATIONS OF A WELL FOR OBSERVATION

FIG. 1: BIRD-EYE VIEW OF A LOCATION OF TK-L3 IN TK1 SITE

: Location of monitoring post

Location of TK-L3

Tokai NPP Tokai Dai-niNPP

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with pavement will reduce public exposure by Gamma-ray from waste materials to safe enough. Retaining walls and dividers will be installed to ensure the safety of emplacement activities of waste materials.

3. The assessment of the public exposure

In the assessments of the public exposure from waste materials disposed in TK-L3, we considered three kinds of exposure scenarios.

(1) Exposure by the migration of radioactivity to the groundwater (FIG. 4)

(2) Exposure by land-reuse as farmland

(FIG. 5).

(3) Exposure by land-reuse to dig the site. (FIG. 6)

We assessed public exposures by each pathway of each scenario by applying appropriate procedure with appropriate parameters [3-6]. Pathways of each scenario are shown in text- boxes colored in blue in FIG. 4, 5 and 6 respectively. Results of the assessment are shown in TABLE 2.

We also evaluated exposure by Gamma-ray from waste materials directly and by sky-shine of Gamma-ray in the operation period of the emplacement by using calculation codes [7-9]. The evaluations are also tabulated in TABLE 2.

TABLE 2 PUBLIC EXPOSURES BY EACH PATHWAY OF EACH SCINARIOExposure by the migration of

radioactivity to the groundwaterExposure by land-reuse as farmland

Exposure by land-reuse to dig the site

Exposure in the operation period

Intake by drinking

Exposure by activities at

the sea shore

Intake of marine

products

Intake of farm

products

Intake of livestock products

Exposure of construction

activities

Exposure by a

residence

Direct Gamma

-raySky-shine

39.0a 3.2x10-6a 5.3a 54.0a 86.0a 13.0a 8.7a 0.14a 21.4a

(1mSv/a)b (10μSv/a) b (1mSv/a) b (300μSv/a) b (10μSv/a) b (50μSv/a) b

a: Unit: μSv/a b: Numerical values in the parentheses are regulation limits.

FIG. 6 Exposure by land-reuse to dig the site

FIG. 5 Exposure by land-reuse as farmland

FIG. 4 Exposure by the migration of radioactivity to the groundwater

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4. Summary and conclusion

JAPC has performed safety assessments to ensure necessary measures for maintaining the TK-L3 safely. Scenarios relevant to public exposure in an operation period and in a post-enclosure period, which is after controlled period, have been considered to develop conceptual and mathematical models. In order to identify scenarios, characteristics of meteorology, geography, hydrology, geohydrology and social environment around the site were investigated.

The assessments provided that public exposures by TK-L3 are low enough for regulation limit except intake by drinking and exposure by a residence. In case of intake by drinking, although the use of the well water at down-stream side in TK-L3 would be possible, the possibility of its occurrence would be low. Furthermore, because the area of Tokai NPP site is not suitable for the residence, the possibility that exposure by a residence would be low. According to the consideration as mentioned here, the assessments show that TK-L3 is a disposal site safe enough.

4. References

[1] The Japan Atomic Power Company, Decommissioning project of Tokai NPP http://www.japc.co.jp/haishi/tokai_haishi.html

[2] Nuclear Regulation Authority of Japan, Regulation for disposal of very low-level waste (in Japanese)

[3] INTERNATIONAL ATOMIC ENERGY AGENCY, Generic Models for Use in Assess-ing the Impact of Discharges of Radioactive Substances to the Environment, Safety Re-ports Series No.19, Vienna (2001)

[4] INTERNATIONAL ATOMIC ENERGY AGENCY, Sediment Distribution Coefficients and Concentration Factors for Biota in the Marine Environment, TECHNICAL RE-PORTS SERIES No.422, Vienna (2004)

[5] INTERNATIONAL ATOMIC ENERGY AGENCY, Generic Models and Parameters for Assessing the Environmental Transfer of Radionuclides from Routine Releases, Expo-sures of Critical Groups, IAEA Safety Series No.57, Vienna (1982)

[6] INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Age-de-pendent Doses to Members of the Public from Intake of Radionuclides: Part 5 Compila-tion of Ingestion and Inhalation Dose Coefficients, ICRP PUBLICATION 72, (1995)

[7] M.L.Couchman et.al., G-33 CODE, NUS-TM-NA-42, Washington D.C. (1965)

[8] W. Engle, et. al, A One Dimensional Discrete Ordinate Transport Code with Anisotropic Scattering, CCC-254, Oak Ridge (1967)

[9] AECL Research, Point Kernel Code System for Neutron and Gamma-ray Shielding Calculations Using the GP buildup Factor, CCC-645, AECL Research (1986)