M. Okoshi, M. Yoshimori, A. Sakai and M. Abe
Japan Atomic Energy Research Institute (JAERI)
Ibaraki, Japan

In Japan, all solid radioactive wastes have to be disposed in terrestrial environments, but the disposal option for highly activated waste (HAW) has not decided yet. To investigate the technical feasibility of the safe disposal, estimation of amounts and activity levels of HAW, container design, repository design, and safety assessments have been performed. In this study, HAW arising from 19 PWRs and 23 BWRs is assumed to be packaged in three types of large waste containers depending on the radioactivity levels. The total volume and weights of waste packages are approximately 27,000 m³ and 196,000 tons, respectively. Two types of repository, tunnel type and silo type, are proposed to compare the safety and cost of those repositories and due to the lack of site-specific conditions. Results from safety analysis have indicated that HAW could be disposed of safely in both repositories.

The technology of reactor decommissioning has progressed in recent years to the point where technical experts agree that the process can be carried out without any unacceptable impact on humans or environments. The JPDR (BWR, 90MWth) dismantling project was used to develop and demonstrate the technologies needed for decommissioning (1). Although dismantling techniques have not yet been used on large power plant, studies have been made of practical feasibility. By comparison, a few studies or experiments have been done on waste disposal, particularly for highly activated waste (HAW).

In Japan, all solid radioactive wastes have to be disposed in terrestrial environments depending on their activity levels and radionuclides. Low-level radioactive waste (LLW) generated from commercial nuclear power plants (NPPs) has been disposed in a near surface disposal facility in Rokkasho-mura, Aomori since Dec. 1994. In this facility, HAW, such as spent control rods and irradiated core internals arising from operation and dismantling of reactors can not be disposed of, because the activity levels of those wastes are greater than the Japanese radioactivity concentration upper bounds for near surface disposal. Therefore, the other disposal options have to be considered for those wastes, but the disposal system for HAW has not decided yet.

The Japan Atomic Energy Research Institute (JAERI) has investigated the disposal systems for HAW under the contract with the Science and Technology Agency. To establish the safe and economical disposal systems, the following elements were considered:

1. Estimation of amounts and activity levels of HAW,
2. Container design,
3. Repository design, and
4. Safety analysis.

In Japan, LLW whose activity levels less than the radioactivity concentration upper bounds, which are shown in Table I, can be disposed in a near surface disposal facility. It is estimated that the concentration of HAW, which is some irradiated core internals, such as a core shroud and core grid plates, and spent control rods, exceeds those limits (See also Table I).

To get a rough idea of the activity levels and amounts of HAW, we look at averaged values for 1.1 GWe PWR and BWR. According to the literature survey (2-4), the dismantling and operation of 1.1 GWe PWR and BWR generate approximately 210 tons and 110 tons of HAW respectively, if their operational periods are 40 years and loading factor is 75 %. The total activities of HAW arising from a PWR and a BWR are about 2.0x10¹⁷ Bq and 2.5x10¹⁷ Bq, respectively at shutdown.

To estimate the total amounts and activities of HAW, it is assumed that 19 PWRs (15 GWe) and 23 BWRs (20 GWe) will be completely dismantled after 40 years of operation. Therefore, approximately 6,500 tons of HAW is generated totally and its total inventory is 9.55x10¹⁸ Bq at shutdown. The inventories of major radionuclides are shown in Table II.

Disposal containers for HAW have been studied in some countries (5-7). Dismantling operations are affected by the size of the disposal container available. For highly radiating items, a large container is better from both economic and radiation protection viewpoints, provided transport and disposal can be achieved.

In this study, to design disposal containers, three criteria were established, namely 1) surface dose rates, 2) external dimensions, and 3) maximum weight. For the first criterion, the Japanese regulations stipulate that the dose rates of waste packages should be less than 2 mSv/h at surface and 0.1 mSv/h at one meter above waste packages, when they are transported outside nuclear facilities. For the second criterion, to place packages compactly in a repository, the external height and width of disposal containers should be set at the same sizes. For the third criterion, the maximum weight of waste packages should be less than 20 tons, because the weight of ropes must be considered to place packages safely in deep caverns by a shaft. It is also assumed that highly activated wastes will be cut into small pieces and packed into containers and solidified with cement mortar on the NPP's site.

Finally, three types of disposal containers were designed according to the radioactivity levels of wastes. These containers will be made of carbon steel or cast iron. The specifications of those containers are shown in Table III. The representative view of Type III container is shown in Fig. 1.

The specifications of disposal containers were defined by radiation shielding calculation for a reference time of 10 years after shutdown, a time that is relatively short in practice from the view point of waste management. In reality the waste will be placed in the center of the container, but in this calculation, radionuclides assumed to be spread uniformly inside the container. These simplified assumptions for planning purposes lead to designs that require too much shielding for the disposal containers if they are meet to the dose criteria (2 mSv/h at surface and 0.1 mSv/h at 1 m).

The summary on total numbers of each waste package to dispose of HAW generated from 19 PWRs and 23 BWRs is given in Table IV. The total volume and weights of waste packages are approximately 27,000 m³ and 196,000 tons, respectively.

Multi barrier system is the basic design concept for LLW disposal. However, the disposal concept depends on many elements such as waste amounts, activity level, and site conditions. As mentioned earlier, HAW could not be allocated to a near surface repository because the concentration of the following nuclides is too high: C-14, Co-60, Ni-63 and Nb-94. On the other hand, the concentration of Sr-90, Cs-137 and actinide elements is not so high. Therefore, HAW does not need to be disposed in deep geological formation in the same way as high-level radioactive waste disposal.

For example, in Finland, the decommissioning waste including HAW will be disposed in a silo at a depth of 70-100 m on the plant site (6). NAGRA in Switzerland plans to allocate HAW with LLW in a horizontally accessed repository in a low-permeability geological medium with >750 m overburden (Type B repository) (7).

There are two possible options to achieve the safe disposal for HAW. The first one is to dispose of them into deep geological formation. The another is to dispose of them in a repository with highly engineered barriers. Therefore, in this study, the depth and the types of repository were considered to dispose of HAW safely and in economical way. Finally, the following two types of repository were proposed to compare the safe and cost of those repositories and due to the lack of site-specific conditions:

• tunnels a few hundreds of meters below the earth's surface (tunnel type), and
• silos with vertical access at a depth of some tenth of meters (silo type).

Tunnels are simply long horizontal vaults of uniform cross section. The proposed layout of tunnel type repository is shown in Fig. 2. In this study small size tunnels are chosen because the construction are not much influenced by the strength of geological formation. If the inner diameter of tunnels is 2.5 m (the cross section of the tunnels accommodates one container), the total length of tunnels is about 16,200 m to emplace all HAW packages. The distance between tunnels and the length of each tunnel are set at about 22.5 m and 280 m, respectively. Therefore, this repository will consist of 60 tunnels and demand the area of about 660 m wide and 1,400 m long.

The waste packages will be placed in tunnels by a vertical shaft and forklift car. When all tunnels are fulfilled by waste packages, tunnels and shafts will be backfilled with cement mortar such that the system is passively safe.

The construction cost for the tunnel type repository is estimated at about 370 million U.S. dollars (1$=110 Yens). This value includes the construction cost for emplacement tunnels, access tunnel and shafts.

Alternatively, silo type repository is designed based on the construction experience for large underground structures such as liquefied natural gas tanks. A silo is a cylindrical opening with the major axis vertical. The silo will be constructed by excavating surface soil layer, so it is easier to access the repository. The inner diameter and height of each silo are 20 m and 30m, respectively. The space between outer and inner concrete wall, the mixture of soil and bentonite will be packed to retard the migration of radionuclides released from waste packages. Each silo will be equipped with a sliding roof for the prevention of rainwater permeation and radiation shielding. And also a drain system will be equipped with on the bottom of each silo to collect and drain the permeated water.

To emplace all HAW packages in this repository, 12 silos will be needed. The required area to construct the silo type repository will be 140 m wide and 190 m long, which is about 1/35 of that for the tunnel type repository, in case of the distance of about 30 m (1.5 times of the diameter) between each silo.

The safety of disposal for the tunnel type repository will be achieved with natural barrier and the four engineered barriers represented by the cement filling in the waste package, the iron shell of the container, the backfilling cement for the tunnel, and the concrete tunnel liner. For the silo type repository, as mentioned before, the mixture of bentonite and soil will be added as the fifth engineered barrier. Because it is assumed to be constructed at the shallower depth than that of the tunnel type repository.

To demonstrate the safety for the disposal of HAW, the safety analysis has been performed. In this analysis, the depths of the repository were changed from 50 m to 300 m, and also the water permeability coefficients of the host formation were changed from 2 m/yr to 20 m/yr. It is assumed that the contaminated groundwater flow into a small river (the annual flow rate is 3x10⁸ m³) and a person takes all drinking water (610 l/yr) from the river. In every alternative, the maximum dose remains below 0.01 mSv/yr, which is the design target for postoperational dose from a repository.

The preliminary studies for technical feasibility of HAW disposal have indicated the following points:

• The expected concentration of HAW (reactor vessel internals and spent control rods) is higher than the radioactivity concentration upper bounds for near surface disposal in Japan. The relevant radionuclides are ¹⁴C, ⁶⁰Co, and ⁶³Ni. The concentration of other radionuclides such as ⁹⁰Sr, ¹³⁷Cs, and actinide elements, is below the upper bounds,
• Operation and decommissioning of 19 PWRs and 23 BWRs would generate about 6,500 tons of HAW. This amount is very small compared with the amounts of LLW arising from operation and decommissioning, which can be disposed in a near surface repository,
• Due to radiation, HAW would be appropriate for packaging in thick walled iron containers that would increase the volume of waste packages and the disposal cost. The use of a returnable shield (cask) is the good way to reduce the volume of waste packages, therefore this option should be investigated in the future,
• To compare the safe and cost of a repository for HAW and due to the lack of site specific conditions, two types of repository (tunnel type and silo type) are proposed in this study. Consideration of the potential doses due to radionuclides releases from these two repositories has indicated that the safe disposal of HAW is technically feasible in both repositories,

Future work on the disposal options for HAW should evolve along with the choice of the main decommissioning options such as time of dismantling and dismantling techniques. There is still much work to be done before practical implementation of NPP decommissioning and waste disposal.

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