1.Introduction

The decontamination should be essentially considered before starting decommissioning of nuclear facilities due to the risk of radiation exposure to the decommissioning workers. But decontamination may not be the best way depending on the case. The benefits of the decontamination should be evaluated with various aspects. In this paper, criteria and main consideration to plan the decontamination are suggested.

Decontamination is defined as the removal of contamination from surfaces of facilities or equipment by washing, heating, chemical or electrochemical action, mechanical cleaning, or other techniques. The objectives of decontamination are:

However, as a result of decontamination, secondary waste would be generated. The higher DF is demanded, the more waste is generated. Waste generation is directly related to cost and benefit of working efficiency. On the other hand, leaving adherent contamination within pipes and components in a dispersed form on the internal metal surfaces can be a good method to reduce both in occupational exposure and cost by simply removing the contaminated system and its components and only performing packaging activities.

A decontamination project may also require a facility capable of treating secondary waste from decontamination. The concentrated waste, representing a more significant radiation source, must be solidified and shipped for disposal in licensed disposal facilities. The optimal waste reduction configuration must be defined after an economic assessment of treatment versus transportation/disposal costs. Each of these additional activities may increase:

These could result in higher doses than those received from removing, packaging and shipping the contaminated system without extensive decontamination. Resolution of this question depends on specific facts, such as the exposure rate of the gamma-emitting contamination, the contamination level, and the effectiveness of the containing component and piping in reducing radiation fields in the work area.

Decontamination is performed to reduce the risk of radiation exposure to the public and workers. In contrast with normal decontamination that is performed during operation, the decontamination process for decommissioning of NPP(Nuclear Power Plant) requires high DF(Decontamination Factor) to keep the acceptance level of the waste disposed. Table 1 shows the typical contamination status according to the surface depth and DF when the layers are removed.

Typical film thicknesses on PWR(Pressurized Water Reactor) stainless steel surfaces are 2-3 μm while those on Inconel surfaces are closer to 1-2 μm. This is partly due to the lower general corrosion rate of Inconel and partly due to the higher velocities over the Inconel surfaces, which tend to limit deposition. Hydrodynamic conditions at the surface are not conducive to particles settling and adhering to the surface. The majority of these films are grown-on oxide. Deposited oxide accounts for <25% of the film. The densities of these films are in the range 3.0-5.0 g/cm³ with 4.0 being a typical average. This is about 75% of the theoretical density of 5.2 g/cm³[1].

Another method that is commonly used to express film thickness is the specific weight, i.e., mg/cm². In many cases, this is a more practical unit of measure since it is obtained directly by scraping or descaling a known area of pipe or fuel. It does not depend upon knowledge of the film thickness and density, both of which require sophisticated laboratory techniques to measure. The specific weight is simply the product of the thickness and density, with the appropriate corrections for units. For the stainless steel film referred to above, the specific weight is:

For the Inconel film, the specific weight is 0.8 mg/ cm²[2].

2.Decontamination Evaluation

2.1.Evaluation Criteria

Various aspects of criteria about the decontamination should be considered before implementing the practical decontamination technique[3].

The radiological safety aspects of a particular cleaning technique can greatly influence its region for use.

Total volume of the waste produced during a decontamination operation must be reduced and properly packaged for storage, transport and disposal. The radioactive waste that is removed from the contaminated surfaces can be expanded in volume by the decontamination agent during the decontamination process. Any increase in the total volume of resultant radioactive waste increases the cost of disposal. The total amount of secondary generated high radioactive waste resulting from decontamination is very important and has a high weighting factor in determining a suitable decontamination method.

Some of the components to be decontaminated would have to be disassembled to some degree to provide access to the interior surfaces. Both the degree of disassembly required and the relative difficulty of disassembly would impact the applicability of a particular decontamination technique. The need for disassembly, along with waste generation, has very high weighting factor in considering a proper technique.

Since the radioactive material will generally be on the inside of the reactor coolant system, the area to be decontaminated must be accessible. Depending upon the technique considered one or more openings may be required to decontaminate the system at hand.

For off-system decontamination, an item that is too large to fit off-system decontamination equipment, e.g. a tank, must be sectioned. The size of the component becomes then an important factor to be considered.

The initial cost of the decontamination equipment and supporting systems, in addition to the expected lifetime of that equipment, can be important factors in the selection of an appropriate decontamination method. The capital cost would include the total cost of all the necessary equipment, amortized over the expected useful lifetime.

This criterion includes labor cost plus the cost of consumable supplies. A labor intensive decontamination technique, and/or one which requires a large amount of expensive material would be unfavorable. It should be noted that the operating cost for decontamination can be quite different from the cost of ordinary industrial cleaning. The costs associated with personnel shielding and waste disposal need to be added considerably to the cost that would result from an application of the same technique to a nonnuclear cleaning project.

The feasibility of reusing a component/system after decontamination can be affected by a number of different factors. One is the cost of decontamination and requalification compared with the cost of replacement. Another factor is the effect of access or sectioning operations that require excessive repair work and requalification tests. Some decontamination techniques result in the removal of a significant amount of metal from the surface being cleaned. Such removal might affect the reusability of the item. The surface quality after cleaning is also important, since a rough surface is much more subject to rapid recontamination. All of these factors can affect requalification.

This refers to the tendency to corrode a surface as a result of the reaction between the decontamination agent and the item being cleaned, both during the decontamination and as a residual effect later on.

This criterion relates to the inherent safety characteristics of the decontamination technique. In deciding on a proper decontamination technique, the level of safety precautions and their effects on the costs need to be considered.

2.2.Factors Influencing Cost

A major consideration in deciding whether or not to decontaminate RCS(Reactor Coolant System) or its subsystem is the economical value the exercise. Not only economical cost but also benefit incurred from applying decontamination should be considered. So, parameters that can be a standard to evaluate benefit/cost are summarized in this section.

2.2.1.Benefit Parameters

The exposure that would be incurred without the decontamination can be estimated by multiplying the radiation exposure taken for each task by DF. Subtracting from this estimated value the value of the actually incurred radiation exposure gives the exposure avoided.[4]

The time for inspection, maintenance and repair is reduced by applying decontamination. The radiation exposure risk is lowered as much as the working time reduced.

This benefit parameter is not related with decontamination for a decommissioning project. In operational NPP, there is a problem about recontamination which is affected by deposit containing radioactivity. The recontamination tends to result from deposit rather than fuels.

There are real benefits to a decontamination which are difficult or impossible to evaluate. The risk level and security level can be lowered by reducing the risk of radiation exposure. Due to the lowered risk level, normal workers and general tools can be put in the workplace where special workers or special tools should have been put before decontamination.

2.2.2.Cost Parameters

This item is the total charge by the decontamination vendor for performing decontamination. It includes transportation of equipment, installation and removal, supply of chemicals and ion exchange resins, performance of decontamination and related documentation. Waste management should be handled separately even if the same vendor handled it[4].

This cost parameter is the most important parameter determining economical efficiency. This item covers waste treatment cost for disposal (drying or solidification of spent resin as well as any other volume reduction methods applied), waste packaging cost (liners or high integrity containers), transportation cost to repository, and disposal cost.

This parameter means costs in supporting the decontamination activities such as staff training and monitoring. It also include the cost of any supplies provided by the utility.

Labor charge and rental fee for equipments is proportional to the working time. The expense can be saved as much as time saved.

Although a majority of radioactivity is reduced by decontamination, small radiation exposure can not be avoided. A proper economical compensation should be paid according to the potential risk and the degree of damage.Table 2

3.Decontamination Planning

3.1.Introduction of Decontamination Technologies

As shown in Table 3, there are various types of decontamination depending on objectives and contaminated items. But only chemical decontamination for RCS and its auxiliary system would be handled in this paper, since chemical decontamination is already proven by experiences and has economical and beneficial advantages compared with other techniques.

Chemical decontamination uses concentrated or dilute chemical reagents in contact with the contaminated item, to dissolve the contamination layer covering base metal. In most cases, required decontamination level would be obtained by repeating the process as long as necessary. In mild chemical decontamination process which has low DF, the process should be non-destructive to the base metal and is generally used for operating facilities. But aggressive chemical decontamination requires high DF since it should remove even some of base metal to mitigate radiation exposure.

In case of decontamination for decommissioning of NPPs, DF should be high to reduce the radioactivity of waste below the acceptance level of disposal. While a DF of 10 is generally used for operating plants, a DF of 100 or more is suitable for decommissioning projects. In this section, techniques of high DF are described[6] and some factors influencing management are proposed.

Among chemical decontamination techniques, there are two proven aggressive decontamination techniques: DFD(Decontamination For Decommissioning) applied to Maine Yankee NPP and CORD D UV(Chemical Oxidation Reduction Decontamination_ Decommissioning_Ultra Violet) to Connecticut Yankee NPP.

Tables 4 and 5 show examples of the decontamination technology which are suitable for permanent shut-down of NPP. DFD developed by EPRI(Electric Power Research Institute) is the more aggressive process than operational- type such as LOMI(Low Oxidation-state Metal Iron), CAN-DEREM(Canadian Decontamination and Remediation process) and CITROX(Citric Oxide acid process) with >1,000 DF [6].

Recently, the EPRI DFDX process that is more efficient and economic technology was developed from DFD. This technology includes processing of radioactive solution using electrochemical ion exchange to produce a minimal metallic waste.

Siemens developed CORD for decontamination of operational facilities. According to the need of high DF, CORD D UV was developed from the CORD for permanent shut-down NPP[6].Tables 6

3.2.Components Volume

For fluid systems evaluations, it is necessary to identify and quantify the volumes of those systems. Volume can be used as a quantity standard of reagent that is consumed to decontaminate. RCS, RHRS(Residual Heal Removal System), CVCS(Chemical Volume and Control System), and DPS(Decontamination Processing System) would be included in decontamination items[7].

If some jumpers like a nozzle dam are utilized to bypass large components, the volume is considerably reduced. Just, pipelines, pressurizer, and RCP(Reactor Cooling/Coolant Pump) need to be considered and analyzed except a reactor and steam generators. According to a plan, whole RCP can be involved in decontamination project or just a part of it can be used. Steam generator can also be considered in the same way. The volume of whole or a part of tubes can be used, or it can be omitted by the plan.

CVCS would be contaminated by radionuclides during operation period. But, depending on the decontamination plan or policy, the volume is different. There are many options like whether pumps or pre-filters are included or not. If pumps are included in the decontamination project, some sealing tool are required to prevent leakage. And, in case of pre-filters, the inner media should be removed to prevent potential pressure drop.

RHRS connected directly with RCS is operated to remove decay heat during operation. So, crud accumulation occurring radiation risk are generated on the inner surface of components.

During decontamination process, additional decontamination equipments should be assembled with RCS. So, its volume also has to be included in the total volume. There are various equipments depending on the applied technology. But, generally, piping and demineralizers take up much portion in the volume of DPS(Decontamination Process System).

3.3.Assembly of DPS

Depending on the technology and plan, connection location of DPS is different. But many cases showed that the area around RHRS is a suitable location to assemble DPS. As shown in Fig. 1, 4 options on RHRS were considered for decontamination of operational NPP [7].

There are several differences between decontamination for decommissioning and decontamination for operational facility. Therefore effect of the location of DPS in case of decontamination for decommissioning also needs to be studied. For example, the components and their structure after a discharge of DPS have an effect on the pressure drop and the temperature change. Furthermore there are high risk of leakage compared with decontaminating operational facilities since strong acid reagent would be used for the decommissioning project. So, many bypass tools or sealing tools would be required to prevent accidents. Especially, pumps need to be considered carefully because of gap between rotating parts and fixed parts.

But the area around RHRS is not always a right answer. As shown in Fig. 3 and 4 in case of Maine Yankee NPP, the jumpers used for bypassing tubes of steam generators were used for connection with DPS. In accordance with the application and cycles, the assembly location of another inlet pipe were changed (App 1: CVCS inlet, App 2: drain/fill line on the Hot/Cold legs). Then, as shown in Fig. 5 in case of Connecticut Yankee NPP, an inlet pipe of DPS was connected with a pressurizer and the discharge line was connected with discharge line of RHRS.

4.Decontamination Methodology

4.1.Decontamination Process

If preparation of technologies and related procedures were finished, these would be applied to the practical fieldwork. Fig. 2 shows the general task flow of the decontamination. Combination of the chemical performance and the number of cycles result in DF eventually. In other words, although the performance of reagent is not good, desired DF can be accomplished by repeating decontamination process. However, in that case, secondary waste generation as well as decontamination time would increase. Eventually, it is a matter of efficiency and cost.

Two decontamination experiences from USA are analyzed to consider the Korean NPPs decontamination in the future. Major parameters of the technologies are showed in Table 7.

4.1.1.Large Components

There are many options for designing of a decontamination process. The first task is determining whether large components is included in decontamination list. As mentioned earlier, trying to reduce radioactivity to clearance level is inefficient since the volume of secondary waste would be increased. Especially, decontaminating entire reactor vessel components is not a proper decision since a reactor consist of many components which have high radioactivity. And, including steam generator tubes in the decontamination list may not have much advantage since only outer about 10 rows contribute the external radiation exposure. The main objective of decontamination before dismantling is making a safe place to protect the workers who would be working in the primary side. To reduce radioactive waste, additional decontamination after dismantling would be needed like electrochemical polishing, ultrasonic wave, chemical immersion method and so on.

4.1.2.Flow Path of Chemicals

If large components are excluded from decontaminated items, nozzle dams have to be installed to connect hot legs and cold legs in RPV(Reactor Pressure Vessel) at first. And, jumper type equipments can be assembled under steam generator to bypass tubes, or a portion of tubes can be used as passages flowing reagent. Fig. 3 and 4 show the flow path of Maine Yankee NPP. Jumpers were assembled under steam generator. The fluid was entered through the pipeline interconnected with the loop #3 jumper and discharged from other pipeline of loop #2 jumper. During application 1, RCS and CVCS were decontaminated as shown in Fig. 3.

During application 2, RCS and RHRS were decontaminated as shown in Fig. 4. One of inlet lines is changed from CVCS line to drain/fill line of loop #1 between hot-leg and cold-leg.

In decontamination of Connecticut Yankee NPP, large components were not included in the decontamination list in comparison with the Maine Yankee case. A portion of tubes(about 20%) was used to bypass rest of tubes in steam generators. As shown in Fig. 5, there is no additional bypass equipments on/under steam generators.

Decontamination equpments would be set up with RCS systems. Existing equipments in NPP can be used with the vendor-supplied equipments. This equipments consist of a circulating pump, heater, filter, chemical injection system, restoration system(ion exchange system in EPRI DFD and UV burning system in CORD D UV) and the connecting tools like pipes and hoses.

Unlike the Maine Yankee NPP case that existing equipments in the NPP were not utilized for decontamination, a combination of existing plant equipment as well as temporary equipment external to the station was used for decontamination of Connecticut Yankee NPP. An RHR pump, the pressurizer heaters, and the plant demineralizers in the plant were used to support decontamination operations. It may mean a single system should be applied to utilize existing equipments since changing pipelines by application is difficult. As shown in Fig. 5, an inlet pipeline is connected with a pressurzier and a discharge line is connected with the RHRS discharge line to utilize components involved in each system. Fig. 6

Tables 8 and 9 show the decontamination schedule of Maine Yankee NPP and Connecticut Yankee NPP[6]. Compared with the Maine Yankee case, more time was spent for decontamination of Connecticut Yankee NPP due to treating failed fuel. Particles of fuels were remained bottom the reactor and crevices on the surface of components. So, those had a negative impact like delaying working time on the process. If there is no delay factor, pure decontamination time is assumed approximately from two weeks to 20 days.

It is required to get the proper value of the fluid velocity. According to the velocity, the status of dead space isolating the fluid is different. And, the velocity effects on the number of decontamination cycles since low flow areas appear to have lower DF. Adequate motive force should be available to ensure good flow to all locations. But, if it is too fast, the decontamination time would be reduced but the risk of leakage is higher and the number of cycles is increased. It means that velocity is in inverse proportion to the number of cycles. So, the optimum value should be studied.

The location of equipments monitoring radioactivity should also be considered in details since MnO₂ is accumulated the bottom of the components and the dead pipeline. MnO₂ is a well-known getter for Co-60. So, the location of detectors is one of important factors for radiation safety.

4.2.Decontamination Waste

Waste are produced from ion exchange equipments that are used to remove metals from deposit dissolution and base metal corrosion. Cation resin is used to remove contaminants involved in water during the overall decontamination process. And, anion resin removes the chemical to clean water after the decontamination and to reduce the concentration of boron before the decontamination.

Cation resin may be recharged to avoid generating GTCC(Greater than class C) waste due to its TRU(Transuranic) loading even if the resin is spent less than half. It is important to keep the decontamination waste as LLW class that can be accepted in near surface disposal facility.

5.Current State of Decontamination Technology in Korea

KAERI(Korea Atomic Energy Research Institute) had developed oxidation-reduction decontamination technologies, such as Regenerative LOMI Process, for maintenance during the operation of nuclear facilities. These technologies had been applied successfully to the decontamination of spent fuel shipping casks and RCP valves. Also electrochemical and chemical decontamination processes using highly concentrated chemical solutions were proven to be effective for decontamination of radioactive metal scraps for recycling[10].

Recently, KAERI has also developed several decontamination technologies such as PFC (perfluorocarbon) decontamination technology and CO2 pellet blasting decontamination technology, which were proven to be effective for decontamination of loosely contaminated particulates. Therefore, essential decontamination technologies for decommissioning have been developed. However, the system decontamination for decommissioning of NPPs has not been fully developed yet in Korea.

6.Conclusions

There are two objectives of decontamination for decommissioning. The first one is to reduce risk of radiation exposure of workers and the public. The decontamination for this objective is applied before dismantling. The second objective is reducing waste generation through reuse or recycling. The decontamination of this case is applied after dismantling. These two kinds of decontamination can be diversely progressed according to the conditions of individual NPP.

Various full system decontamination technologies such as LOMI, CANDEREM, CORD, and DFD have been applied in nuclear facilities. It is possible to apply these technologies to the NPP decommissioning project since the desired DF can be obtained by repeating decontamination cycles. But the desired DF is not the only value that should be considered. Secondary waste generation containing highly concentrated radioactivity needs to be controlled because it causes additional disposal concern, management costs, risk of secondary radiation exposure and so on. Highly concentrated reagents are used in these technologies since not only contaminated layer on the surface of components but also some of the base metal should be removed to get high DF. From the analysis of EPRI DFD and CORD D UV decontamination technologies, key considerations for decontamination planning are derived as follows:

Most of chemical and electrochemical processes have been effective in decontaminating reactor components to free release limits. However this work should be performed in fixed-base center so that the material is free-released. In many overseas cases, this is more cost effective than near surface disposal of the contaminated components. After removal of the bulk of radioactivity through a full system decontamination at NPP site, the rest of radioactivity would be removed in the fixed-base facility to free release limits. Although the total decontamination is not feasible, the components may be sent to less expensive disposal facility for VLLW or LLW class waste.