Reactor components, decontamination

Reactor Component Assembly and HX Decontamination. DPSOP 25-2, E.I. duPont de Nemours and Company, Aiken, SC, November 1987. 

In decontaminating a reactor component, there is always the problem that after a relatively short period of continued plant operation redistribution of radionuclides from the non-decontaminated surfaces of the other circuits and components will result in a recontamination of the cleaned regions. This effect can only be reliably precluded by decontamination of the whole reactor system, possibly with the fuel in place. Such decontamination operations require additional preparatory work as compared with the decontamination of components, due to the large volumes to be treated which require large amounts of decontamination solution, resulting in large amounts of radioactive waste. Further, additional preparatory work is necessary because different materials are exposed simultaneously to the decontamination solution. Since the mid-1960 s, a number of reactors have been decontaminated in this way more details will be given in Section 4.5.3. 

The following sections will concentrate on procedures which are applied for operational decontamination of reactor components, systems and circuits, i. e. decontamination of materials that are going to continue to be used, often for decades a summary description of the relevant topics has recently been published by IAEA... 

If the central off-site workshop is used for maintenance of large components, the operating organization should ensure that the same quality assurance requirements are applied at snch a facility as at the nuclear power plants. Reactor components shipped to a central facility may need more stringent standards of decontamination and, when such components are dismantled, appropriate off-site radiological controls shonld be provided. 

Dismantling of decommissioned nuclear reactors requires special procedures. The outer parts can be handled like normal industrial waste, whereas the inner parts, mainly the reactor vessel and some core components, exhibit high radioactivity due to activation. Radioactive deposits on the inner surface of the reactor vessel may be removed by chemical decontamination. Altogether, the relatively large volumes of LLM and MLW which are obtained by dismantling are further processed and then preferably enclosed in concrete or bitumen. 

Most of the waste from the dismantling of decommissioned nuclear reactors is non-radioactive. Also, large volumes can be decontaminated and "declassified which permits them to be treated as normal industrial waste. Of the remaining waste, most contains low levels of relatively short-lived nuclides similar to wastes from normal reactor operation. Hence, it can be treated and disposed of in the same manner, e.g. put in bitumen or concrete, and stored in repositories for LLW and ILW ( 21.9). The volumes may be rather large, however. Some internal parts of the reactor have relatively high activity. The induced activity is limited to part of the pressure vessel (closest to the core) and some core components. 

Principal Component REACTOR SERVICES GROUP (Continued) Decontamination Services Tanks Pumps... 

Figure 1 shows the location of the facility for sodium cleaning in the nuclear island. The core components are handled by fuel handling machines where as the primary components are handled by respective flask. The components that are to be decontaminated are removed from the reactor when the reactor is in shutdown condition. The handling of the core components is based on fixed time duration and interval in the reactor. For the primary components, it is as and when required. 

From comparative measurements performed at 10 US BWRs where different chemistry measures and materials pretreatment procedures had been employed, it was concluded that electropolishing and pre-oxidation of the materials, plus subsequent addition of zinc to the reactor water, results in a slower contamination buildup, whereas electropolishing alone did not show any significant effect (Duce, 1990). These findings apply to decontaminated pipework just as for new pipes installed in the course of the replacement of components. 

The chemical composition and the structure of the contamination layer oxides in the primary systems depend highly on the reactor type (PWR, BWR) and also on the operational history of the plant. The optimum decontamination process to be applied, therefore, must be selected and, if necessary, optimized to meet the task in question. In particular, when components and systems are to be decontaminated, different structural materials and, consequently, oxide layers of different composition are involved, resulting in more stringent demands on the decontamination procedures to be applied. 

In the past years a large number of decontaminations have been carried out, most of them in nuclear power plants in the USA, with the fields of application ranging from components which have been removed from the primary circuit to subsystems and to full reactor primary systems. In the following section only a few examples will be cited with special respect to the procedures applied a more detailed survey of decontamination applications in water-cooled reactors was given by the IAEA (1994 a). 

Riess, R. German experience including chemical, electropolishing and decontamination for decommissioning. Paper ANS Executive Conf. E>econtamination of Power Reactors The Costs, Benefits and Consequences. Springfield, Ma., 1984 Sasaki, T, Kobayashi, T, Wada, K. Method and apparatus for regenerating an acid electrolyte that has been used in the decontamination of components with radioactively contaminated surfaces. European Patent Specification 0 141 590 B. 1 (30.1.1991)... 

Wille, H., Bertholdt, H. O. Recent developments in component and system decontamination. Proc. 5. BNES Conf. Water Chemistry of Nuclear Reactor Systems, Bournemouth, UK, 1989, Vol. 1, p. 163-167... 

After the reactor is shut down by control rods, cooled down and after-cooled during 30-90 days, the plant is brought to a nuclear-safe condition by removing the fuel. The heat exchangers of circuits E-Ill, the TEG, the drives and other components are disassembled in succession. The shielding plug and then the reactor cover are removed using a special container they are later decontaminated and placed in special rooms at an elevation of minus 14.2. 

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