Physical characterization

Detailed and complete physical characterization include:

• description of the current status of the plant (number of units, type of reactor, connections to existing infrastructure, operational history)

• description of the main technological systems and equipment

• material inventory (masses, material types, shape, location)

• description of structures, including protective barriers and an estimation of their residual life).

Basic technical parameters of WWER-440 reactors are given in Appendices 1 and 2. A plan of a multi-unit site is given in Fig. 4.

7.2.1. WWER construction and operational features relevant to decommissioning

The following paragraphs indicate features of WWER-440 reactors, arising from design or operating history, that can influence the approach to decommissioning [33] (see also Table X).

Table X. WWER features related to decommissioning based on Greifswald experience

Facilitating Complicating

Design & construction

• on-site independent fuel storage normally available

• liquid and solid waste storages not designed for retrievability

• enough space in controlled area for easy dismantling

• twin-unit and multi-unit sites with several systems in common, i.e. dismantling of one unit difficult

• primary loop components exchangeable • tube storage module for activated components in reactor hall

• low-Co and low-Ni steels used • building construction very simple (concrete slabs, etc.)

Operation

• low contamination due to water chemistry • SG leakage has often led to contamination of secondary loop

• low level of fuel leakage • often fuel pool leakages

• borate precipitation in liquid waste storage tanks

(a) All WWER-440 NPPs have been constructed as so called twin units. The main feature of this type of construction is the structural arrangement of two reactors in one building with a common reactor hall.

(b) Most WWER-440 sites are multi-unit sites, i.e. at NPP site there are several twin units.

Some of the technological systems serve more than one twin unit building.

(c) A majority of the WWER-440 sites have sufficient space for further development (nuclear or non nuclear). Thus there is little pressure to complete decommissioning rapidly to enable such development.

(d) Due to the nature of building constructions, i.e. prefabricated slabs and no containment, the boundary of the potential safe enclosure structure is not evident. A detailed study is necessary in order to define the optimum boundary.

(e) Any leakages into secondary circuits (for example steam generator tube leaks) will require more thorough radiological investigations and radiological classification of the equipment and systems in the turbine hall.

(f) The ventilation systems of WWER-440 plants have to be modified for decommissioning purposes. Due to the lack of pressure regulated control systems and filtering in certain important areas (e.g. reactor hall), special care has to be taken to avoid airborne contamination and release of radioactivity to the environment.

(g) The spent fuel is stored in the storage pools located in the vicinity of the reactor within the reactor hall. From the nuclear safety point of view this is a factor limiting the operation termination activities and early stages of decommissioning. For safety reasons a storage of at least 3 years is required before the fuel assemblies can be removed from the reactor hall. Any damaged or leaking fuel assemblies will complicate this process.

(h) Radioactive waste processing technology was not included in the original design of the plant. Therefore, operating NPPs have accumulated large volumes of liquid (concentrates, sludges, spent ion exchange resins, contaminated oil) and solid radioactive waste. Waste is stored at the NPP either unconditioned or partially processed (evaporated, compacted) to meet the available storage capacity. Problems can be expected during removal of solid waste from storage vaults where the waste may not have been placed in an orderly fashion. This material may be difficult to remove due to potential biological degradation, gas generation and waste volume changes.

(i) Storage tanks contain precipitated borates. Chemical regime, high concentration of salts in evaporator concentrate and boric acid content are the main factors resulting in precipitation of salts. Removal of borates from the storage tanks requires the application of specific techniques.

(j) Activated parts are stored in the reactor hall pits. Removal of these activated parts will require special tools and transport containers.

(k) Leakage of contaminated liquid (e.g. from the storage pools) has occurred in some cases so that contaminated concrete must be removed under radiological controls.

(l) Stability of barriers can be another issue. According to current WWER440 decommissioning studies (Paks, Jaslovske Bohunice, Kozloduy) it is intended to use existing hermetic compartments for long term protection of activated and contaminated components. Stability of these barriers or components will have to be demonstrated for this purpose.

7.2.2. Material inventory

Aspects of WWER-440 design and construction (no containment, 6 circulation loops, 2 turbines per reactor) mean that the material inventory of WWER differs from western PWRs.

Additional differences result from local conditions (e.g. absence of cooling towers, size of site). Almost all activated and contaminated materials are situated in the controlled zone of the reactor building and the auxiliary building. Only a small part of the sanitary building belongs to the controlled zone. The turbine hall is classified as a monitored area. Auxiliary buildings and sanitary buildings are connected to the reactor building by corridors (ee Fig. 4).

Characteristic data of structures (NPP Paks, V-213 type) [18] are given in Appendix 2.

The total volume of materials arising from the dismantling of a double unit WWER-440 V-213 type technological equipment (detail parameters of the main components are shown in Appendix 3), auxiliary and sanitary buildings as estimated for the NPP Paks are summarized in Table XI [18].

The total quantity of material to be dismantled at the NPP Greifswald units 1–8 have been estimated as 1.8 million tonnes. From the total decommissioning mass, the non-contaminated plant parts (mainly turbine hall as well as the non-contaminated building structures and remaining materials) represent 1.2 million tonnes. The remaining material quantity of approximately 0.5 million tonnes and their classification are described in [34].

Table XI. Material balance calculated for NPP Paks (Unit I, II) [18]

Stainless Main building (see Appendix 2)

Activated 700 300 - - 200 -