Operational waste from NPPs

The original Soviet design concept for the waste management of Dukovany WWER reactors envisaged the collection, pre-treatment and interim storage of all radioactive waste produced at the site during the whole NPP operational period. Such a concept postponed the final decisions on conditioning and disposal of operational waste to the decommissioning stage. Dukovany was therefore equipped with huge liquid storage capacity: 5,320 m3 for concentrates and 920 m3 for spent ion-exchange resins. A very similar concept was designed at Temelin, but with smaller storage capacity: 520 m3 for concentrates and 200 m3 for spent ion-exchange resins.

The disadvantage of the system of wastewater treatment in Dukovany NPPs, and partly also in Temelin NPP, is mixing of all sorts of wastewater, which complicates recycling of separate wastewater streams. Wastewater is routed to drain sump tanks from which it is pumped to a sedimentation tank and then through an overflow tank to wastewater holding tanks. Wastewater from laundry and laboratories is collected in control tanks and if it is not possible to discharge it, it is pumped to the radioactive drain sump tanks. Wastewater in holding tanks usually contains 0.5-2 g of salt/l with pH ~ 8 (Kulovany, 2001).

Temelin NPP adopted design changes, which allowed the reduction of the volume of wastes produced, dividing the draining system for multiple inde­pendent systems, so that inactive water can be discarded out of the primary circuit.

In both NPPs, evaporation is used to increase the salt concentration in the waste liquid by -50. The basic equipment consists of a layer rotor evapo­rator with a vertical double-shell drum (rotor part of evaporator, produced by Czech company VUCHZ, a. s., is shown in Fig. 12.2). Prior to concentra­tion, it is necessary to increase the alkalinity of the wastewater by addition

image176

12.2 Rotor part of a layer rotor evaporator used for increasing the salt concentration in the waste liquid and bituminisation of liquid radioactive waste.

of sodium hydroxide to pH at least 11 to reach solubility at least 60 g of boric acid/l. The evaporator bottom contains a great amount of nitrates coming mainly from the recovery of cation-exchange resins by nitric acid. The problem facing the operators is to keep the boric acid in a soluble state to prevent its crystallisation. The evaporator bottom concentrates are solidi­fied by bituminisation. The asphalt and liquid concentrate are tangentially sprayed to the upper part of the evaporator. Drained bitumen product, containing 30-40% of salts, flows down the evaporator wall and through heated piping to drums, which are transported to the near-surface, fully engineered disposal facility at Dukovany (Fig. 12.3). A special thermal resistance test of bitumen compound with evaporator bottom has to be carried out before bituminisation to prevent fire hazard. For this reason, the limit for manganese content in bituminised salt is 15 g/l and for pH 11.5.

Most ion-exchange resins are currently stored in storage tanks, but the State Office for Nuclear Safety (SONS) has recently approved their condi­tioning using geopolymers developed in the Slovak Republic (Majersky et al., 2007) and the first drums (20 m3 of 307 m3 stored) were already dis­posed of in the near-surface repository at Dukovany.

Liquid radioactive wastes of organic origin (oils) are stored in 200 l metal­lic drums. Currently, approximately 1.3 m3 of this type of waste, are stored.

The solid waste bags are collected before treatment and sorted for detailed segregation of active and nonactive wastes. The activity of bags is measured using the system Merlin (Envinet, a. s.) equipped with three Canberra HPGe Big Mac detectors. The Merlin system and procedure for releasing the waste to the environment or landfill sites from nuclear facili­ties must always be approved by the Czech regulatory body.

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12.3 Fully engineered repository at Dukovany.

The only available technology for solid waste treatment is low-pressure compaction, waste crusher and cable insulation ripper. High-pressure com­paction was used to minimise the final volume of solid waste in 1996 (using a rented high-pressure compactor). Intermediate-level waste is only frag­mented (if practicable) and stored under control in the storage facility for radioactive items. Some part of the organic solid waste has recently been incinerated in Studsvik and only the ash will be disposed of at the Duko — vany repository. The average volume of RAW from operation of Czech NPPs Dukovany and Temelin is given in Table 12.2.

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