GB2295483A - An electrolytic method for removing surface radioactivity from nuclear reactor cooling circuit ducting and associated components - Google Patents

Abstract

An electrolytic process is described for decontaminating radioactivity from the inside surfaces of cylindrical shaped nuclear reactor cooling circuit components. A cylindrical section 1 to be decontaminated is placed on its side on a bed fitted with powered rollers 7, allowing the cylinder to rotate. A rubber sealed end cap 9 rotates with the cylinder and is fitted with a stationary central section 15 via a rotating seal 18. The central stationary section houses the cathode 3 support/conductor/gas-input assembly and a number of services access points. A rubber sealed end cap 21 rotates with the cylinder and is filled with a cathode bearing 22 and a capped access point 23. This arrangement allows large inside surfaces of cylinders to be decontaminated using the minimum of electrolyte and electrical power while completely containing any radioactivity released in the process. The identical process can be used on non-radioactive nuclear reactor or other cylindrical vessels to produce super-clean surfaces prior to use. <IMAGE>

Description

AN ELECTROLYTIC METHOD FOR REMOVING SURFACE RADIOACTIVITY FROM NUCLEAR REACTOR COOLING CIRCUIT DUCTING AND ASSOCIATED COMPONENTS This invention relates to an electrolytic method for removing radioactivity contaminating the inner surfaces of the ducting and heat exchanger casings which form the cooling circuits of decommissioned nuclear reactors.

Nuclear reactors are decommissioned at the end of their useful life. At a total of about 3000 tonnes, the cooling circuit ducting and heat exchangers form the largest mass of radioactive steel in a defunct reactor. The ducting and heat exchangers are in the general geometric form of large thick walled steel cylinders; the ducting having typical diameters of two metres and the heat exchanger boiler casing typical diameters of six meters.

The radioactivity is deposited in the anti-corrosive graphite loaded paint coating and underlying metal of the inside surface of the cylindrical sections. The radioactive surface layer is restricted to a combined paint/metal depth not greater than 50 Mm (micro metres). Apart from a very thin layer of radioactive metal of less than 20 Um thick, the vast mass of metal is in its original state and almost entirely free of reactor produced radioactivity.

By removing the thin radioactive graphite paint and metal layer in a safe and cost effective manner the bulk of steel can be reduced to a level of specific activity of less than 0.4 Bq.g-l, that is, below regulatory concern, and released to industry for recycling, thus avoiding the immense cost of radiological disposal and long term storage of untreated reactor metal.

The concentrated radioactive material removed from the cylindrical sections and requiring radiological storage facilities consists of a relatively small mass of paint flakes and iron dust.

Electrolytic methods for the cleansing of radioactive contaminated metal have been proposed in the past, but not widely adopted. This new method significantly differs from those previously proposed in that: the process is remotely controlled and conducted within its own sealed enclosure, ensuring that no radioactive or toxic emissions are released to the environment and that radiation doses to workers are kept to a minimum. The unique arrangement of the electrodes uses a very small amount of electrolyte to decontaminate large surface areas, thus keeping the concentrated radioactive residue to a minimum and the costs relatively low.

Although this process is mainly restricted to large cylindrical shaped metal vessels contaminated with radioactivity on the inside surface, this is exactly the shape and condition of the bulk of radioactive metal in nuclear reactors.

This same process can be used to produce extremely clean surfaces in nuclear reactor or other cylindrical vessels requiring super purity surfaces prior to use.

The invention consists of a self enclosed electrolytic bath utilising a novel arrangement of the electrolyte and the electrodes. The process involves the dismantled nuclear reactor radioactive ducting, in the form of lengths of flanged and unflanged cylinders, being placed on its side on a bed fitted with power driven rollers allowing the whole cylinder to rotate.

A suitable length of ducting to be decontaminated itself forms: a) the anodic electrode, b) the bath containing the electrolyte and c) its own sealed enclosure.

To retain the electrolyte, end plates are fitted to the cylindrical section and rotate with the cylinder. One end plate is fitted with a stationary supported central plate and is attached to the rotating plate via a rotating seal. The stationary section facilitates access for internal services.

The stationary metal cathode can be either a solid bar or a hollow tube with fine holes along its length. This latter arrangement allows air or nitrogen gas to be injected under pressure between the cathode-anode space thus clearing the electrical current path of detritus and purging the interior of hydrogen.

The considerable advantages of this process are: a) a large radioactive surface area is treated with a small amount of electrolyte, thus avoiding a large quantity of radioactive liquid waste which itself would require further expensive treatment prior to disposal and storage, b) the electrolyte and electrical power costs are low, c) man-handling in preparing the sample for treatment is minimal, thus avoiding man-dose and manpower costs normally required for cutting the cylindrical sections into small parts for storage or disposal, d) the process is operated remotely thus avoiding man-dose, e) the process is completely self-enclosed, thus avoiding radioactive gases or particulates releasing to the environment, f) the usual incurred enormous disposal and long term storage costs are avoided and the vast bulk of the metal can be sold for scrap and recycled.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Fig. 1. shows in perspective a dismantled flanged section of reactor cooling circuit ducting placed in position on its side on rollers; Fig. 2. illustrates the rotating metal end plates and stationary services access plate; Fig. 3. shows a cross section view of the electrode/electrolyte geometry; Referring to Fig. 1, a flanged 6 cylindrical section of reactor cooling circuit ducting 1, is placed on its side on powered rollers 7 allowing rotation 8. The end plates 9 and 21 (see Fig.

2) are fitted to the flanges 6. A small amount of paint stripper is placed in the bottom of the cylinder via access point 23 (see Fig 2.) The cylinder is then rotated 8 allowing the paint stripper to cover the whole inside surface area. When the paint has been sufficiently softened and detached, a small quantity of electrolyte is introduced into the cylinder, via the access point 23 (see Fig 2). The anodic cylinder 1 is connected to the positive terminal 2, and the cathodic bar 3 is connected to the negative terminal 4 of an external low voltage high current power supply. The cathode 3 is submerged in a small amount of electrolyte 5. The resulting electrolytic action removes metal from the inside surface together with paint flakes.

Referring to Fig. 2, the end plate 9 with bolt holes to match existing bolt holes on the existing cylindrical flange 6 fitted with a rubber gasket 10, rotates with the cylinder, thus containing the pool of electrolyte. The stationary section 15, which is externally supported and stabilised by appropriate means, is attached to the rotating end plate 9 via a rotating seal 18.

The cathode electrical conductor 4 when formed by a hollow copper tube, is also the access point for air or nitrogen gas used for electrolyte agitation, internal ventilation flow, and purging of the hydrogen gas produced during the operation of the process.

The stationary section 15 is fitted with a number of sealed services access points, namely, a central insulator 19 holding the cathode supply conductor 4, a gaseous exhaust connection 16 connected to a flexible hose and extraction system fitted with a HEPA (High Efficiency Particulate Air) filter, a sludge removal connection 20 connected to a flexible hose to a special shielded holding tank, and a monitoring point 17 incorporating special sealed electrical connections for monitoring internal temperature, condition of electrolyte, radioactivity, hydrogen gas etc.

The secondary end plate 21 prepared with bolt holes to match existing bolt holes on the flange 6 and fitted with a rubber gasket 24 totally rotates with the cylindrical section. It incorporates a central insulated bearing 22 which supports the cathode bar 3. A capped access point 23 is available for access and inspection purposes etc.

Referring to Fig. 3, the intense electrical field 11 is shown in cross section, the electrolytic action is most intense immediately under the cathode bar 3. The chemically disrupted radioactive paint and underlying metal surface is shown 12, as is the decontaminated surface 14. Electrolysis takes place most efficiently at the edge of the disrupted layer of paint 13.

Claims (10)

1 A decontamination process using electrolysis for removing surface radioactivity from the inside of large metal cylindrical shaped nuclear reactor cooling circuit components, the cylindrical sections being placed horizontally on a power driven roller bed on which the cylinder can be rotated. Special end caps are provided to retain the electrolyte, support the cathode, supply access service points to the interior of the cylindrical section and maintain interior enclosure integrity.

2 A decontamination process as claimed in Claim 1 wherein electrical power supply means are provided on the cylinder casing for a sliding heavy current electrical connection to maintain connection while the cylinder rotates, facilitating the cylinder casing acting as a rotating anode.

3 A decontamination process as claimed in Claim 1 or Claim 2, wherein electrical power supply means are provided on a bar or hollow tube to connect it as a stationary cathode.

4 A decontamination process as claimed in Claim 1 or Claim 3, wherein means are provided to support the stationary cathode in position close to the inner surface of the cylindrical section.

5 A decontamination process as claimed in Claim 1 or Claim 3 or Claim 4, wherein the cathode is provided as a hollow metal tube with small holes penetrating the wall of the tube to facilitate clearance of inter-electrode detritus.

6 A decontamination process as claimed in Claim 1 or Claim 3 or Claim 4 or Claim 5, wherein the cathode is provided with gas pressure input means to facilitate clearance of inter-electrode detritus, input ventilation, and purging of hydrogen gas.

7 A decontamination process as claimed in Claim 1, wherein rotating end caps are provided to retain the electrolyte, maintain internal enclosure integrity, in which one end cap is provided with a central stationary section wherein facilities are provided for an electrically insulated bearing to support the cathode bar assembly, an access point for sludge removal, an access point for controlled output ventilation, sealed electrical connections for data collection on the sealed environment within the cylindrical section.

8 A decontamination process as claimed in Claim 1 or Claim 7, wherein one end cap rotates and is fitted with an insulated bearing to support the cathode bar and a capped access point for introducing electrolyte or for inspection purposes.

9 A decontamination process as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 or Claim 6, wherein specially CLAIMS cont.

shaped bars are added to the cathode, and the rotation is temporarily stopped to enable longer and more intense treatment of welds, which tend to retain radioactivity at greater depths than the normal surface.

10 A decontamination process as claimed in Claim 1 wherein instead of removal of radioactivity, a super-clean surface is obtained in nuclear reactor or other cylindrical vessels requiring super-pure surfaces prior to use.