Experience of chemical decontamination3 August 2002
Chemical decontamination has been an effective method for reducing piping system and component dose rates at nuclear plants. Fully qualified, field-proven processes are available for nearly every application, from in-situ decontamination of the reactor coolant system to ex-situ decontamination of components. By Ron Morris
To achieve the optimum results when decontaminating reactor systems and components, their precise chemistry must be established. Where possible, representative remnants, which have been exposed to plant water chemistry during reactor operations, are tested as part of a programme to accomplish this goal. For those times when representative remnants are not available, reactor operating conditions, system materials and chemical decontamination experience play a major roll in selecting the optimum chemical decontamination process.
Several recent applications and results of Westinghouse subsidiary PN Services' chemical decontamination projects demonstrate available chemistries and technologies and the types of systems to which they have been applied.
The experiences provide evidence that qualified, field-proven processes are available for nearly every application, and unique applications can be engineered to achieve excellent results.
For Susquehanna and Laguna Verde it was the introduction of a specialised level control system in combination with the LOMI-NP chemical decontamination process that proved successful. Remnant testing at River Bend and application of the CITROX-AP chemistry proved to be the best formula for the removal of biological fouling and iron and copper deposits found on the residual heat removal (RHR) heat exchanger tubes. At Big Rock Point and Maine Yankee the EPRI Decontamination for Decommissioning (DfD) process, along with some innovative concepts and engineering designs such as the "Spider and the Snowflake", proved optimum for the decommissioning decontaminations. The online RHR decontamination at Pilgrim showed that it is not always necessary to use critical path time for chemical decontaminations during scheduled refuelling and maintenance outages. And the demonstration at Los Alamos showed that commercially used chemical decontamination processes can be successful for the decontamination of non-reactor systems.
Susquehanna & Laguna Verde
Susquehanna and Laguna Verde are 1100MWe and 654MWe General Electric (GE) BWRs, respectively. Chemical decontamination of the reactor recirculation system (RRS) piping and components at Susquehanna in 2000 and at Laguna Verde in 1998 were performed primarily to reduce general area radiation levels in the drywell to facilitate outage maintenance activities. In both cases, the EPRI LOMI process in combination with nitric acid permanganate was used resulting in decontamination factors of 14.4 and 34.3 respectively. The application of the primary coolant loops took place with fuel in the vessel to reduce the outage critical path time required for the decontaminations.
Shorter maintenance and refuelling outages have made chemical decontamination less attractive since most of the work has been performed during scheduled outages. To reduce the impact on critical path, PN Services has devised a patented level system in combination with vented RRS suction and discharge nozzle plugs. This allows chemical decontaminations to be performed without draining the reactor vessel. The approach was used at both Susquehanna and Laguna Verde, and decontamination factors were achieved that compared favourably to those where the fuel had to be removed from the vessel and the vessel drained for the decontamination.
River Bend is a 939MWe General Electric BWR. In 1997 heat transfer testing of the residual heat removal (RHR) system indicated that fouling existed on the shell side of the Division II heat exchangers. Further visual examination and remnant testing of scrapings taken from the heat exchangers confirmed that a biological and iron/copper deposit had formed on the surfaces of the copper-nickel tubing and support plates.
In conjunction with plant personnel, PN Services performed several tests in the laboratory to determine the optimum chemistry to dissolve the corrosion deposits and destroy the biological fouling. Ultimately PN Services' CITROX process was chosen in combination with the alkaline permanganate oxidation chemistry. Hydrogen peroxide was also a contender for the application but prior experience at the plant with the AP chemistry and materials compatibility made the decision for the plant personnel.
The application of the CITROX-AP chemistry restored the heat transfer characteristics of the RHR heat exchangers to a level where the plant could be safely returned to service after the outage. On the subsequent outage the Division I heat exchangers were cleaned, using similar chemistry and application methodology, with similar results. Because of a favourable outage schedule window PN Services was requested to re-clean the Division II RHR heat exchangers as part of the heat loss technical evaluation being performed at the plant. Additional deposit was removed from the Division II heat exchangers. An added benefit for the year 2000 cleanings was the decontamination factors of 3.9 (Div I) and 9.8 (Div II) achieved during the applications.
Big Rock Point
Big Rock Point is a 67MWe General Electric BWR that was permanently shut down on 29 August 1997. As part of Consumers Energy's decommissioning plans, a chemical decontamination of the primary cooling system, reactor water cleanup system and the shutdown cooling system was scheduled using the EPRI DfD process. This was the first full system decontamination for decommissioning using the DfD process.
The primary cooling system (PCS), comprising the reactor vessel, steam drum, PCS pumps and piping, and the tube side of the of the regenerative heat exchangers was decontaminated using a six-cycle high temperature DfD process applied at 90ºC. PN Services supplied decontamination equipment to augment plant equipment during the chemical decontamination process. However, for the entire duration of the decontamination the two plant PCS pumps were utilised giving approximately 150,000l per minute recirculation flow through the primary system. At the conclusion of the high temperature process the system was cooled to 30ºC and the shutdown cooling system and the full reactor water cleanup system were added to the flowpath. Details of decontamination flowpaths are given in Figures 1 and 2.
At Big Rock Point an overall dose reduction factor of >=27 was achieved for the DfD decontamination and specified contact radiation dose levels were reduced to less than 100mSv/hour (10mR/hour). The decontamination resulted in less radiation exposure to plant employees and a decreased potential for the spread of contamination.
Maine Yankee is an 860MWe Combustion Engineering PWR. In the spring of 1998 PN Services performed a Full Loop chemical decontamination for decommissioning of the Maine Yankee power station. This was the first application of the EPRI DfD process to a PWR, and the second application to support the dose reduction goals for a decommissioning plant. Other reasons for performing the decontamination were to:
• Improve worker safety by lowering dose and activity levels.
• Eliminate many locked high radiation areas.
• Maximise benefit to all in the future and to ensure the decommissioning did not exceed the generic environmental impact statement (GEIS) limit of 1115 person Rem (11.15 person Sv).
• Allow the decommissioning operations contractor to proceed unimpeded with dismantling.
The decontamination for decommissioning was performed in two distinct applications. The first application included the charging system, pressurizer spray system, letdown system, high and low pressure seal injection systems, portions of the loop fill and drain lines, and portions of the RCS. The second application included the residual heat removal system, the remaining loop fill and drain lines and the remaining RCS system. Because the reactor pressure vessel was bypassed during decontamination, this decontamination was known as a "Full Loop" application.
The Full Loop was first introduced by PN Services in the early 1990s. The initial concept was introduced to decontaminate PWR RCS loop piping where plants did not have loop stop isolation valves. This initial concept was later developed for the Maine Yankee decontamination for decommissioning when Maine Yankee requested that the vessel be eliminated from the decontamination flow-path to reduce secondary waste produced during decontamination and to ensure that the resultant waste was kept from exceeding Class C (see Figure 3/figures currently not available).
As part of the design of the Full Loop decontamination concept PCI, a sister company of PN Services, designed and fabricated a six-legged plug assembly which became known as the "Spider and the Snowflake." The design entailed a series of flow-through plugs supported from the core barrel flange seating surface of the reactor pressure vessel by a steel frame. The Spider and Snowflake concept was so successful at Maine Yankee that a similar design was used by Connecticut Yankee for its Full Loop decontamination for decommissioning in 1998. Figure 4 shows the Spider and the Snowflake assembly mounted in the reactor vessel in readiness for the start of the decontamination at Maine Yankee.
The decontamination at Maine Yankee maximised the dose benefit for the ongoing decommissioning and supported dismantling operations. Using 48 predetermined survey points, an average decontamination factor of 31.5 was achieved, which corresponds to an activity removal of 96.8%. For fifteen radiologically significant points, where the initial field was >=100 mRem/hour (1mSv/hr), the contact decontamination factor was 89.5 with an activity removal of 98.9%. This eliminated many of the locked high radiation areas at the plant which were above 1Rem/hour (10mSv/h).
Pilgrim is a 670MWe General Electric BWR located in Massachusetts. In 1998, the Pilgrim residual heat removal (RHR) system was decontaminated using the CITROX-AP-CITROX process. Two separate decontaminations were performed, one on each RHR loop, by invoking limiting conditions of operations (LCOs) for applicable system functions. This allowed the decontaminations to be performed in a specific period of time while the reactor was in operation. An overall decontamination factor of 7 was achieved by measuring survey points through installed insulation and shielding.
Prior to the application an engineering evaluation was performed to determine the optimum RHR connection points to be used for decontamination. This concluded that only one additional connection to the RHR system would be necessary to support decontamination operations and to apply the decontamination chemistry to the RHR system. This connection was made during reactor operations and prior to PN Services setting up equipment for the decontamination.
Los Alamos National Laboratory
In February 2000, a chemical decontamination demonstration was performed at the Los Alamos Neutron Science Center in New Mexico. The demonstration was performed by PN Services and members of General Atomics and Los Alamos National Laboratory to demonstrate the effectiveness of commercially available decontamination technologies on accelerator and spallation target heat removal systems. This was part of the Accelerator Production of Tritium Target/Blanket Engineering Demonstration and Development programme.
The Area A He Loop Cooling Water System components were used to recirculate and sample the decontamination chemistry. Only minor modifications were required, including installation of a chemical mixing and injection system; a 7.5kW heater; a corrosion-monitoring vessel; and one additional cation exchange resin vessel.
For this system and other water-cooled spallation targets, the dose rate from plateout activity is dominated by decay of Be-7, which is produced from spallation reactions occurring directly in the cooling water. Two applications of PN Services' proprietary NITROX-E process were applied during the demonstration, and an average Be-7 decontamination factor of 200 was achieved. The average decontamination factor for all gamma-emitting radionuclides was 53.
The decontamination demonstration performed over a three-day period was successful in removing activity throughout the system.
|Chemical decontamination processes|
|LOMI – is an acronym for Low Oxidation State Metal Ion and was developed by the scientists at the Central Electricity Generating Board (CEGB) in England in the late 1970s and early 1980s. The process incorporates vanadium (II) as a reducing agent and picolinic acid as the complexing or chelating agent. LOMI has been the most successful process for the removal of deposits where zinc and hydrogen water chemistries (HWC) have been employed during reactor operations. It is also the only process approved by the Electrical Power Research Institute (EPRI) and General Electric (GE) for use on GE designed reactor systems including the reactor pressure vessel and fuel. LOMI-2 – similar to the properties outlined above for LOMI, but adjusted to be applied in a regenerative mode. The process, developed by EPRI in the late 1990s, reduces the secondary waste produced from the decontamination. CANDEREM – a regenerative process comprised of citric acid and EDTA was developed by the Atomic Energy of Canada (AECL) in the mid-1980s. The CANDEREM process was used for the full system chemical decontamination performed by PN Services at Indian Point 2 in the mid-1990s. The process is now approved by Westinghouse for full system decontaminations, with fuel in place, for Westinghouse PWRs. CITROX – a dilute regenerative process to be applied to both PWR and BWR reactor piping and system components developed in the 1980s. The CITROX process comprises citric acid and oxalic acid. NITROX – a proprietary chemistry of PN Services was developed in the mid-1990s for the chemical decontamination of reactor coolant pumps (RCPs). The cyclic process containing nitric acid, oxalic acid and potassium permanganate was modelled on the CITROX process and was developed to minimise secondary waste. The NITROX process was qualified by Westinghouse specifically for the chemical decontamination of Westinghouse RCPs. NITROX-E – similar to the properties outlined above for the NITROX process but adjusted to destroy the chelating species during the process. The NITROX-E chemistry has been applied very successfully to both reactor coolant pumps and contaminated systems since its inception in the late 1990s. REMCON – is a family of chemistries employed by PN Services for very specific customer applications. AP and NP – alkaline permanganate (AP) and nitric acid permanganate (NP) are oxidation processes applied when radioactive deposits contain high levels of chromium. These processes were developed in the early 1980s and are used when the presence of chromium in the deposit renders the deposit insoluble by simple acidic dissolution. Remnant testing prior to the chemical decontamination, or samples taken during the process, can determine when these chemistries need to be applied. DfD – or Decontamination for Decommissioning was developed by EPRI primarily for the decontamination of reactor systems and components for free release. The process was developed in the late 1990s and used by PN Services for the full system decontamination for decommissioning at Big Rock Point in Michigan and at Maine Yankee.|