Harnessing the power of bubbles for repair

18 August 2015



In a world first, ultra-high pressure cavitation peening is set to be performed on the reactor vessel closure heads at the Byron and Braidwood nuclear plants in the USA.


Today 437 nuclear power plants are in operation in 30 countries. Nearly 80% of these reactors are over 20 years, and 50% are over 30 years old. The worldwide fleet is ageing. Most operators have already engaged in the process of extending the lifetime of their nuclear power plants and they are actively seeking the most effective means of managing the ageing of their power generation asset.

One of the biggest concerns related to nuclear plant ageing stems from primary water stress corrosion cracking (PWSCC). This can lead to increased costs for operation, maintenance, assessment, repair and replacement of components in boiling water reactor (BWR) and pressurised water reactor (PWR) plants.

PWSCC is caused by a combination of three factors:

  • Tensile surface stresses at the exposed, wetted surface;
  • A susceptible material (alloy 600 base metal and filler metals 82 and 182);
  • A corrosive environment (borated water at a high temperature and pressure).

Alloy 600/82/182 materials are widely used in PWR systems. Consequently, PWSCC has been reported in the reactor pressure vessel top-head penetration nozzles, reactor vessel bottom-mounted nozzles, dissimilar metal welds in the primary system piping, steam generator tubes and plugs, pressuriser heater sleeves, pressuriser nozzles and reactor hot-leg piping instrumentation nozzles. Many of the components have been repaired or replaced to mitigate the issue. However, there are some locations where repair or replacement is not a cost effective option.

Consequently, a third option that provides in-situ material remediation is needed.

Fortunately, there is a solution: ultra-high pressure (UHP) cavitation peening. This technique creates compressive surface stresses, effectively eliminating one of the three contributing factors of primary water stress corrosion cracking. Extensive testing has demonstrated that PWSCC will not initiate on treated materials, even in severe test environments.

How does cavitation peening work?

Cavitation is the result of a physical phenomenon. It is the formation of vapour bubbles in a liquid, as a result of process pressures dropping below the vapour pressure of the fluid. After the vapour bubbles are formed, ambient pressure forces them to implode while still submerged. Each bubble collapse or implosion creates a significant shockwave.

The UHP cavitation peening process creates this physical phenomenon and delivers the cavitation bubbles to the surface of the susceptible material, harnessing the power of the shock waves to improve the condition of the material.

The process uses water from the refuelling canal, so there are no foreign materials or dilution concerns.

The water is pumped using custom pumps to an ultra-high pressure and forced through a very small orifice of proprietary design, exiting at a very high velocity. This high-velocity flow creates a low vapour pressure region through the orifice, which generates cavitation bubbles. The bubbles are effectively 'sprayed' across the surface of a susceptible material. As these vapour bubbles collapse on the material surface, shock waves travel into the material and create compressive residual stresses on the surface and into the material. Minimum compressive stress depths are demonstrated to meet or exceed Electric Power Research Institute (EPRI) guidelines. Eliminating the pre-existing tensile weld residual stresses mitigates the material's susceptibility to PWSCC.

Used preventatively, cavitation peening removes many causes of ageing, since by creating deep residual compressive stresses it improves fatigue life and damage tolerance. The process is commonly used in aerospace industries to improve metal properties. AREVA has been applying peening technologies to improve the repair life of susceptible materials since 2001, addressing over 110 RV head nozzles. The company has recently begun offering cavitation peening to nuclear utilities to mitigate PWSCC on reactor pressure vessel top head penetration nozzles, reactor vessel bottom mounted nozzles and dissimilar metal welds of the reactor vessel inlet and outlet nozzles. The process is safe, forgiving and easily adaptable to complex geometries. To pave the way for the nuclear industry, AREVA has demonstrated the process to EPRI and the US Nuclear Regulatory Commission (NRC) and has responded to their questions.

The process

AREVA will work with a utility to determine the best timing to complete the cavitation peening process during a scheduled refuelling outage. It will use diagrams of the laydown space in the containment to determine the optimal position for the ultra-high pressure pumps and routing the hoses used in the process. To minimise personnel radiation dosage, the equipment is installed and operated remotely, using a programmed control system. The mitigation zone must be confirmed and programmed into the delivery system. The weld fabrication drawings related to the components being peened are thus required to determine the required peening coverage.

After setup, it takes a few hours to complete the peening process for each nozzle. Operators enter data for a particular nozzle or part to be peened and carefully monitor the system's progress during the peening process. Quality is ensured through the monitoring and recording of key process variables and periodic testing. To ensure chemical integrity, the plant's cavity water is used and returned directly to the reactor cavity.
Reactor pressure vessel top-head penetration nozzles can be treated on their head stand during a typical refuelling window. The internals are removed to peen the reactor vessel bottom-mounted nozzles and dissimilar metal welds of the reactor vessel inlet and outlet nozzles.

Qualification

To qualify this technology, newly used in the nuclear industry, AREVA will meet regulatory guidelines and demonstrate control of the process.
Major milestones have already been reached. Process and tooling was approved by a multi-national design review board. It was followed by the definition of the parameters and fixing the process variables. AREVA has been actively participating in industry peening events. This includes hosting several demonstrations and meetings at the AREVA training centre along with the participation of EPRI MRP and the NRC throughout the first and second quarter of 2015.

The process has been demonstrated to meet or exceed EPRI guidelines for compressive depths. Now site-specific qualification is under way, with the aim of first implementation in early 2016.

The following steps will be used for site-specific qualification. The peening process effectiveness will be demonstrated on full-scale representative mockups at the nominal, maximum and minimum ranges of essential variables used for site implementation. The full-scale nozzle mockups will also reproduce material and geometry constraints. X-ray diffraction will be used to determine the residual stress on peened full-scale representative mockups. The inspection must confirm that the desired peening coverage has been achieved and that the desired depth of compression has been achieved.

A qualification report will be provided to the customer that documents how all requirements have been met for the Topical Report/Relief Request/ASME Code. This topical report is a document that addresses a technical topic related to nuclear power plant safety, which the industry submits for review and approval by the NRC before publishing for use in the licensing process by other licensees.

An extensive 'field hardening' campaign is planned for the second half of 2015 to prepare tools and operators for the first reactor vessel head cavitation peening implementation with Exelon in early 2016.

What are the benefits of cavitation peening?

For utilities, many benefits result from performing cavitation peening on reactor components. The primary purpose for using peening is to extend the life of reactor components, extending the life of the plant. The material benefits of cavitation peening have been demonstrated to last indefinitely.

Incorporating this process into a maintenance schedule has minimal impact during a service outage. Peening can typically be completed within a few days and in parallel with other maintenance work. Since AREVA's process operates at a higher pressure than other providers, the company claims the process achieves better results and faster work completion, leading to a lower overall implementation schedule.

In many ageing plants, one of the biggest expenses is replacing components due to wear and degradation, such as the reactor vessel closure head. By proactively peening such components, utilities can mitigate the effects of ageing and extend the life of the component. Even better, cavitation peening all nozzles on a component is often less expensive than repairing a single nozzle.

PWSCC susceptible materials are subject to greater inspection requirements than are non-susceptible materials. EPRI's Materials Reliability Program (MRP) is leading the industry effort with the NRC to provide inspection credit after peening for reactor vessel closure heads and reactor vessel primary inlet and outlet nozzles. The basis for this credit is documented in ASME Code Cases draft N-729-5 and N-770-4. These mechanised, remote inspections can be costly, so reduced inspection frequencies will reduce operation and maintenance expenses

 

How the cavitation preening process works
Model showing cavitation peening process


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