The energy and power generation markets have rarely left the spotlight, whether it is the price of crude oil or plans to instigate or reinitiate nuclear expansion programmes.
The main driver behind such media attention is consumer and business demands for affordable energy accompanied by the negative aspects of global warming caused by carbon dioxide (CO2) emissions.
As published by the Nuclear Energy Institute (NEI), Nuclear generation offers operators the lowest operating cost per MWh accompanied by minimal CO2 emissions when compared with fossil fuels but still suffers from lengthy refuelling outages.
To combat this drawback, efforts to maximise generator productivity in nuclear plants have focused on reducing plant refuelling outages.
There have been many advances in reactor shutdown techniques with the NEI recently reporting that average outage times had fallen by 64 per cent from over 100 days (1990) to around 40 days (2007).
Refuelling outages are typically completed every 15-18months with reactor shutdown and start-up as the start- and end-points.
During reactor shutdown and start-up, accurate measurements of both oxygen (O2) and hydrogen (H2) within the reactor coolant system (RCS) are critical before the reactor is depressurised and opened for maintenance to avoid an explosive mix.
Most new methods to accelerate this process rely on continuous real-time oxygen and hydrogen measurement using Orbisphere instruments rather than intermittent sampling using gas chromatography or other offline methods.
During recent trials, a top five global nuclear operator evaluated a number of different methods used in Pressurised Water Reactor (PWR) shutdown and start-up in order to find a balance between speed, efficiency and safety.
The results were staggering when considering how each could be implemented and the benefits it could bring across a nuclear fleet. The two methods discussed here are physical degassing and hydrogen-peroxide (H2O2) injection.
The first method, physical degassing using nitrogen, was an earlier method used to remove excess hydrogen but can be slow and costly due to the cost of storage and treatment of the nitrogen and other radioactive waste gases before release.
For a 1300MW block with a theoretical reactor shutdown time of 10-13hrs dependent on nitrogen flow rate, it is generally accepted that 24hours is required - this is obviously faster for smaller (900MW) reactor blocks.
Hydrogen peroxide injection, the second of those trialled, absorbs hydrogen to create water. This can be used to reduce the level of hydrogen (illustrated in Fig.1) however it relies on the use of real-time instrumentation for precise feedback on oxygen and hydrogen levels within the RCS. It has been demonstrated that rather than a 24 hour shutdown window, it is possible to complete this step within a two hour window (four hours being the adopted norm by tracking oxygen and hydrogen real-time within the reactor). This significantly reduces plant shutdown timeframes whilst balancing safety and maximizing operator control of the shutdown process.
The Hach Ultra Orbisphere instrumentation line has been developed over the years with nuclear requirements in mind and all instrumentation is nuclear certified (hence tried and tested in highly radioactive environments).
The most recent breakthrough in oxygen measurement is the Orbisphere G1200 - a luminescent ppb oxygen sensor certified and validated for use within radioactive environments.
Promoting ALARA principles (As Low As Reasonably Achievable) to limit human radiation exposure, the sensor only requires five minutes maintenance every 18 months (maintenance can be co-ordinated with plant outages giving a 100 per cent sensor uptime).
For hydrogen measurement, the patented Orbisphere TC (Thermal Conductivity) sensor was developed to give continuous, real-time ppm or ppb hydrogen readings in a gas or liquid phase with the sensor requiring an annual maintenance.
During a plant shutdown or start-up sequence, a significant margin of error is introduced during time lapses on periodic sampling thus increasing risk.
These unique features of Orbisphere sensors combined with the fact that Orbisphere sensors are designed to last for many years (ie: sensors are not consumables/disposable) reduces any radioactive waste to be handled by plant personnel and reduces disposal costs for the plant.
hen considering operation in nuclear plants, Orbisphere instruments for oxygen and hydrogen measurement have been installed in all operational plants in the USA with 90 per cent of EDF's plants in France and in over 350 plants globally.
And what does reduced outage mean for operators: primarily more time connected to the grid and with that, more generating dollars or generating capacity available.
But how much more can the use of Orbisphere instrumentation for real-time oxygen and hydrogen measurement bring to plant operators?
Using recent figures from the EDF 2008 H1 Financial Report (average realized base-load prices), a 20 hour uptime gain results in $2.5m of generating revenue for every plant shutdown on a typical 1330MW reactor in France.
So considering EDF's generating fleet of 58 reactors, that's a staggering $145m of additional revenues over 18 months through faster plant shutdown during plant outages; or the equivalent of 117MW additional capacity at near zero cost.
Hach Ultra - a wholly owned subsidiary of Danaher Corporation - service offices and authorised distributors operate in more than 80 countries. The company has four research and development centers located in Europe and the United States.
The company's research programmes ensure that customers meet regulations and optimize quality throughout their processes.
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Stephen Bell is Market Manager - Power, Hach Ultra - Orbisphere Vésenaz, Switzerland. www.hachultra.com
