Ontario Power Generation
PICKERING UNIT 1 START-UP DETAILS
Entering the Home Stretch
Moderator Fill
Feeder Tube Inspections
Heat Transport System Pressurization
Reactor Building Pressure Test
Guaranteed Shutdown State (GSS) Removed and Unit Goes Critical
First Electricity
Unit 1 Returned To Service
Entering the Home Stretch
In the countdown to the completion of the Unit 1 project, there were literally thousands of tasks to complete and hundreds of modifications to test and test again. Behind each of the major milestones was a complex and carefully managed schedule of interrelated tasks to be completed in sequence before the milestone could truly be celebrated.
Moderator Fill
Drained of all its heavy water (D2O) in February 2004 in anticipation of major construction work on pumps, valves and piping, OPG started the meticulous job of refilling the refurbished Moderator System for the first time over the Victoria Day weekend.
This is the system that manages the heavy water used to facilitate a useable nuclear reaction for in the core of the reactor. For about 18 months the Moderator System had been pretty much dismantled-with many major pieces of equipment taken out of the Moderator Room and returned as equipment was refurbished or replaced. In the interim, 20 large valves had been replaced (each roughly the size of an oil drum), five new pump motors installed, two new, 21,000 kg heat-exchangers installed, and countless lengths of piping replaced.
As equipment was installed, it was tested. But putting the D2O back into the system was the first step toward testing the system as a whole and thus an important step toward the lifting of the Guaranteed Shutdown System and returning the unit to commercial service. The work plan behind the refilling process itself was akin to a complex military campaign. The 83-page test plan defined and outlined each task in the long line of requirements leading up to the first flick of the switch to start the fill-and the careful testing of piping, water quality and pumps that follow.
Feeder Tube Inspections
Feeder tubes move the hot pressurized water created in the nuclear reactor to the steam generators or boilers. The resulting steam is then used to drive a turbine-generator that produces electricity.
There are 780 feeder tubes in Unit 1 and four of these were replaced in January as part of the construction project. Subsequent analysis of the removed tubes indicated it would be prudent to follow up with extensive and sophisticated inspections of a large sample of feeder tubes in order to be certain that thinning of the tube walls was not an issue.
The measurements required technicians to climb into exceedingly tight spaces inside the reactor building (which means, of course, layers of protective clothing and careful monitoring of worker exposure to radiation) and to use two different types of tools to measure the thickness to very precise measurements. The data was then extensively analyzed by expert technicians.
This was a large campaign that had to be managed by OPG's Construction and Maintenance and Operations groups which fine tuned the schedule in order to work around the hundreds of additional tasks.
Heat Transport System Pressurization
As part of this activity, Unit 1's heat transport system reached an operating temperature of 265 degrees Celsius (commonly referred to as "zero power hot") for the first time in many years. This was the reason for the puffs of white steam local residents may have seen spiraling into the atmosphere from the unit. Virtually every valve, pump and pipe was extensively overhauled-work designed to enhance safety and improve productivity. Because most of the system is inside the reactor building, the more than 100,000 hours in this sub-project were among the most challenging in the almost 2 million hour project schedule.
The heat transport system carries heavy water, which is pumped constantly through the fuel chambers in the reactor. Its purpose is to take the intense heat created by the fission process and carry it up to the boilers above the reactor, where the heat is used to boil water to make steam. The steam is piped out of the boilers and over to the turbine hall and here it drives the huge turbine generator that makes electricity. The pressure in the system is regulated so that the heavy water coolant never boils. This makes the heat transport system part of the "cool and contain" boundary for nuclear safety.
Reactor Building Pressure Test
One of the last things in the countdown to the removal of the Guaranteed Shutdown State (GSS) was the reactor building pressure test. On June 24, engineers put Unit 1's Reactor Building (RB) through its pressure test. The structure passed- indicating everything is working well within design specifications.
Guaranteed Shutdown State (GSS) Removed and Unit Goes Critical
OPG received the formal letter from the Canadian Nuclear Safety Commission (CNSC) on Thursday, July 28 allowing for the preparations for start up to begin. Although there was still much work to be done to commission and test the plant under operating conditions following GSS removal, this was the milestone that everyone working on the project had been focusing on over the past few months. It signified the successful completion of all 115 regulatory pre-requisites relating to the upgrading and refurbishment to virtually every system in the unit. During the subsequent testing phase, the unit produced power that was fed into the grid.
First Electricity
When the reactor is producing enough heat to create water that is hot enough for the boilers to produce steam, the turbines turn (or roll) and the generators create electricity that goes through the output transformers and out into the province's electricity grid.
From this point on in the commissioning process, Unit 1 provided electricity to the province. This is clearly an important milestone as it proved all the plant systems are working in synch. In the run up to the first turbine roll there were hundreds of tasks that needed to be completed, including filling the generators with hydrogen, testing the condensers and flushing out the feedwater system.
Unit 1 Returned To Service
The final step in the countdown to completion of the project is the production of electricity on a commercial basis. This followed a long list of tasks that tested critical plant systems, including testing the shutdown system enhancements at high power and testing responses to the sudden loss of power at 60% full reactor power. Pickering A Unit 1 was considered to be back in commercial service and dispatchable by the Independent Electricity System Operator on November 3, 2005.
