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What does it take to refuel a commercial nuclear power plant?

4/29/2014

8 Comments

 
By Chris Wagener

Nuclear power plants are very different than their fossil fuel counterparts both in how the energy is generated and in the day to day operations. Both nuclear power plants and fossil fuel power plants rely on generating steam to spin a turbine to generate electricity but that is where the similarities end. Fossil fuel plants rely on burning hydrocarbons, such as coal and natural gas, to generate heat to boil water while nuclear power plants rely on fission. Fission generates substantially more energy than the burning of fossil fuels, especially on a pound per pound basis. Inside of a commercial nuclear power plant, fission is the splitting of a uranium and plutonium atoms into smaller atomic pieces.  By splitting these atoms, a large amount of energy is released. A fossil fuel plants works differently by chemically burning hydrocarbons into mainly water and carbon dioxide.

Have you ever seen a train carry loads of coal from a mine to a coal power plant? For a coal power plant to continue to generate electricity, coal must be constantly provided to the boiler.  However, for most commercial nuclear power plants, new fuel is added to the reactor every 12 to 24 months, which denotes the cycle of the reactor. Each cycle, between a quarter and a half the fuel assemblies, which contain the uranium and plutonium for fission, are removed and replaced with new fuel assemblies.  After the reactor is refueled, no fuel is added to the reactor and all the energy that will be generated during that cycle is already there, it only needs to be extracted.  Since the fuel will be in the same location in the reactor for up to 24 months, placement within the nuclear reactor is very important.

For each cycle, engineers start preparing up to a year and a half in advance to determine where the fuel assemblies should be located within the core. The first step in the process is to determine how many new fuel assemblies are required and what the uranium enrichment should be. This is the most important step since the manufacture and delivery of the new fuel takes a long time to ensure that the fuel is properly manufactured and inspected prior to use. Once the fuel is built and delivered, the fuel inventory is set for the cycle and it is very difficult to make major changes to the design.  During this phase, engineers will use computational tools to model the reactor core, which is made up of the fuel assemblies, to determine key characteristics of the cycle.  Once a candidate design has been determined, the safety analysis for the cycle can be performed.
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Nuclear fuel shown in a smaller research reactor.
For each cycle, engineers start preparing up to a year and a half in advance to determine where the fuel assemblies should be located within the core. The first step in the process is to determine how many new fuel assemblies are required and what the uranium enrichment should be. This is the most important step since the manufacture and delivery of the new fuel takes a long time to ensure that the fuel is properly manufactured and inspected prior to use. Once the fuel is built and delivered, the fuel inventory is set for the cycle and it is very difficult to make major changes to the design.  During this phase, engineers will use computational tools to model the reactor core, which is made up of the fuel assemblies, to determine key characteristics of the cycle.  Once a candidate design has been determined, the safety analysis for the cycle can be performed.

Due to the unique nature of nuclear power, a wide variety of safety analysis calculations are performed to ensure that the core will meet all the safety requirements as defined by the Nuclear Regulatory Commission (NRC). The calculations performed for each plant are documented in the plant’s Final Safety Analysis Report, describing all the work done to ensure the safety of the plant for a multitude of accident scenarios. Each time the reactor is refueled, engineers will perform calculations using core and plant models to show that in the event of an accident, the regulations set out by the NRC will be met. These regulations exists to ensure the safety of not only the public, but also the workers at the plant. Once the calculations have been completed, engineering reports are generated for submission to the NRC demonstrating that refueling the core continues to meet all the regulations and requirements. The safety analysis calculations that are performed each cycle are done to demonstrate that the plant can operate safely in the worst possible scenarios and during an accident.  
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After demonstrating that the core can safely operate for a cycle, engineers will generate the information necessary to operate the plant. This will include information for the plant to show to predict how the reactor will behave when intentional changes are made, such as going up and down in power. In addition, it provides information for the reactor operators to benchmark against their measurements. This is used to help ensure that the models used for the safety analysis were accurate. If any issues arise that indicate that the models were inaccurate at predicting the actual operation of the core, engineers will have to determine whether the plant can still operate safely given the new information. The final set of information provided to the plant before refueling the reactor, is all the information necessary to do what is called startup physics testing.

Startup physics testing is a series of tests performed after each refueling as the reactor is brought to power to demonstrate that the reactor is actually as it was modeled. This shows that the reactor is behaving as predicted and validates the safety analysis. Specific calculations are done each cycle and compared to the results of the startup physics testing. If the plant is within a certain tolerance to the predictions, it demonstrates that the safety analysis is bounding and the plant and continue to operate. If outside of the predictions, engineers must investigate to determine what the issue is and whether or not the plant can continue to operate.

After startup physics testing, the plant will continue to operate until the end of the cycle, assuming that the predictions remain accurate.  During this time, reactor operators will continue to check and ensure the overall safe performance of the reactor.  Once the startup is completed, engineers will restart the process to prepare for the next reactor refueling. Overall, nuclear power plants operate on a cyclical basis with the refueling of the reactor being the main goal. Once the refueling is performed, reactor operators monitor the performance of the core to ensure safe operations while other engineers prepare to refuel the core again.  The breakdown of this cyclical nature is to generate a design for the reactor cycle, demonstrate that the design can operate safely for the entire cycle, provide the necessary data for plant operations, including startup information, and then finally perform startup physics testing to validate all the work performed. After this process is completed, the reactor operators will operate the plant and ensure the safety of the reactor until the next refueling is needed.
8 Comments
anon
4/29/2014 05:53:56 am

Did you proofread this before posting?

Reply
Lenka Kollar link
4/29/2014 06:06:55 am

I did, but it looks like the copy-paste from word included the tracked changes. Thanks for letting me know.

Reply
Thomas Clegg III
5/3/2014 11:56:30 pm

Chris at my site Indian Point we change 1/3 of the core on a refuel. That means the fuel is good for 6 years. Breaking down what a cost savings nuclear is. 1,000 mega watts powers about 1 million homes here are the ways and what it takes to produce that much power.
oil- 40,000 barrels of oil a day(42 gallons=1 barrel
coal- 9,000 tons of coal a day
gas- 200,cubic feet of gas a day
nuclear- 16 pounds of uranium a day

Reply
Anon2
5/28/2014 06:41:46 am

Thomas, are you sure you work at Indian Point? Indian Point operates on two year cycles. Because of that, they replace half of the core every cycle, not 1/3. Because of that, most of the assemblies are only in the core for 2 years. On average, only 4 or 8 assemblies go for 6 years at Indian Point. If you want to get specific, then get your facts straight. Otherwise, leave the specifics out and stay general.

Reply
Tahir
12/24/2014 07:26:22 pm

I want to know the complete steps to refuel a nuclear reactor.??
Thanks

Reply
THE PROFESSOR
10/31/2015 08:48:20 pm

16LB a day? I haven't done the math on it yet.... but if that is correct. there are massive inefficiencies in that plant. 1+2≠16

but I fear these inefficiencies are correct, and not with that plant alone. We harness the power of the sun and..... we boil water???

I hope I'm not alone in saying that something is wrong there.

Reply
Joe
11/3/2015 05:33:48 am

Just conflating Fission with Fusion.

Reply
Kaleb Stone link
4/21/2021 07:36:31 pm

Loved reading thhis thanks

Reply



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