Career Path > Nuclear Operator

Switching between PWR and BWR

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TVA:

--- Quote from: Bonds 25 on Nov 10, 2018, 08:26 ---I was under the impression it was Pu-239.....

--- End quote ---


You are correct. Just saw my typo!

TVA:
Commercial PWR operate all rods out then dilute as the core ages. Eventually you get to the point where you have to add so much water to maintain power you put in a clean demin periodically to remove boron to maintain power. Eventually that stops working so you let power coast down. Iirc its about .75% a day. Once we got to around 75% power it was outage time

Bonds 25:


--- Quote from: TVA on Nov 10, 2018, 08:28 ---

You are correct. Just saw my typo!

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 ;)

scurrilous:

--- Quote from: TVA on Nov 10, 2018, 05:32 --- I can and will answer any questions you might have.


--- End quote ---

No real questions, just curious what people had to say. I've heard people say it's hard to switch from PWR to BWR and vice versa, but personally I don't think it could be that bad... If you could license at one plant I'm sure you could do it at another, just a question of whether you want to go through class again.

TVA:
Actually knowing what to study isn’t hard. Once you license the exam process is the same.
When I got my first license I studied too much.
An example is I would memorize power supplies to everything.
I came to realize what I call the Big Pump Little Pump theory. There are only about 10 big safety related pumps. Half will be off a big safety related power supply. Half from the other. A Train pumps will have a suffix A or C. Same with Busses
B Train are B and D.
On exams I would look for the A and match it with the Big Bus that had an A in it.
Little pumps same theory.
Works like a charm.

The theory was a bit different but only on the reactor side. Thermal limits for a BWR are more complicated and they change as the core ages. PWR Boron changes things too.

The major difference was Emergency Procedures. BWR use flow charts and rely on more decision making by the Operators and the CRS. Once you are done with something you leave it. If equipment gets restored you simply use it.

PWRs have a family of accidents and events.
So the EPs are a lot different.
First you enter a wide ranging procedure called E0. You do this for every scram. A BWR does not require entry on all scrams. Only if you meet certain conditions
Anyway you enter E0 and if you meet certain conditions you go to another EOP
You are only in ONE EOP at a time.
While doing this an individual is taking data for status trees. Each has it’s own color
Red: Priority 1. If you are in red you leave whatever procedure and go to the red procedure. Once done with a red you go back to the procedure you were in.
Orange: If you are in a procedure and an orange path comes up you enter it UNLESS you are already in a red path and have not entered it.
Once done you go back to the last procedure you were in.
Yellow: Yellow paths are optional.
However if you elect to enter a yellow path you leave whatever procedure you are in perform the yellow path and once complete you go back to the procedure you were in when you entered the yellow path.
Red and Orange paths have priorities too
Say you are in EO
You have a safety injection so you leave EO and enter E1 because thats what E0 tells you to do.
About 5 minutes later the STA tells you that you meet the conditions to enter say Red Path 3.
You leave E1 and enter Red Path 3.
You are midway through it and the STA says you meet the conditions for Red Path 1 which is the highest priority procedure.
You leave Red Path 3 and enter 1
When you are done with 1 you go back to 3 where you left off even if the conditions no longer exist.
Once done with 3 you go back to E1 wherever you left it.

The issue I had is the first 5 to 7 steps of each procedure is repetitive. You might have already done it in a previous procedure. You still do it anyway. The reasoning is you can enter a red path from virtually anywhere so you have to ensure the items get done. Even if you have done them you still do them anyway.
Coming from a BWR that was unusual for me and I had to break the BWR habits.

The way power changes in a PWR and BWR is totally different.
In a BWR you simply lowered core flow till around 68% power.
Then you put in certain control rods
Then flow
Then Rods
At about 20% power you scrammed the plant.
Going up same way

PWR: You raised or lowered turbine load.
When after temperature reached a certain point you either added pure water or boron OR you move control rods to a certain position.
20% power you scrammed if shutting down for an outage.

If something happened that required lowering power rapidly aka a runback BWR was simple. Recirc flow simply lowers and power and the turbine follow it.

PWR the turbine runs back. The reactor follows.
Control rods insert automatically to keep reactor temperature within a certain temperature range around Turbine First State Pressure aka TRef
Iirc at SQN it was within 1.5 degrees.
The issue is for any given power in a PWR rods must be above a given position to prove the reactor can be shutdown. A commercial PWR can literally reach 45 % power at the beginning of a cycle if the rods are in and there is no boron in the core.
So if you are in the proper temperature band and you get an alarm for rods too far in you have to add borated water at a certain rate to compensate. From the time you start until boron starts having an affect is around 5 or so minutes.
You know when it starts having some bite as you can hear the indicators click as the rods move out. It’s not a science. Takes a lot more time to address a run back on a PWR than a BWR.

What I described was for a Westinghouse 4 Loop Reactor.

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