Design flaw in Canadian Maple Reactor

Andrew Mason

I am trying to figure out why the design problem with the Maple1 and Maple 2 reactors cannot be fixed. This Globe and Mail article explains the decision to end the projects.

These reactors have been built. During commissioning, they became aware of the problem with a positive power coefficient of reactivity, which means that reactor power output increases with temperature. Obviously that has to be corrected. But why would it present such an insoluble problem that the reactors must be scrapped?

AM

vanesch

I don't understand this. How come that was not noticed upon design simulation ??

Andrew Mason

That appears to be a $592 million question.

It was designed to have a negative power coefficient of reactivity of –0.12 mk/MW. But in operation it was measured at +0.28 mk/MW. The units are some kind of measure of the increase in ratio of neutrons produced to neutrons used per MW increase in power output.

AM

Astronuc

That's the question. It depends on what assumption are made particularly with respect environmental (boundary) conditions.

I wonder what neutronics/core simulator was used in the design process, and what cross-section library, and how it was collapsed.

Perhaps the methods lead to faulty predictions in moderation or resonance absorption (Doppler coefficient), which ostensibly could be fixed with an appropriate burnable absorber or fuel composition.

28-06-2006 - INVAP to perform nuclear calculations for Canadian company
http://www.invap.net/news/novedades-...20060628192056

An article on the design of Maple and the fuel. Ostensibly, they could redesign the fuel.
http://anes.fiu.edu/Pro/s4ma1.pdf

Homer Simpson

I heard somewhere that to save on design effort they essentially just scaled up a previous smaller fully engineered design. I'm not too sure how true that is.

Morbius

Homer,

Of course that's what the Soviets did when they built the RBMK reactors like those at Chernobyl.

The Soviet RBMK reactor is a smaller Soviet nuclear weapons material production reactor scaled
up by a factor of 2. The mistake the Soviets made was not redesigning the fuel to go along with
the reduced neutron leakage afforded by the larger RBMK.

The Soviet RBMK thus had the wrong feedback characteristics; which led to the infamous accident.
If they have a reactor that has the wrong feedback characteristics; it's best not to pursue startup
unless the problems are corrected. Perhaps the mistakes in the design are too integral to the design
that they can't be fixed without essentially scrapping the original design.

This means that they really don't have a good model for their design simuation. Somebody made a
BIG MISTAKE somewhere - be it in processing the nuclear data, the transport simulation software;
the design calculations....who knows where the error is - but evidently the error is large.

I hope someone follows up on this; I'd be curious as to what part of the design process was faulty.

Dr. Gregory Greenman
Physicist

Astronuc

I was thinking more about this aspect. Could it be in the design phase, they made assumptions about the moderator (heavy water) and it's decrease in density if the temperature changed, whereas the reactor is a pool type so like the Trigas, convection will allow for a flow of moderator which will have a lower temperature than in a static case.

Or, they could improve the Doppler coefficent by changing out the U-silicide fuel with UO₂, which has lower thermal conductivity, so the fuel runs hotter and that would produce more negative reactivity at a given power level.

Morbius

Astronuc,

One would hope that they have a "coupled physics" simulation code; neutron transport coupled with
a thermal-hydraulics response solver. That way they don't have to "assume" anything about the density
or temperature of the heavy water moderator. The thermal-hydraulics code will calculate that for them.

Dr. Gregory Greenman
Physicist

vanesch

Ooopsa ! That's centimeter, not inches !

Morbius

vanesch,

You mean remeniscient of the miscommunication between NASA and Lockheed on the
Mars Orbiter mission due to a confusion over whether the units were metric or English:

http://www.cnn.com/TECH/space/9909/30/mars.metric/

Dr. Gregory Greenman
Physicist

Astronuc

One would hope. But then in this day and age, I've seen some rather unbelieveable practices or mind-bogglingly stupid assumptions, which left me wondering 'what were they thinking?' The miscommunication between NASA and Lockheed - and no one bother to check the interface to assure consistency in the calcs - is a striking example.

It would seem pretty easy to figure out what's wrong with the initial design and develop a remedy. Afterall, TRIGA fuel was redesigned from HEU to LEU, but still had to meet all the same tech specs. It should be a simple proposition to modify the driver fuel.