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Tuesday, June 15, 2010

Fuel Element Failures - see previous post...

Let's take a moment to look more closely at something I mentioned in my previous post - failed fuel elements in nuclear reactors.

As I wrote earlier, you do expect fuel elements to fail in any number of ways from minor cladding or canning imperfections and defects to total rupture. It's going to happen, and so long as you don't have a very catastrohic failure that blocks channels or widespread fuel element swelling you may continue operation. In fact, the design analysis that takes place for any reactor includes a well thought out, deliberately conservative number of fuel element failures which would take place over the expected core lifetime. Using that expected failure number results in, for example, a given amount of radioactive content in circulation in the coolant (and largely deposited in crud) and this results in calculable exposure levels which are then used for many further considerations. The point is that it's really expected to occur and that it's usually in reality far less than the design basis.

We need look no further than the Reactor Protection Analysis performed for the very first commercial nuclear plant in the US to be approved - the Shippingport Atomic Power Station. This plant was officially launched as a project in 1954 and placed in operation in December 1957; this was a Westinghouse pressurized water plant (which in fact was often referred to in early parlance as the PWR plant -- as if there wouldn't be other PWR's!)

If we look at the design considerations for a major loss of coolant we see that the designers considered that when this happened they'd also assume the maximum credible activity in the coolant from, of course, the maximum credible number of failed fuel elements. Right from the book: "The release of 100% of the reactor coolant to the outside atmosphere would not result in a biological hazard at the site boundary even if the core had been operated for 3000 hr at a power level of 275 MW and the coolant contained the maximum expected activity, ie the activity caused by imperfections in the cladding of about one percent of the UO2 fuel elements (1000 defected elements during the lifetime of the core.)"

I could go on and on, but this whole "radiation leak" (author cringes) stuff is hoo-ha if based on a predicted, expected fuel element defect or some such. There is clearly a wide spread of expected problems from minor imperfection to major failure.

Again- you can have a serious element failure and a major spike in coolant activity. That's something much more serious, like if for example a fuel plate swells and cuts down coolant flow, furthering the swelling until it ruptures. Even so, this can happen and operation can sometimes continue. Shippingport had a complicated FEDAL system, short for Fuel Element Detection And Location, designed to tap all of the coolant outlets of all the blanket fuel elements to see if any one had experienced a failure in its assemblies. They were ready for this in the 1950's. Let's hope the press keeps itself informed on this sort of thing as nuclear power begins its predicted climb back to the fore of US energy policy.

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