Blastout during scram with leaking oxide fuel at high power density

So it does not happen often, but it has happened. At a recent meeting of the NRC's ACRS, one member wondered if a rapid shutdown could induce a PCI failure and the expert replied that, "We've never seen that."

Of course, the expert is likely correct that a PCI cladding opening has never been induced by a reactor scram. However, a reactor scram has induced severe additional failure of defected fuel that was operating at high power density. The readers who have the patience may scroll down to my entry of June 13, 2008, in which I detail events at GETR on July 12, 1959, along with hot cell photographs of blasted fuel.

Here are a few sentences from the recent meeting of the ACRS subcommittee:

ADVISORY COMMITTEE ON REACTOR SAFEGUARDS
SUBCOMMITTEE ON MATERIALS, METALLURGY AND
REACTOR FUELS
+ + + + +
TUESDAY
MARCH 3, 2009
+ + + + +
ROCKVILLE, MARYLAND

MEMBER RAY: Is there -- everything you talked about, Sam, had to do with raising power, ramp rate, increasing power. I just kept thinking to myself is there symmetrical problem on the downside with a unit trip or something like that in terms of stress levels?

CHAIR ARMIJO: No. Actually, I have never known a fuel rod to fail when you reduce power. You reduce power as a result of failure, that's typical. Actually it unloads. Everything unloads. As you drop power the pellets contract thermally so the stresses, whatever stresses are there and there is just no -- this is stress corrosion cracking mechanism. Unless there is a mechanism to put stress on the cladding, it just won't happen.

MEMBER RAY: Well, that's what I was thinking about. Is there a mechanism on a down-power, rapid down-power that has the same affect?

CHAIR ARMIJO: We've never seen that.

Well, during summer 1959, GETR had a wild time with blastout of a leaking oxide fuel element that had center melting. On July 20, 1962, a recurrence was avoided by by complying with the following Leyse procedure. I addressed this note to Kornblith with a copy to Thorburn. Thorburn sent it to O’Rourke who sent it to Akin who returned it to O’Rourke with this note. The copy came back to me and I filed it.

This procedure made life a lot easier at GETR on July 20, 1962, when test fuel operating at very high power density developed a leak. When the fuel was removed from the PWL, a large blasted zone was not present, although the some fission products were released during removal of the fuel.

As usual, the regulators (then the AEC) did not appreciate initiative from the GETR staff as they followed this guidance. An inspector asked, “Are the operators afraid to scram this reactor?” Click on each slide to enlarge and use the return arrow to get back here.