Modeling the Potential Dose Rates of Dropped Irradiated Hardware in BWR-5 Plants

[JTG]

I work in a BWR-5 plant and this outage we will be swapping 36 control rod blades. We currently restrict access to the upper elevations of the drywell during irradiated hardware and fuel moves because of the potential dose rates if we were to drop something. This has a significant schedule impact because of the work restriction, and also takes quite a bit of dose posting and deposting.

Has anyone done a MCNP model of a dropped CRB, which is pretty much the worst case? I'm not even going to investigate dropping irradiated fuel. Has anyone developed a case (for or against) working the upper elevations during irradiated hardware moves?

Before I get too much more invested in modeling this thing, just thought I'd ask.

 

[Astronuc]

I don't think anyone here has done such a calc.

Is there a reactivity issue? If so, what is the worth of a fresh/used blade compared to the - reactivity of the core? The effective worth or differential worth will be a function of where it gets dropped, e.g. dropping vertically into the core and having it tip (tilt) against other assemblies, which presumably have their CRB inserted. On the other hand, an irradiated CRB will not have much worth.

When replacing blades, are just those cells where the blades are to be replaced empty? Are the blades replaced after the fuel shuffle and fresh fuel assembies are inserted?

CRB's are effectively inert, except for the T inventory, and I don't imagine one will necessarily break.

Is the plant using Marathons, Duralifes or CR99's?

One could contact the BWROG RCMC representative at one's utility.

 

[JTG]

Astronuc,

It's not a reactivity issue, it's a health physics issue. The question is if I drop a blade and it somehow lands on the reactor vessel flange, what would be the dose rates in the upper elevations of the drywell.

Thanks for the help,

JTG

 

[Astronuc]

i.e. what is the activity coming from an irradiated (activated) blade dropped on the reactor flange. I guess one assumes max design snvt's (exposure). I doubt that anyone here has done that either.

I suppose one could compare that activity with the activity of the core (fuel and blades) and UGS. What is the difference in elevation between UGS and flange?

 

[gtn]

JTG,

I have not modeled this. A Google search yielded this link within the Energy Citations Database http://www.osti.gov/energycitations/purl.cover.jsp?purl=/10159708-T0sPgi/. It seems that INL has characterized a number of decommissioned reactor internals, including BWR CRBs. The report is from 1993. Their data shows contact gamma dose rate ranging from 6000 R/hr (approximately 1.75 feet from the velocity limiter) to 20 000 R/hr (approximately 11 feet from the velocity limiter). I could not find the irradiation history of the CRB they characterized.

-Gary