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

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Discussion Overview

The discussion revolves around modeling the potential dose rates associated with dropped irradiated control rod blades (CRBs) in BWR-5 plants. Participants explore health physics concerns related to access restrictions during irradiated hardware moves, particularly focusing on the implications of dropping a CRB and its impact on dose rates in the drywell's upper elevations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about existing MCNP models for dropped CRBs, suggesting that this scenario represents a worst-case situation.
  • Another participant raises questions about the reactivity implications of dropping a CRB, noting that the effective worth of a blade depends on its location when dropped.
  • Concerns are expressed regarding the health physics implications of a dropped blade landing on the reactor vessel flange and the resulting dose rates.
  • There is a suggestion to compare the activity from an irradiated blade with the activity of the core and other components.
  • A participant references a report from INL that characterizes gamma dose rates from decommissioned reactor internals, including CRBs, but notes the lack of irradiation history for the specific CRB studied.

Areas of Agreement / Disagreement

Participants generally agree that the primary concern is health physics rather than reactivity issues. However, there is no consensus on the specific dose rates or modeling approaches, and multiple viewpoints regarding the implications of dropping a CRB remain unresolved.

Contextual Notes

Participants express uncertainty about the irradiation history of the CRBs referenced in the INL report, which may affect the relevance of the dose rate data. Additionally, assumptions about maximum design exposure levels and the specific conditions of a dropped blade are not fully established.

JTG
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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.
 
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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.
 
Last edited:
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
 
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?
 
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
 

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