[ RadSafe ] BWR radiation safety issues

[Olsson Mattias :MSO]

Hi,

During operation the turbine hall and any area near the steam lines and turbines hold elevated dose rates due to short-lived isotopes such as the N-16 you mention (activation product of oxygen). Elevated enough not to allow just anyone to access such rooms. After shut-down the dose rate goes down quickly and access does not need to be restricted (more than it is normally anywhere in the controlled area).

During an outage the dose to staff that works near the steam lines is generally dominated by Co-60. The inside of the steam lines is contaminated from droplets of reactor water that are carried over with the steam. A well dried steam at the exit of the reactor tank will minimize this issue. I am not aware of any problems with the condensate or feed water. Radioactivity in those systems is low. If you want to know the typica radioactivity levels in your plant systems you should be able to find them in your plant SAR.

Fission products are released during operation with failed fuel, but they can also be formed directly outside the fuel cladding from uranium contamination on the fuel assemblies. There is always a small contamination from the factory to begin with, and severe fuel failures may cause further uranium contamination. Uranium on core surfaces will fission just as happily as it does inside the fuel. Unless there are severe fuel failures or core contamination, iodine should not be a problem even though it would be elevated in the reactor water. The radioactive noble gases will end up in your  off-gas system (as will all non-condensable gases that are sucked from the condensor). Those gases may pass through a delay system to allow them to decay before release into the air. Some delay systems can even deal effectively with the more long lived Xe-133, but Kr-85 would be more difficult.

Corrosion products that enter the reactor tank, either from the feed water or from the corrosion of materials inside the tank or other connected systems, will be activated by the neutron flux of the core. They will then spread to system surfaces to some extent (including by carry-over to the steam lines an mentioned above). Activated corrosion products on system surfaces, such as in many pipes in the reactor building, will cause dose rates that far exceed that from just the reactor water inside such pipes. Co-60 usually dominates. At my plant I've seen contact dose rates from such pipes up to about 10-15 mSv/h. To minimize this problem you could use system decontaminations, and you should try to avoid the situation. This can be done by wise material choices, good cleaning systems (the condensate clean-up being really important since a turbine system may release hundreds of kilograms of corrosion products per year - those should not be allowed to enter the reactor) and a good plant water chemistry. Some elements that you do NOT want to use in your plant in contact with the primary system would be cobalt, antimony and silver. They get activated especially nastily into Co-60, Sb-124, Ag-110m...

Mattias Olsson
Forsmark NPP (3 x BWR)


-----Ursprungligt meddelande-----
Från: radsafe-bounces at health.phys.iit.edu [mailto:radsafe-bounces at health.phys.iit.edu] För Kulp, Jeffrey B
Skickat: den 5 december 2011 19:38
Till: The International Radiation Protection (Health Physics) Mailing List
Ämne: [ RadSafe ] BWR radiation safety issues

Hello Radsafers,

I would like to call on the expertise of any HP's who have experience with boiling water reactors; I am interested in radiation safety issues associated with boiling water reactors while operating and shutdown. My interest is primarily in the steam, condensate and feed systems. I know about N-16 carryover to the steam side of the plant, but I read an article recently that discusses problems with Xe, Kr, and Iodine as well; these are fission products, are the authors saying that the fuel cladding is not able to contain fission product gases? Another article I read talks about activated corrosion products in the condensate and feed systems. How do the corrosion products become activated, is the condensate and feed systems in a BWR plant subject to a neutron flux? Any clarification offered will be appreciated.

Thanks,

Jeff Kulp
Washington State University
Radiation Safety Office
Pullman, WA 99164-1302
(509) 335-8175

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