Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
February 1, 1990
Information Notice No. 90-08: KR-85 HAZARDS FROM DECAYED FUEL
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors and holders of licenses for permanently shutdown facilities with
fuel on site.
Purpose:
This information notice alerts addressees to potential problems resulting
from the accidental release of Kr-85 from decayed fuel. It is expected that
recipients will review the information for applicability to their facilities
and consider actions, as appropriate, to avoid similar problems. However,
suggestions contained in this information notice do not constitute NRC
requirements; therefore, no specific action or written response is required.
Description of Circumstances:
During the licensing reviews for the Oconee independent spent fuel storage
installation, and in the decommissioning of the La Crosse and Dresden Unit 1
power reactors, the NRC staff analyzed the radiological hazards associated
with the gases in decayed spent fuel. The age of the nuclear power industry
and the lack of a permanent repository for spent fuel have resulted in the
accumulation of decayed spent fuel. Decayed spent fuel is manipulated after
long shutdowns of operating reactors, during spent fuel pool re-racking,
during movement to alternate reactor sites or independent spent fuel storage
installations, and during decommissioning. Analysis of hypothetical
accidents involving decayed spent fuel has focused attention on potential
difficulties that could be associated with the exposure of onsite personnel
to an accidental release of Kr-85. Kr-85 is a noble gas fission product
that is present in the gaps between the fuel pellets and the cladding. It
has a 10.76-year half-life, and, as a result of the considerably shorter
half-lives of virtually all other gaseous fission products (I-129 being the
exception, but in low abundance), Kr-85 becomes increasingly the dominant
nuclide in the accident source term for gap releases as decay times
increase. After 2 weeks of decay, Kr-85 is a significant nuclide in the
source term, and after 190 days of decay, it is the predominant gaseous
nuclide for a gap release. The unusual decay characteristics of Kr-85 give
cause for focusing attention on the onsite consequences of a gap release
from decayed fuel.
9001260198
.
IN 90-08
February 1, 1990
Page 2 of 2
Discussion:
Kr-85 emits beta radiation with a maximum energy of 0.67 MeV for
99.6 percent of the decays and 0.51 MeV gamma radiation for 0.4 percent of
the decays. Consequently, direct exposure to this gas would result in a
dose to the skin approximately 100 times the whole-body dose. Analysis of
the relative consequences (in terms of radiological doses) of a cask-drop
accident as a function of decay time of the fuel is illustrated in Figure 1.
In the event of a serious accident involving decayed spent fuel, protective
actions would be needed for personnel on site, while offsite doses (assuming
an exclusion area radius of 1 mile from the plant site) would be well below
the Environmental Protection Agency's Protective Action Guides.
Accordingly, it is important to be able to properly survey and monitor for
Kr-85, and to assess the skin dose to workers who could be exposed to Kr-85
in the event of an accident with decayed spent fuel.
Licensees may wish to reevaluate whether Emergency Action Levels specified
in the emergency plan and procedures governing decayed fuel-handling
activities appropriately focus on concern for onsite workers and Kr-85
releases in areas where decayed spent fuel accidents could occur, for
example, the spent fuel pool working floor. Furthermore, licensees may wish
to determine if emergency plans and corresponding implementing procedures
address the means for limiting radiological exposures of onsite personnel
who are in other areas of the plant. Among other things, moving onsite
personnel away from the plume and shutting off building air intakes downwind
from the source may be appropriate.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate NRR project
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Charles S. Hinson, NRR
(301) 492-3142
Robert A. Meck, RES
(301) 492-3737
Attachments:
1. Figure 1, Dose Consequences of a
Spent Fuel Drop Accident
2. List of Recently Issued NRC Information Notices
.ENDEND
