Agenda - 11-17-1998 - 5a - Laserfiche WebLink

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- ~..: - c=~ w- _i~ - -``y- ~ ~: .. 20 !~: ~~. Chapter Eight • Spent Fu°! Risks ~; •"d~materiaL'I11e average gap size was 1-1 /2 inches, ~+•ith the lamest measuring near- Iy four inches. The gaps were discovered in the upper t+vo-thirds of the cell l~g~ ~ z .- During testing of selected South Texas Project Electric Generating Station Y~ ~ Unit 1 storage racks in August 1994, 20 of the 37 storage cells tested sho++~ed some evidence of gaps or significant neutron absorption panel dew adation. The neu- :~. tron absorption panels contain a polymer matrix of boron carbide and silicon rub- . ~ ber The large area degradations (up to 3 or 4-1 /2 feet in length) represented ~, accelerated dissolution of the BorafIex material caused by spent fuel pool ~+•ater flowing through the poison enclosures. Samples of spent fuel pool ++•ater re+•ealed increased silica amounts. Research determined that once the silicia concentration ;r~ reaches equilibrium, the neutron absorption panel dissolution rate effectively stops. Removal of the silicia by the fuel pool cleanup system therefore sustains W the degradation phenomenon. However, failure to remove the silica can adverse- ; Iy affect reactor coolant chemistry during refueling outages when the refueling :;cavity is directly connected to the spent fuel pool. Silica depositing on the fuel '~• assemblies in the reactor core can adversely affect the heat transfer rate across the h^.- • fuel rod cladding, causing fuel centerline temperatures to ineaease. k ~ During the licensing process to obtain NRC's approval to install high-den- 8 -, sity storage racks, utilities frequently committed to test the adequacy of the neu- r:. tron-absorbing material contained within the rack structure. This testing has led ~rto the discovery of the deficiencies described above. Depending on the extent of ~,_ the problem, the affected locations within a storage rack may be administrati+•e- .-~ Iy barred from being used or the entire storage rack may be unloaded and ~• •- ~. replaced. As spent fuel pools are filled with irradiated fuel assemblies, these <~.'".options may be restricted. For example, at some point a nuclear po++•er plant may s.'~-' :• fir:.; Iose the capability to completely unload a storage rack due to the unavailability ~~~'of sufficient em locations in other racks. ,.. PtY ~~~ Spent fuel pool criticality is managed exclusively through passive design ' features and administrative controls. Critiglity in the spent fuel pool ++~ould yield ,~„~sigivficant consequences if it ever were to occur. First, nuclear power plants are . `~ ~ not equipped with instrumentation to directly monitor subaiticality in the spent -fuel pool Second, nuclear power plants are not equipped with features to miti- gate spent fuel pool criticality if it occurs, with the possible exception of PVVRs . •~~that can borate the spent fuel .pool water. And finally, nuclear po+ver plants are ':~ not designed to shield against spent.fuel pool criticality. The risk management is -solely dependent on criticality preventipn. =Passive Storage_ '~ The previous sections discussed the spent Fuel risk from an initiating e+•ent ~snch as the inadvertent drop of a fuel assembly or the failure of the fuel pool cool- sng systeat. The risk from spent fuel Y,as5eatblies under normal storage conditions '-aLso has been evaluated. Understanding this passive risk is important because ~t °.~ x~Jt~ ~' i 19 .. .::: \„ ~' i,pi- ,:•