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

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i Chapter Eioht • Spent Fuel Risks ~-. P the turbine could damage the spent fuel pcwl. The conditional probability ~~f this occurrence, given the turbine failure, was estimated to be 1.Ox1U, per year. = ~'+~ ~+~+zth the cask drop event, ++•hile the consequences from a turbine generated mis- sale striking the spent fuel pool are significant, the probability that such an e+~ent would occur vas considered to be sufficiently low as to effectively control this risk factor. • Spent fuel pools are designed to handle a loss of fuel pool cooling. This ini- -~ bating event culminates in appreciable loss of spent fuel pool ~+•ater in+•entun~ ' only when the spent fuel pool boils ~~ithout makeup. This failure mode 11as been discounted in safety studies due to the extended period (relative to traditional reactor acddent analysis time fumes) available to restore cooling or pro+•ide makeup. On January 25, 1994, Commonwealth Edison Company disco+•ered cun~id- erable water in the basement of the containment structure at its Dresden L'nit 1 •plant. Dresden Unit 1 shutdown in October 1978 and remains +•irtuallv aban- Boned next to the operating Dresden Unit 2 and 3 plants. A service ti•ater system pipe in the unheated Unit 1 containment had frozen and ruptured, draining : about 5,000 gallons from the system into the basement. Commomvealth Edison determined that piping in the spent fuel pool transfer system vas also suscepti- ble to freezing. If this piping had broken, the spent fuel pool would ha+•e drained to two feet below the top of the 660 irradiated fuel assemblies in the storage racks. ''At that level, the dose rate at the spent fuel pool railing vas estimated at X33 ;.Rem/hr, radiation levels that could have impaired operations on Dresden Units 2 and 3." Dresden Unit 1 vas not equipped ++•ith spent fuel pool le+•el instru- mentation to detect in+•entorv loss." This event had significant potential radio- _. logical consequences even though only 660 irradiated spent fuel assemblies =_ resided in the spent fuel pool and these assemblies had undergone over 1~ years •' of radioactive decay. Failure of inflatable and mechanical seals is the must frequent reason that ~ spent fuel pool water inventory is lost. Figure S-1 illustrates various seal applica- ". lions used in BWRs. Mechanical seals are used between t}~e reactor pressure ves- ~sel and the containment structure (labeled "RPV to Drvv,-eIl Bellows Seal" in ~~ Figure 8-1) and between the drywell and the refueling cavity (labelled "Drv~vell `i to Reactor Building Bello++'s" in Figure 8-1). Inflatable seals are used around '~ removable gates (labeled "Gates" and "Double Gates" in Figure 3-1 ). Inflatable -seals are like bicycle fire intertubes-when filled ~•ith air, they form a nearly leak 1 tight bamer. The problem occurs when the inflatable seal loses air pressure and ;`the barrier becomes rather porous. ;fix ' The refueling cavity water mechanical seal (comparable to the "Dn~+•ell to Reactor Building Bello++•s" shown in Figure S-1) at tl~e Haddam tieck plant suf- fered a gross failure in August 19b4 when mechanical interference significantl~~ =;.displaced the seal. At the time cif the failure. the refueling ca~•ity was tluc~ded in preparation for refueling. The refueling ca+•it+• ~+•ater le~•el decreased ~ ~ feet tc~ the is ~~ 113 "c: y ~;Y- 4 .~ - W Yi'T • ~~i' .. 14