Resolution of Generic Safety Issues: Issue 187: The Potential Impact of Postulated Cesium Concentration on Equipment Qualification ( NUREG-0933, Main Report with Supplements 1–34 )
NRC regulatory requirements include the estimation of individual offsite dose from the design basis LOCA. The fission product source term in TID-1484473 which is referenced by the regulations, has historically been used for this application. As an update to the source term in TID-14844,73 the NRC developed NUREG-1465,1465 which provides a more realistic source term based on two decades of severe accident research; its use in offsite dose analysis provides safety and cost benefits. Accordingly, the NRC issued a new regulation, 10 CFR 50.67, allowing licensees to implement an alternative source term. Together with the issuance of 10 CFR 50.67, the NRC issued a regulatory guide which states that one acceptable alternative source term (AST) is the gap and in-vessel releases described in NUREG-1465.1465 In the implementation of NUREG-14651465 for estimating offsite dose, the issue arose1777 as to whether any additional requirements were needed with respect to estimating doses for equipment exposed to sump water.
NRC regulatory requirements also include the environmental qualification of equipment for the duration that it is needed to perform its safety function. This includes qualification for radiation, temperature, pressure, and humidity. Regulatory Guide 1.8991 states that it is acceptable to use the TID-1484473 source term for this application. Regulatory Guide 1.89 also states that, for equipment that must be qualified for more than thirty days, a source term that incorporates considerable quantities of cesium, as suggested by the accident at TMI-2, may produce doses greater than those estimated by TID-14844,73 which includes a 1% release of cesium. The gap and in-vessel releases described in NUREG-14651465 include a 30% release of cesium.
The SNL report Evaluation of Radiological Consequences of Design Basis Accidents at Operating Reactors Using the Revised Source Term, dated September 28, 1998, showed that, for equipment exposed to the containment atmosphere, the TID-1484473 source term and the gap and in-vessel releases in the AST produced similar integrated doses. This report also showed that, for equipment exposed to sump water, the integrated doses calculated with the AST exceeded those calculated with TID-14844,73 after 42 days for a PWR and 145 days for a BWR, because of the 30% vs. 1% release of cesium.
The staff concluded that there was no clear basis for backfitting the requirement to modify the design basis for equipment qualification to adopt the AST. There would be no discernible risk reduction associated with such a requirement. Licensees should be aware, however, that a more realistic source term would potentially involve a larger dose for equipment exposed to sump water for long periods of time. Longer term equipment operability issues associated with severe fuel damage accidents, (with which the AST is associated) could also be addressed under accident management or plant recovery actions as necessary. Thus, the issue was DROPPED from further pursuit.1776