Illinois Department of Nuclear Safety GOOD SCIENCE VS UNCERTAIN REGULATORY GUIDANCE OR HOW CHI/Q AFFECTS NUCLEAR SAFETY Ken Evans Illinois Department of Nuclear Safety 1035 Outer Park Drive Springfield, Illinois 62704 217-558-0499 evans@idns.state.il.us

Introduction X/Q Calculations used to be simple As a result of tracer studies more realistic models were developed Regulatory Guidance has not always followed the best science

Development of ARCON-96 Regulatory Guide 1.145 recognized the existence of plume meander and building wake There was no attempt to develop an accurate model of these two terms NUREG/CR-5055 was a first attempt to develop a more realistic model ARCON-95 was the first realistic model to predict the effects of plume meander and building wake.

ARCON-96’s Results The results of the new X/Q are spelled in on page 44 of ARCON-96 “…there are numerous instances where ARCON-96 predicts concentrations that are more than a factor of 10 lower than the Murphy-Campe model predictions.”

The Source Term Issue NUREG-1465 Published in 1995 It received much attention because source terms were known to be overly conservative. Conservatism in X/Q’s not as well understood

Clinton’s RAST Team Clinton Power Station decided to form a team to evaluate the new source term. The Revised Accident Source Term (RAST) team’s mission was to look for economic benefits. The RAST Team also evaluated the effect of ARCON-95

First Case The timing aspects of the new source term only were used. Other aspects of the new source term such as chemical species and accident analysis were more difficult to calculate. In 1996 it was not clear what methodology the NRC would approve for revised design basis calculations.

Results For Timing Only Reduction in dose less than a factor of 2. Did not produce enough benefit to pursue further.

New X/Q Case Take credit for Suppression Pool Scrubbing Calculate new X/Qs using ARCON-95 Use current Reg Guide 1.3 Source Term Results Approximately a factor of 10 reduction

Control Room X/Q The Clinton calculations for Control Room were reduced by a factor of 4 for occupancy. The new guidance in DG-1111 allows a factor of 10 for Clinton’s design of air intakes.

Reg Guide 1.183 Reg Guide 1.183 provides guidance on use of new source term It is silent on use of ARCON-96 for other than control room dose Defense in Depth must be maintained, i.e charcoal trains can not be eliminated.

SUMMARY AND CONCLUSIONS ARCON-96 as well as the NUREG 1465 Source Term result in lower dose when performing design basis calculations. The design of nuclear plants has been based on conservative analysis The regulatory dilemma becomes recognizing improvements in the science without obviating the required safety systems that are now in place.