LETTERS TO THE EDITOR COMMENT ON ' CONTRIBUTION 2. D. DUBE, M. S. KAZIMI, and L. M. LIDSKY, "Thermal Response of Fusion Reactor Containment to Lithium Fire," 3rd Topi. Mtg. on OF ACTIVATION PRODUCTS TO Fusion Reactor Technology, May 1978. FUSION ACCIDENT RISK: PART I. A PRELIMINARY INVESTIGATION" 3. M. S. TILLACK and M. S. KAZIMI, "Development and Verifica- tion of the LITFIRE Code for Predicting the Effects of Lithium Spills In the first issue of Nuclear Technology/Fusion, in Fusion Reactor Containments," PFC/RR-80-11, Massachusetts Holdren1 assessed conceivable radioactivity releases from Institute of Technology (July 1980). a fusion power plant under severe accident conditions. 4. M. S. KAZIMI and R. W. SAWDYE, "Radiological Aspects of These conceivable releases were estimated to be 20% of the Fusion Reactor Safety: Risk Constraints in Severe Accidents," J. total radioactivity in the stainless steel first wall and blanket Fusion Energy,!, 1, 87 (Jan. 1981). of the reference design, UWMAK-I. As noted at the end of the paper, ongoing work at Massachusetts Institute of Technology has addressed the REPLY TO "COMMENT ON CONTRIBUTION problem of the potential for accidental release of structural OF ACTIVATION PRODUCTS TO FUSION activation products. In doing so, we have examined the ACCIDENT RISK: PART I. A PRELIMINARY temperatures that may be realistically generated as a conse- INVESTIGATION' " quence of a lithium fire.2'3 As expected, the theoretical maximum (adiabatic) flame temperature of 2400 K was 1 found extraconservative. Because of the inevitable radiation Kazimi describes accumulating evidence that the tem- cooling and the limitations on the rate at which oxygen may peratures experienced in actual lithium fires could be very much lower than the adiabatic flame temperature used in reach the flame, the maximum temperature of the flame 2 3 under severe accident conditions was found limited to my article and in some earlier work on fusion reactor acci- ~1400K, not enough to melt the structural materials. dents. His results are encouraging. If they stand up under Further, the gas temperature in a containment with no the needed further experimental investigation of the full provision for emergency cooling was predicted to be limited range of conditions that could be encountered in severe to a maximum of 1000 K. These findings by our model accidents, then the maximum release fractions from stain- even appear conservative when compared with scoping ex- less steel structure in such accidents will indeed be smaller periments in lithium fires performed at Hanford Engineering than the 20% I used as a "worst case" for the most mobile Development Laboratory.3 isotopes. (The assumed release fractions from stainless steel for the less mobile isotopes were stated in the article to be Thus the mobility of 20% of the structural activity by 4%.) melting or vaporization seems to be arbitrarily high. Our How much smaller depends not only on what the actual estimate for the maximum fraction that may be mobilized temperatures turn out to be, but also, as my article empha- for UWMAK-I amounts to 0.2% of the total structural 4 sized, on the effectiveness of formation and mobilization of radioactivity. We share the opinion that strategies that volatile oxides from structural material at temperatures will further reduce the potential for activation product below its melting point. (An ameliorating factor is the frac- accidental release should be pursued. However, the inherent tion of mobilized material that would plate out or fall out limitations on the activation product release should not be before reaching potential victims, which I stated I was ne- underestimated. glecting for low-melting-point materials, and which some experts think was likewise underestimated in the Rasmussen Mujid S. Kazimi report's analysis of light water reactor accidents.4) Kazimi's Massachusetts Institute of Technology estimates of possible mobilization fractions do account for Department of Nuclear Engineering oxidation from solid as well as from molten material, but 138 Albany Street I think he would have to agree that making these estimates Cambridge, Massachusetts 02139 required quantitative characterization of physical phenom- ena that are poorly understood theoretically and very March 16, 1981 scantily investigated experimentally. In this situation, the associated uncertainties are bound REFERENCES to be large, and the question of how to define the "worst case" becomes not only a matter of technical judgment but 1. JOHN P. HOLDREN, "Contribution of Activation Products to also of philosophy. The history of technology assessment Fusion Accident Risk: Part I. A Preliminary Investigation," Nucl. and regulation of nuclear fission reveals both a tendency to TechnolJFusion, 1, 79 (1981). place on the technology's proponents a substantial burden 300 NUCLEAR TECHNOLOGY/FUSION VOL. 1 JULY 1981 300 .