Main steam line break reload licensing analysis using RETRAN-02

The response of the Yankee plant to postulated main steam line break accidents was analyzed as a part of reload licensing. A post-processor to the RETRAN code was developed to predict transient boron concentrations, thus allowing credit or the negative reactivity effects of borated water injected by the emergency core cooling system (ECCS). All of the cases analyzed show substantial margin to recriticality for the limiting end-of-cycle conditions. In addition, both the sensitivity studies and a comparison of the results of different cases show the minimum negative reactivity obtained to be relatively insensitive parameter. This insensitivity is due, in large part, to the loss of driving force for the transient as the steam generator pressure approaches atmospheric pressure. Thus, as temperatures drop below about 3000F, the effectiveness of heat removal by liquid boiling and steam flow out the break rapidly decreases. The heat removal from the Primary System via sensible heat to cooler incoming feedwater is much less effective than boiling due to the combination of lower film coefficients and lower enthalpy rise on the secondary side of the steam generators. The net effect by these phenomena is a relatively slow cooldown of the primary system once temperatures approach 3000F. The minimum negative reactivity for the most limiting case was approximately 1%Δrho for the break from hot zero power initial conditions with off-site power available. Thus, it can be concluded that recriticality would not occur, and hence, no fuel damage would result from the postulated main steam line breaks at any time during the cycle