USE OF AN INTEGRATED CONTAINMENT AND ULTIMATE HEAT SINK (UHS) RESPONSE APPROACH TO EVALUATE NUCLEAR POWER PLANT MODIFICATIONS

Transcription

1 USE OF AN INTEGRATED CONTAINMENT AND ULTIMATE HEAT SINK (UHS) RESPONSE APPROACH TO EVALUATE NUCLEAR POWER PLANT MODIFICATIONS M. C. Wetzel A. T. Vieira D. C. Pattn ABSTRACT CA Detailed cntainment and Ultimate Heat Sink (UHS) perfrmance evaluatins ften are required t supprt majr plant mdificatins, such as pwer uprates and steam generatr replacements. These UHS and cntainment pressure and temperature respnse evaluatins are interrelated. Nt nly is the cntainment heat lad t the UHS a factr in these evaluatins, but ther heat lads, such as thse frm the spent fuel pl, may change as a result f the plant mdificatin and impact cntainment r UHS respnse. Our experience is that if an integrated cntainment/uhs respnse mdel is develped prir t the feasibility evaluatins fr such plant mdificatins, significant savings in engineering hurs can be achieved. This paper presents an verview f such a frnt-end engineering tl that has been develped and used t supprt engineering evaluatins. Rigrus calculatins have been perfrmed t supprt the design f the cntainment and the UHS fr all nuclear plants. This wrk, requiring thusands f engineering hurs, has been reviewed in detail by the U.S. Nuclear Regulatry Cmmissin (USNRC) during the licensing f the plant. These evaluatins have established the perfrmance requirements fr cntainment systems and the cntainment equipment qualificatin envelpe and have served as a basis fr technical specificatins and plant perating prcedures. During the subsequent peratin f the plant, numerus re-evaluatins and sensitivity studies are perfrmed n the interrelated analyses t assess plant perating prcedures, plant degradatin, revised limits n plant peratin, and changes t the licensing basis f the plant. Fr a majr plant mdificatin, reevaluating the entire system f interrelated cntainment and UHS perfrmance analyses becmes a repetitive, cumbersme and jbhur intensive task. In tday's perating plant envirnment, nearly all plant mdificatins are driven by increasingly tight schedules and reduced budgets. A plant-specific cmputer mdel, cupling the cntainment respnse directly t the perfrmance f the UHS and auxiliary heat lads, increases the quality and cnsistency f the engineering evaluatin fr a majr plant mdificatin. Additinally, the studies can be perfrmed in much less time and fr cnsiderably less cst than the riginal evaluatins. Further, the integrated apprach allws the engineering team and management t assess the sensitivity f prpsed changes and t enhance the credibility f any assciated licensing submittal. Bechtel Pwer Crpratin

2 USE OF AN INTEGRATED CONTAINMENT AND ULTIMATE HEAT SINK (UHS) RESPONSE APPROACH TO EVALUATE NUCLEAR POWER PLANT MODIFICATIONS M. C. Wetzel A. T. Vieira D. C. Pattern 1. INTRODUCTION The Ultimate Heat Sink (UHS) f a nuclear pwer plant is a cmplex cling water system which serves the plant during a variety f bth nrmal and emergency perating scenaris. As set frth by the USNRC in Regulatry Guide 1.27 (Reference 1), the UHS must be designed t: dissipate the heat f a design-basis accident f ne unit plus the heat f a safe shutdwn and cldwn f all ther units it serves prvide a thirty-day supply f cling water at r belw the design-basis temperature fr all safety-related equipment be capable f perfrming under the meterlgical cnditins leading t the wrst cling perfrmance and the cnditins leading t the highest water lss The cntainment fr a nuclear pwer plant has several emergency safety features which perate during a Lss f Cling Accident (LOCA) r Main Steam Line Break (MSLB). These systems prvide cre and cntainment cling and pressure suppressin. The perfrmance f these systems, which include the cntainment air clers, cre injectin, cntainment sprays and residual heat remval heat exchangers, is interdependent with the perfrmance f the UHS. This interdependency can be different fr each accident and can change during the curse f an assumed accident. The technical analyses perfrmed in the design phase f the cntainment and the UHS fr a nuclear plant and the subsequent detailed review by the USNRC during the licensing phase have helped t establish cntainment system perfrmance requirements, cntainment equipment qualificatin envelpes, bases fr technical specificatins, and plant perating prcedures. These analyses, dne during the design and initial licensing phases f the plant, were typically perfrmed separately frm ne anther with cnservative simplifying assumptins made cncerning their areas f interdependency. Fr example, the cntainment pressure/temperature was calculated with certain heat exchanger perfrmance and cling water temperatures assumed. The heat exchanger perfrmance was calculated using assumed heat rejectin rates frm the cntainment and assumed cling water temperatures. Additinally, the UHS perfrmance was calculated based n an assumed heat rejectin rate frm the plant. Changes t any f these systems

3 required an iterative prcess f calculatin and evaluatin f impact n the ther systems. Over time, as a plant matures, age-related changes in the plant perfrmance, revised limits n plant peratin, and changes t the licensing basis f the plant necessitate the re-evaluatin f these analyses. Fr a majr plant mdificatin, reevaluating the entire system f interrelated cntainment and UHS perfrmance analyses can becme a repetitive, cumbersme and jbhur intensive task. This paper details the develpment f a plant-specific cmputer cde, COPATTA- CT, which cuples the cntainment respnse directly t the perfrmance f the UHS and auxiliary heat lads. The Bechtel Standard Cmputer Cde fr cntainment analysis, COPATTA (Reference 2), is integrated with a UHS perfrmance mdel, including the effects f mechanical draft cling twers, t assess prpsed changes such as the engineering feasibility f a steam generatr replacement. COPATTA-CT, like COPATTA, prepares a histry f pressure and temperature in the cntainment atmsphere frm the mment f the accident up t any specified later time. COPATTA-CT, unlike COPATTA, als has the capability f mdelling a dual cling train with cling twers. 2. COPATTA, A CONTAINMENT ANALYSIS CODE The COPATTA (Cntainment Pressure and Temperature Transient Analysis) Cde is a pressurized water type nuclear reactr plant design and analysis tl and is dcumented in Tpical Reprt BN-TOP-3 (Reference 3). It prvides capability fr calculating the pressure and temperature within the reactr cntainment building as functins f time fllwing the release f water and/r steam t the cntainment due t a pipe rupture. The cde prvides fr cnsideratin f the effects f reactr system blwdwn, decay pwer energy release, metal-water reactin energy release, sensible heat release frm the reactr system piping, and engineered safeguards including air clers, cntainment sprays and reactr cre safety injectin. COPATTA uses the implicit finite difference methd t calculate the heat absrbed by the cntainment structure and equipment. Examples f prblems slved by COPATTA include: Lss f Clant Accident (LOCA) Main Steam Line Break (MSLB) in cntainment any high energy line break (HELB) in a single cmpartment, and assessment f equipment thermal respnses during accidents fr the purpse f equipment qualificatin

4 2.1 General COPATTA Mdel COPATTA perfrms a stepwise iteratin between thermdynamic statepints based n the laws f cnservatin f mass and energy in calculating the transient cntainment respnse. A three regin cntainment mdel is used which cnsists f the cntainment atmsphere (r vapr regin), the sump {r liquid regin), and the water cntained in the reactr vessel. Mass and energy are transferred between the liquid and vapr regins by biling, cndensatin, and liquid drput frm the cntainment atmsphere. 2.2 Thermdynamic Assumptins The sump, vapr, and vessel regins are treated as pen systems in a thermdynamic sense since mass flws acrss bundaries fr all three regins. The first law f thermdynamics fr such a system is: i + dt y dt y i dt where: U is the internal energy f the system (Btu) Q is the heat energy transferred t the system (Btu) h is the enthalpy f the mass entering r leaving the system (Btu/lbm) m is the mass entering r leaving the system (Ibm) t is time (hr) This equatin is integrated fr each regin frm the start f the transient t any later specified time, yielding the thermdynamic cnditins frm which the state pint prperties f pressure and temperature can be determined. The fllwing assumptins are inherent in COPATTA: At the break, the discharge flw flashes int a steam prtin that is added t the atmsphere and a liquid prtin that is added t the sump. The water prtin is at the saturatin temperature crrespnding t the ttal cntainment pressure, while the steam prtin mixes with the cntainment atmsphere. Each regin is assumed hmgeneus, but a temperature difference can exist between regins. Any misture cndensed in the vapr regin during a time increment is assumed t fall immediately t the sump. Mass and energy are transferred as steam frm the liquid regins (sump and reactr vessel) t the atmsphere by biling if the cntainment pressure is less than the saturatin pressure crrespnding t the liquid temperature. The sump regin cntains n water at the beginning f the transient.

5 2.3 Atmsphere and Sump Regins Typically, the cntainment system is assumed t be initially at nrmal perating cnditins with water vapr and air ccupying the entire free vlume f the cntainment. During subsequent time increments, pipe break mass and energy is added t the cntainment. The transient pressure and temperature calculatins are made by cnsidering the mass, vlume, and energy equatins fr water, steam, and air in the atmsphere and sump regins. These equatins are slved iteratively fr the vapr and sump temperatures after each time increment until a specified cnvergence criterin is met. The ttal cntainment pressure is cmputed frm the sum f the partial pressures f steam and air at the cntainment atmsphere temperature. 2.4 Reactr Vessel Regin Fr the evaluatin f a pstulated lss f clant accident, the rates f mass and energy released t the cntainment are typically supplied by the reactr manufacturer. Data is prvided fr the initial blwdwn and reflding phases, up t the time when the reactr vessel is in pressure equilibrium with the cntainment. At that pint, usually a mass and energy balance is initiated fr the reactr vessel. The reactr vessel regin is mdelled t include the effects f decay heat, metal-water reactin, transfer f sensible heat frm the reactr clant system metal, and ECCS peratin. If the vessel water internal energy is less than saturatin at the cntainment ttal pressure, n steam is transferred frm the reactr vessel t the cntainment atmsphere. If it is greater than saturatin, water is biled ff t the cntainment until the vessel water internal energy is reduced t saturatin cnditins. 3. PDAP AND UHSSIM, THE MECHANICAL DRAFT COOLING TOWER CODES The cmputer cdes PDAP and UHSSIM, written by William E. Dunn f the University f Illinis, tgether mdel the perfrmance f the mechanical draft cling twer. The Perfrmance Data Analysis Prgram (PDAP) calculates the cling twer perfrmance characteristic, KaV/L, based n the design data frm the manufacturer. The Ultimate Heat Sink Simulatin Prgram (UHSSIM) calculates the evaprative water lss, basin inventry, and basin temperature fr a given set f meterlgical data, heat lads, and cling twer perfrmance data. The rate f heat additin t the basin is the difference between the plant heat lad and the rate f heat rejectin by the twer, determined using a mdel f cling twer peratin. The rate f evapratin is als determined frm the cling twer mdel, and the assumptin f n make-up water is emplyed. The slids cntent f the basin is fund frm the basin mass and the initial cnditin based

6 n a simple cnservatin f slids principle. 4. COPATTA-CT, THE INTEGRATED CODE In the creatin f COPATTA-CT, the cling twer cde mdules were incrprated int the existing versin f COPATTA. These mdules cnsist f the prgram UHSSIM and its supprting functins and subrutines. A schematic f a typical COPATTA-CT mdel is shwn in Figure 1. This figure shws the COPATTA three regin mdel (sump, vessel, and cntainment atmsphere). In COPATTA, safety injectin (SI) is added directly t the vessel regin. Additinally, cntainment sprays and air clers can be mdelled. The surce f the SI and the spray water initially is frm the Refuelling Water Strage Tank (RWST). At recirculatin, the pumps are realigned such that water is drawn frm the sump regin. Depending upn plant design, the SI and/r the sprays are cled by the primary heat exchanger. Auxiliary heat lads, including thse frm the cntainment air clers, can be mdelled as being cled by the intermediate lp r by the secndary heat exchanger lp. The UHS cling twer mdel in COPATTA-CT evaluates the resulting UHS respnse based n the transient plant emergency heat lad. A separate prgram, PDAP, is run prir t running COPATTA-CT. PDAP uses asbuilt infrmatin n the mechanical draft cling twers and design meterlgical cnditins t calculate the cling twer perfrmance characteristic, KaV/L. The cling twer perfrmance characteristic, KaV/L, is part f the input required by COPATTA-CT. The UHSSIM prtin f COPATTA-CT calculates the time dependent evaprative lsses, basin inventry, and basin temperatures given the twer perfrmance characteristic, meterlgical data, and the COPATTA-CT calculated heat lad. The UHSSIM calculatins are perfrmed every fifty COPATTA-CT calculatinal timesteps since the twer water temperature changes slwly cmpared t the cntainment temperature. The cling water t the secndary heat exchangers has a temperature equal t that f the twer basin. Hence, heat exchanger calculatins are dne explicitly, eliminating the need t iterate n basin temperature and the calculated heat lad. The enhanced features f COPATTA-CT are: tw independent cling trains the ptin fr a cling twer Ultimate Heat Sink (UHS) n each cling train. The cling twer cde mdules are based n UHSSIM and its supprting functins and subrutines. The basic calculatinal algrithm f

7 COPATTA is nt changed. Critical variables and algrithms in the UHSSIM related mdules are retained in their riginal frm, the air cler clant temperature can be ne f three ptins: (1) a cnstant value, (2) the UHS cld water (return) temperature, r (3) the intermediate lp clant temperature meterlgical data is input fr use in cling twer calculatins the ptin fr cling twer makeup water can be chsen such that the amunt f water in the twer basin remains at a cnstant level (i.e. at its riginally specified value). The makeup water is assumed t be added at a cnstant temperature as specified by the user. If tw trains f engineered safeguards features are mdelled, the fllwing simplifying assumptins are made: the heat exchanger cnfiguratin and heat exchanger types are identical fr each train, tw cling twers are specified and the basin temperature is assumed t be the same fr bth trains. (If ne train is used, ne cling twer must be specified.) the cnfiguratin f the air cler cmpnent cling water piping is identical fr each cling lp. T facilitate the interpretatin and dcumentatin f results, the COPATTA-CT prgram uses input frm a single input file and prduces fur utput files: (1) the nrmal utput file with wrd descriptins f the utput parameters, (2) a file cntaining cntainment, sump, and reactr vessel data. This file is suitable fr use by a pst-prcessr fr cnstructing tables r plts, (3) a file cntaining primary and secndary heat exchanger data, and (4) a file cntaining the UHS twer results. The integrated cde is PC-based and runs under the WINDOWS envirnment. Figure 2 shws a sample family f curves illustrating the type f results that can be btained using COPATTA-CT. The particular study invlved engineering supprt fr a steam generatr replacement and a fuel cycle change. In Figure 2, the lng term cntainment pressure and temperature respnses are shwn. These curves are used fr evaluatin f cntainment design and assessment f equipment qualificatin. The third figure shws the calculated transient heat lad f the secndary heat exchanger lp. The shape f the curve is characterized by the initiatin f the cntainment air clers at 35 secnds and the subsequent start f Sl/spray recirculatin at 3050 secnds. The temperatures assciated with these heat lads were used t evaluate thermal stresses in the intermediate and secndary cling water systems. The last curve shws the actual heatup f the UHS cling twer basin. The basin temperature remains apprximately cnstant

8 until the significant increase in heat lad assciated with the start f recirculatin. This curve als shws that in the lng-term, the basin temperature is driven by the daily cyclic change in wet bulb temperature. The COPATTA-CT mdel used t generate the respnses shwn in Figure 2 has als been utilized t assess UHS cling twer basin inventry and makeup capability. The apprach eliminates unnecessary cnservatism because the mdel integrates the cntainment respnse with the perfrmance f the heat exchangers, cntainment air clers, and the UHS cling twer. The use f the integrated apprach has als resulted in at least a 50 percent savings in bth the cst and schedule required t perfrm the assciated engineering. This same technlgy has been successfully applied fr plants with a cling pnd, a spray pnd and fr plants with a time-varying bay temperature as a UHS. 5. CONCLUSION This paper presents an verview f hw a frnt-end engineering tl which cuples the cntainment respnse directly t the perfrmance f the UHS and auxiliary heat lads has been develped and used t supprt engineering evaluatins. The plant-specific cmputer mdel, COPATTA-CT, has been fund t significantly increase the quality and cnsistency f related engineering evaluatins. Additinally, studies can be perfrmed in much less time and fr cnsiderably less cst than the riginal evaluatins. The integrated apprach allws the engineering team and management t assess the sensitivity f prpsed changes and t enhance the credibility f any assciated licensing submittal. 6. REFERENCES 1. U.S. Nuclear Regulatry Cmmissin Regulatry Guide 1.27, "Ultimate Heat Sink fr Nuclear Pwer Plants," Revisin 2, January Bechtel Standard Cmputer Prgram COPATTA (MAP-175), "Cntainment Pressure and Temperature Transient Analysis Cde," Versin G1-14, Nvember Tpical Reprt BN-TOP-3, "Perfrmance and Sizing f Dry Pressure Cntainments," Revisin 4, March 1983, Bechtel Pwer Crpratin.

9 Primary Heat Exchanger Auxiliary Heat Lad Figure 1: COPATTA-CT Mdel Schematic

10 Cntainment Temperature vs. Time OO.O 1 1 I f «'i «" r TTtl ''' 1OO.O 10 " E 10 -I 1O 1O e 1O* 1O * 1O 6 Time (secnds) 1O " Cntainment Pressure vs. Time O.O 20.0 \ I IJ / / J / / I 1O.O 1O " * 10 3 Time (secnds) 10 * 1O 6 1O a 1 y \ m in Figure 2: Sample COPATTA-CT Results

11 Ttal Heat Lad in Cling Water System 2.50E-I E+8 1.5OE + 8 T3 CO 3 l.ooe-t-8 CO <D c 5.00E+7 O.OOE-+-O m Mill 10 " s * Time (secnds) -! \ i \ '! UHS Basin Temperature i» = M f \ (V z " z 10~ l s 10* " Time (secnds) Figure 2: Sample COPATTA-CT Results, cncluded