ML19221A645

Task 10a:Modes of Operation During Long-Term Cooling

ML19221A645
Person / Time
Site:	Crane
Issue date:	04/18/1979
From:	Selbrig C INDUSTRY ADVISORY GROUP
To:
References
OLS-790418, TASK 10A, TASK-10A, NUDOCS 7905230509
Download: ML19221A645 (10)

Text

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e e j .) MODES OF OPEPATION CURING LONG TERM COOLING ~ THREE MILE ISLNiD INDUSTRY ADVISORY GROUP O e 166 150 Q

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SUMMARY

Three modes of long. tem cooling are,seen as possible without taking benefit for RHR _with, natural circulation being the most desirable of the three modes. The three modes are: (1) natural circulation without boiling, (2) boiling with a closed system and (3) boiling with an open system. Water makeup to the primary of some amount would probably be required for both 2 and 3 because of system leakage. As long as the system remains in natural circulation there should be no bulk boiling. Changes in the rate of heat removal from the primary system; e.g., failure / loss of both A and B loop secondary closed cooling systems, could cause loss of natural circulation and lead.to a boiling mode. Other factors, such as reverse flow in the B loop (primary side), may influence the transition to natural . circulation and/or may actually ultimately result in loss of natural circulation once established. Thus moving into a boiling mode is not necessarily unlikely in the long-term. It is believed that the possible boiling modes represent a viable. way to operate the primary system if natural circulation cannot be maintained. In fact, if primary system pumping is lost altogether, it is believed that the plant should be allowed to go into natural circulation, and if necessary, a boil-ing mode instead of using the High Pressure Injection to provide forced cooling. The major uncertainty with going into the boiling modes of cooling is the concern for the effect of non-condensa'oies und whether they will block the flow process of steam from the core to the heat exchanger. This effect was evaluated and reported.in the IAG Memo Report #IA-10c. Based on that report, it was concluded ~ that non-condensables will not prevent achievement of an acceptably stable boil-ing mode. b SUM 4ARY OF COOLING MODES There are three basic modes of heat rcmoval envisioned for possible long-term cooling of Three Mile Island Unit 2. These configurations are: 1. Natural Circulation - Natural circulation in the primary with all hquid in both the primary and secondary 2. Boilina with a closed system - A boiling mode in the primary system with boiling occurring in the core and condensing in steam generators and the pressurizer relief valve closed. 3. Boiling with an open system - Same as Number 2 with the relief valve on the pressurizer open. These cooling modes are listed in the order of desirability. The natural circu-lation mode is the most desirable, however, certain events may cause transition frcxn one of these modes to a less desirable, but adequate mode. All three modes assume that the secondary side of at least one steam generator can continue to function; i.e., even if lost for some period that it could be brought back into service. The boiling modes are, however, less sensitive to the loss of secondary cooling particularly at low decay heat. O 166 15\\

s. / / / BfiARY OF COOLING MODES (Cont.)- The natural circulat; ion mode might possibly p to a boiling mcde if the heat removal capability of both steam generators is lost for a period of time. The pressure in the primary system would build up due to volume expansion so that the relief valve on the pressurizer would have to be opened. The ensuing blowdown frcxn the pressurizer would allow steam fonnation in the primary system and eventual transition into a boiling (RB) mode. If makeup water is not supplied to the reactor in the open system boiling mode, the water level will decrease over a long period of time until the core is un-covered. System leakages are such that even the closed system acts somewhat like an open system tec. The generated steam may continue to cool the core 'until the water level goes below the heated length. At this point, a water seal will exist at the bottom of the care which would inhibit circulation of non-condensible gas around the system.. In order to inhibit the change from natural circulation to a boiling mode, the mass flow on the secondary side of one steam generator is required. If the secondary is all liquid, then the flow rate on the secondary must be large enough to remove the heat. A plot of the flow rate Se9" ired " the 5'c "d3rY (or. primary) to remove sufficient heat assuming a 10 F temperature rise as a function of decay heat is shown in Figure 1. In order'to inhibit the production of non-condensables if the plant is in a boiling mode, the relief valve on the pressurizer should be closed as scon as the steam generators are restored to working order. If the relief valve is left - ) open, the primary system will be in the open and will tend to attain to a lower / equilibrium pressure. Eventually, with the valve open, hydrogen and oxygen will be generated by radiolysis, stripped out of solution, and enter the vapor region. Eventually (possibly within a week) enough H2 will be produced to cause the vapor transfer from the core to the steam generator to be diffusion limited and could possibly stop the steam generators from removing the heat. This case is evaluated in IAG Memo Report IIA-loc. If a boiling mode occurs and the pressurizer relief valve is closed when the steam generators are restored, the vapor region could be made to collapse by removing excess heat ar.d adding makeup water. Due care should be taken to ensure that water hammer does not damage the system during the collapse. If the rton-condensables have built up by the time the steam generators are restored, the pressurizer relief valve could be closed to increase the rate of presstre build up. When the pressure builds up to a sufficiently high value, the solu-bility of the water to the non-condensaoles will be increased to the valJe where a sufficient quantity will dissolve in the water in the steam generator and unblock the steam flow from the core and allow it to condense in the steam genera tors. If none of these systems function, a very unlikely event, the core uncovered mode will occur and would probably be unsatisfactory. An analysis of this case is presented in Appendix 1. A discussion of each of the modes of long-term cooling is presented in the re- ) mainder of this report to show how each will function. 166 152

's,/' {g,p/ NATURAL CIRCULATION MODE

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INEL has made calculations of natural circulation with use of the RELAP4 code. These results are sumnarized in Table 1. On paper, it appears to be diff'icult to leave the natural circulation mode with the steam generators in operat?on. A computer run was made where the pressurizer vent was opened during one of the steady state natural circulation runs. The natural circulation remained during the blowdown and the pressurizer was completely voided without any steam beiny generated in the upper reactor vessel or candy cane. Thus, on paper, it appears difficult to thwart natural circulation. This calculation is, however, not too consistent with naval reactor experience which indicates that such systens are very sensitive changes in rate of heat renoval during natural circulation. Thus, , loss of the natural circulation mode cannot be ruled out. BOILING MODE This mode of heat transfer appears difficult to control. The secondary side should only remove enough heat to offset the heat generated in the core. Excess heat removal will cause the voids to collapse. Insufficient heat removal will cause the void to expand. Basically, this made will cause the entire primary system to be behave like a pressurizer. Preferred operation would appear to be with a closed system at 10-20 atmospheres of pressure. Although, operation with the pressurizer relief valve open may accomplish similar results. The basic reason desiring some pressurization is that radiolysis is low, H resolution is higher and boiling is more stable in a 2 ,J pressurized system. Figure 2 shows t a supposed configuration of the liquid and gas phases. Increasing the pressere on the system will allow the non-condensables to dis-solve in the water and allow the process to proceed. The heat transfer coeff.cient will degrade due to the presence of non-condensables. The required steam generator area is shown in Figure 2. Other processes which were considered included: 1. Streaming of steam through the center of the candy cane due to buoyancy. 2. Heat less from the pipe causing condensation. These processes do not appear sufficient to aid the process. Subsequent work reported in IAG Memo #IA-10e indicate the on-condensables should not be a long-tenn problem. O 166 153

5 f Ted Mott .f APPENDIX 1 ANALYSIS OF, CORE UNC0VERED MODE ~ 1. Vapor blocks natural circulation. 2. Water in core heats to boiling point and core now generates steam and hydrogen pressure rises and steam and H system is loosing mass with no make L, 2 vent to containment - primary 3. Steam is generated in the core - part is vented and part condenses - water level drops. During this time, hydrogen concentration is low in vent flow, however, hydrogen concentration in steam generator builds, since the steam is being condensed there. / ef 0 -~ 3f/sec W 10,000 #/hr. di / mH2 = 36 f/ day = 1.5 f/hr 4. As H 2 concentration builds up in SG, condensation slows - core now drys out ) 5. Rapient heat tracsfer from the core evaporated enough water to break seal. 6. At a pressure of 14.7 psia free convection of hydrogen will not cool core. At 147 psfa free convection will cool core (if loss coefficienth.4100) with 3000 F A T - Fuel surface temp 2 4000 F 0 CONCLUSION 4 If vapor blocks natural circulation, the system pressure should be raised as high as possible as soon as possible. circulation of hydrogen and prevent a melt down.This would, in the limit, pennit natural 8 i 166 154 l i 1

TABLE I SUfNARY OF INEL' NATL)RAL CIRCULATION CALCULATIONS ~ Percent blockage 0. 40 85 95 Core Mass Flow (1bm/sec) 429 429 318 100 Th-Tc ( F) 10.9 10.9 14 47 244 244 244 244 c Secondary side of Steam Generator A is steaming, 8 is isolated J 166 155 .u

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