Evaluation of UF6 Vapor Release in Postulated Accident

Journal of NUCLEAR SCIENCE and TECHNOLOGY, 15[6], pp. 455~460 (June 1978). 455

TECHNICAL REPORT

Tsuyoshi OKAMOTO and Ryohei KIYOSE

Department of Nuclear Engineering, Faculty of Engineering, University of Tokyo*

Received February 23, 1977 Revised November 21, 1977

The rupture of UF6 gas line connected to hot UF6 cylinder, being one of various accidents in UF6 vapor leak-out, is considered as a postulated accident for uranium enrichment plants. For this type of rupture, we will estimate the amount of UF6 vapor release based on a simplified calculation model and then make an evaluation of UF6 vapor release through a ventilation system of feed vaporization facility. Assuming an instantaneous steady state for the change of UF6 states, an unsteady state thermodynamics process is solved. Numerical examples show that about 52% of the initial UF6 quantity are vaporized at 80dc (the temperature of the liquid UF6 in the cylinder). Furthermore, by using the amount of released UF6 vapor and the collection capacity of HEPA filter for HF gas, the amount of gaseous UO2F2, HF which may be dissipated to the environment are conservatively estimated. KEYWORDS: uranium enrichment plants, uranium hexafluoride, cylinders, UO2F2, HF, HEPA filter, triple point, ventilation, convergent nozzle, safety analysis, ruptures

state. In order to simplify the mathematical I. INTRODUCTION treatment of vaporization phenomenon, the When we go into concrete design of ura- assumption of instantaneous steady state is nium enrichment plants, safety evaluation will adopted and a simplified model is made. Using be required to a potential accidents which this simple model, calculations were made to may be caused in the plants. The UF6 vapor determine the amount of UF6 vapor release leak-out in a postulated accident is analyzed and the releasing time for the liquid UF6 in a from the viewpoint of safety evaluation. A cylinder to solidify. pigtail or a valve failure of hot UF6 cylinder Upon release, the UF6 vapor becomes hy- in a feed vaporization facility, being one of drolyzed from contact with water vapor in the accidents of a potentially greater severity for air, forming UO2F2 and HF. If the ventilation UF6 vapor leak-out but having a very low system is operated in the rupture accident, probability of occurrence, may be considered the most of gaseous materials (i.e., UO2F2 and for the plants"'. However, because of the HF) released in the vaporization room would fact that only the amount of released UF6 be exhausted through a scrubber, a high effi- vapor resulting from this type of failure or ciency particulate air (HEPA) filter and out rupture accident have been reported, first and the stack. For this case, we estimate con- foremost, we consider a calculation model of servatively the amount of materials dissipated UF6 vaporization. to the environment. Finally, the relation The heat and mass transfer caused by * Hongo vaporization result in a system with unsteady , Bunkyo-ku, Tokyo.

— 67 — 456 TECHNICAL REPORT (T. Okamoto, R. Kiyose) J. Nucl. Sci. Technol., between the collection capacity and the failure reaches the triple point, is referred to Process time of HEPA filter is discussed. I . Assuming that the changes of the UF, state is made instantaneously throughout the II. CALCULATION MODEL UF6 cylinder, the quantity of heat which is OF UF6 VAPORIZATION lost through the UF6 cylinder during the time t should be equal to the difference between After passing through the triple point the heat capacity of the initial liquid UF6 and (64.0dc, 1,137.5mmHg), the state of UF6 in the that of the liquid UF6 at the time t. The cylinder will completely change from the liquid following equation can be derived by the heat UF6 to the solid UF6. When the temperature capacity difference DMI in Process I. of the solid UF6 has decreased to the sublimation point (56.4dc, 760.0mmHg), the UF6 vapor pressure in the cylinder equilibrates with atmospheric pressure, with net release of the UF6 vapor ceasing. Figure 1 illustrates (1) a schematic diagram of equilibrium state where Mt: Quantity of liquid UF6 in cylinder between two phases of UF6. The state vari- at time t (kg) ables (temperature T and vapor pressure P) Tt: Temperature of liquid UF6 at time change every moment along the P—T curve. t (dc) The process in which the initial point A mI: Weight flow rate of UF6 vapor through rupture hole in Process I (kg/sec) passes through the liquid-vapor region and tI : Time required to reach triple point (sec). The significations of other signs are shown in Table 1. The first term of the right-side in Eq. (1) represents the total heat loss by vaporization during the time t. The material balance in this Process I is

(2)

The liquid UF6 will begin to solidify after the time tI. If the liquid UF6 finished solidification at a future time tII, the heat capacity which is required to have phase develop-HII

Fig. 1 Releasing process of UF6 ment should be zero at that time. Therefore, vapor in phase equilibrium the following equation is satisfied :

(3)

68 Vol. 15, No. 6 (June 1978) TECHNICAL REPORT (T. Okamoto, R. Kiyose) 457

In particular, the vaporization and sublimation where phenomena are simultaneously considered in m : Weight flow rate of UF6 vapor (kg/sec) F: Cross-sectional area of exit (m2) this equation. It is clear that the solidification Pout : Vapor pressure in F plane (kg/m2) time (tII-tI) will be equal to the time inter- Po: Vapor pressure in UF6 cylinder (kg/m2) g val when the state of the UF6 is in existence : Specific heat ratio 1.06 on the triple point. g : Acceleration of gravity 9.8 (m/sec2) After this Process II, the solid UF6 loses vo: Specific volume of UF6 vapor in cylinder (m3/kg). the heat by the sublimation phenomenon. Process III is the process that the state of In case of Pout=Pc (critical pressure), the fol- the solid UF6 reaches the terminal point B, i.e., lowing equation is used the sublimation point. In the same manner as Process I , the heat capacity difference DHm (9) between the heat capacity at the time tII and that at the time (tII+t) is expressed as follows : III. CHARACTERISTICS OF UF6 VAPOR RELEASE

A computation code UFLEAK is prepared by programming the series of equations de- (4) scribed in the previous chapter. The input where mIII : Weight flow rate of UF6 vapor data for the postulated accident, used in this in Process III (kg/sec) code, are listed in Table 1. tIII : Time required to reach sublimation Table 1 Input data for hot UF6 cylinder point (sec). rupture accident

The material balance in this Process III is

( 5)

To solve the fundamental equations (1)~ ( (5) the following equations are provided"). The vapor pressure of the liquid UF6 and of the solid UF6 are calculated from Eqs. (6) and (7), respectively.

(6)

(7) And the equations of convergent nozzle(8) are used to calculate the flow rate of UF6 vapor release through the rupture hole :

Data of these items are used as parameter. (8) Thirty times as much as a standard (about 3,000cm2 cross-section) glass fiber filter (Ref. (4)).

69— 458 TECHNICAL REPORT (T. Okamoto, R. Kiyose) J. Nucl. Sci. Technol.,

Parameter survey is made for the items with asterisk sign in Table 1. While a parameter survey on one item is made, the other items are fixed on the input data listed. The UF6 cylinder installed in the feed

vaporization equipment (autoclave) is connected to a UF6 gas line and the equipment is closed. Low-pressure steam is used as the

source of heat to melt the solid UF6. After a preheat of some hours, the feed valve is opened and the liquid UF6 is vaporized at a controlled rate. Model 30A cylinder in which the nominal net weight of UF6 is 2,180 kg is selected for the object of acci-

dent. Let us consider the following postulated rupture accident. It is assumed that the rupture of UF6 gas line connected to hot UF6

cylinder occurs in the vaporization room. This rupture accident, however, has a very

low probability of occurrence as compared Fig. 2 Characteristics of UF6 vapor release in with the rupture accident within the auto- rupture accident of hot UF6 cylinder clave. The rupture hole is 2.54 cm dia. and

is above the liquid level. When the rupture time when the state of UF6 enters into Proc- accident occurred, the supply of low-pressure ess II, on the other hand, an open arrow •ª

steam is stopped. Even if the supply of steam indicates the time when the state of UF6 is is stopped, the UF6 vapor is released furiously free from Process II. The time interval be- in all directions through the rupture hole. It tween the solid arrow and the open arrow is

is assumed that the specific heat and the heat very long. From this fact, it is obvious that of vaporization or sublimation of UF6 is con- the amount of UF6 vapor release depends stant in each temperature range considered strongly on Process II. This tendency appears in Processes I and III The flow rate of UF6 more remarkably as the initial temperature of

vapor is obtained as a function of time for liquid UF6 becomes lower. The linear part the rupture accident. The calculation results of each curve represents the constant release

are shown in Fig. 2 where the initial temper- rate of 0.525 kg/sec. Because the overall re- atures of liquid UF6, T0 are indicated as pa- leasing time settles down roughly between 30 rameter. If the initial temperature of liquid and 35 min, it can be seen that there is almost

UF6 is 80dc, the releasing time in Process I no effect of the liquid UF6 temperature on is 9.9 min, Process II 22.3 min and Process HI the overall releasing time. It should be noted

2.3 min, respectively. Therefore, for all proc- that the released fraction becomes constant, esses, the overall releasing time is 34.5 min, 52.2% at To=80dc, independent of the initial during which time the amount of released quantity of liquid UF6 in the UF6 cylinder. UF6 vapor is 1,138.4 kg. If the released fraction is defined by (an amount of released UF6 IV. DISSIPATION OF UO2F2, HF /an initial quantity of UF6) x 100%, it is 52.2% in this case. Under the initial temperature The following condition is assumed in corn- of 100dc, the overall releasing time is 33.9 min putations that all of the released UF6 vapor and the amount of released UF6 vapor is 1,138.4 kg, react on water vapor in the air. 1,430.9 kg. Then the released fraction is 65.6%. If the ventilation system is stopped in the A solid arrow •ª on this figure indicates the rupture accident, the HF quantity of 258.8 kg

70— Vol. 15, No. 6 (June 1978) TECHNICAL REPORT (T. Okamoto, R. Kiyose) 459 and the UO2F2 quantity of 996.2 kg may be time for 95, 90 and 99% removal rate. One accumulated in the vaporization room. can be seen from the figure that in case Let us consider the case that the ventila- of 2.0 kg-HFmax the failure time of HEPA tion system is operated in the rupture accident. filter is 25.3 min for 99% removal rate and Removal rates of the scrubber to be operated 28.2 min for 90% removal rate. only in the rupture accident and of the HEPA filter are summarized in Table 1. The removal rate for HF is assumed to be equal to that for UO2F2. Since there is a limit to the collection capacity of HEPA filter for HF(4), the maximum collection capacity is denoted by HFmax. It follows that when the collection capacity exceeds a HFmax, the removal rate is zero, that is, the function of HEPA filter will be damaged. The amount of gaseous UO2F2, HF which may be dissipated to the environment are shown in Fig. 3.

Fig. 4 Effect of maximum collection capacity on failure time of HEPA filter and on amount of gaseous UO2F2, HF dissipation

If the removal rate is 90% and the HFmax, >2.35 kg, the HEPA filter will not be failed. In this case, the amount of dissipated UO2F2, HF become 1.0, 0.26 kg, respectively. For 95% removal rate, it is estimated that the necessary collection capacity of HEPA filter for HF is >2.5kg-HFmax in the rupture accident of Model 30A UF6 cylinder (designed operating temperature 80dc).

V. DISCUSSION AND CONCLUSIONS

The amount of released UF6, in the rupture Fig. 3 Dissipation curves of gaseous UO2F2, accident is approximately estimated by solving HF through ventilation system having the instantaneous steady state equations. damaged HEPA filter However, it may be desirable to treat exactly As seen in Fig. 3, if the HFmax of HEPA the release of the UF6 vapor as the enthalpy filter is 2.0 kg, the HEPA filter will be dam- balance in the unsteady state. Although the aged in 26.6 min (failure time), and then the enthalpy values of the UF6 can be calculated amount of UO2F2, HF dissipation rapidly in- by using the enthalpy-temperature equations crease up to 2.26, 0.59 kg, respectively. For given in Ref. (2), in general, more computing the HFmax of 2.5 kg, no damage can be read time is required for the mere estimation of off the figure. Figure 4 shows the failure the release of the UF6 vapor. Comparing

71 460 TECHNICAL REPORT (T. Okamoto, R. Kiyose) J. Nucl. Sci. Technol.,

with the calculation results based on the kg, the initial temperature of UF6, 80dc enthalpy balance, we would expect there is and the rupture hole, 2.54 cm in diameter, no large difference in our results from the it was estimated that the overall releas- simple model proposed. ing time is 34.5 min and the amount of Considering the heat capacity of UF6 cyl- released UF6 vapor 1,138.4 kg. In this inder, it is conservatively estimated that fur- case, the released fraction is 52.2%. ther additional quantity of UF6 vapor release (2) Because the amount of UF6 vapor re- becomes to be ~,90 kg at To-=80dc lease depends strongly on Process II, It is assumed that the nozzle efficiency in further study is necessary as to dealing convergent nozzle is equal to unity. Since with the triple point. there is the friction between a fluid flow and (3) In the rupture accident of Model 30A an inside wall in the nozzle, the isentropic cylinder, if the HFmax is >2.5 kg for the flow is not realizable. Consequently, the HEPA filter of 95% removal rate, the actual releasing time would be longer than HEPA filter will not be failed. the obtained time. All of the gaseous UO2F2 and HF is exhausted from the vaporization ACKNOWLEDGMENTS room. However, considering that there are The authors are indebted to Dr. N. Katoh holding times in the ventilation system, it of University of Tokyo for many suggestions takes some time to dissipate the UO2F2 and in the preparation of the manuscript. HF. The amount of dissipated UO2F2 and HF REFERENCES- must be less than the computed values, stick- (1) U.S. AEC: Environmental survey of the uranium fuel cycle, WASH-1248, (1974). ing to a floor and a duct on the way to (2) DEWITT, R.: Uranium hexafluoride, A survey exhaust. And building containment features of the physico-chemical properties, GAT-280, and appropriate emergency measures should (1960), Goodyear At. Corp. confine a large part (perhaps 40%) of the (3) BRANSOM, S.H. : "Applied Thermodynamics", UO2F2 to the building where it could be re- (1961), D. Van Nostrand Co. Ltd. covered(5). (4) SCHMETS, J.J. : Safety aspects of non aqueous reprocessing, KR-126, (1967). The conclusions drawn from this study can (5) U.S. ERDA : Environmental statement expan- be summarized as follows : sion of U.S. uranium enrichment capacity, (1) If the initial quantity of UF6 is 2,180 ERDA-1543, (1975).