GOOD NEWS ABOUT IODINE RELEASES Sidebar 3:

ne of the elements vital to the reactor in Cumberland, England resulted in assumptions should be regarded, not as a proper functioning of the hu- escape to the atmosphere of more than conservatism, but as an error. Measurements man body is iodine. This ele- 20,000 curies of iodine-131 and a maximum of both xenon-133 and iodine-131 showed ment, in trace quantities, is es- radiation dose to the public (observed in the that, although the core inventories of both sentialo for the synthesis of thyroid glands of several children) of 5 to 15 isotopes were roughly comparable (154 mil- metabolism-regulating hormones by the rads. Despite its rather minimal public con- lion curies of xenon-133 and 64 million thyroid gland. To produce these hormones sequences, this accident may have had a curies of iodine-131), the quantity of iodine as needed, the thyroid gland selectively ab- determining influence on the assumptions that escaped to the atmosphere (13 to 18 sorbs iodide ions from the blood, accumulat- adopted for regulatory purposes in the early curies) was less than that of xenon (2,4 to 13 ing and storing 25 to 45% of the body’s 1960s by the Atomic Energy Commission million curies) by a factor of 105 to 106. normal intake of iodine. The thyroid gland is and later by the Nuclear Regulatory Com- In a letter of August 14, 1980 to the thus particularly susceptible to damage by mission. It is assumed that 25% of the core Nuclear Regulatory Commission, A. P. radioactive iodine isotopes, should these be inventory of iodine would be distributed as Malinauskas and D. O. Campbell of Oak available to the body. volatile species within the containment as a Ridge National Laboratory and W. R. Strat- Such isotopes are present among the fis- result of the rupture of a major coolant pipe ton of Los Alamos National Laboratory sion products within the fuel rods of a and, should the containment be breached, have proposed an explanation for this great reactor, and the possibility of their escape to would escape to the atmosphere without disparity.* They suggest that iodine exits the atmosphere from damaged fuel rods has diminution. A similar fate is assumed for the from damaged fuel rods predominantly as dominated considerations of reactor acci- total core inventory of inert gases, such as cesium iodide (CsI) rather than as volatile dents and the design ’of safety systems. The xenon-133 and krypton-85, but these species such as molecular iodine (12). The isotope iodine-131 is of particular concern chemically inert materials pose a con- reducing environment of a water-cooled re- because of its relatively high fission yield siderably lesser danger to human health. actor during a loss-of-coolant accident sus- (2.77%) and significantly long half-life (8.07 Information obtained during the accident tains this chemical state and also converts days). at the Three Mile Island Unit 2 reactor other iodine species, should they be present, In 1957 an accident at the Windscale indicates that, in the case of iodine, these to iodide ions. The escaped CsI will readily

At 63 minutes, the primary system pressure rises because the This calculation shows that the automatic safety systems would steam generators have dried out and no longer remove heat bring the reactor to quasi-stable conditions without any intervention. from the primary coolant. However, actions by the operators can prevent core voiding or At about 66 minutes, the relief valve on the pressurizer opens reduce the severity of the accident. Below we list some conclusions and begins to discharge steam. based on TRAC analyses regarding successful management of the By 80 minutes, water begins to flow through the relief valve accident. because the increased temperature in the primary system has caused the coolant to expand. The pressure remains fairly 1. If, within the first hour, the operators notice a drop in the constant, but the temperature continues to increase. water level of the steam generators and are able to restore at At 96 minutes, the emergency core-cooling system is actuated least 30 per cent of the auxiliary feedwater supply, no voiding by a containment overpressure signal. will occur in the primary system and the core will be adequately At 120 minutes, the coolant in the primary system is saturated. cooled. The coolant begins to boil, the upper part of the vessel voids, 2. If auxiliary feedwater cannot be restored, the operators can the primary system pressure rises, and safety valves on the prevent boiling only by initiating the complex sequence of pressurizer open briefly. manipulations known as feed-and-bleed cooling near the begin- By about 130 minutes the partially voided core has begun to ning of the transient. This cooling technique consists of refill; thus, the system is recovering. alternately injecting emergency coolant with the high-pressure

88 LOS ALAMOS SCIENCE condense on available metal surfaces at o Of those compounds that could be the atmosphere than has been as- temperatures at or below 673-773 kelvin formed by iodine within fuel rods of sumed. (750-930° Fahrenheit) and will enter into water-cooled reactors, CsI is thermo- This hypothesis must be strengthened by solution as cesium and iodide ions upon dynamically the most stable. Further, information about the fundamental encounter with water or condensing steam. because the fission yield of cesium is chemistry—under the conditions within a This situation will persist in the absence of larger than that of iodine by a factor of reactor—of cesium and iodine and of fission an oxidizing atmosphere. Thus the amount 10 to 11, cesium is always available in products in general, In response to this issue, that could escape to the atmosphere from a great excess for reaction with iodine. the Nuclear Regulatory Commission and the Used fuel rods have been made to fail Department of Energy have sponsored stud- lower than has been assumed. in experiments simulating accident ies to pinpoint those areas of research that conditions in water-cooled reactors, In contrast, during the accident at the should be pursued. and the iodine released has been re- air-cooled and graphite-moderated Wind- If further study confirms that cesium and covered predominantly as CsI rather scale reactor, metallic fuel and (probably) iodine behave in the manner proposed, many than as I . 2 criteria for reactor safety must be re- The chemistry of iodine is such that, if evaluated and the reactor systems for fis- water is accessible, iodine species such and I2. sion-product control must be reexamined. In as CsI react with the water so that the In further support of their hypothesis, the addition, and most importantly, the public iodine concentration in the gas phase is scientists cite the following observations. could then be assured that the danger posed very much smaller than its concentra- by even a very severe reactor accident may o Iodine and cesium escape at the same tion in the water. be significantly lower than previously esti- time from leaking fuel rods in pressur- 0 An investigation, still continuing, of mated. ■ ized-water reactors during normal incidents involving fuel-rod damage at power transients. This behavior is com- other water-cooled reactors indicates H. J. Kouts of Brookhaven National Laboratory pletely different from that of the inert that, as at Three Mile Island, much has independently developed a similar hypothesis gases. smaller amounts of iodine escaped to about the behavior of cesium and iodine.

injection system and venting steam through the pressurizer relief but will not materially alter its ultimate course. valve. However, if the containment has been isolated auto- This particular accident and all related accidents, such as loss of matically by an overpressure signal due to vented coolant, use feedwater with stuck-open pressurizer relief valve (the Three Mile of feed-and-bleed cooling is severely restricted because the Island accident) or a loss-of-feedwater with stuck-open atmospheric compressed air that operates the relief valve cannot be replen- relief valve, have very characteristic signatures that can help the ished. operators to diagnose the situation. Not all multiple-failure accidents 3. After the steam generators dry out, the operators will see have such characteristic signatures, and in some cases additional increases in the pressurizer water level and in the primary instrumentation may be needed for proper identification. system pressure and temperature. They should respond by The SASA program is currently focused on accident sequences at initiating feed-and-bleed cooling. If the containment is not large two- and four-loop pressurized-water reactors. The emphasis at isolated and feed-and-bleed cooling begins between 1 and 2 the Laboratory is on plant-specific accident delineation, early hours, some core voiding will occur but the system will recover accident recognition, early accident management, and definition of much faster than it would otherwise. critical times and actions. By improving the operational safety of 4. If the primary pumps were not tripped at the start of the reactors, the severity of multiple-failure accidents, and thus the risk accident, leaving them running until the emergency core-cooling to public health, can be reduced. ■ system actuates automatically will prolong the accident slightly

LOS ALAMOS SCIENCE 89