<\/p>\n
<\/p>\n
0. \u00a0Introductory Note: Rasmussen and the CANDU<\/small>\u00a0Reactor<\/a><\/span><\/p>\n I. \u00a0Excerpts from the Executive Summary (WASH-1400<\/small>)<\/a><\/p>\n II. \u00a0Excerpts from the Main Report<\/a><\/span><\/p>\n V. \u00a0\u00a0Steam Explosions: Molten Materials Contacting Water<\/a><\/span><\/p>\n <\/p>\n Both\u00a0AECL<\/small>\u00a0and Ontario Hydro, in submissions to the Ontario Royal Commission on Electric Power Planning (the “Porter Commission”), cited the Rasmussen Report (WASH-1400<\/small>) as an essential part of their argument that risks associated with Canadian\u00a0CANDU<\/small>\u00a0reactors are acceptable.\u00a0Porter Commission, Exhibit 158, pp.14-15; Exhibit 2, pp.43-44<\/i><\/small>.<\/b><\/span><\/p>\n G.A. Pon, Vice President of\u00a0AECL<\/small>\u00a0Power Projects, said of\u00a0WASH-1400<\/small>:<\/span><\/p>\n “Although the study was prepared in the\u00a0U.S.<\/small>\u00a0assessing the risks associated with their light water nuclear power plants, the findings should not be significantly different for the\u00a0CANDU<\/small>\u00a0reactor.” \u00a0Porter Commission, Exhibit 28, p.5<\/i><\/small><\/span><\/p>\n In sworn testimony before the Cluff Lake Board of Inquiry into Uranium Mining in Saskatchewan, Dr. Norman Rasmussen — the principal author of\u00a0WASH-1400<\/small>\u00a0— commented about\u00a0CANDU<\/small>\u00a0meltdown possibilities:<\/b><\/span><\/p>\n “although the Canadian design philosophy differs in some of its approaches . . . it achieves, in my judgment, about the same safety level as far as I can tell.”<\/span><\/p>\n So what is the bottom line as described in the Rasmussen Report?<\/span><\/p>\n \u00a0<\/b><\/span><\/p>\n Nor are these the worst consequences that can be imagined.<\/b><\/span><\/p>\n Although the Rasmussen Report is flawed in many respects, and in many ways tends to understate the dangers from nuclear reactors, nevertheless there is much in the report that is illuminating and important. The excerpts given here were cited by Dr. Gordon Edwards following the Three Mile Island Accident, when testifying to the Select Committee on Ontario Hydro Affairs on the subject of potentially catastrophic accidents in\u00a0CANDU<\/small>\u00a0nuclear plants.<\/b><\/span><\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/b>The only way that potentially large amounts of radioactivity could be released is by melting of the fuel in the reactor core.<\/p>\n The fuel that is removed from a reactor after use and stored at the plant site also contains considerable amounts of radioactivity. However, accidental releases from such used fuel were found to be quite unlikely and small compared to potential releases of radioactivity from the fuel in the reactor core.<\/p>\n The study has examined a very large number of potential paths by which potential radioactive releases might occur and has identified those that determine the risks. This involved defining the ways in which the core could melt and the ways in which systems to control the release of radioactivity could fail.<\/p>\n <\/a>2.7. \u00a0\u00a0HOW MIGHT A CORE MELT ACCIDENT OCCUR?<\/b><\/p>\n It is significant that in some\u00a0200<\/small>\u00a0reactor-years of commercial operation of reactors of the type considered in the report there have been no fuel melting accidents. To melt the fuel requires a failure in the cooling system or the occurrence of a heat imbalance that would allow the fuel to heat up to its melting point, about\u00a05000\u00a0o<\/sup>\u00a0F<\/small>\u00a0[2800\u00a0o<\/sup>\u00a0C<\/small>] .<\/p>\n To those unfamiliar with the characteristics of reactors, it might seem that all that is required to prevent fuel from overheating is to promptly stop, or shut down, the fission process at the first sign of trouble. Although reactors have such\u00a0[fast shutdown]<\/span><\/small>\u00a0systems, they alone are not enough since the radioactive decay of fission fragments in the fuel continues to generate heat (called decay heat) that must be removed even after the fission process stops. Thus, redundant decay heat removal systems are also provided in reactors. In addition, emergency core cooling systems (ECCS<\/small>) are provided to cope with a series of potential but unlikely accidents, caused by ruptures in, and loss of coolant from, the normal cooling system.<\/p>\n The Reactor Safety Study has defined two broad types of situations that might potentially lead to a melting of the reactor core: the loss-of-coolant accident (LOCA<\/small>) and transients.<\/p>\n In the event of a potential loss of coolant, the normal cooling water would be lost from the cooling systems and core melting would be prevented by the use of the emergency core cooling systems (ECCS<\/small>). However, melting could occur in a loss of coolant if the\u00a0ECCS<\/small>\u00a0were to fail to operate.<\/p>\n The term “transient” refers to any one of a number of conditions which could occur in a plant and would require the reactor to be shut down. Following shutdown, the decay heat removal systems would operate to keep the core from overheating. Certain failures in either the shutdown or the decay heat removal systems also have the potential to cause melting of the core.<\/p>\n <\/a>2.8\u00a0\u00a0\u00a0WHAT FEATURES ARE PROVIDED IN REACTORS TO COPE An essentially leaktight containment building is provided to prevent the initial dispersion of the airborne radioactivity into the environment. Although the containment would fail in time if the core were to melt, until that time, the radioactivity released from the fuel would be deposited by natural processes on the surfaces inside the containment. In addition, plants are provided with systems to contain and trap the radioactivity released within the containment building.<\/p>\n Even though the containment building would be expected to remain intact for some time following a core melt, eventually the molten mass would be expected to eat its way through the concrete floor into the ground below. Following this, much of the radioactive material would be trapped in the soil; however, a small amount would escape to the surface and be released. Almost all of the non-gaseous radioactivity would be trapped in the soil.<\/p>\n It is possible to postulate core melt accidents in which the containment building would fail by overpressurization or by missiles created by the accident. Such accidents are less likely but could release a larger amount of airborne radioactivity and have more serious consequences.<\/p>\n <\/a>2.9\u00a0\u00a0\u00a0HOW MIGHT THE LOSS-OF-COOLANT ACCIDENT Loss of coolant accidents are postulated to result from failures in the normal reactor cooling water system, and plants are designed to cope with such failures. The water in the reactor cooling systems is at a very high pressure (between 50 to 100 times the pressure in a car tire) and if a rupture were to occur in the pipes, pumps, valves, or vessels that contain it, then a “blowout” would happen. In this case some of the water would flash to steam and blow out of the hole. This could be serious since the fuel could melt if additional cooling were not supplied in a rather short time.<\/p>\n The study has examined a large number of potential sequences of events following\u00a0LOCA<\/small>s\u00a0[loss-of-coolant accidents]<\/span><\/small>\u00a0of various sizes. In almost all of the cases, the\u00a0LOCA<\/small>\u00a0must be followed by failures in the emergency core cooling system for the core to melt.<\/p>\n <\/a>2.10\u00a0\u00a0\u00a0HOW MIGHT A REACTOR TRANSIENT LEAD TO A CORE MELT?<\/b><\/p>\n The term “reactor transient” refers to a number of events that require the reactor to be shut down. These range from normal shutdown for such things as refuelling to such unplanned but expected events as loss of power to the plant from the utility transmission lines.<\/p>\n The reactor is designed to cope with unplanned transients by automatically shutting down. Following shutdown, cooling systems would be operated to remove the heat produced by the radioactivity in the fuel. There are several different cooling systems capable of removing this heat, but if they all should fail, the heat being produced would be sufficient to eventually boil away all the cooling water and melt the core.<\/p>\n In addition to the above pathway to core melt, it is also possible to postulate core melt resulting from the failure of the reactor shutdown systems following a transient event. In this case it would be possible for the amounts of heat generated to be such that the available cooling systems might not cope with it and core melt could result.<\/p>\n <\/a>2.11\u00a0\u00a0\u00a0HOW LIKELY IS A CORE MELT ACCIDENT?<\/b><\/p>\n The Reactor Safety Study carefully examined the various paths leading to core melt. Using methods developed in recent years for estimating the likelihood of such accidents, a probability of occurrence was determined for each core melt accident identified. These probabilities were combined to obtain the total probability of melting the core.<\/p>\n The value obtained was about one in\u00a020,000<\/small>\u00a0per reactor per year. With\u00a0100<\/small>\u00a0reactors operating, as is anticipated for the\u00a0U.S.<\/small>\u00a0by about\u00a01980<\/small>, this means that the chance for one such accident is one in\u00a0200<\/small>\u00a0per year\u00a0[or about 1 in 10 over a period of 20 years]<\/span><\/small>.<\/p>\n <\/p>\n <\/p>\n <\/a> <\/a> A\u00a0LOCA<\/small>\u00a0[loss-of-coolant accident]<\/span><\/small>\u00a0would result whenever the reactor coolant system (RCS<\/small>) experiences a break or opening large enough so that the coolant inventory in the system could not be maintained by the normally operating makeup system. Nuclear plants include many engineered safety features (ESF<\/small>s) that are provided to mitigate the consequences of such an event.<\/p>\n The specific\u00a0LOCA<\/small>\u00a0initiating events analyzed in this study are:<\/p>\n <\/p>\n a. \u00a0Large pipe breaks b. \u00a0Small to intermediate pipe breaks c. \u00a0Small pipe breaks d. \u00a0Large disruptive reactor vessel ruptures.<\/p>\n e. \u00a0Gross steam generator ruptures.<\/p>\n f. \u00a0Ruptures between systems that interface with the <\/p>\n The basic purpose of the\u00a0ESF<\/small>s\u00a0[engineered safety features]<\/span><\/small>\u00a0is the same for both PWR\u00a0[pressurized-water reactors]<\/span><\/small>\u00a0and BWR\u00a0[boiling-water reactor]<\/span><\/small>\u00a0plants. However, the nature and functions of\u00a0ESF<\/small>s differ somewhat between PWRs and BWRs because of the differences in the plant designs.<\/span><\/p>\n A number of the\u00a0ESF<\/small>s\u00a0[engineered safety features]<\/span><\/small>\u00a0are included in a group termed the emergency core cooling system (ECCS<\/small>) whose function is to provide adequate cooling of the reactor core in the event of a\u00a0LOCA<\/small>\u00a0[loss-of-coolant accident]<\/span><\/small>. Other\u00a0ESF<\/small>s provide rapid reactor shutdown and reduce the containment radioactivity and pressure levels that result from escape of the reactor coolant from the\u00a0RCS\u00a0[reactor coolant system]<\/span><\/small>.<\/p>\n In early power reactors the power level was about one tenth that of today’s large reactors. It was thought that core melting in those low power reactors would not lead to melt-through of the containment. Further, since the decay heat was low enough to be readily transferred through the steel containment walls to the outside atmosphere, it could not overpressurize and fail\u00a0[i.e. cause a failure in]<\/span><\/small>\u00a0the containment. Thus, if a\u00a0LOCA<\/small>\u00a0[loss-of-coolant accident]<\/span><\/small>\u00a0were to occur, and even if the core were to melt, the low leakage containments that were provided would have permitted the release of only a small amount of radioactivity.<\/p>\n However, as reactors grew larger, several new considerations became apparent.<\/p>\n The decay heat levels were now so high that the heat could not be dissipated through the containment walls. Further, in the event of accidents, concrete shielding was required around the outside of the containment to prevent over-exposure of persons in the vicinity of the plant. Finally, it became likely that a molten core could melt through the thick concrete containment base into the ground. Thus, new sets of requirements came into being.<\/p>\n <\/p>\n The major goal behind these changes were to attempt to provide\u00a0ESF<\/small>s\u00a0[engineered safety features]<\/span><\/small>\u00a0designed so that the failure of any single barrier would not be likely to cause the failure of any of the other barriers. For example, if the\u00a0RCS\u00a0[reactor coolant system]<\/span><\/small>\u00a0were to rupture, ECC\u00a0[emergency core cooling]<\/span><\/small>\u00a0systems were installed to prevent the fuel from melting and thereby protect the integrity of the containment.<\/span><\/p>\n Other features were added to aid this positive objective. For example, additional piping restraints and protective shields were required to lessen the likelihood of\u00a0ESF<\/small>\u00a0[emergency safety features]<\/span><\/small>\u00a0damage that could result from pipe whip following a large break in the\u00a0RCS\u00a0[reactor coolant system]<\/span><\/small>.<\/p>\n Knowledge that large natural forces such as earthquakes and tornadoes could cause multiple failures led to design requirements that attempted to reduce the likelihood of dependent failures from such causes. Appendix IX provides more detailed descriptions of the above and many more of the safety features in current nuclear plants.<\/p>\n <\/a>MORE\u00a0ABOUT FUEL MELTING<\/b><\/p>\n Prior studies have indicated that a core meltdown in a large reactor would likely lead to failure of the containment (Ref. 1,2). Thus, a commonly held opinion regarding core melting is that such an event would result in a very serious accident with large public consequences. This is evidently one of the reasons that major safety efforts have been devoted to the prevention of core meltdown and little attention has been directed toward the examination of the potential relationships between core melting and containment integrity.<\/p>\n At two key stages in the course of a potential core meltdown there would be conditions which would have the potential to result in a\u00a0steam explosion<\/a>\u00a0that could rupture the reactor vessel and\/or the containment. These conditions may occur<\/p>\n\n
\nTO COPE WITH A CORE MELT ACCIDENT?<\/a><\/li>\n
\nLEAD TO A CORE MELT?<\/a><\/li>\n\n
\nSTORAGE POOL<\/a><\/li>\n<\/ul>\n
\nIII. \u00a0Excerpts from Appendix VIII<\/a><\/span><\/p>\n\n
\nIV. \u00a0Excerpts from Appendices VII and VI<\/a><\/span><\/p>\n\n
\nReactor Core at the Time of the Hypothetical Accident (Appendix VI)<\/a><\/li>\n\n
\n
\n
\n<\/a>0. Introductory Note:<\/span>
\nRasmussen and the\u00a0CANDU<\/small>\u00a0Reactor<\/span><\/h2>\n
\n\n
\n
\n<\/a>I. Excerpts from the Executive Summary (pp. 6-7)<\/h2>\n
\n<\/a>2.6. \u00a0\u00a0HOW CAN RADIOACTIVITY BE RELEASED?<\/b><\/span><\/p>\n
\nWITH A CORE MELT ACCIDENT?<\/b><\/p>\n
\nLEAD TO A CORE MELT?<\/b><\/p>\n
\n<\/a>II. Excerpts from the Main Report (WASH-1400<\/small>)
\nre Fuel Melting (pp. 25-29<\/small>)<\/h2>\n
\nThe identification of all significant sources of radioactivity, the fact that a gross release of radioactivity can occur only if fuel melts, knowledge of the factors that affect heat balances in the fuel, and the fact that mechanisms that could lead to heat imbalances have been scrutinized for many years, all provide a high degree of confidence that those accidents of significance to risk have been identified.<\/p>\n
\n<\/b><\/a>3.3\u00a0\u00a0\u00a0LOSS OF COOLANT ACCIDENTS<\/b><\/p>\n
\n(6 inches to approximately 3 feet equivalent diameter).<\/p>\n
\n(2 inches to 6 inches equivalent diameters).<\/p>\n
\n(1<\/sup>\/2<\/sub><\/small>\u00a0inch to 2 inches equivalent diameter).<\/p>\n
\nRCS\u00a0[reactor coolant system]<\/span><\/small>.<\/p>\n\n
\n