The Impact of the Use of Mixed-Oxide Fuel on

                              The Potential for Severe Nuclear Plant Accidents in Japan

                                                             Edwin S. Lyman, PhD

                                                                 Scientific Director

                                                            Nuclear Control Institute

                                                                   October 1999

                                                                    Introduction

            There are many lessons to be learned in the aftermath of Japan's worst nuclear accident, the criticality event at the JCO, Co. fuel conversion plant in Tokai-mura.  Perhaps the most important is that complacency is dangerous where nuclear technology is involved.  Although the Science and Technology Agency (STA) was quick to lay the blame for the accident on the unfortunate workers who initiated it, it is clear that the real culprits were the plant managers and government regulators who believed that criticality accidents were impossible.  This attitude was responsible for the environment of utter carelessness that made this accident possible. 

            Unfortunately, the same foolish attitude seems to pervade other aspects of the Japanese nuclear program, from the numerous shipments by land and sea of large quantities of radioactive materials to the regulation of nuclear power plants.  One can only hope that Japanese government will learn the primary lesson of the accident at Tokai-mura and revamp the foundations of its nuclear regulatory system. 

            For decades, nuclear power plants in the United States were designed, sited and built according to the belief (on the part of the industry) that an accident severe enough to breach the containment and cause the release of large amounts of radioactive materials was essentially impossible.  This belief was shaken in the mid 1970's when a massive report, the Reactor Safety Study (RSS), was released by the newly created U.S. Nuclear Regulatory Commission (NRC).  In the RSS, nuclear plant accident sequences were identified that could result in a meltdown of the core and a breach or bypass of the containment.  However, the RSS provided some comfort by arguing that the probability of such accidents, while not zero, was extremely low.  Therefore, the NRC did not believe that there was an urgent need for action to upgrade the safety systems at existing nuclear power plants, although they began to analyze the consequences of such accidents and what kind of actions (i.e. evacuation or sheltering) could protect the people living near nuclear plants.

            Less than five years later, in 1979, the kind of accident that the RSS had predicted would occur once every twenty thousand years occurred at the Three Mile Island plant in Pennsylvania.  NRC finally had to take these accidents seriously, and imposed new regulations on existing and new nuclear plants.  In addition, it developed new guidelines for emergency planning based on the potential for severe accidents that could result in off-site radiation doses.  Finally, numerous organizational and procedural reforms were undertaken at NRC itself to strengthen its inspection and enforcement functions.

            Today, it is well understood in the United States that severe nuclear power plant accidents can result in major radiological releases, causing dozens of prompt fatalities (PFs) due to acute radiation exposure and hundreds to thousands of latent cancer fatalities (LCFs).  These accidents may involve violent events, such as steam explosions, hydrogen explosions or fuel fragmentation.  These are highly energetic events that can release not only volatile and semi-volatile radionuclides like iodine-131 (I-131) and cesium-137 (Cs-137), but also low-volatile radionuclides that do not vaporize easily from the molten fuel, like lanthanum-140 (La-140) and isotopes of the actinides plutonium (Pu), americium (Am) and curium (Cm). 

            The release of the actinides is of particular concern because most are alpha-particle emitters that have relatively large radiotoxicity if inhaled or ingested.  According to the U.S. NRC, releases of up to 5% of the actinide inventory of an PWR core are possible in severe accidents, and up to 10% of the actinide inventory of a BWR core. 

                                                   Severe Accidents and MOX Use

            Japanese utilities are on the verge of beginning a large-scale program to load mixed plutonium-uranium oxide (MOX) fuel in existing light-water reactors (LWRs).  MOX fuel has been delivered to the Fukushima 3 and Takahama 4 reactors, and it is planned to be loaded soon.  The Kashiwazaki-Kariwa 3 reactor is slated to be the next to use MOX.

            An LWR loaded with reactor-grade (RG) MOX fuel contains substantially greater quantities of actinides than an LWR loaded with conventional low-enriched uranium (LEU) fuel.  This is because of the presence of plutonium in high concentrations in the fresh fuel.  Irradiation of MOX fuel also results in greater accumulations of other actinides, such as Am-241, Cm-242 and Cm-244, than does irradiation of LEU fuel. 

            In Table I, we present the results of a calculation performed with the U.S. computer code ORIGEN-S that shows the relative quantities of actinides in a full RG-MOX core and an LEU core at the end of an operating cycle.  The calculations were based on a typical RG-Pu isotopic composition,[1] and assumed a total plutonium enrichment in the fuel of 8.3%.  (In Japan, Pu enrichments of up to about 13% have been authorized.)  Actinide inventories are five to nearly twenty-two times greater in the MOX core for all actinides except Np-239.  However, Np-239 is a beta-particle emitter and is much less hazardous than alpha-emitters.    

                 TABLE I End-of-Cycle Actinide Core Inventories in LEU and RG-MOX cores

            The increased actinide inventories in a RG-MOX core can greatly increase the consequences (PFs and LCFs) resulting from a severe loss-of-containment accident, compared to those resulting from the same accident if only LEU fuel were present.  Using the values for radionuclide release fractions (RFs) that have been estimated to occur during a severe accident, one can calculate the increase in consequences. 

            Table II provides the results of a calculation, performed with the U.S. computer code MACCS2, of the consequences of such an accident within an area of 113 kilometers around a 870 MWe pressurized-water reactor similar the Takahama 4 plant.  The release fractions used were taken from a recent U.S. NRC publication.[2]  A population density of 550 persons per square kilometer was assumed, similar to the average population density within a 110 kilometer radius of Takahama.    

            The three cases evaluated, medium (M), high (H) and low (L), correspond to three different possible magnitudes of the plutonium release fraction.  For each case, both a full MOX core and a one-quarter MOX core were considered.  Kansai Electric Power Company (KEPCO) plans initially to use only a one-quarter core of MOX fuel, but intends to eventually reach a one-third MOX core.  However, in the future, Japan intends to use full cores of MOX, and plans are proceeding to build an advanced boiling-water reactor (BWR) that will use a full MOX core in Aomori Prefecture.

                     TABLE II  Consequences of Severe Accidents Involving RG-MOX Cores

                                                   (Pu RF = plutonium release fraction)

            The data in Table II clearly illustrates the greatly increased risk to the Japanese public associated with the loading of RG-MOX into LWRs.  For a one-quarter core MOX loading, the number of LCFs resulting from a severe accident are 42-122% higher than for LEU cores, and the number of PFs are 10-98% higher, depending on the actinide release fraction.  For full-core MOX, the number of LCFs are 161-386% higher, and the number of PFs are 60-480% higher.  Depending on the MOX core fraction and actinide release fraction, thousands to hundreds of thousands of additional LCFs could result within a 110 kilometer radius of the plant.  (This distance was selected for calculational convenience --- there of course will be impacts outside of this area as well).    

            These calculations were performed under the assumption that the release fractions (the fraction of the core inventory that is released during the accident) are the same for LEU and MOX fuel, and the differences in consequences are only due to the difference in inventory.  However, this may not be the case.  There is evidence that the fractional release of volatile radionuclides, like cesium, is significantly greater from MOX fuel rods that have been irradiated to burnups of greater than 40 gigawatt-days per tonne (GWD/t) than from LEU rods of the same or higher burnup.  In particular, in a test called VERCORS in France, in which spent fuel was held at a temperature of 1780 K for one hour, the cesium release fraction from a MOX fuel rod with a burnup of 41 GWD/t was 58%, compared to only 18% for an LEU rod with a burnup of 47 GWD/t.[3]  Moreover, there are indications from another test in France (known as Cabri) that high-burnup MOX fuel has a greater propensity to rupture and disperse actinide-bearing solid fuel particles than high-burnup LEU fuel.  

            Given the magnitude of the increased hazard associated with MOX use, one may well ask how prefectural and national regulatory authorities are able to justify this program.  The answer can be found in the magazine Atoms in Japan, published by the Japan Atomic Industrial Forum (JAIF).  According to the article entitled "MITI, STA Explain MOX Use in Fukushima,"

            "a citizen attending a public forum on MOX use asked `is it true that an accident at a MOX-burning reactor would be four times worse than conventional ones?'  The reply was that an accident would result in a large scale of damage only if fuel were scattered outside the plant.  Since the MOX pellets are sintered, it would be virtually impossible for them to become powdered and be carried outside the site, meaning that the safety of MOX fuel in an accident was deemed to be the same as that as uranium fuel."

            This response summarizes the flawed logic by which the Nuclear Safety Commission judged that utilities that planned to use MOX did not need to evaluate the consequences of accidents that would lead to plutonium releases off-site.  It conveniently allows Japanese authorities to sidestep the serious safety issues associated with the vastly larger actinide inventories in MOX cores.   

            This reasoning, which is indefensible on a technical basis, is clearly a result of the same mindset that believed that criticality accidents at the uranium fuel processing plant at Tokai were also impossible.  As explained above, MOX fuel, just like LEU fuel, can be dispersed in fine aerosol form in a severe accident with core disruption.  One mechanism which has been under study in the U.S. is high-pressure melt ejection (HPME), in which the reactor vessel ruptures at high pressure after melting of the core.  This causes the core to be ejected into the containment in the form of fragments, which rapidly heat the containment and in principle can cause it to fail, leading to radiological release. 

            The use of MOX can also increase the probability that severe accidents will occur.  Two examples are loss-of-coolant accidents (LOCAs) and station blackout events (SBOs), which in U.S. PWRs are the largest contributors to the risk of early containment failure.  The probability that these events will progress to core damage depends largely on both the extent to which fuel cladding is damaged prior to startup of emergency core cooling.  The thermal conductivity of MOX fuel is about 10% smaller than that of LEU fuel, and the centerline temperature of MOX fuel is about 50C higher, so that the stored heat in MOX fuel rods is greater than in LEU fuel.  Because the centerline temperature and stored energy of MOX fuel is higher than LEU fuel, the rise in cladding temperature and rate of cladding oxidation during the initial stages of a LOCA can be greater than for LEU fuel,[4] and it may be harder for MOX cores to meet regulatory requirements for LOCA mitigation. 

                                                                    Conclusions

            In the U.S., it is estimated that the average risk of a nuclear accident resulting in a large radiological release before evacuation of the local population can take place is between five in a million and ten in a million per reactor year.  Since the U.S. has about 100 nuclear power plants, this corresponds to an annual overall risk of 0.1% per year.  The NRC has recently introduced guidelines that would restrict the allowable increases in risk at nuclear power plants to low levels.  It is highly doubtful that the large increases in risk associated with RG-MOX use would be acceptable under these U.S. guidelines. 

            It is foolish for Japanese authorities to think that their own nuclear plants have significantly lower risks than plants in the U.S, especially if they consider the severe inadequacies of their own regulatory system that have been revealed by the JCO accident.  Consequently, it is imperative that Japan reconsider its plan to begin loading MOX fuel in LWRs.  It should follow the example of the U.S., accept the fact that severe, loss-of-containment accidents are possible in Japan, as they are everywhere else, and evaluate the risks of MOX fuel use in that context.  If this is done rigorously and honestly, the authorities will have to come to the conclusion that the increased risk associated with MOX use is too great a burden for the Japanese public to bear, and that the focus of the Japanese nuclear industry in the future should be to concentrate on operating their existing nuclear plants safely with conventional LEU fuel.