Patent Number: 049922327
Section: summary

This invention relates to boiling water nuclear reactors (BWRs) operating under hydrogen water chemistry conditions. More particularly, a technique for restricting the levels of increase of radiation in the main steam lines due to N-16 production where hydrogen water chemistry is utilized to minimize intergranular stress corrosion cracking (IGSCC) is disclosed. BACKGROUND OF THE INVENTION Boiling water reactors (BWRs) operating over long periods of time have stainless steel components which are subject to IGSCC. The injection of hydrogen into the feedwater of BWRs has been demonstrated as an effective means of suppressing the stress corrosion cracking of these stainless steel components. Under normal water chemistry conditions, the oxygen concentration is approximately 200 parts per billion (ppb) and the hydrogen concentration is approximately 10 ppb. Under hydrogen water chemistry conditions, the concentrations necessary to prevent ISGCC are in the range of 2-15 ppb oxygen and 100 ppb hydrogen. These concentrations are approximate and vary among reactors. To inspect for the presence of stress corrosion, non-destructive testing is used. Such non-destructive testing of piping joints requires plant shutdown while the inspection occurs. Thus, even the threat of IGSCC costs the plant expensive down time. Unfortunately, and coincident with this hydrogen treatment, higher levels of radiation in the main steam lines and turbines have been observed. These higher levels of radiation in the more heavily shielded plants have not caused a significant problem. Heavily shielded turbines, condensers and steam piping have prevented the radiation from finding its way through to operating personnel and occupied areas. Unfortunately, many plants include heavy shielding in the turbine, condenser and steam piping side of the plant which is only adequate to limit dose rates under normal water chemistry. This being the case, the increased levels of radiation have tended to limit the use of hydrogen water chemistry to prevent stress corrosion. BWR operation under normal water chemistry produces a small fraction of N-16 formed by the n,p reaction of 0-16 and exists in a chemical form which tends to be volatile. As this fraction is transported in the aqueous phase in the reactor and the water coolant is converted to steam, a portion of the volatile fraction is swept into the steam phase and transported to the turbine. N-16 is a radioactive nuclide whose half-life is approximately 7 seconds. In its decay, high energy gamma radiation of 6 and 7 MeV is emitted. Thus, during normal plant operation a significant radiation field emanates from the steam lines and turbine. Because of the intensity and relatively high energy of the gamma radiation, significant shielding is required to limit the radiation field intensity. In spite of the shielding, the influence of the N-16 source can be measured even at significant distances from the source. We have discovered as a part of the present invention that the observed radiation levels are caused at least in part by the N-16 being converted into volatile nitrogen compounds, including ammonia, which are transported in the steam phase. Under hydrogen water chemistry conditions, a larger fraction of the N-16 is converted to a volatile form. Thus, the radiation levels in the steam phase increase significantly when compared to the levels without hydrogen addition. Dependent on the reactor, the levels have been measured by us to increase from 1.2 to 5 times at the hydrogen concentration necessary to prevent IGSCC in the recirculation system. For some plants the increase is sufficient to exceed safety dose rate limits not only close to the source, but also in surrounding buildings and grounds and at site boundaries. This is perceived as one of the most detrimental aspects of hydrogen water chemistry. Thus, it would be highly desirable if a method could be found to limit the N-16 volatility, i.e., the quantity transported to the steam. SUMMARY OF THE INVENTION The present invention generally provides a method for operating boiling water reactors with hydrogen water chemistry under conditions limiting the level of released radiation which has heretofore accompanied such chemistry. More specifically, by inhibiting the transfer of gaseous nitrogen compounds from the liquid phase to the steam phase, the release of radioactive N-16 into less shielded portions of the reactor system may be reduced. Various approaches for inhibiting such transfer are employed, depending in part on the operating mode of the reactor. Three operational modes are employed providing varying levels of hydrogen protection. First, the boiling water reactors may be operated with full plant protection where feed water hydrogen concentration is sufficient to prevent IGSCC or irradiation assisted stress corrosion cracking (IASCC) in all parts of the primary reactor system, including all stainless steel components in both the recirculation system and in the reactor vessel. Second, the reactors may be operated with selective protection where hydrogen is introduced only at certain critical regions within the reactor, providing localized protection within those regions. Operating with selective protection allows less total hydrogen to be introduced, generating less N-16 than is generated with full protection. The critical regions for hydrogen introduction will generally include (a) the recirculation system, (b) the core bypass region, i.e., the region not contained within the boundary of the fuel assemblies and the region immediately above the fuel core, more particularly the region of the top fuel guide, and (c) the lower plenum region of the reactor vessel which includes the bottom of the vessel up to the region just above the fuel support plate. Third, the boiling water reactor may be operated with partial protection where the hydrogen concentration is lower than that required to attain the lowest electrochemical potential in order to completely inhibit IGSCC. Such mode of operation partially arrests crack growth and decreases the fraction of volatile N-16. In the first category, the approach is either to chemically decrease the quantity of volatile N-16 species transferred into the steam phase, to physically decrease the quantity of volatile N-16 species transferred into the steam phase, or to delay transport of the volatile N-16 species in the steam phase to the main steam line to the turbine for a sufficient time to allow substantial decay of the radiation. The second and third categories involve the use of less hydrogen and hence less volatile N-16 formation. FULL PROTECTION First, formation of volatile nitrogen compounds may be chemically inhibited. This may be accomplished by altering reaction paths leading to the formation of volatile N-16 with small amounts of additives particularly free-radical scavengers and/or increasing the pH of the reactor feed water to an acidic level. Thus, the present invention provides for the introduction of trace quantities (ppb concentrations) of a species, to inhibit, suppress, or alter the reaction path of the radioactive nitrogen leading to the formation of a volatile species, e.g., ammonia, and/or enhancing the reaction path leading to a non-volatile species, e.g., a nitrite. Examples of possible additives are nitrous oxide, carbon dioxide, nitrite, nitrate, low molecular weight alcohols, or ketones, copper, zinc or vanadium, etc., not excluding other possibilities. Alternatively, a slight increase of the pH is effective to reduce the volatility of the ammonia. A change in pH may also alter the reaction paths leading to the formation of volatile nitrogen compounds. In this latter instance, the formation of volatile N-16 species is inhibited. Usually, the pH of the boiler feed water will be increased to the range from about 7.0 to 8.6 as measured at room temperature (normally, the feedwater will have a pH in the range from about 6.1 to 8.1). This may be accomplished by appropriate balancing of the anionic and cationic ion exchange resin used in the boiler water treatment facility. Secondly, the N-16 transport to the steam phase can be limited by physical means. It is recognized that the N-16 is formed in two regions of the core: (1) within the envelope of the fuel bundle channel, i.e., the in-channel region, and (2) in the region outside the fuel bundle region, i.e., the bypass region. Essentially all the boiling occurs within the in-channel region and essentially none in the bypass region. In addition, the flow rate is much faster and hence the residence time in the in-channel region is much shorter than that in the bypass region, while the volumes of water in both regions are comparable. Thus, a significant portion of the N-16 formation occurs in the bypass region. Because the residence time of the water in the bypass region is significant compared with the half life of N-16, a decrease in the flow rate will decrease the total production of N-16 at the core exit, which constitutes one physical method of control. A second physical method involves limiting the contact time between the steam and water. This will decrease the net transfer of the volatile N-16 species from the liquid to the steam phase and, hence, limit the N-16 steam phase concentration. This approach may be achieved by physically altering the region above the core. A third method involves increasing the retention time of the N-16 in the steam by several seconds which is significant compared to the half-life. This may be accomplished by a physical means of providing an increased volume for the steam as close to the reactor vessel as possible. This volume could be shielded. An alternate method may involve a chemical method of adsorption onto a matrix material for a long enough time (seconds) to effect a significant delay relative to the second half-life. SELECTIVE AND PARTIAL PROTECTION The concentration of hydrogen required to achieve protection of the recirculation system stainless steel components varies significantly among plants. (See, FIG. 2). Such variation is attributed to differences in the effectiveness of hydrogen utilized to promote hydrogen-oxygen recombination in the downcomer region. Thus, methods which can promote the efficiency of hydrogen utilization will decrease the total hydrogen utilization and diminish the amount of volatile N-16 generated and transported into the steam phase. Reducing the hydrogen concentration required can be beneficial in reducing the N-16 steam phase activity depending on the ultimate hydrogen concentration required (see FIG. 3). This can be accomplished by methods such as catalyzing the hydrogen-oxygen recombination reaction, for example by increasing radiation in the downcomer region, or possibly by surface catalysis. It is also recognized that hydrogen injected into the feed water, the usual region of injection, is only partially available to the downcomer region. That is, the hydrogen bearing water partitions at the top of the jet pump such that a portion goes into the jet pump and hence directly into the core, which bypasses the downcomer region. Therefore, if the hydrogen is injected below the inlet to the jet pumps, the total quantity is available for the downcomer region. Since this can result in a corresponding decrease in the total amount of hydrogen added (and going into the core region) less volatile N-16 will be formed. To ensure an adequate concentration of hydrogen in the bypass region of the core while limiting the volatile N-16 production, hydrogen can be injected directly into bypass regions. Such hydrogen injection in combination with the techniques described for enhancing hydrogen utilization in the downcomer region, will reduce the overall hydrogen addition and hence reduce the volatile N-16 concentration. Under such operation, both the recirculation system and the core bypass region would be protected from IGSCC and IASCC with only relatively small increases in the volatile N-16 concentration. In the lower plenum region, a sufficiently low oxygen concentration may be attained in this region by the addition of hydrogen e.g., via feedwater addition, and catalyzing the recombination reaction by either increased radiation in this region or in the vicinity of the jet pumps or by surface catalysis, as in the downcomer region described above. For partial protection, it is possible to operate at a somewhat higher electrochemical potential so that the crack growth rate is reduced but not stopped. This would require less hydrogen and hence decreased formation of volatile forming of N-16. Under either full or partial suppression, it also appears very likely that the fraction of ammonia (N-16) transferred to the steam phase can be affected by other changes in physical parameters. Among these, changes in the water level in a reactor core may be effective in reducing the N-16 level in the steam phase to compensate at least partially for the increase in radiation levels brought about by operation under hydrogen water chemistry. Other parameters include but are not limited to recirculation flow rate, axial power distribution, and axial steam void distribution.