Patent Number: 052710521
Section: summary

TECHNICAL FIELD OF THE INVENTION This invention pertains to a nuclear reactor control system which employs the use of an enriched boric acid solution in which the boron-10 isotope to boron-11 isotope ratio is greater than 19.8:80.2 as is found in naturally occurring boron acid solutions. This invention also pertains to operating a nuclear reactor plant utilizing an enriched boric acid solution for its primary reactor coolant solution during power operations and a natural boric acid solution for refueling functions in which the amount of make-up enriched boric acid solution needed for the reactor coolant system after refueling is minimized. BACKGROUND OF THE INVENTION A nuclear reactor must be provided with a system to control the reactor output. A number of ways of controlling the excess reactivity that is consciously designed into a nuclear power reactor core are known. These include the use of neutron absorbing control rods that can be inserted into or withdrawn from the reactor core, the adjustment of moderator temperature which changes the density and therefore both the fast neutron moderation and the thermal neutron absorption rates of the hydrogen in the light water coolant/moderator, and the use of solid and dissolved neutron absorbing poison materials incorporated either directly in the reactor core lattice (as burnable poison rods or fuel pellet coatings) or dissolved in the primary coolant/moderator as a "chemical shim". The chemical shim is commonly a boric acid solution. Systems employing such a boric acid solution for control of the nuclear reactor are discussed in Loose U.S. Pat. No. 3,380,889 and Gramer et al. U.S. Pat. No. 3,666,626. These coolant systems utilize natural boric acid solutions, which contain a maximum boron-10 (B-10) to boron-11 (B-11) atomic ratio of 19.8:80.2. The prior art has refined processes for concentrating the natural boric acid solutions used as chemical shims in reactor coolant systems. This concentration is necessary due to the need for a highly concentrated solution of neutron capturing compounds at the start of the reactor cycle and to compensate for the loss of B-10 material (nuclei) during the reactor cycle, and to minimize waste water streams containing radioactive wastes. Van der Schoot U.S. Pat. No. 4,073,683 discloses an ion exchange system to reconcentrate a natural boric acid solution while also producing a dilute natural boric acid solution to control the reactivity in the reactor core. Brown et al. U.S. Pat. No. 4,225,390 discloses a joint ion exchange and evaporative system to control the reactivity of the reactor core wherein the chemical shim is also natural boric acid. U.S. Pat. No. 4,225,390 also discloses how to load follow the reactor using a natural boric acid solution. These processes deal with "concentrating" a solution of natural boric acid, that is, they raise or lower the amount of natural boric acid in a solution, but do not disclose how to operate a nuclear reactor which utilizes "enriched" boric acid as the primary reactor coolant. The term "enriched" refers to a boric acid solution in which the B-10 to B-11 atomic ratio is above the naturally occurring ratio of 19.8:80.2. It is known that the B-10 isotope is the only isotope in boron-based poisons that contributes materially to the absorption of excess thermal or near thermal neutrons in reactor configurations. This is due to its relatively large neutron capture cross section in the thermal range. Also, it is known that the presence of any of the boron-based poison compounds in a typical power generating nuclear reactor leads to known deleterious effects, such as corrosion and wear, on other material components of the reactor core and of the associated nuclear steam supply system. Therefore, it follows that marked advantages over the prior art reactor coolant systems containing a chemical shim could be obtained if the B-10 to B-11 isotope ratio could be raised, thereby allowing a significant reduction in the total quantity of the boron-based poison material in the primary reactor coolant system at all times during power operations. Such a system would allow for the control of the nuclear reactor and also would be less deleterious on the physical components constituting the nuclear reactor. SUMMARY OF THE INVENTION The invention provides a pressurized water reactor coolant system (RCS) which differs from prior systems in that it contains a boric acid solution which is enriched in the boron-10 isotope to control the excess reactivity that is consciously designed into a nuclear power reactor core. This boric acid solution--referred to herein as EBA--is enriched in the boron-10 isotope and has a B-10 to B-11 atomic isotope ratio in excess of the natural ratio of 19.8:80.2. An EBA solution is preferred as a reactor coolant system solution since such a solution allows for a lower overall boric acid concentration in the reactor coolant system. This lower overall boric acid concentration means that a lower amount of lithium hydroxide is necessary to control the pH in the reactor coolant system. Such a reactor coolant system has a milder chemistry than those of the prior art and allows for a lower minimum temperature to be kept in the boric acid storage system. The lower concentration of the boric acid and the lithium hydroxide may lead to prolonged life for the components constituting the nuclear reactor coolant system. Therefore it is an object of the present invention to provide an apparatus for controlling the excess reactivity of a nuclear reactor utilizing an EBA solution in the reactor coolant system during the power producing operation of the core cycle. It is also an object of this invention to provide a process in which a nuclear reactor can be operated employing a reactor coolant system containing an EBA solution and minimizing the intermixing between the reactor coolant system and the refueling water system which contains a natural boric acid solution. The inventive nuclear reactor control system comprises a nuclear reactor which has a primary reactor coolant solution circulating through the reactor core. The coolant solution is comprised of an isotopically enriched boron-10 boric acid, or EBA, solution. The EBA solution has a boron-10 to boron-11 atomic isotope ratio of greater than 19.8:80.2, and as great as 95:5 at the start of the reactor cycle. The inventive control system employs a refueling water storage system containing a natural boric acid solution. This storage tank is connected to the reactor vessel and employed during a refueling operation. The control system design of the present invention is operated during the normal power operation mode by diverting a quantity of the primary reactor coolant which is an EBA solution from the coolant system to a boron-10 storage system. When the reactivity of the reactor core diminishes, the boric acid concentration of the EBA solution is decreased and enriched boric acid is stored in the boron-10 storage system. At the end of the reactor core cycle the concentration of boron-10 in the coolant solution is near 0-10 ppm. Therefore, most of the boron-10 material is in the boron-10 storage system. The inventive process for operating the EBA solution coolant system also provides that minimal mixing of the EBA solution and the refueling water solution will occur during and after refueling. During the refueling, the refueling water storage tank solution of natural boric acid--referred to herein as NBA--mixes with the reactor coolant solution (containing 0-10 ppm boron-10) in the reactor coolant system. After refueling, and when the fuel rods have been replaced, the vessel is closed. The boric acid solution in the reactor coolant system is then replaced with enriched boric acid. The invention provides for an apparatus and procedure to reduce the dilution of the EBA solution during this replacement step. The replacement of the boric acid solution in the reactor coolant system with the enriched boric acid is accomplished in the following manner. The solution within the refueling canal is drained via the drain system to the refueling water storage tank. A portion of the solution still within the reactor vessel and the reactor coolant system is drained to a tank. The remaining solution in the reactor vessel and the reactor coolant system is displaced by the replacement EBA solution, preferably under plug flow conditions to minimize intermixing. The reactor is now in a condition to begin the next cycle. A preferred method of minimizing the intermixing of the refueling water storage solution and the EBA solution is provided. Once the solution in the refueling canal is transferred via the drain system to the refueling water storage tank, then a portion of the remaining solution in the vessel is drained to the reactor hold-up tank by means of the residual heat removal system (RHR system). This RHR system is connected to the reactor coolant loop which connects the reactor vessel to the steam generation system. After an amount of the reactor coolant is drained from the reactor coolant system via the RHR system, and most preferably to a level such that the minimum amount of coolant is left in the reactor vessel to maintain core cooling and shutdown margin, displacement may begin. The displacement with the EBA solution is preferably carried out by employing the RHR system piping to both direct the incoming EBA solution and the outgoing remaining vessel coolant solution. The displacement can be monitored by use of temperature sensitive devices since the temperature of the incoming EBA solution is significantly cooler than the exiting vessel solution. Monitoring may also be accomplished by a B-10 isotope analyzer. The EBA solution for displacing the solution in the reactor vessel following refueling can be supplied by directing a heated solution through the ion exchange resins or by utilizing the concentrated EBA solution from the evaporative system. A make-up supply of EBA solution can be supplied to the reactor coolant system to compensate for EBA dilution during this displacement process.