Takaaki Mochida

Improvements in boiling water reactor uranium utilization and operating experience with burnup increase

A boiling water reactor (BWR) core design for better uranium utilization is presented, and its validity is demonstrated through simulation and operation data. Together with the axial power flattening obtained by an axially zoned enrichment core, uranium utilization improvement techniques such as an axial blanket for neutron leakage reduction, a low leakage loading pattern, an improved local enrichment distribution in the fuel bundle, and spectral shift operation method are promising design features to be applied to the BWR core. Quantitative studies for the amount of burnup increase and power peaking rise are made to estimate a level of effective uranium utilization. The improvements in uranium utilization are confirmed not only in the computational core design study, but also in empirical data from a commercial BWR. Operating experience proves the adequacy of the core design. A uranium utilization improvement of >10% is obtained without a loss of load factor.

A new boiling water reactor core concept for a next-generation light water reactor

In this paper a boiling water reactor (BWR) core concept that meets various requirements for a next-generation light water reactor is proposed. This BWR core can be operated as either a high-burnup core or a high-conversion core simply by replacing the fuel assemblies and control rods. The high-burnup core is suitable for a once-through nuclear fuel cycle and has a low fuel cycle cost due to the adoption of advanced spectral shift technology. The high-conversion core is suitable for nuclear fuel recycling and reaches a high-conversion ratio by adopting a tight-lattice arrangement of mixed-oxide fuel rods in the fuel assemblies and using control rods with a zirconium follower. The reactor structures are essentially identical, and they are designed to be as simple as the current BWR to achieve high reliability. The reactor core also has high operability due to the spectral shift water rods that are operated with all control rods withdrawn. At reactor shutdown, the core has a large reactivity control capability due to the cruciform control rods with wider blades and has an ample safety margin.

Development of a High-Conversion Boiling Water Reactor

Light water reactors (LWRs) are expected to be a primary source of electrical power in Japan into the 21st century. A next-generation LWR must be developed that efficiently uses uranium resources and improves fuel economy. In this paper a high-conversion boiling water reactor (BWR) core design is proposed that conserves natural uranium through a high conversion ratio that is achieved through efficient utilization of the vapor void in the BWR core. The proposed reactor concept employs fuel bundles with a square channel box and cruciform control rods, which are commonly used in conventional BWRs. Thus, it is possible to use current BWR core internals and vessel designs with minimal modifications, which makes the entire reactor system design more feasible.

Technology Development Issues for High-Conversion BWRs Using Island-Type MOX Fuel

Abstract The high-conversion boiling water reactor (HCBWR) has been studied as one of the next-generation BWRs. The HCBWR can be improved by the use of island-type fuel, which has mixed-oxide rods in the bundle interior and uranium rods only in the small region of the periphery, to have inherently negative void coefficient (i.e., negative void coefficient in infinite lattice configuration). The proposed reactor concept also has the sustainability to extend the light water reactor’s period by ~180 yr and the compatibility with a conventional BWR system such that only substitution of fuel bundles and control rods is required. As an example case, the high-conversion advanced boiling water reactor II (ABWR-II) is evaluated.

Operating experience with the multienrichment initial core of the boiling water reactor Kashiwazaki-Kariwa Unit 5

The multienrichment boiling water reactor (BWR) initial core design was first applied to the Kashiwazaki-Kariwa Nuclear Power Station Unit 5 [1100-MW (electric) BWR] in Japan. This core is designed to improve fuel discharge exposure, capacity factors, and operability. The design study shows that three types of fuel bundles with different enrichments are suitable to achieve the design targets. Three bundle enrichments are selected to simulate each of the following: fresh bundles, once-burned bundles, and twice-burned bundles in the reload core. Although the heterogeneity of multienrichment design increases the complexity of the design analysis, both the initial criticality test and the moderator temperature coefficient measurement showed good agreement with the prediction. Subsequent full-power operation verified the expected core performance. Average discharge exposure for the total initial fuel is {approximately}10% larger than that for the conventional single-enrichment BWR initial fuel with reinsertion of discharged fuel at the end of the first cycle. These experiences verified the effectiveness of a multienrichment initial core for the improvement of fuel utilization, capacity factors, and operability.

Spectral shift rod for the boiling water reactor

Abstract A Boiling Water Reactor core concept has been proposed using a new fuel component called spectral shift rod (SSR). The SSR is a new type of water rod in which a water level is formed during core operation. The water level can be controlled by the core recirculation flow rate. By using SSRs, the reactor can be operated with all control rods withdrawn through the operation cycle as well as that a much larger natural uranium saving is possible due to spectral shift operation than in current BWRs. The steady state and transient characteristics of the SSRs have been examined by experiments and analyses to certify the feasibility. In a reference design, a four times larger spectral shift width as for the current BWR has been obtained.