Examples of Research Reactor Conversion Assessment of Alternatives

Transcription

1 Examples of Research Reactor Conversion Assessment of Alternatives Benoit Dionne, Ph.D. Section Manager - Conversion Analysis and Methods Nuclear Engineering Division, Argonne National Laboratory National Academy of Sciences Committee Meeting Oak Ridge, TN, June 24 th, 2015

2 Introduction DOE/NNSA Office of Material Management and Minimization (M3) Reactor Conversion Program mandate to Argonne is clear: LEU (< 20 w/o of U-235) solutions are to be considered Conversion should maintain most of the experimental capabilities of the reactor and minimize impact on the cost to the facility Objective: Clarify the role of alternative fuel assembly designs in the conversion process. Outline: Overview of the low-enriched uranium (LEU) fuel assembly design process Examples of alternative fuel assembly designs 2 National Academy of Sciences Meeting, Oak Ridge, TN

3 Overview of LEU Fuel Assembly Design Process Determine mass of U-235 necessary to obtain a core excess reactivity and cycle length similar to the HEU core. Higher enrichment is effective to introduce large amount of U-235 with a low uranium density. LEU fuel assembly mass of U-235 must be equal to the HEU fuel assembly plus a certain amount (typically >=15%) to compensate for the parasitic absorption in U- 238, alloying components and other reactor-specific considerations. Broad estimates can be made by adjusting the fuel uranium density according to above rule-of-thumb (e.g RERTR paper from J. Matos) but reactor-specific analysis is necessary to define required fuel characteristics and assembly design. Increasing uranium density or fuel meat thickness have diminishing returns due to a phenomena called self-shielding. For dispersion fuel, a larger uranium density requires a larger volume fraction of uranium in the matrix which impacts fuel performance under irradiation. 3 National Academy of Sciences Meeting, Oak Ridge, TN

4 Overview of LEU Fuel Assembly Design Process In almost all cases, a reactor-specific LEU fuel assembly has to be designed and qualified. The required LEU density as well as the fission rate and fission density are compared to the already available qualified fuel system. When no appropriate fuel system is adequate for a direct replacement (simplest conversion), historically, this has implied that a new LEU fuel system also needed to be developed and qualified. Alternative LEU designs are also often needed to address other constraints. It is not always possible to identify all constraints at the beginning of a conversion project. Some constraints may require additional work (cost and schedule) and increase the risk to the conversion project. 4 National Academy of Sciences Meeting, Oak Ridge, TN

5 Overview of LEU Fuel Assembly Design Process Constraint Excess reactivity and cycle length Power peaking (safety margins and fuel qualification) Reactivity management (shutdown margin/excess reactivity) Structural integrity Dimension restrictions Possible and cost effective fabrication Impact on the design Change in fuel geometry - Increase fuel meat volume to reach target mass - Modification to fuel-to-water ratio - Change in fuel meat geometry - Change in burnable absorber - Change in fuel meat geometry - Change in burnable absorber - Limit on plate/meat/cladding thicknesses - Limit on fuel length - Limit on number of fuel plates and/or assemblies - Limit on cladding thickness (and other geometry limitations) - Limit number of fuel systems 5 National Academy of Sciences Meeting, Oak Ridge, TN

6 RHF, France Basis information Current fuel uranium density is 1.2 gu/cm 3 Compact core (requires a larger increase in U-235 mass) Rule-of-thumb : LEU density larger than 6.4 gu/cm 3 HEU cycle lengths are typically 50 days at 52 MW 45 days at 57 MW Constraints: Dimension restrictions (fuel plate length, number of fuel plates) Power peaking (safety margins and fuel qualification) 6 National Academy of Sciences Meeting, Oak Ridge, TN

7 RHF, France HEU and reference LEU fuel assembly designs 7 National Academy of Sciences Meeting, Oak Ridge, TN

8 RHF, France Reference LEU design (TOUTATIS 2010) U7Mo dispersion fuel at 8 gu/cm 3 (HERACLES). Extended active fuel length to 880 mm by removing borated zones to meet cycle length. Burnable absorber belt with B-10 mass 4x larger than HEU to control power peaking. Cycle length at 57 MW: ~48 days. HERACLES backup solutions: Very high density U 3 Si 2 U10Mo monolithic 8 National Academy of Sciences Meeting, Oak Ridge, TN

9 RHF, France Decreasing number of fuel plates (change fuel-to-water ratio) would require hydraulics retesting and results in marginal gains in cycle length. Study of an alternative designs for U10Mo monolithic backup solution is planned. Alternative designs for qualified LEU U 3 Si 2 (4.8 gu/cm 3 ) deemed unacceptable due to significant reduction in cycle length: With meat extended length (~26 days). With meat extended length and increased thickness (0.67mm) (~41 days). 9 National Academy of Sciences Meeting, Oak Ridge, TN

10 RHF, France Alternative designs for very high density U 3 Si 2 considered as backup solution to U7Mo dispersion: 6.5 gu/cm 3 with meat extended length. Deemed unacceptable since Significant reduction in cycle length (~42 days). Never fabricated commercially and/or irradiated. 6.0 gu/cm 3 with increased meat thick. (0.61 mm) and length (880 mm) (~49 days). This option has potential as a backup solution but there are some reservations: Same volume fraction U7Mo dispersion at 8 gu/cm 3. Limited fabrication and irradiation data. Successful under RHF irradiation envelop but not under HERACLES envelop (might require different solution for BR2 or additional fuel development work). 10 National Academy of Sciences Meeting, Oak Ridge, TN

11 RHF, France Reference RHF LEU design in the feasibility study (TOUTATIS ) required an increase in boron loading in burnable absorber belt (factor of 4 compared to HEU design). Due to helium production and absorption rates, higher risks of blistering (based on previous tests performed at ILL and EVITA fuel qualification program for JHR). Additional modeling has been performed that shows that under more realistic power density discretization (3mm x 3mm zone), TOUTATIS LEU design overestimated the boron loading necessary in the belt to reduce the axial power peak. 11 National Academy of Sciences Meeting, Oak Ridge, TN

12 RHF, France Additional risk mitigation activities are in progress: Improve understanding of RHF local power peaking impact on plate swelling. Alternative burnable absorber design. Current Modeling Over-conservative Modeling B-10 Nominal Peak Power Density (W/mm 3 of meat) Heat Flux with Uncertainty (W/cm 2 ) Heat Flux with Uncertainty (W/cm 2 ) 4xHEU xHEU xHEU National Academy of Sciences Meeting, Oak Ridge, TN

13 RHF, France Additional risk mitigation activities are in progress: Improve understanding of RHF local power peaking impact on plate swelling. Alternative burnable absorber design. 463 W/cm W/cm W/cm 2 13 National Academy of Sciences Meeting, Oak Ridge, TN

14 BR2, Belgium Basis information Current fuel density is 1.3 gu/cm 3. Rule-of-thumb : LEU density ~7 gu/cm 3. Current HEU fuel uses integral absorber (B 4 C and Sm 3 O 2 dispersed in the Al matrix) for reactivity management. Power (variable): typically between 60 and 70 MW (up to 100 MW). Variable core configuration (7 to 40 fuel assemblies). Cycle length (variable): typically ~28 days. Constraints: Dimension restrictions: No changes to fuel assembly design due to complex flow pattern within the core. Core configuration and fuel management representative of current and foreseen use of reactor. Reactivity management (equilibrium LEU core and HEU to LEU transition cores). 14 National Academy of Sciences Meeting, Oak Ridge, TN

15 BR2, Belgium HEU and reference LEU fuel assembly designs HEU Representative core configuration 15 National Academy of Sciences Meeting, Oak Ridge, TN LEU

16 BR2, Belgium Reference LEU design (2011) LEU U7Mo with a density between 7.5 and 8.5 gu/cm 3 is feasible. Replace integral burnable absorber by cadmium wires (D= mm) wedged between fuel plates and side plates. Predicted cycle lengths at 59 MW HEU: 26 days, LEU: 34 days. HERACLES: LEU U7Mo dispersion fuel at 8 gu/cm 3. HERACLES backup solutions: Very high density U 3 Si 2 U10Mo monolithic. Control Rods Position (mm) Widthdrawal Insertion HEU Minimum CR position = 565mm LEU Minimum CR position = 432mm Operating time (days) HEU core LEU core 16 National Academy of Sciences Meeting, Oak Ridge, TN

17 BR2, Belgium Alternative designs considered: 4.8 gu/cm 3 (maximum density currently qualified ). UMo dispersion fuel with densities between 7 gu/cm 3 and 8.5 gu/cm 3. The feasibility of the conversion strongly depends on the choice of appropriate burnable absorbers (material, geometry and location). For fuel with a high volume fraction of uranium, there are fabrication concerns related to the difficulty of achieving an homogeneous mix of absorber in the matrix. The use of integral burnable absorbers for the UMo-Al fuel system is not considered as part of LEONIDAS and HERACLES fuel development programs. 17 National Academy of Sciences Meeting, Oak Ridge, TN

18 BR2, Belgium Different burnable absorbers (B 4 C, Cd, Gd 2 O 3, Er 2 O 3 ) were also considered in different geometries: 1. Wires inserted in the side plates' grooves where fuel plate are wedged. 2. Plates inserted in the side plates. 3. Homogeneously mixed boron in side plates. Selection process was based on several criteria Reactivity at beginning-of-cycle (BOC) Burnup of burnable absorber (should be fully consumed by the assembly end-of-life) Simple and proven fabrication Concerns over fabrication, behavior under irradiation and burnup achievable for options 2 and 3 18 National Academy of Sciences Meeting, Oak Ridge, TN

19 BR2, Belgium Different burnable absorbers (B 4 C, Sm 2 O 3, Cd, Gd 2 O 3, Er 2 O 3 ) were also considered in different geometries: Wires wedged between fuel plates and side plates B 4 C, Sm 2 O 3, and Er 2 O 3 achieved insufficient absorber burnup Change in reactivity at BOC for LEU core compared to HEU HEU Cd-wires D=0.5mm U7Mo 8.5 gu/cm 3 Cd-wires D=0.5mm U7Mo 7.5 gu/cm 3 Cd-wires D=0.4mm U7Mo 7.5 gu/cm 3 Gd 2 O 3 -wires D=0.4mm U 2 Si gu/cm 3 Cd-wires D=0.08mm +0.88$ +1.73$ +0.36$ -1.47$ -3.96$ Cadmium wire was deemed the most appropriate choice for the burnable absorber for the proposed LEU fuel assembly. 19 National Academy of Sciences Meeting, Oak Ridge, TN

20 Other Examples Alternative LEU fuel assembly designs are often considered to address different constraints. A few other examples: Constraint Enrichment larger than 20% (FRM-II prior to construction) Enrichment larger than 20%, core size restriction (FRM-II after construction) Minimal cladding/plate thickness to reliably fabricate fuel plates with U-10Mo foils Alternative design considered Increase core volume and number of fuel plates (using different involute) would have allowed the use of qualified LEU U 3 Si 2 fuel. U8Mo monolithic fuel (HERACLES) with increased active length and meat thickness (current alternative design still require U-235 w/o > 20%) A series of more 30 alternative designs were considered for MURR. Selected LEU design has reduced number of fuel plates (23), increased fuel assembly U-235 mass (1507 g) and improve radial heat flux profile (5 unique foil/plate combinations) 20 National Academy of Sciences Meeting, Oak Ridge, TN

21 Other Examples Alternative LEU fuel assembly designs are often considered to address different constraints. A few other examples: Constraint Reactivity management Minimal cladding thickness to reliably fabricate fuel plates with U-10Mo foils Alternative design For HEU to LEU transition cores, MURR alternative design make use of borated side plates to taper the reactivity of an core loaded with all fresh LEU fuel assemblies. A large space of alternative designs was considered to develop an MITR fuel assembly without fins. The most attractive design contains 19 plates (54 mils thick.), full-thickness fuel meat of 25 mil in the interior plates, and variable thicknesses for exterior plates to control power profile. 21 National Academy of Sciences Meeting, Oak Ridge, TN

22 Summary The suitability of a fuel system depends on more than its performance under irradiation (fission density vs fission rate). Core excess reactivity and cycle length typically guide the selection of a base fuel system. Alternative to the HEU and LEU base designs are usually necessary to take into account constraints such as: Lack of qualified LEU fuel Power peaking (safety margins and fuel qualification) Reactivity management (shutdown margin and excess reactivity) Structural integrity Dimension restrictions Possible and cost effective fabrication 22 National Academy of Sciences Meeting, Oak Ridge, TN