Reactivity Control (1) Reactivity Requirements Reactivity requirements can be broken down into several areas: (A) Sufficient initial reactivity should be installed to offset the depletion of U 235 and buildup of fission products with burnup and achieve the required cycle life. (B) Sufficient reactivity should be installed to allow operation at power, overcoming negative reactivity offsets from fission product buildup (mainly Xe 135 and Sm 149 ) and power feedback. Reactivity requirements associated with availability and load follow capability do not enter into the fuel reactivity requirements, but do influence the reactivity control system. Core reactivity maximized => reduced fuel w% (enrichment) required => reduced fuel cycle cost by a combination of (a) Proper selection of structural materials, minimizing neutron loss via capture (b) Proper (water/fuel) ratio, minimizing neutron loss by excessive over or under moderation (c) Low flux on core peripheral, minimizing neutron loss by leakage.
Reactivity Balance Reactivity 0 Supercritical keff 1 0 Critical keff 0 Subcritical A reactor can be critical at any power level!! The net reactivity is composed of several components 0 CR BP SP VOID MOD RX FP 0 reactor critical and at steady state 0 = Excess reactivity CR BP SP VOID MOD = Reactivity from control rods = Reactivity from fixed burnable poisons = Reactivity from soluble poisons (boron) = Reactivity from coolant voids = Reactivity from moderator temperature changes ( T T ) 0 MOD MOD avg avg0 MOD RX = Reactivity from fuel temperature changes (Doppler) ( T T ) 0 RX RX RX RX 0 RX FP = Reactivity from fission products (Xe and Sm) In moving from one critical state (power level) to another, the individual reactivity components are adjusted such that the sum is zero.
BOC Xenon Reactivity After Shutdown The reactivity contribution from Xe and Sm is directly related to their number densities, and as such a function of operating and shut down time.
PWR Total Power Defect at BOL and EOL The Power Defect reflects the reactivity contributions of both moderator and fuel temperature.
A modern LWR requires about 20%/15% excess reactivity to be installed to achieve a cycle length of 1 2 1 years for PWR/2 years for BWR (of operation without refueling). To offset this large excess reactivity, a reactivity control system must be designed. One can separate the reactivity control system requirements into two categories: (1) Long-time reactivity control to offset gradual fuel depletion Shim control (2) Short-time reactivity control to: (i) assure plant subcriticality for accidents after trip (seconds) (ii) follow Xe 135, and to a lesser extent Sm 149, transient concentration behaviors (hours) (iii) offset power feedback associated with load changes (minutes) Reactivity Control Approaches Several alternatives are available for reactivity control (1) Control Elements: A highly absorbing (thermal) neutron material, encased in a metal structure, which can be inserted in the core to introduce negative reactivity. It should not deplete rapidly with exposure. PWR: Rod Control Cluster Assembly (RCCA) Cylindrical control rods, grouped into clusters with 5 to 24 fingers, that are inserted in vacant cell locations of one or more assemblies simultaneously. Materials: Control => Ag-In-Cd, B4C or Hafnium Clad => Stainless Steel BWR: Cruciform control blade inserted in inter-assembly gap region Materials: Control => B4C Clad => Stainless Steel
Assemblies Cruciform Advantages: Quick working and have capability to do power shaping (good and bad) Disadvantage: Very costly (pressure vessel penetrations and drives), power shaping capability can be disadvantageous and local power peaking problems (water holes/slots) must be overcome. Usage: PWR: several control elements which are located in radially symmetric positions are programmed to move together [control bank or group], avoiding radial power tilting. Basically almost totally withdrawn at HFP via soluble boron adjustment. BWR: Individual control element motion allowed but attempt to preserve radial symmetry.
(2) Burnable Control Elements: A less highly absorbing neutron element which is affixed in the core for the entire cycle or F/A life. Designed to deplete as the fuel also depletes, introducing positive reactivity. Concepts: (A) Burnable control material blended into or coated on the fuel, thereby reducing fabrication cost of a separate component, eliminating disposal problems and minimizing local power perturbation. Material: Blended Gd (used in BWRs and some PWRs) or Er2O3 (Erbium); Coated B 10 (IFBA), (B) Have burnable control element distinct from fuel, clad in metal (only used in PWRs). Material: B4C pellets or Borosilicate (Pyrex) glass tube (WABA) Form: Cylindrical rods which are placed in vacant unit cell locations Advantage: cheap, do not distort power profile nearly as much as control element since can use many, burns out as fuel burns out [not exactly]. Disadvantage: No prompt reactivity response, fuel cycle cost penalty since it does not completely burn out and for distinct BP rod displaces water (for undermoderated core decreases reactivity) and clad captures neutrons. (3) Soluble Poison: A highly absorbing substance which is dissolved in the primary coolant and: (A) has a large thermal absorption cross-section (B) soluble in water has no appreciable detrimental chemical or radioactive properties. [If have RCS leak, boric acid crystals formed which chemically attack metal.] Can only be used in a PWR since high voiding of water in BWR would make usage unattractive Material: Natural Boron (Boric Acid) Advantage: Cheapest and most uniform means of control [no power peaking problems]. Disadvantage: Requires time to change concentration and at high concentrations, moderator temperature reactivity coefficient can become positive violating basic design requirement
Resulting Reactivity Control Systems PWR BWR Long-term Control (Shim) Short-term Control Comments on Choice: Soluble Poison Burnable Poison RCCA Soluble Boron Burnable Poison Cruciform Control blade Void Fraction Cruciform Control Blade Void Fraction PWR: Burnable poison is used in a PWR to bring the soluble poison concentration down to produce a negative reactivity moderator temperature coefficient. BP also provides power shape control. BWR: Burnable poison is used in a BWR to satisfy cold shutdown margin requirement, i.e. degree of subcriticality at cold conditions with control rods inserted. BWRs are also able to use the large negative void coefficient to adjust reactivity by adjusting the core flow rate Some Typical Values: PWR Soluble Poison Content: 1800-0 ppm at rated power decreasing as the fuel burns 14-0% Δρ BP Rods Cycle 1: 500 1500, increasing as plant size increases, for an out-in fuel management, where they are not dominantly used for power shape control, 4.5-7.5% Δρ Reload Cycles: Number depend on desired cycle length RCCA 30 50 in number increasing as plant size increases, which comes to 8 10% Δρ BWR Burnable Poison: 5 10% Δρ Cruciform Control Elements: 100-150, which corresponds to 17% Δρ (for 137)
Boron Concentration versus Burnup with/without Burnable Poison Rods
Burnable Poison Loading Pattern
Rod Cluster Control Assembly Pattern
PWR- 4 Loop 3411 MWth Reactivity Requirements for Rod Cluster Control Assemblies (Preliminary) Beginning of Life (First Cycle) End of Life (Equilibrium Cycle) Reactivity Effects, percent 1. Control Requirements Fuel temperature (Doppler), %Δρ 1.30 1.30 Moderator temperature, %Δρ 0.20 1.25 Void, %Δρ 0.01 0.05 Redistribution, %Δρ 0.50 0.85 Rod Insertion Allowance, %Δρ 0.50 0.50 2. Total Control, %Δρ 2.51 3.95 3. Estimated Rod Cluster Control Assembly Worth (53 rods) a. All full length assemblies inserted, %Δρ b. All but one (highest worth) assemblies inserted, %Δρ 8.00 7.00 7.30 6.20 4. Estimated Rod Cluster Control Assembly credit with 10 percent adjustment to accommodate uncertainties (3b-10 percent), %Δρ 6.30 5.58 5. Shutdown margin available (4-2), %Δρ 3.79 1.63 [a] [a] The design basis minimum shutdown is 1.6%