Dose uniformity of radiation sterilization high-voltage power supplies in textile disinfection and pet food treatment

 

　　The accelerator tube is a vacuum tube, 1 m long, with a thermionic cathode at the top and a titanium foil window at the bottom. The foil is 50 μm thick, and the electrons pass through the foil to the product below. The tube vacuum is 1e-4 Pa, maintained by a 200 L per second turbomolecular pump. The tube electrodes are stainless steel, 100 mm diameter, spaced 50 mm apart. The high-voltage supply connects to the cathode electrode, and the anode is grounded.

 

　　The beam current is measured by a Faraday cup at the tube exit, and the signal feeds back to the supply control board. The control board adjusts the acceleration voltage to hold the beam current at the setpoint. The feedback loop bandwidth is 100 Hz, fast enough to compensate for beam current fluctuations from the cathode temperature. The loop once overshot when the cathode emission dipped, and the beam current surged to 15 mA, 50 percent above the setpoint. The overshoot was corrected by adding a lead compensator to the loop, and the overshoot dropped to 10 percent.

 

　　The product conveyor is a belt type, 1 m wide, moving at 0.1 m per second. The pallets are 1 m by 1 m, holding 50 kg of textiles or 20 kg of pet food. The pallet passes under the beam window, and the dose is integrated over the pallet transit time. The dose is measured by dosimeter films placed on the pallet, and the films are read out after the run. The dose uniformity is calculated from the nine film readings, and the non-uniformity must stay below 10 percent.

 

　　The high-voltage cable connects the supply to the accelerator tube, through a 5 m shielded cable. The cable is a 120 kV silicone type, with the outer jacket grounded at both ends. The cable once developed a hairline crack in the jacket, and the leakage current rose from 2 μA to 25 μA. The leakage did not trip the 100 μA overcurrent threshold, but the extra 23 μA added noise to the beam current measurement. The cable was replaced at the next scheduled maintenance, and the leakage returned to 2 μA.

 

　　The supply cabinet is 800 mm wide, 1000 mm deep, 1800 mm tall, with IP54 protection. The cabinet contains the inverter, transformer, rectifier, divider, and control board. The cabinet cooling fans run on a thermal switch, starting at 35 degrees and stopping at 25 degrees. The fans once failed in summer, and the cabinet temperature reached 55 degrees. The electrolytic capacitors on the DC bus reached 65 degrees, above the 60 degree rating. The fan fault alarm now routes to the central monitoring room, and the fan is replaced within 10 minutes of alarm.

 

　　The control board uses a 16-bit ADC, sampling the output voltage at 1 kHz. The ADC reference is a buried-zener type, with a drift of 2 ppm per degree Celsius. The cabinet temperature is held at 25 plus or minus 2 degrees by the fan and heater, and the reference drift over a 24 hour operation stays below 10 ppm, equivalent to 0.1 V at 100 kV. The drift is within the 0.1 percent acceptance limit, and the reference is recalibrated annually against a 0.01 percent standard divider.

 

　　The safety interlock on the irradiation room door was tested during commissioning. A technician opened the door while the supply was running, and the output dropped to zero within 20 ms. The residual charge on the output capacitor bled through a 10 MΩ resistor within 100 ms. The interlock response time was logged at 18 ms, within the 25 ms design limit. The door carries a radiation warning label, and entry requires a radiation badge. The non-authorized personnel are not permitted inside the irradiation room.

 

　　The maintenance log lists routine tasks. Each shift, the operator records the output voltage, beam current, and cabinet temperature. Weekly, the technician inspects the cable jacket and the tube window. Monthly, the insulation resistance is measured with a 2500 V megohmmeter, and the reading should exceed 1000 MΩ. Quarterly, the divider is calibrated against a 0.1 percent standard. Annually, the ADC reference is recalibrated. The log is signed at each step, and the record is filed in the plant archive.

 

　　The input side of the supply draws 400 V three phase at 20 A. A 100 kVA isolation transformer sits between the plant bus and the supply, to isolate the accelerator ground from the plant ground. The isolation transformer has a 2500 V insulation rating between primary and secondary. The input cable is copper, 16 mm², rated for 50 A, routed through a cable tray. The input breaker is a 63 A molded case type, with electronic trip set to 1.05 times rated current long delay. The input power factor is 0.96, measured with a power analyzer during commissioning.

 

　　The high-voltage transformer inside the supply cabinet is oil insulated, with a 20 L oil volume. The oil temperature reaches 45 degrees after 8 hours of continuous operation at 100 kV and 10 mA, against an ambient of 25 degrees. The silica gel breather on the transformer tank changes color after 12 months, and the gel is replaced before the color reaches half saturation. The oil breakdown voltage is tested annually, and the last measurement showed 42 kV per 2.5 mm gap, well above the 25 kV replacement threshold. The transformer sound level is 50 dB at 1 m, measured with a sound level meter during commissioning.

 

　　The output divider is a series chain of 100 MΩ high-voltage glass glaze resistors, totaling 2 GΩ. The divider tap at 1:2000 feeds the control board ADC. The divider is calibrated quarterly against a 0.1 percent standard divider. The calibration drift over 12 months was 0.3 percent, equivalent to 300 V at 100 kV. The drift is within the 1 percent acceptance limit, but the divider is scheduled for resistor replacement at the 24 month mark.

 

　　The accelerator tube cooling system is a closed loop with deionized water, conductivity below 1 μS per cm. The water flow is 10 L per minute, and the water temperature is held at 20 degrees by a chiller. The chiller once failed, and the water temperature rose to 30 degrees. The tube electrode temperature reached 60 degrees, and the beam current dropped 10 percent. The chiller was repaired within 4 hours, and the beam current returned to normal. The water conductivity is measured weekly, and the ion exchange resin is replaced when the conductivity exceeds 1 μS per cm.

 

　　The product conveyor speed is controlled by a variable frequency drive, set at 0.1 m per second. The speed feedback comes from an encoder on the drive shaft, and the speed error is within 1 percent. The conveyor belt is a food-grade type, 1 m wide, 5 m long. The belt tension is adjusted monthly, and the belt is replaced at 5000 hours of operation. The pallet position is detected by a photoelectric sensor, and the signal triggers the beam scan start.

 

　　The customer acceptance test was run over three days. Day one covered the textile sterilization at 25 kGy, measuring the dose uniformity and the sterilization efficacy. Day two covered the pet food sterilization at 10 kGy, measuring the dose uniformity and the product quality. Day three covered the safety interlocks, opening the door and verifying the shutdown response. The test report was signed by the customer representative, and the supply entered the 12 month warranty period.

 

　　The plant environment around the supply cabinet is held at 23 plus or minus 3 degrees Celsius, with 45 to 65 percent relative humidity. Winter heating raises the room temperature to 26 degrees, and the cabinet fan runs continuously. Summer cooling failure once pushed the room temperature to 32 degrees, and the cabinet internal temperature reached 50 degrees. The ADC reference drifted 0.3% during that week, equivalent to a 300 V error at 100 kV. The drift was corrected after the cooling unit was repaired, and the supply was recalibrated. The plant floor is epoxy coated, cleaned daily with a vacuum cleaner, and the illumination is 300 lux with LED fixtures at 24 V.

 

　　The product dose measurement is done with dosimeter films, placed at nine points on each pallet. The films are a radiochromic type, changing color when exposed to ionizing radiation. The films are read out by a scanner, and the dose is calculated from the color change. The dose uniformity is calculated from the nine readings, and the non-uniformity must stay below 10 percent. The films are stored in a cool, dark place, and are discarded after one use.

 

　　The irradiation room safety audit is conducted annually, by the radiation safety officer. The audit covers the radiation shielding, the interlocks, and the personnel monitoring. The audit report lists the findings, and the corrective actions are scheduled within 30 days. The last audit found two minor findings: a worn light curtain, and a faded radiation sign. Both findings were corrected within the scheduled time. The audit record is filed in the quality system.

 

　　The irradiation room spare parts inventory includes: dosimeter films, conveyor belts, fuses, and survey meter batteries. The inventory is stocked at 20 percent of the annual consumption, and is reviewed monthly. The inventory is stored in a locked cabinet, and the key is held by the room supervisor. The parts are logged out when used, and the log is reviewed by the quality department. The inventory was once depleted when a batch of films expired, and the stock was replenished within 3 days.

 

　　The irradiation room operates two shifts per day, 6 days per week. The day shift runs the textile sterilization, and the evening shift runs the pet food sterilization. The product changeover happens between shifts, and the pallet is swapped. The shift handover includes a status report, listing the completed runs, the dose measurements, and the conveyor speed. The handover log is reviewed by the room supervisor, who allocates resources for the next shift.

 

　　The irradiation sterilization service is qualified per ISO 11137, with the dose validated for each product type. The dose validation is conducted annually, with dose measurements at the minimum and maximum dose locations. The validation report is filed in the quality system, and the customer receives a copy. The service has maintained the qualification for 8 years, with no major findings in the last 3 audits. The service crew carries the certification, and the certification is checked by the customer before each batch.

 

　　The irradiation room conveyor system is maintained by the maintenance department, with weekly inspection and monthly preventive maintenance. The conveyor belt is replaced at 5000 hours of operation, and the belt tension is adjusted monthly. The conveyor drive motor is a variable frequency type, with the speed feedback from an encoder. The conveyor system has a 98 percent uptime target, and the downtime is logged in the maintenance record. The system once had a belt tear, and the belt was replaced within 4 hours, with no production loss.

 

　　The irradiation room operator training is conducted by the radiation safety department, with a 40 hour course covering the radiation safety rules, the dosimeter use, and the equipment operation. The operator must pass a written exam and a practical exam, before being authorized to run the irradiation. The operator certification is renewed annually, with a refresher course. The training record is filed in the quality system, and the certification cards are carried by the operators. The training has been conducted for 8 years, and 12 operators have been certified.

 

　　The titanium foil window developed a pinhole after 11 months of two-shift operation, and the tube vacuum dropped from 1e-4 Pa to 1e-2 Pa within a 2 hour shift. The pinhole measured 0.3 mm across, traced to thermal fatigue at the foil edge where the copper seal clamp concentrates stress. The foil is cooled by a 3 m per minute air knife on the product side, and the air knife once skipped a maintenance stop, allowing the foil surface to run 35 degrees hotter than design. The vacuum interlock tripped before any product was irradiated with the breached window, and the foil was replaced in 6 hours during the next weekend window. A replacement foil costs 1800 dollars in material plus 6 hours of labor, and running the line with a known thinning foil risks a full tube vent that would cost 2 days of down time. The foil edge is now inspected weekly with an endoscope, and the air knife flow is logged on a shift checklist. The helium buffer gas behind the foil was raised from 0 to 0.05 MPa overpressure after the rupture, so a pinhole pushes helium out instead of pulling atmosphere in, buying about 20 minutes of warning time before the vacuum gauge alarms. That margin is not enough to keep running, but the warning window is long enough for a controlled beam-down ramp instead of an abrupt trip.

 

　　The supply has run for 15 months. The high-voltage cable has been replaced once. The output capacitor shows a 3 percent capacitance loss, still within the 10 percent threshold. The unresolved issue is the dose non-uniformity at the pallet edges. The non-uniformity is 12 percent at the edges, above the 10 percent acceptance limit. A beam scan correction magnet is under evaluation, and the upgrade is scheduled for the next tube replacement.