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IMPROVEMENTS IN IRRADIATED FUEL HANDLING AT DHRUVA FOR ENHANCED SAFETY

HEMANT GUJAR and SWARAJ AGARWAL Dhruva, Bhabha Atomic Research centre, Trombav, Humbai, India.

ABSTRACT

Dhruva is a 100 MWt research reactor located at the Bhabha Atomic Research Center. Trombay, Mumbai, India. The reactor achieved its first criticality in August 1985 and full power operation in the year 1988. Heavv water is used as moderator. primary coolant and reflector in the reactor. The fuel assemblies are made of natural metallic uranium pins clad in aluminium. The reactor provides facilities for basic and applied research. material testing, production of radioisotopes and training of manpower. Operation of Dhruva for over sixteen years has an excellent safety record. Continued efforts are made towards enhancing operational. safetv of the reactor by way of reviewing incidences and implementing necessary, modifications including changes in operating procedures. This paper highlights some of the improvements carried out in irradiated fuel handling at Dhruva for enhanced safety.

1. INTRODUCTION

India's fifth research reactor DHRUVA which became critical on August 81985, Is a natural uranium fuelled, heavy water moderated and cooled thermal neutron research reactor with an operating power level of 100 MWt and a maximum thermal neutron flux of 1.8 X 1014 n/cM 2/sec. The 'indigenously designed and built reactor is located on the east Coast of Trombay Island at the Bhabha Atomic Research Centre (B.A.R.C.), and is about 10 km north- east of Mumbal city. The reactor has a vertical core and employs natural metallic uranium seven-pin cluster fuel assemblies installed in Zircalloy guide tubes in stainless steel reactor vessel. Heavy water is used as the moderator, pimary coolant and eflector. Helium is used as cover gas. Reactor power regulation is achieved by moderator level control. Fast shut down of the reactor is effected by nne Cadmium Shut off rods with simultaneous dumping of heavy water moderator. Heat from pmary coolant is transferred to a secondary closed loop system recirculating dernieralised light water in a set of heat exchangers. The secondary coolant in turn is cooled by sea water another set of heat exchangers. The sea water coolant is drawn from the Mumbal harbour bay and flows through the heat exchangers on once-through basis.

2. THE REACTOR FUEL CYCLE

The fuel cycle for Dhruva Reactor is represented fig. I.Tbe basic source material is uranium ore. Te concentrated ore is shi ped to Uranium Metal Plant (UMP). At UMP uranium ore is converted into uranium metal. Te uranium metal is transported to Atomic Fuel Division (AFD) for fabrication of fuel. Dhr-uva fuel p is three meter length and 12.7 mm in diameter. These pns are clad with Imm. thick alummilum. Seven pins are assembled inside an alununium flow tube to form one cluster sub assembly. Alurniium spacers are fixed to the central rod of the fuel cluster and distributed over the length. Each fuel pn and clad tube is subjected to Eddy current testing, Radiography and Visual inspection to ensure integrity of fuel pin and its cladding. Fuel pns are subjected to Glycol testing before assembly. These fuel

188 DHRUVA REACTOR FUEL CYCLE

URANIUM ORE

URANIUM METAL PLANT

ATOMIC FUEL DIVISION (FUEL FABRICATION)

DHRUVA REACTOR

ASSEMBLY OF FUEL

FUEL REACTOR

SPENT FUEL STORAGE

FIG 1. OUTLINE OF COMPLETE URANIUM FUEL CYCLE.

189 cluster sub assemblies are transported to Dhruva Reactor for final assembly and loading in to reactor.

At Dhruva each fuel cluster is subjected to visual inspection and dimensional checks. Fuel cluster sub assembly, alurnimurn shield subassembly and seal-and-shield plug subassemblies are assembled to form a fuel assembly. The fuel assembly is 92 meter length. 'Me alurnimum shield sub assembly and seal-and-shield sub assembly forin the extension of fuel cluster sub assembly up to the top of the reactor and provide shielding and means for locking the fuel assembly to its channel. The full-length assembly is flow tested, dried and tagged cert ying its pile worthiness. All fuel assemblies are stored vertically.

Refueling is carried out by 300 Te fuelling machine having layers of lead and borated paraffin shielding to protect against primary gamma, photo neutrons and secondary gamma rays. The machine has two barrels, which are 'indexed to load and uload ftesh and irradiated fiiel in to and from the core. All operations of machines are carried out remotely by ydraulic drives from machine console. All drives are backed up by manual drives. Limit switches 'dicate actuations of various operations and these indications are used in iterlocks for ensuring sequential operations of machine dves. The fuelling machine is designed to have its on arrangement for cooling fuel with heavy water. Provision also exists for cooling the fuel machine with light water exigencies. Necessary arrangement is provided machine to Mminiise tritium exposure during fuel handling. A test channel is provided to quality the machine por to the fuelling operation. All dives are hydraulically operated and set suitably to ensure the limited force on fuel, coolant channel and machine components.

Refueling is carried accordance with approved agenda. The fuelling engieer-Lin-charge prepares the agenda. The station reactor physicist certifies the load changes and Dhruva Reactor Superintendent approves the agenda. Each movement of a fuel assembly is recorded in Transfer Slips to ensure proper movement and inventory of fuel assemblies.

The fuel assemblies are removed from the core after completion of scheduled irradiation and transferred by the fuelling machine into fuel transfer buggy which carries it through an underwater fuel transfer trench to the adjoining spent fuel storage building (SFSB). In the SFSB the fuel assembly is bisected separating the reusable seal and shield sub assemblies from the fuel sub-assembly. The fuel sub-assemblies are stored vertically under water. Towards ensuring safety of fuel and operation staff following provisions exist

• Bays are built above ground level and extend up to pile elevation. This facilitates to minimise dry handling duration of irradiated fuel assembly while transferring from machine to SFSB. • Bays are lined with Stainless Steel to provide leak tightness and ease in decontamination. This also obviates te problem of water table exertmg pressure on the stainless steel lining. • The fuel sub-assembly storage bay cannot be drained completely which ensures fuel submergence. • SFSB is divided into cutting bay, storage bay and 'inspection bay. Provision exists to isolate each bay for any repair ob. • Under water tools cannot be lifted beyond specified eight by design to ensure adequate water shielding. • All under water equipments are subjected to regular preventive maintenance checks and surveillance testing to ensure the tended functioning. • Bay water chemistry and specific activity is maintained by use of ion-exchanger resin. • Ventilation is provided to ensure personal safety, air curtain is maintained above water level to minimise air activity in the SFSB.

190 3. IMPROVEMENTS IN FUEL HANDLING

ne following are some of the improvements carried. out fuel handling systems and operating procedures based on experience:

3.1 During itial commissioning tals with dummy fuel, it was realized that possibility of fuel assembly remaining unlocked from a channel couldn't be ruled out. In Dhruva upward force exerted on the fuel by the coolant flow when main coolant pumps are in operation is more than the weight of the fuel and an unlocked fuel can get lifted up. In view of this apprehension, provisions were made to deny access to fuel channels wen main coolant pumps are in operation in the form of physical barriers and interlocks. In addition special lockmg gadget was provided on top of each fuel assembly whose design does not permit the assembly to be unlocked while coolant pumps are in operation. [I]

3.2 During transfer of rradiated fuel from reactor to the underwater fuel transfer buggy, the fuel is cooled by heavy water cooling system of the fuelling machine. Just prior to discharge of fuel, heavy water from the machine is dumped in to storage tank and the fuel is quickjy lowered into the buggy. Detailed procedural checks ensure that the irradiated fuel assembly is discharged within the "stipulated dry time". Provision also exists for dousing the fuel assembly with light water if there. is a possibility of exceeding the "specified dry time". Based on experience finther provisions have been made to make available additional ctical information to filellmg engineer and also to enhance reliability of light water dousing system.

3.3 A pool site inspection facility is being provided to carry out post-irradiation examinations of routine nature at the reactor site itself avoiding the need for frequent transportation of irradiated fuel to hot-cells 21

3.4 For canning of fuel elements with clad failure, the flow-tube itself is used as a can. For achieving this, significant development work had to be done towards work'mg out the design of plugs and the plugging tools. 2]

3.5 Adequate storage capacity for irradiated fuel has been provided for storage of fuel for few years. The irradiated assemblies are stored vertically. Being metallic fuel, if fuel clad falls during storage the recovery of uranium powder becomes difficult task. To recover any debris/uranium powder, removable trays have been provided below the underwater fuel storage racks. With this the bay floors remain clean and the trays can be removed for cleaning when required. 21

4. CONCLUSION

Enhancement of safety is an on going process. From over 16 years of experience the field of handling and storage of irradiated fuel from Dhruva we have incorporated several changes towards enhancing safety and improving system perforrilance. These include design modification, procedural changes, provision of physical barrier, development of in house inspection facility and development of canning facility for fuel.

5. ACKNOWLEDGMENT

We are thankful to Mr. S.K. Sharma, Director Reactor Group and Mr. S. Sankar Associate Director Reactor Group for their guidance and suggestions provided preparations of this paper. We also gratefully acknowledge valuable discussions with Mr. R.Chowdhury, Head. Rod, and Mr. S.Duralsamy, ARS. for informative input for this paper.

191 6. REFERENCES

[11 N.Veeraraghavan, ntI.Symposium On Research Reactor Safety, Operations and Modifications, October 23-27,1989 p. 295. [2] S.K.Sharma, IAEA-AGM on Database Management and storage of spent Nuclear Fuel ftom Research reactor, IAEA, Vienna, November 14,1994.

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