N E W S L E T T E RFounder’s Day Special Issue October 2016

Improvements to Gang type Spent Fuel Chopper at TRP, Tarapur

Abhishant, A.K. Jha, K. Agarwal

NRPSED, NRB, Mumbai

Abhishant is the recipient of the DAE Scientific &

Technical Excellence Award for the year 2014

Abstract

Spent Fuel Chopper (SFC) is the most critical equipment for a reprocessing plant. SFC is one of the equipment in operating reprocessing plants which does not have a standby. It’s down time directly affects the productivity of the plant. The earlier SFC design at PREFRE-1, Tarapur and KARP, Kalpakkam was based on progressive cutting whereas the design of SFC at TRP, Tarapur is based on ‘Gang Chopping Concept’, which cuts one PHWR spent fuel bundle into multiple pieces in one stroke. During the initial phase of operation, few teething problems related to design emerged which resulted shortfall in the name plate plant capacity. This paper is based on problem case studies and troubleshooting backed up by solution implementation after mock test trials with minimum affecting the plant operations.

Background:

Spent fuel chopping is a first stage activity for reprocessing of nuclear power reactor’s spent fuel as it enables the fuel material to interact with the reagents.

The chopping of spent fuel is done by a shearing machine called “Spent Fuel Chopper”. The shearing machines installed at PREFRE-1, Tarapur & KARP, Kalpakkam were based on progressive cutting, where fuel bundle was cut in sequential steps of pushing, gripping and shearing.

This process as well as maintenance of shear machine was time consuming & hence a new chopping system based on ‘Gang Chopping Concept’ was developed & installed at TRP, Tarapur where a fuel bundle is chopped in a single stroke, thus reduces time of chopping considerably.

This machine avoids other aid like fuel gripping etc. Additionally this shearing machine has remotely replaceable modules which reduces the downtime & thereby increase the plant throughput. During initial phase of operation, many technical challenges arose which were overcome in a time bound manner to surpass the plant annual targets year by year.

The problem statements:

Following problems were encountered during the operation of machine in the first few years:

• Interferences of bundle with Component Transfer Tube (CTT).

• Erratic/stuck CTT movements • Abortive bundle transfers by CTA • Premature failure of cutting tools • Time & labour intensive cutting tools replacement

activities. • High man-rem expenditure during shear module’s

maintenance. • Pusher link stuck up at home position.

The solution statements:

Each of above problems was analyzed in detail and technically feasible solutions were implemented by modifying the existing design of corresponding components. These design modifications were mock tested up to the extent possible at the existing Component Testing Facility (CTF), at Tarapur before implementing the same in radioactive environment. The major components which needed design modifications were (1) Component Transfer Assembly, (2) Shear Module Assembly and (3) Pusher Link Assembly.

1. Component Transfer Assembly:

The Component Transfer Assembly (CTA) has a pneumatically actuated tube, which reciprocates between moving and fixed blades to receive one fuel bundle at a time from fuel feed magazine. The CTA is locked with the body of SFC by 450 rotation.

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The frequent problems of fuel bundle interfering/getting stuck up during pushing and jamming of CTT was affecting the machine availability. Sometimes bundle failed to move along with CTT retraction stroke resulted in obstruction for successive bundle pushing in to the shear zone, which also affected the productivity. Apart from above, a sticky situation resulted when CTT fails to retract due to stuck up after receiving bundle from fuel feed magazine henceforth:

• CTA could not be unlocked from SFC as it was required to be rotated.

• Shear module could not be removed as CTT was extended in space between the blades.

• Fuel bundle could not be retrieved from shear area through CTA port.

On analysis, it was observed that original CTT ( Fig. 1(a)) was circular in shape with a slot for bundle stopper. Internal diameter of CTT was less than that of opening port of fuel feed channel but CTT ID bottom level was equal or lower than that of Fuel Feed Magzine (FFM) level. It was also revealed that CTT was over constrained by various closed tolerance guiding elements. Further it was established that transfer of bundle from FFM to CTT was not positive and only dependent upon the relative frictional forces exerted on bundle by CTT and FFM. (Fig. 1(c))

Fig. 1(a): Original Component Transfer Assembly

Fig. 1(b): Modified Component Transfer Assembly

Fig.1(c): Original bundle transfer from FFM to CTT

Fig.1(d): Modified bundle transfer from FFM to CTT

To overcome these problems, the design of CTT was modified (Fig. 1(b)). A semi-circular tube with increased and eccentric ID was introduced. The modified CTA design addressed the problem of over constrained by providing a bit liberal fits in the components subjected to relative motions without affecting the functional requirement.

The level of CTT ID was elevated by one mm w.r.to FFM ID level to ensure positive pushing by ensuring no physical contact between bundle and FFM as soon as bundle center of gravity falls in CTT. (Fig. 1(d))

In order to ensure the assembly of CTA with SFC within the prescribed tolerances inside the radioactive cell, a full scale mock set up was made in Component Testing Facility (CTF) & several iterations were tried to suit the cell environment. These modification on CTA has helped in reducing down-time of spent fuel chopper drastically. The semi circular cross section of CTT has also helped to overcome the sticky situation as CTA can rotate, unlock and remove even after receiving the spent fuel bundle from FFM.

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N E W S L E T T E RFounder’s Day Special Issue October 2016

With the introduction of new CTT, the feasibility of chopping of even slightly flared irradiated fuel bundles with one or two pins dismantled is also possible.

2. Shear Module Assembly:

Shear Module Assembly (SMA) accommodates the stationary and moving cutting tools. It is located in shear cavity of SFC with the help of four wedges. The movement of moving tools is by virtue of actuation of a hydraulic ram.

Frequent premature tool failures seriously hamper the SFC availability and thus resulted in a suppressed plant throughput. Replacement of damaged tool assembly with a fresh one by dismantling and assembling of SMA resulted in a highly time and man-rem consuming operation.

A comprehensive study was initiated to find out the root cause of failures. Several brainstorming sessions were held with experts from the field of design, analysis, material, maintenance and manufacturing. Strain gauging experiments were conducted for tools to simulate the actual cutting conditions followed by finite element analysis. Even a destructive testing was performed to validate the design parameters.

It was concluded that the cut pieces were accumulating in to the inter blade spaces after the prolonged cutting operations. This accumulation results in the induction of high peak stresses at critical locations of cutting blade much beyond the threshold limits of fracture (Fig. 2(a) and Fig. 2(b)). Need of a modified SMA design was also emerged which can enable remote, quick and easy maintenance and tool replacement.

Fig.2(a): Anatomy of tool failure due to accumulation

Fig.2(b): Stress distribution in tool during bundle cutting

To mitigate these effects several measures were adopted:

• Upgraded the tool material from AISI D2 to AISI M2.

• Addressing of tool profile w.r.t. stress concentration point of view.

• Providing more clearances to cut pieces for falling under gravity.

• Incorporated features in SMA by which dismantling and assembling is possible by a distance with an ease and within a reasonable time span. (Appendix-A)

• Included administrative checks for monitoring and cleaning of SMA, specially the inter blade spaces.

3. Pusher Link Assembly:

Pusher link assembly, angularly hinged to a horizontally guided rigid chain, allows spent fuel bundles to push through Transfer Port Assembly (TPA) to Fuel Feed Channel (FFC) by retracting itself up to its home position. Subsequently it moves down from its home position to push the bundle's column against bundle stopper for creating further pushing reference. The frontal part of pusher link assembly is a pusher pad which has rotational degree of freedom.

When pusher link approaches to its home position, a pair of guide roller attached to it, ride on a ramp and rotate the pusher link. At the same time, pusher pad also tilts after tilting rod slides on a horizontal rail of FFC.

Above a threshold sliding friction between pusher pad and horizontal rail, the pusher link assembly stuck at its home position with the combined forces at ramp and horizontal guide of chain rollers. The sliding friction is a

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N E W S L E T T E RFounder’s Day Special Issue October 2016

function of surface conditions of tilting rod & horizontal rail of FFC and magnitude of reaction force.

The problem was studied analytically and established mathematically that control over frictional forces could resolve the issue. ( Fig. 3(a))

As a remedy, a rolling element, track roller, was introduced in tilting rod of pusher pad to interact with horizontal rail of FFC during lifting & tilting. The modification was resulted in trouble free operations of pusher link assembly since implementation. ( Fig.3(b) & Fig.3(c))

Fig.3(a): Force and reaction diagram of pusher pad

Fig.3(b): Original arrangement for pusher pad tilting

Fig.3(c): Modified arrangement for pusher pad tilting

Conclusions:

Design modifications in CTA, SMA and pusher link assembly of SFC have resulted in an increased throughput of the plant by virtue of improved availability of the shearing machine. The modified CTA design has virtually eliminated the breakdown of SFC by reasons attributed to CTA. Similarly measures for SMA also results in an enhanced tool life and a maintenance friendly experience whenever needed, if any. In the last few years, the performance of the plant has comfortably surpassed the targets mainly due to improved performance and availability of the SFC.

Acknowledgement:

The authors gratefully acknowledge the contribution of Shri S. Pradhan, Chief Superintendent, TNRP, Tarapur, Shri K. Dubey, Plant Superintendent, TRP, Tarapur and Shri F. T. Qureshi, Maintenance Superintendent, TRP, Tarapur for encouraging and motivating during conceptualization and successful implementation of these design modifications.

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N E W S L E T T E RFounder’s Day Special Issue October 2016

Appendix- A

Comparative statements between original and modified shear module design features.

Original design features Modified design features

In event of cutting tool failure, post retainers of fixed tool holder plate have to remove by accessing from inside

the shear module using Allen keys. Maintenance crew has to work in proximity of shear module & for long time period.

Orientation of post retainers has changed in such a way that it can be handled from outside

of shear module. Fixing of the same has modified in such a manner that it can be done by using an extended tool from a distance and in a short span of time.

After removing of fixed tool holder plate from guide posts, cutting tools has to be removed from fixed or moving tool holder plate depending upon which side of cutting tool has failed. Shear module can resume its operation only after replacement of failed cutting tool by new cutting tool to its respective tool holder plate.

Fixed or moving tool holder plate along with tools can be replaced with a spare depending upon which side of cutting tool has failed. The shear module can resume it operation and in parallel replacement of failed tool can be initiated.

In case of cutting tool failure in any tool holder plate, the clamp screws have to manipulate with the help of small Allen keys in the proximity of tool holder plate due to limited space available with original profile of cutting tools.

With the modified profile of cutting tools, the manipulation of clamp screws can be done by an extended tool from a distance and in a short span of time.

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