R2.30/96 Assess Residual Lifetime RBMK

The structural integrity of RBMK fuel channels and their interaction with the Graphite blocks governs the safe lifetime of the reactor core.
The objective of this project was to assess the degradation of the zirconium tubes and graphite blocks and the safety consequences of subsequent operation.
A critical degradation mechanism considered was the clearance between the zirconium tubes and graphite blocks. This gas gap clearance changes through life due to irradiation induced dimensional changes. The project considered the extent and consequences of gas gap closure, compared it with the other degradation mechanisms and applied the resultant correlations to the Smolensk 1 NPP reference plant to predict the useful lifetime of channels.
To compliment the analytical methods described above, a second objective of the project was to develop a prototype device to measure the gas gap directly.
Multiple pressure tube rupture could occur if a single pressure tube ruptures and causes a chain reaction of ruptures in adjacent channels by mechanical shock propagation through the graphite block matrix. The consequences are cavity over-pressurisation, containment failure and release of radioactivity. The final objective of this project was to design a multiple pressure tube rupture experiment to validate the claims made in RBMK safety reports.
The project therefore consisted of five complementary tasks:
Task 1: Project Management.
Task 2: Assessment of the influence of degradation mechanisms on the life time of fuel channels.
Task 3: Development of means to determine the gas gap between the pressure tube and the graphite column.
Task 4: Determination of the consequences of gas gap closure on pressure tube and graphite blocks integrity, replacement of pressure tube.
Task 5: Experimental facility for validating tube rupture scenarios and tube rupture propagation possibilities.
The Terms of Reference (Appendix A) as defined by the Beneficiary ROSENERGOATOM and TACIS were implemented through the nominated subcontractor RDIPE and the Electrogorsk Research and Development Institute. The prime contractor was AEA Technology plc.
Task 3 (development of the gas gap device) was executed with TECNATOM and Westinghouse Atom TRC (formerly ABB TRC), RDIPE, VNIIAES, SNITMASH and MIFI.

Task 2:
Task 2 confirmed that the gas gap closure is the highest priority factor for fuel channel life time. The estimated time to commencement of re-tubing Smolensk 1 is after 18 years. In the case of stage-by-stage re-tubing, channels with a low diametrical deformation rate could be left to operate up to 30 years. For such channels the most important factor will be fracture resistance.
Task 3:
Task 3 tested several devices to measure gas gap directly. Since none of the devices met the desired accuracy of + / -0.05 mm further development was recommended before plant trials should be undertaken. The measuring head developed by Westinghouse Atom TRC and extensively tested in-pile at Ignalina NPP was modified by the team to overcome interference from thick ferritic deposits anticipated inside Russian RBMK channels. The final meeting agreed that this concept should be further optimised and tested in-pile.
Task 4:
Task 4 confirmed that gas gap closure is detrimental both to safety and operability. However it is evident that operation for one year after gas gap closure can be tolerated. The service life of channels which are in operation after secondary swelling of the graphite begins, and do not reach the gas gap closure limit (a diameter of 81.5mm) will not be limited by the gas gap criterion. An approach to optimise stage-by-stage re-tubing was developed.
Task 5:
Task 5 redesigned the Multiple Tube Rupture Rig (TKR), the experimental programme and the conceptual framework for subsequent analyses. The main results were:
The full scale TKR should be supplemented by a number of smaller scale rigs to get a better understanding of the accident through more visible, cost effective and quicker sensitivity analyses.
Difficulties remain in designing instrumentation to measure channel displacement and acceleration within the full scale RBMK rig, as is measurement of the pressure wave propagation through the graphite stack.
The graphite stack should be superheated to simulate vaporisation of the liquid component of the escaping coolant. These phenomena will accentuate the pressure surge making the accident more severe.
A number of conceptual solutions to these problems were designed. Currently the TKR building superstructure is complete and some process equipment installed. TKR components have been imported from Ukraine. The small-scale models were 80% complete at completion of the present project.