Molten Salt Reactor (MSR) which use Thorium as its main fuel. Operating pressure around atmospheric pressure, fuel and coolant freezes into solids at temperatures below 400 degrees Celsius are the main safety features of this type of reactor. Commercial plant is currently in the process of pre-licensing in Canada.
CHALLENGES: readiness of regulations for generation IV reactors, readiness of the Indonesian research community to cooperate with generation IV reactor developers to build prototypes for fullfilling requirements in licensing by test/experiments.
DISCUSSION ON MSR SAFETY REVIEW AND INDONESIAN NUCLEAR REGULATION: Regulatory readiness greatly influences readiness of supplying electricity or process heat (heat for process industries) from nuclear energy with a high level of safety in Indonesia.
Technological developments have achieved safety features based on natural phenomena, no longer relying on engineered safety features.
This development is in the form of several Generation IV nuclear power plant designs. The generation IV design which is currently prepared to deploy in Indonesia, is TMSR (Thorium Molten Salt Reactor) designed by Thorcon.
According to the attachment 1 of BAPETEN No. 3 of 2011 concerning SAFETY DESIGN OF POWER REACTOR PROVISIONS. PIE – Postulated Initiating Event are events that initiate a series of events and which result in:
1. AOO – Anticipated Operational Occurrences
2. DBA – Design Base Accident
3. Severe Accidents.
Similarly, according to the IAEA GS-G-4.1 “Format and Content of the Safety Analysis Report for Nuclear Power Plants”.
It is stated in Article 3 that the Head Regulation of the BAPETEN applies to water-cooled power reactors built on land.
But based on the analysis and the results of the MSRE, accidents scenarios that have been identified are consistent with the category of events in the SAR (Safety Analysis Report) according to the IAEA GS-G-4.1.
Given the promised safety advantages in MSR, the application of regulation number BAPETEN No. 3 of 2011 needs to be carried out with a “graded approach” mechanism.
The implementation of a “graded approach” is proposed by applying “risk informed licensing”. Thus the grading process is based on:
1. Safety analysis,
2. Regulatory requirements
3. Engineering judgment.
Regarding the safety analysis, APRONUKI proposes a Review Area Focus nr 7 which contains the following review steps:
1. Purpose of analysis
2. Analysis of hazard threats
3. Deterministic safety analysis
4. Safety assessment based on stages in risk informed design:
4.1. Accident scenario analysis
4.2. Failure Modes & Effect Analysis (FMEA).
4.3, Deterministic analysis to identify failure modes, FMEA is used as the basis for developing event trees by PRA analysts.
4.4. PRA (Probabilistic Risk Assessment) to identify dominant failure modes
4.5. Addition of safety features for mitigation or prevention of dominant failure modes
4.6. Steps 4.3 and 4.4 are repeated to meet the acceptance criteria for safety system design
Regarding the analysis of hazard threats or PIEs (according to Appendix 1, BAPETEN Nr. 3 of 2011 and the IAEA GS-G-4.1), PIEs in the MSR have been identified as follows:
1. AOT-abnormal operating transients:
The efficiency of the non soluble fission products extraction by helium bubbling, which is a key-point for the core neutron balance and safety concern.
The helium circulation in the reactor core will affect the reactivity (void effect).
2. DBA – Accident Base Design:
2.1. Power increase accident or RIA (reactivity initiated accident). PIE according to NRC and EPRI in RIA’s “Technology Assessment of a Molten Salt Reactor Design (2015)” is: Unintentional control rod withdrawal.
2.2. Flow decrease accident (pump trip accident)
2.3. Fuel-salt leak accident (primary loop break accident)
Besides the 3 RIAs, APRONUKI also proposes:
a. Fuel salt channel blockage
b. Drain tank cooling mechanism design deficiency
It is necessary to review whether the following events are relevant to add as RIA:
a. Loss of coolant salt flow
b. Solar salt / Intermediate Coolant Salt Heat Exchanger failure
Another PIEs which can cause core damages according to NRC and EPRI in “Technology Assessment of a Molten Salt Reactor Design” (2015):
a. Breakage of one or more graphite tubes (e.g. due to Improper handling of graphite tubes during maintenance or inspection)
b. Improper or inadequate cooling of the drained fuel salt
c. Partially thawed piece of salt plug or solid mass obstructs piping to the drain tank. e.g. Freeze valve design deficiency
3. Severe accidents
The source term:
Tritium production and the different ways to reduce it and to trap the tritium in the primary and secondary circuits.
The MSRE remains the principal source of knowledge of fission product behavior in molten fluoride salts. There, it was observed that:
(1) noble gasses (e.g., xenon, krypton) will bubble out freely in to an off-gassing system,
(2) many fission products (including caesium) form stable fluorides and are permanently retained in the salt, and
(3) certain noble metals and tellurium will not form fluorides nor remain in solution, but rather plate out on the surfaces of the primary circuit.
MSR has a Small Accident Source Term due to:
• Reduced radionuclide source term
• Low pressure
• Off-gas system selectively removes volatile components
• Salt chemistry limits release of radionuclides under reactor accident conditions
− Low iodine release potential
− Low cesium release potential
MSR has potential for processing accidents:
Off-Gas System
• Significant radionuclide inventory
− Xenon (with high decay heat)
− Krypton
− Other
The krypton and xenon fission product gases have a very low solubility in the fuel salt and will naturally bubble out.